UNLV Retrospective Theses & Dissertations

1-1-1995

Determination of soluble mineral content in Las Vegas soils Determination of soluble mineral content in Las Vegas soils

Mark Alan Leonard University of Nevada, Las Vegas

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313/761-4700 800/521-0600

DETERMINATION OF SOLUBLE MINERAL CONTENT

IN LAS VEGAS SOILS

by

Mark Alan Leonard

A thesis submitted in partial fulfillment of the requirements for the degree of

Master of Science

in

Civil and Environmental Engineering

Department of Civil and Environmental Engineering University of Nevada, Las Vegas

May 1995

UMI Number: 1374894

UMI Microform 1374894 Copyright 1995, by UMI Company. All rights reserved.

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI300 North Zeeb Road Ann Arbor, MI 48103

The Thesis of Mark Alan Leonard for the degree of Master of Science in Civil and Environmental Engineering is approved.

Chairperson, Moses KarakouzianffPh.D., P.E

4 s A u f r ________4-ZC-3S"'Examining Committee Member, Gerald Frederick, Ph.D., P.E.

4 - Z o ~ f lExamining Committee Member, Samaan Ladka D., P.E.

jraduate Facylty Representative, Brian Johnson, Ph.D.

1 .Interim Graduate Dean, Cheryl L. Bowles, Ed.D.

dT-S-fS

University of Nevada, Las Vegas May, 1995

ABSTRACT

Lower basin areas in the Las Vegas Valley contain soils with large

amounts of soluble minerals. Dissolving and leaching processes due to

water intrusion into foundation soils containing soluble minerals can cause

a volume reduction of the soil which in turn causes structural stress. Soil

improvement recommendations for soluble soils are based upon the

amount of soluble minerals of a parent soil. This study reports the results

of research to develop a correlation between the electrical conductivity and

the concentration of soluble minerals in aqueous soil extracts.

Correlations were developed by measuring the total dissolved solids and

conductance of aqueous soil extracts of water to soil ratios from 1 0 0 : 1 to

2:1. A best fit correlation curve was developed from the results that relates

the soluble mineral concentration to measured conductance of soil

extracts. An optimum water to soil ratio that ensures that all minerals are

dissolved in solution prior to measuring conductance was identified. Finally

a test procedure utilizing the best fit correlation curve is proposed.

TABLE OF CONTENTS

A BSTRACT................................................................................................. iii

LIST OF FIG U RES.............................................................................................vii

LIST OF T A B L E S ..................................................................................... ix

ACKNOWLEDGMENT......................................................................................xiii

CHAPTER 1 IN TRODUCTION ............................................................. 1

CHAPTER 2 B A C K G R O U N D ............................................................. 4Source and Accumulation of Soluble M inerals .......................... 5Soluble Mineral C onstituents....................................................... 7Factors Affecting S o lu b i l i ty ....................................................... 9

Mixed S o lv e n ts .......................................................................... 10T e m p e r a t u r e .......................................................................... 14Pressure ................................................................................ 16

Measuring the Soluble Mineral Content Of Soils . . . . 17Local Test M ethods.................................................................... 19Electrical Conductivity C o r r e l a t io n ..................................... 20

CHAPTER 3 METHOD OF STU D Y ..............................................................25Experimental A p p r o a c h ....................................................................25Sampling and Geotechnical P ro p e rtie s ........................................... 26Test P r o c e d u r e s ................................................................................28

Sample P r e p a r a t i o n ..............................................................28Determination of Soluble Metals (Cations) . . . . 29Determination of Nonmetallic Inorganic

Constituents ( A n io n s ) ........................................................31Determination of the Physical Properties of Soluble

C o n s titu en ts ..........................................................................32

CHAPTER 4 R E S U L T S ................................................................................33Soil C lassification ................................................................................33

I V

Soluble Metals (C a tio n s ) ....................................................................34Calcium - Standard Method 3 1 1 C ..................................... 34Potassium - Standard Method 322B - Flame Photometer 35 Sodium Standard Method 325B - Flame Photometer 35Metals - Standard Method 3120B Inductively Coupled

Plasma (IC P ).......................................................................... 36Metals - Standard Method 3111B - Atomic Absorption

Spectrom etry..........................................................................40Nonmetallic Inorganic Constituents ( A n io n s ) ............................... 42

Chloride - Standard Method 407A - Argentometric . . 42Sulfate - Standard Method 426C - Turbidimetric 43Alkalinity - Method 403 H2 SO4 T itra tio n ............................... 44

Physical Properties of Soluble C o n s t i t u e n t s ............................... 46pH Value - Method 423 ....................................................... 46Conductivity - Method 2510 B ................................................. 46Total Dissolved Solids at 180°C - Method 2540C 47

Summary Of R e s u l t s ..........................................................................49

CHAPTER 5 DISCUSSION OF R E SU LT S..................................................55Accuracy of Test M e t h o d s ..............................................................55

Charge N e u tra l i ty ....................................................................55Identification of Soluble Com pounds..................................................63Comparison of Specific And Equivalent Conductances 6 8

Conductivity and Soluble Mineral Content Analysis . . . 71Conductivity as Function of Total Dissolved Solids . . 72Conductivity as Function of Mineral Content as % Dry

Soil W e ig h t .......................................................................... 78Conductivity as Function of TDS and Mineral Content 82Proposed Correlation And Test M ethod........................................... 8 8

Conductivity and Total Dissolved Solids Correlation 8 8

Proposed Test M e t h o d ........................................................90Comparison to Existing Correlations..................................................92

Agricultural Handbook No. 60 - United States SalinityL a b o ra to r y ..........................................................................92

Atlas Chemical Consultants Empirical Correlation . 95

CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS . . . 98

v

A P P E N D IX ....................................................................................................... 102

B IB L IO G R A PH Y ........................................................................................... 112

V I

LIST OF FIGURES

Figure 1. Solubility of Gypsum in Aqueous Sodium Chloride Solution 11

Figure 2. Solubility of Calcite in Carbonated Water at 10°C . . . 13

Figure 3. A Model of Dissolution of Calcium Carbonate in CarbonDioxide S o lu t io n ...................................................................16

Figure 4. Total Dissolved Solids of Single Salt Solutions as Relatedto C o n d u c t iv i ty ...................................................................21

Figure 5. Concentrations of Soil Extracts as Related to Conductivity 23

Figure 6 . Specific Conductance vs. Total Dissolved Solids of 5:1 SoilE x t r a c t s ...............................................................................70

Figure 7. Total Dissolved Solids as Function of Conductivity(Soils A & L ) .........................................................................73

Figure 8 . Total Dissolved Solids as Function of Conductivity(Soils M, O & D ) ...................................................................74

Figure 9. Total Dissolved Solids as Function of Conductivity forExtract with Single Salt C om pound.................................... 76

Figure 1 0 . Soluble Mineral Content as Function of Conductivity(Soils A & L ) .........................................................................79

Figure 11. Soluble Mineral Content as Function of Conductivity(Soils M, O & D ) ...................................................................80

Figure 12. Soluble Mineral Content as a Function of Conductivity forExtract with Single Salt C om pound.................................... 81

V I 1

Figure 13. Soil A Total Dissolved Solids and Mineral Content as %Weight of Soil as Function of Conductivity............................... 83

Figure 14. Soil L Total Dissolved Solids and Mineral Content as %Weight of Soil as Function of Conductivity............................... 84

Figure 15. Soil M Total Dissolved Solids and Mineral Content as %Weight of Soil as Function of Conductivity............................... 85

Figure 16. Soil 0 Total Dissolved Solids and Mineral Content as %Weight of Soil as Function of Conductivity............................... 8 6

Figure 17. Soil D Total Dissolved Solids and Mineral Content as %Weight of Soil as Function of Conductivity............................... 87

Figure 18. Best Fit of Total Dissolved Solids as Function ofConductivity for all Extracts of Soils A, L, M and D . 89

Figure 19. Comparison of Las Vegas Valley Soils to U.S. Handbook No. 60 Approximate Average Line of Soluble Soil Extracts as Related to C o n d u c tiv ity ........................................................94

Figure 20. Comparison of Atlas Empirical Values from 5:1 Extractsand Best Fit Correlation of Various Soil Extracts . . . 97

v i i i

10

15

15

18

19

20

24

27

33

34

35

36

37

LIST OF TABLES

Solubility of Salt Compounds in Water at 20°C . . . .

Dependence of the Solubility of Sodium Chloride (NaCI) on T e m p e ra tu re ...................................................................

Dependence of the Solubility of Gypsum in Pure Water, on T e m p e ra tu re ...................................................................

Classification of Soil Solubility and Recommended Remediation A c tio n s .............................................................

Soluble Soil Classification C r i te r ia ....................................

Local Geotechnical Firms Solubility Test Procedures . .

Conversion Factors for Converting Electrical Conductivity ((imhos/cm) of Solution to Total Dissolved Solids (mg/l) .

Sample Locations and Depth of Sam pling........................

Soil Sample Index Properties and Classification . .

Calcium (Ca) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 311C - EDTA Titration . . . .

Potassium (K) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 322B - Flame Photometer

Sodium (Na) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 325B - Flame Photometer

Soil A Cation Concentrations of 5:1 Extract Test Method 3120B Inductively Coupled Plasma ( I C P ) ........................

I X

38

39

39

40

41

41

42

42

43

44

45

46

47

48

Soil L Cation Concentrations of 5:1 Extract Test Method 3120B Inductively Coupled Plasma ( I C P ) ........................

Soil M Cation Concentrations of 5:1 Extract Test Method 3120B Inductively Coupled Plasma ( I C P ) ........................

Test Blank Cation Concentrations Test Method 3120B Inductively Coupled P la sm a .................................................

Calibration Standards & Blank Test Method 3120B Inductively Coupled P la sm a .................................................

Magnesium (Mg) Composition of 5:1 Extracts APHA- AWWA Method 311B Atomic Absorption Spectrometry .

Calcium (Ca) Composition of 5:1 Extracts APHA- AWWA Method 311B Atomic Absorption Spectrometry .

Sodium (Na) Composition of 5:1 Extracts APHA- AWWA Method 311B Atomic Absorption Spectrometry .

Potassium (K) Composition of 5:1 Extracts APHA- AWWA Method 311B Atomic Absorption Spectrometry .

Chloride Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 407A - A rgen tom etric ..............................

Sulfate (S 0 4'2) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 407A - A rgentom etric.......................

Measured Alkalinity of 5:1 Extracts APHA-AWWA 15th Edition Method 403 - H2SO4 T itr a t io n ..............................

pH Values of 5:1 Extracts APHA-AWWA 15th Edition Method 423 .........................................................................

Measured Conductivity of Various Extracts APHA-AWWA 15th Edition Method 251 OB - C onductance.......................

Soil A - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

X

48

48

49

49

50

51

52

53

54

57

58

59

60

61

62

65

Soil L - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Soil M - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Soil O - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Soil D - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil A Constituents.........................................

Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil L C o n s t i tu e n t s ....................................

Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil M C o n s t i tu e n ts ....................................

Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil O C o n s t i tu e n ts ....................................

Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil D C o n s t i tu e n ts ....................................

Soil A Ratio of Cation to Anion of 5 : 1 Extracts . . .

Soil L Ratio of Cation to Anion of 5:1 Extracts . . .

Soil M Ratio of Cation to Anion of 5:1 Extracts . . .

Soil O Ratio of Cation to Anion of 5 : 1 Extracts . . .

Soil D Ratio of Cation to Anion of 2:1 Extracts . . .

Soil A, L & M Ratio of Cation to Anion of 5:1 Extracts

Soil A Soluble Minerals Composition by Gravimetric Analysis of 5:1 E x t r a c t .......................................................

x i

Table 44.

Table 45.

Table 46.

Table 47.

Table 48.

Table 49.

Table 50.

Soil L Soluble Minerals Composition by Gravimetric Analysis of 5:1 E x t r a c t ..............................................................6 6

Soil M Soluble Minerals Composition by Gravimetric Analysis of 5:1 E x t r a c t ..............................................................6 6

Soil O Soluble Minerals Composition by Gravimetric Analysis of 5 : 1 E x t r a c t ..............................................................67

Soil D Soluble Minerals Composition by Gravimetric Analysis of 5:1 E x t r a c t ..............................................................67

Calculated Equivalent and Specific Conductance . . . 69

Comparison of Measured Concentrations to U.S. Salinity Laboratory Correlation V a l u e s ..................................................93

Comparison of Measured Concentrations to Atlas Chemical Consultant Empirical Correlation Values of 5:1 Extracts . 96

xii

ACKNOWLEDGMENTS

First and foremost, I would like to thank Dr. Moses Karakouzian for his

encouragement, and guidance in the completion of this study. Also I would

like to thank him for the willingness to share his time and knowledge

throughout my graduate studies. His instruction has been invaluable in my

pursuit of academic and professional goals.

The author is indebted to Dr. Brian Johnson for his time and patience in

instructing this chemistry novice in the various testing methods used in this

study. His numerous discussions and guidance were beneficial toward the

completion of this study.

A special thanks goes to Mr. Robert Summers for the generous

contribution of his time, knowledge, reference library and laboratory

facilities. His experience in chemical analysis of Las Vegas soils was

invaluable to developing the experimental approach for this study.

Finally I would like to extend my thanks to Dr. G. Frederick and Dr. S.

Ladkany, who served on my graduate committee.

x i i i

CHAPTER 1

INTRODUCTION

Soils containing large amounts of soluble minerals have been

encountered in the Las Vegas Valley during geotechnical investigations.

Foundations sensitive to differential settlements are susceptible to the

volume reduction that occurs when the moisture content of these soils are

increased. As water migrates through the soluble soil, it leaches soluble

minerals from the soil structure, thereby increasing its voids with an

accompanying decrease in strength. The leaching or dissolution of soluble

constituents from foundation soils is promoted by excessive landscape

watering, poor surface drainage, water pipe leakages and changes to local

groundwater environment. If soils containing water-soluble minerals are

identified prior to construction, then soil improvement recommendations

can be made to prevent the potential for foundation distress due to

differential settlement. Typically, soil improvement recommendations for

soluble soils are based upon the amount of weight loss of a parent soluble

soil due to the dissolution of its soluble constituents. For geotechnical

engineering applications a simple, reliable and accurate method for

determining the soluble mineral content by weight of its parent soil is

1

2

needed. The amount of soluble minerals can be determined by measuring

the weight loss of soil after leaching minerals from a soil sample with water

or measuring the amount of soluble minerals in the resulting solution. The

latter is easily done by measuring the conductivity of the minerals in

solution with a conductivity meter. Therefore, the objective of this study is

to develop a correlation between electrical conductivity of aqueous soil

extracts from Las Vegas Valley soils and their soluble mineral content by

dry weight of the soil.

This study is presented in five sections. The first section provides

background information on the origin of soluble soils, the major soluble

minerals encountered, factors affecting the dissolution of the soluble

minerals from soils, and a review of existing methodology used to

determine soluble mineral content of soils. The second section presents

the experimental approach and the test procedures used to obtain data

necessary to develop a correlation between soluble mineral content and

conductivity. The method of study section is followed by the results

section. This section shows how the test results were computed and

presents them. Next, the discussion of the results section depicts how the

soluble metals and soluble inorganic nonmetallic constituents contribute to

the conductivity of the aqueous extract of a soluble soil. A correlation

between conductivity and total dissolved soluble solids is developed from

the test results. A test procedure to determine the total soluble minerals by

3

weight of soil is presented. To end the section, comparisons between the

results obtained from the study and results obtained from existing

correlations that utilize conductivity measurements to determine soluble

mineral content of soils are made. Finally, all observations from this study

and recommendations for further areas of study are presented in the

conclusions section.

CHAPTER 2

BACKGROUND

The dissolution of soluble minerals from soils has been responsible for

distress to many civil engineering works incurring costly investigations and

remedial work. A complete engineering assessm ent of the potential

effects of dissolution of soluble minerals upon a foundation soils requires

the geotechnical engineer to ( James, 1992):

• Determine if soluble minerals exist in the foundation soil and in what

relevant amounts.

• Analyze the chemistry of the ground water or surface water entering

the foundation soil.

• Determine the solution potential of the soluble minerals ( the m ass of

substance that will dissolve in a unit volume of the water entering the

foundation so ils)

• Quantify the solution rate of the soluble minerals ( rate of m ass loss per

unit of surface area that the water is in con tact).

4

5

• Determine the mode of dissolution of the foundation material

(Velocities of various seepage flows such as fissure, and intergranular

affect the rate of dissolution of foundation soil).

• Perform physical, chemical or mathematical models using the above

data to quantify the overall amount and rate of dissolution of the

foundation soil.

This study shall focus on the first step of the engineering assessm ent

by investigating a method to quantify the soluble mineral content of soils.

Background information discussed includes: the source and accumulation

of soluble minerals, soluble mineral constituents, factors affecting solubility

of the minerals, and existing methods used to quantify the soluble mineral

content of soils.

Source and Accumulation of Soluble Minerals

The original source, and to some extent the direct source, of soluble

minerals (salts) is from the process of chemical weathering of the primary

minerals found in the soils and exposed rocks of the earth’s crust. During

this process, which involves hydrolysis, hydration, oxidation, and

carbonation, the mineral components are gradually released and

transformed into salt solutions (Blatt et. al.,1980, Petrukhin, 1993). The

salt solutions produce secondary accumulations of salts as they move

6

through sedimentary deposits, soil, underground and surface waters. The

accumulation of salts from this source alone is not sufficient to form a

soluble soil. Soluble soils usually occur in areas where the direct source

of salts are from other upland locations and where surface flows,

concentrated channel flows and ground water flows are the primary

carriers (Richards et. al.,1954, and Rogers et, at., 1994). The

concentration of dissolved salts from this source depends upon the salt

content of the soil and geological materials with which the transport water

has been in contact. For example the concentration of soluble salts from

waters in contact with soils where the parent material consists of marine

deposits transported in earlier geological periods would not exceed 5 - 8 %

(Petrukhin, 1993). Regardless of the source, the concentration of salts of

sufficient quantity to produce a soluble soil is a result of the process of

accumulation of the salts within the soil structure that occurs after they are

deposited.

The greatest accumulation of soluble salts within a soil structure

typically occurs in regions with semi-arid and arid climates where there is a

negative moisture balance in the soil; i.e., the amount of evaporative

moisture exceeds the amount of atmospheric precipitation (Sonnenfield,

1984). In humid regions there is available rainfall to leach and transport the

salts accumulated in soil by the above processes into the groundwater

where they are ultimately transported by streams into the ocean. In arid

7

regions leaching is local and soluble salts may not be transported far. This

occurs not only due to the lack of rainfall, but because of the high

evaporation rate of arid regions that tend to concentrate salts in the soil

structure and in surface waters (Berner, 1971. and Blatt et. al.,1980).

Poor drainage in conjunction with low permeability of the soil is an

additional factor that accumulates soluble salts. Due to the low rainfall in

arid regions, surface drainage pathways may be poorly developed. The

result is extensive inland drainage basins that have no outlets to

permanent stream s (Richards, 1954 & Petrukhin, 1993). During rainfall

events the drainage of salt concentrated waters away from the higher

elevations of the basin raise the lower basin groundwater level and cause

temporary flooding. Under this condition an increased accumulation of

salts in the soil occurs due to the separation of salt crystals from the

supersaturated pore fluids as they evaporate in the higher capillary zone

created from the upward movement of the salt concentrated ground water

level and as the salts accumulate from the evaporation of the surface

water (Petrukhin, 1993). Many of the soluble soils found in the desert

playas of the Great Basin were formed this way (Richards, 1954).

Soluble Mineral Constituents

The soluble salts that accumulate in soils consist mostly of various

portions of the cations (metals): magnesium (Mg+2), calcium (Ca+2), sodium

(Na+), and anions (non metallic constituents): chloride (Cl") and sulfate

(S 0 4‘2). The constituents which ordinarily occur in minor amounts are the

cation potassium (K+) and the anions: carbonate (CO3"2), bicarbonate

(HCO3"1), nitrate (NO3 ') and phosphate (PCV3). (Jam es 1992, Richards

1954, Blatt et. al. 1980) Although mineral forms of the complex silicate

anion (SiCV4) account for the majority of naturally occurring inorganic

compounds in the earth’s crust, soluble forms are infrequently encountered

(Rogers, et.al., 1994). The soil particles adsorb and retain the cations on

their surfaces. The cation adsorption is attributed to sands and fine

grained soils such as silts and clays. Cation adsorption occurs mostly with

clay. Even though the adsorbed cations combine chemically with the soil

particles, they may also be replaced with other cations that occur in soil

solutions. Sodium, calcium and magnesium are readily exchangeable,

while other cations like potassium are readily fixed (Richards, 1954).

Magnesium and calcium are the predominate cations in normal soils and

solutions in arid regions. When excess soluble sodium accumulates in the

soil solution the sodium cations exchange with the calcium and magnesium

to become the predominate cation. Ultimately calcium and magnesium

compounds are precipitated to the surface due to evaporation. White

crusts of salts on the soil surface often mark the occurrence of this

process (Richards, 1954).

9

The predominate soluble minerals in arid and semi-arid soils can be

classified on the basis of their degree of solubility in water, as readily

soluble, moderately soluble and weakly soluble (James, 1992, Petrukhin,

1993). The readily soluble soils are generally chlorides; sodium (NaCI),

magnesium (MgCI2), calcium (CaCI2) and sulfates; sodium (Na2 S 0 4),

magnesium (MgS04) and of other elements. Of less occurrence are

sodium bicarbonate (NaHC03 ) and sodium carbonate (NaC03). The

solubility of these salts in pure water at 20°C range from 71 grams (for

Na2C 0 3) to 357 grams ( for NaCI) of the compound per liter of aqueous

solution. The moderately soluble salts are mainly in the forms of gypsum

(C aS0 4 -2H2 0), and anhydrite (C aS04). Carbonates such as calcite

(CaC03), dolomite and magnesite (CaMg(C03)2) are examples of weakly

soluble salts. Carbonate salts are virtually insoluble in water ( .0014g per

100 ml of solution) (Petrukhin, 1993). The solubility of various salts are

presented in Table 1.

Factors Affecting Solubility

In a civil engineering context, solubility is the capacity of water to

dissolve soluble minerals from soil and rock. Solubility of minerals is

affected by other dissolved salts in mixed solution, temperature, and in

some instances pressure. These factors are discussed below.

10

Table 1. Solubility of Salt Compounds in Water at 20 °C(After Lide, 1993)

Compound Formula MolecularWeight

Solubility(grams/liter)

Readily SolubleSodium chloride NaCI 58.44 357Calcium chloride CaCh 110.99 745

Magnesium chloride MgCb 95.21 542.5Sodium sulfate Na2 S 0 4 142.04 47.6

Magnesium sulfate M gS04 120.36 260Potassium sulfate k 2 s o 4 174.25 1 2 0

Sodium bicarbonate NaHCCh 84.1 69Sodium carbonate Na2C03 105.99 71

Moderately SolubleCalcium sulfate

(Anhydrite)C aS 0 4 136.14 2.09

Calcium sulfate (Gypsum)

C aS 0 4 -2H20 172.17 2.53

Weakly SolubleCalcium carbonate C aC 0 3 100.09 .0141

Mixed Solvents

Solutions containing mixtures of other dissolved salts have the greatest

effect on the solubility of minerals. Sodium chloride, NaCI, in solution

affects the solubility of many salts, while it is scarcely affected by other

salts. Of significant importance is the effect of NaCI upon the solubility of

calcium sulfate because many groundwaters in C aS 0 4 environments

contain NaCI (James, 1993). The effect of sodium chloride on the solubility

of calcium sulphate is depicted in Figure 1 ( Shternina, 1960). This figure

Calc

ium

Su

lfate

(g

ram

/lite

r)

11

8

7

6

5

425 C & 100 kPa

3

2

1

00 50 150100 200 250 300

Sodium Chloride ( gram/liter )Figure 1. Solubility of Gypsum in Aqueous

Sodium Chloride Solution (After Shternina, 1960)

12

shows that the solubility of gypsum increases to a maximum of 7.3

grams/liter in a NaCI solution. Further increases in NaCI concentration

beyond this point results in a decrease in the solubility of calcium sulfate.

Within a temperature range between 0°C (32°F) to 60°C (140°F), the

concentration of sodium chloride has a greater effect on the solubility of

calcium chloride than does an increase in temperature ( Sonnenfield,

1984).

The solubility of other salts are affected by mixed water solutions. For

instance calcium carbonate, CaC 0 3 , has a low solubility, 13 mg/l, in pure

water. In the presence of CO2 its solubility is increased. The solubility of

calcium carbonate, in the form of calcite, is shown in Figure 2 (Weyl,

1958). The effect of solubility of this mixed system has little significance in

normal geotechnical environments. Rainwater would be the primary source

of CO2 solutions. Rainwater contains approximately 2.5 mg/l of dissolved

C 0 2. The solubility of CaCCb in rainwater is about 8 mg/l. This

concentration is significantly lower than those shown in Figure 2. Other

sources of carbon dioxide solutions are from deep artesian and deep

underground water tables (James, 1993) that would not be encountered in

most civil engineering environments.

Con

cent

ratio

n of

Calc

ium

Ca

rbon

ate

(mg/

l)

13

350

300

250

200

150

100 Solubility line

0 50 100 200150 250Concentration of Carbon Dioxide (mg/l)

Figure 2. Solubility of Calcite in Carbonated Water at 10 C (After Weyl, 1959)

14

Temperature

In general, the solubility of most solid substances in a liquid solvent

increases with an increase in temperature (Hamilton, 1969) . Temperature

ranges in geotechnical environments are restricted and therefore have

limited effect on solubility (James, 1993). The maximum solubility of

sodium chloride, NaCI, in pure water at 20°C is 360 grams/liter or 6.159

moles/liter. Table 2 shows the dependence of the solubility of sodium

chloride on temperature. The solubility continues to increase with an

increase in temperature. In the range between 0°C (32°F) to 50°C (122°F),

a practical range for most geotechnical environments, the solubility of NaCI

is increased only 4%. For the range between 50°C (32°F) and 100°C

(212°F), temperatures outside of normal geotechnical environments,

solubility is increased only by another 8%. In some cases temperature has

a limiting effect on the solubility of some minerals. For instance, the

solubility of gypsum in pure water at 20°C is 2.001 grams/liter and reaches

a maximum solubility of 42°C (107°F) where an increase in temperature

results in a decrease in solubility as low as 2.047 grams/liter at 100°C

(212°F). Again, within the range of normal temperatures likely to be

encountered in geotechnical environments, the change in solubility due to

an increase temperature is small. Table 3 shows the effect of temperature

on the solubility of gypsum and anhydrite.

15

Table 2. Dependence of the Solubility of Sodium Chloride (NaCI) on Temperature ( After James, 1993)

Temp°C

erature° F

Solubility(grams/liter)

0 32 35610 50 35820 68 36030 86 36340 104 36650 122 37060 140 37370 158 37880 176 38490 194 390100 212 398

Table 3. Dependence of the Solubility of Gypsum in Pure Water, on Temperature (After Robie, 1978)

Temperature Solubility°C 0 F as CaS04 (g/l) as CaS04-2H20 (g/l)0 32 1.759 2.22410 50 1.928 2.43820 68 2.001 2.53130 86 2.09 2.64340 104 — —

50 122 2.097 2.65260 140 2.047 2.58970 158 1.974 2.49680 176 — —

90 194 — —

100 212 1.619 2.047

16

Pressure

Pressure does not affect the solubility of solids in two- phase systems,

one solid and water for most engineering environments (James 1993).

The solubility of gypsum is scarcely increased at pressures exceeding one

hundred bars ( Manikhin & Krykow, 1968). An exception is on three phase

systems with a gas as one phase. For example solubility is affected by

increased carbon dioxide concentrations at higher pressures in calcium

carbonate - carbon dioxide - water systems. A dissolution model of

calcium carbonate in a carbon dioxide system is shown in Figure 3.

H20 + C 0 2 (Water and Carbon Dioxide)

▼

-t* (Acidic Condition)

C3CO3

Solid Calcium Carbonate

Figure 3. A Model of Dissolution Of Calcium Carbonate in Carbon Dioxide Solution ( After Loewenthal, 1976)

Calcium carbonate in pure water has low solubility because it has

predominately covalent chemical bonds which have little affinity for water.

Ca8' C 0 3s+-----------► Ca++ + 2HC03‘

Polarized Molecule; Calcium & BicarbonateWeakened Bond ions in solution

17

The introduction of carbon dioxide creates a slightly acidic solution with

free hydrogen ions that increase the electrical charge of the system. The

C aC 03 molecules become slightly polarized electrically. The hydrogen ions

attach themselves to the carbonate parts of the molecules and split off as

bicarbonate ions balanced electrically by the creation of calcium ions

(Loewenthal & Marais 1976).

Measuring Soluble Mineral Content of Soils

There are many field methods and laboratory tests used to identify the

various soluble minerals. Shearman (1979) proposed a field test to identify

the presence of gypsum that involved heating soil and observing whether

a white powdery product develops as a result of the dehydration of

gypsum. Samples dissolved in distilled water after filtration can be

analyzed by a variety of laboratory tests such as titration with silver nitrate

to determine chloride, and atomic absorption spectroscopy to identify

sodium and potassium. These methods have limited use since multiple test

methods would be required to identify and quantify the numerous minerals

that normally accumulate in the same soil structure. Furthermore,

geotechnical engineers, in order to gauge the settlement properties of

foundation soils due to the dissolution of minerals in water having access

to them, are more concerned with the amount of soluble minerals present

in the soil than the type found. For instance, local geotechnical firms

18

classify the severity of the potential dissolution of minerals from the soil

and base the recommended remediation on the amount of soluble minerals

present in a soil. Local classifications based on the amount of soluble

minerals expressed as a percentage of the dry weight of the foundation

soil as determined by the filter method as discussed later, are shown in

Table 4. Similar classifications exist for soils in other geographic locations.

Petrukhin (1993) provided classification of soil solubility for soils in Central

Asia, Kazakhstan and the Ukraine. According to the classification

proposed by Petrukhin, various soil types must contain a minimum

content of soluble minerals as a percentage of dry soil weight in order to

be classified as soluble ( refer to Table 5). Generally, all classifications

rely upon simple and inexpensive test methods to quantify the soluble

mineral content of soils. Procedures used to quantify the soluble mineral

content of soils are discussed below.

Table 4. Classification of Soil Solubility and Recommended Remediation Actions ( Cibor ,1983)

Solubility % Dry Weight of Soil

Classification Recommendation

0-1 Negligible No action required1-2 Low Mix w/ import in 1:1 ratio2-4 Medium Mix w/ import in 1:2 ratio4-6 High Mix w/ import in 1:3 ratio>6 Critical Remove from site

19

Table 5. Soluble Soil Classification Criteria (Petruhkin, 1993)

Soil Type Minimum Salt Content Required to Classify

Soil as Soluble (% Dry Weight of Soil)

Detrital With Sand Filler < 40% 2

Detrital With Clayey Filler < 30% 2

Detrital With Sand Filler > 30% .5

Sandy Soil .5

Sandy Loam & Loam 5

Clays 10

Local Test Methods

Local geotechnical firms have developed their own rudimentary test

procedures to determine the amount soluble minerals present in soil. The

test methods vary between firms (See Appendix A for specific procedures),

however the basic procedures consist of inundating an oven dried soil

sample in a filter apparatus with water and then redrying the sample to

determine the weight loss due to the dissolution of soluble minerals. The

soluble mineral content is expressed as a percentage of the original dry

soil weight. The test procedures vary between one another by the

minimum sample weight, the volume, temperature and type of water used

as indicated in Table 6.

20

Table 6. Local Geotechnical Firms Solubility Test Procedures

Firm Sample

Weight

(grams)

Water: Soil

Ratio

(ml:grams)

Water

Type

Water

Temperature

(°F)

Oven Drying

Temperature

(°F)

A 150 12.5 A Tap Varied 140

B 200 19:1 Distilled 68 115

C 300 6:1 Deionized 68 140

D 150-200 25:1 Distilled 68-75 140

Electrical Conductivity Correlation

The United States Salinity Laboratory has used electrical conductivity

measurements of water extracts of soils to estimate the soluble salt

content of soils. Electrical conductance is expressed in mhos/cm. This

measurement is convenient since electrical conductivity increases with

increases of soluble salt content of soils. The relationship between the

electrical conductivity and the salt content of various single-salt solutions

produced in the laboratory is shown in Figure 4. The concentrations shown

in Figure 4 are expressed in grams of salt per 100 gram of water

(gm/100ml or 100mg/l). The curves for the chloride salts ( MgCI2, CaCI,

and NaCI) and sodium sulphate (Na2 S 0 4 ) almost coincide, indicating

Tota

l Di

ssol

ved

Solid

s ( g

ram

per

100

gram

w

ater

)

21

CaCl

MgCI2

CaSO,

NaHCO

NaCI

0.010.1 1 10 40

Conductivity (millimhos/cm)

Figure 4. Total Dissolved Solids of Single-Salt Solutions as Related to Conductivity ( After Richards, 1960)

22

similar conductivity at equivalent concentrations. The moderately soluble

salts, MgS04, C aS 0 4 and NaHC0 3 have lower conductivities at equivalent

concentrations.

The National Salinity Lab developed a correlation between conductivity

and total salt concentration, Figure 5, for soils from widely separated areas

in the western United States. The concentration range for the soluble salts

was higher than the single salt solutions shown in Figure 4. The

concentrations were obtained from direct laboratory measurements of the

soluble salt content of saturation extracts of the soils. The soluble mineral

content for other soils can be estimated from Figure 5. A soil extract is

obtained from a mixture of water to soil at a 5:1 ratio. The extract can be

filtered with the use of a vacuum. The conductivity of the remaining

aqueous extract is measured with a conductivity meter. The conductivity

is used to estimate the soluble salt content from the curve in Figure 5.

A local chemical consulting firm, Atlas Chemical Testing Laboratories,

Inc., has developed empirical conversion factors correlating the electrical

conductivities (jumhos/cm) of aqueous extracts from local soil samples to

the concentration of the soluble minerals expressed in milligrams per liter

(mg/l). The correlation was based on a large data base developed from

chemical analyses performed on soils for geotechnical firms executing site

investigations of local projects. The factors were determined

experimentally by comparing the measured electrical conductivities of

Solu

ble

Min

eral

Con

cent

ratio

n (m

eq/l)

23

1000Approximate Average Line

100

100 1000 10000 100000Conductivity ( micromhos/cm)

Figure 5. Concentrations of Soil Extracts as Related to Conductivity ( After Richards, 1960)

24

numerous 5:1 (water to soil) filtered extracts of soil to their measured total

dissolved solids (mg/l). The American Public Health Association and

American Water Works Association (APHA-AWWA) Method 2540 C and

APHA-AWWA 205 Standard Methods were used to determine

concentrations of dissolved solids in solution and electrical conductivity

respectively. The conductivity of the aqueous soil extract is measured

directly with a conductivity meter. The total dissolved solids are

determined by evaporating the water from the extract in an 180°C oven

and weighing the residue. Table 7 gives the electrical conductivity ranges

and the corresponding conversion factors. The soluble mineral

concentration is obtained by the following relationship:

Total Dissolved Solids = Conductivity (^mhos / cm) x Conversion Factor

Table 7. Conversion Factors for Converting Electrical Conductivity (pmhos/cm) of Solution to Total Dissolved Solids (mg/l)

(Atlas Chemical Consultants)

Electrical Conductivity

((.imhos/cm)

Conversion Factor

0 -2 0 0 0.55200 - 400 0.60400 - 700 0.65700-1000 0.701000-1500 0.751500-2000 0.802000 - 2500 0.852500 - 3000 0.90

>3000 0.95

CHAPTER 3

METHOD OF STUDY

The method of study section covers three areas: experimental

approach, determination of basic geotechnical properties of the soils

studied and test procedures. The test procedures section contains four

sub-sections: sample preparation, the determination of soluble metals

(cations), the determination of soluble inorganic nonmetallic constituents

(anions) and the physical examination of the soluble minerals.

Experimental Approach

The objective of this study is to investigate the correlation between

electrical conductivity of aqueous soil extracts and their soluble mineral

content. Methods 2540C and 205 (Standard Methods of Analysis for

Water and Wastewater, 15th Edition, APHA-AWWA) were performed on

filtered 5:1 (water, 100 ml to soil , 20 gram) aqueous extracts for five soils

studied to identify the individual anions and cations concentrations. From

these tests the observation of the contribution of each ion to the overall

conductivity of the soil solution is made. Additionally, from gravimetric

25

26

analysis of the ion content a determination of the soluble salt compounds

are made. The identification of the compounds allowed for further

qualification of the correlations developed between conductivity and

soluble mineral content.

The data used to develop a correlation between conductivity and

soluble mineral content are obtained from physical measurements of the

aqueous soil extracts. They include the direct measurement of

conductivity and the total dissolved minerals in the soil extract. To model

the effects of the concentration upon conductivity and total dissolved

mineral content, physical examinations of various soil to water extracts

were made. For each of the five soils studied, the water to soil extracts,

from 2:1 to 100:1, were tested to determine the total dissolved solids at

180°C.

Sampling and Geotechnical Properties

Five soluble soils from the Las Vegas Valley were tested. Three soils

designated as soils A, L, and M were collected from projects where local

engineering/geotechnical firms identified them as containing soluble

minerals during site investigations. Two soil samples, designated soil D

and O, were obtained from random sampling in order to have a soil

sample from each of the major geographical areas in the Las Vegas Valley

where soils with soluble minerals occur. Table 8 shows the general

27

information of soils collected. All samples were disturbed samples

excavated by hand except for soils L and M, which were collected from

spoil piles deposited by mechanical excavators.

Table 8. Sample Locations and Depth of Sampling

Sample Location Geographical Area Sample depth

Soil A Jimmy Durante &

Stephanie

East 1 -3

Soil L Lake Las Vegas East 4

Soil M Paradise & Russell Central 5

SoilO Lamb & Cheyenne North 1-3

Soil D Windmill & Green

Valley Parkway

South 1

The field samples were prepared according to ASTM C 702-87 ,

Reducing Field Samples of Aggregate to Testing Size prior to performing

engineering index test or preparing samples for soil extracts. Index test

methods performed were:

(1) Liquid Limit, Plastic Limit, and Plasticity Index of Soils - ASTM D 4318

(2) Amount of Material in Soils Finer than No. 200 Sieve - ASTM D1140

(3) Test Method for Particle Size Analysis of Soils - ASTM D422

28

(4) Laboratory Determination Of Water Content of Soil and Rock - ASTM

D2216

Soils were classified according to ASTM D2478 -93, Classification of Soils

for Engineering Purposes ( Unified Soil Classification System).

Test Procedures

Sample Preparation

All tests performed utilized extracts from soil solutions. All soil

specimens used to prepare the extracts were oven-dried at 60°C and then

sieved through No. 10 sieve. The soil solutions were prepared by mixing

soil and distilled water at specified water to soil ratios (ml:grams) in plastic

sample flasks using a mechanical shaker at low speed for fifteen minutes.

The specimens were vacuum filtered through a No. 1 Whatman paper filter

and then through a .45 micron glass filter in a Buchner funnel and a

vacuum flask. The extracts were then placed in 150 ml plastic sample

containers. Three 5:1 (100ml:20g) extracts were prepared for soils A, L,

and M to measure the repeatability of the tests. Single 5:1 (100ml:20g)

extracts were prepared for soils O and D. All cation and anion tests were

performed on the same extract for these samples unless otherwise noted.

Upon completion of cation testing the remainder of the extract was

preserved with 5 milliliters of ultra pure nitric acid (HN03) to keep the

29

dissolved salts in solution. A 50 ml test blank of the distilled water used in

preparing the extracts was also acidified. The cation and anion test

procedures consumed a significant part of the extract; therefore separate

extracts were prepared for measuring the total dissolved solids at 180°C.

Determination of Soluble Metals (Cations)

Four tests were performed on the extracts to determine the

concentrations of the predominate soluble metals found in soils;

magnesium, calcium, sodium and potassium. The standard test methods

performed were:

(1) 325B Sodium Flame Emission Photometric

(2) 322B Potassium Flame Emission Photometric

(3) Calcium 311C - EDTA Titration

(4) 3120 Inductively Coupled Plasma (ICP)

(5) 3111B - Atomic Absorption Spectrometry

Methods 325B, 322B and 311C were performed on separately

prepared 5:1 extracts for all soil samples to provide a preliminary

measurement of the sodium, potassium and calcium concentrations in

order to prepare standards of similar concentrations used in method 3120

and 3111B.

A Perkin-Elmer Plasma 40 Emission Spectrometer was used to

determine the concentration of the four cations by Standard Test Method

30

3120B. Mixed calibration standards of the four metals were prepared from

stock solutions. The low end standard concentrations were 10 mg/l of Mg

and Ca; and 20 mg/l of Na and K. The high end standards concentration

were 60 mg/l and 100 mg/l of Mg and Ca; and Na and K respectively. A

separate Na standard of 1000 mg/l concentration was prepared based on

the results of the Na concentration determined by the flame photometric

method. Soil extract samples A, L, and M were tested. Two replicates,

used to check repeatability of results, were run for each of the three 5:1

extracts prepared for soils A, L and M. The test blank cation

concentrations were also measured by this method. Extracts for soils O

and D were not tested by this method. Direct measurements of the cation

concentrations in mg/l were obtained from this method.

The metal concentrations in mg/l were obtained from a Perkin Elmer

Model 5000 flame atomic absorption spectrometer, using Standard Test

Method 3111B to determine the cation concentrations for Soils A, L, M, O

and D. Four mixed solution calibration standards of the four cations

tested for were prepared from stock solutions. The standards are shown

in tabular form in the appendix. Based on the concentrations obtained by

the ICP and flame photometer test, the samples were diluted in order to

fall within the range of the test standard concentrations. Direct

measurements of the cation concentrations in mg/l were obtained from this

method.

31

Determination of Nonmetallic inorganic Constituents (Anions)

Four separate tests were performed on the extracts to determine the

concentrations of the predominate soluble nonmetallic constituents found

in soils; chloride, carbonate, bicarbonate, and sulphate. The standard test

methods performed were:

(1) Chloride 407A - Argentometric

(2) Alkalinity 403 - H2 SO 4 Titration

(3) Sulfate 426C - Turbidimetric

(4) 411 OB Ion Chromatography with Chemical Suppression of Eluant

Conductivity

Method 411 OB was performed on separately prepared 5:1 extracts for

Soils A, L and M. This method was utilized to determine if anions other

than chloride, sulphate, bicarbonate and carbonate were present in the

extracts. A suppressed anion chromatograph, with a 200 micro-liter sample

loop was used. The eluant flow rate was 2 ml/min. The eluant was 1.8

millimole/liter of Na2 C0 3 and 1.7 millimole/liter of NaHC03 in ultra pure

water (resistivity > 1 8 |nfi-cm). Chromatographs were obtained on a strip

chart recorder, with peak heights measured manually.

A standard curve with four different concentrations: Cl (2 mg/l) , NO3

(3mg/l), P 0 4 (4mg/l), and SO4 (4mg/l) was run for quantitative purposes. A

1000 to 1 dilution of each extract for Soil A, L and M were run.

32

Comparable peaks for NO3 and PO4 were insignificant, indicating

negligible amounts of these anions.

Determination of the Physical Properties of Soluble Constituents

Three separate test were performed on the extracts to measure the

conductivity, pH and the total dissolved solids of the extracts. The

standard test methods performed were:

(1) Conductivity - Method 2510 B

(2) pH Value - Method 423

(3) Total Dissolved Solids Dried at 180°C - Method 2540 C

CHAPTER 4

RESULTS

SOIL CLASSIFICATION

The engineering soil classifications obtained from the index tests for

each soil is shown in Table 9. The samples represent a variety of soil

classifications. Half of the fine grained inorganic soils and a good portion

of the sand classifications are represented.

Table 9. Soil Sample -Index Properties and Classification

Soil Plasticity

Index

In-situ Moisture

Content (%)

USCS Classification

A 7 3.2 CL-ML with sand

L 2 2.6 ML

M Non Plastic 13.7 SM

0 25 4 CL

D Non Plastic 10 SW-SM

33

34

Soluble Metals (Cations)

Calcium - Standard Method 311C

The results of test method 311C is presented in Table 10. Calcium

concentrations were calculated from the amount of .01 Molar solution of

EDTA that was titrated in 10ml of the 5:1 extract with ten drops KOH and

blue indicator from the following equation:

« . . , .... . (ml 0.01 Molar EDTA)(400.8)Calcium (mg/liter) = —--ml of sample

The calcium composition as a percentage of the original soil extract

concentration is calculated by the following equation:

Calcium (%) = C aC onc^ration_m g / 1 X V o lu m a Wa te r (L) (10Q%)Weight sample (g) X 1000 m g/g v '

Table 10. Calcium (Ca) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 311C- EDTA Titration

SAMPLE

(#)

EDTA

(ml)

CONCEN

(mg/l)

CALCIUM COMPOSITION*

(%)SOIL A 10 14.4 577.2 0.29

SOILL 11 13.3 533.1 0.27

SOILM 12 13.8 553.1 0.28

SOIL O 38 12.4 496.0 0.248

SOIL D 13 14.2 568.0 0.114

* Ca compositions measured for the 5:1 extract may represent a solution saturated with Ca and should not be considered the total calcium content of the soil.

35

Potassium - Standard Method 322B - Flame Photometer

The results of test m ethod 322B is presented in Table 11. Potassium

concentrations in meq/l were obtained from a direct reading flame

photometer. The concentration in mg/l is calculated by the relationship:

Potassium (mg/liter) = m eq/l x 39.1 m g/ meq (eguiv.weight of K)

The potassium composition as a percentage of the original soil extract

concentration is calculated by the following equation:

0 . ,n/ x K Concentration (mg / 1) X Volume Water (L) xPotassium (%) = .................................... ...’------------------------- ^(100%)Weight sample (g) X 1000 mg /g v ’

Table 11. Potassium (K) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 322B - Flame Photometer

SAMPLE

(#)

CONCEN

(meq/l)

CONCEN

(mg/l)

POTASSIUM COMPOSITION

(%)

SOIL A 10 10.2 397.8 0.199

SOILL 11 1.7 66.47 0.033

SOILM 12 0.3 11.7 0.0059

SOIL O 38 1.5 58.65 0.029

SOILD 13 2.0 78.20 0.0156

Sodium - Standard Method 325B - Flame Photometer

The results of test method 325B is presented in Table 12. Sodium

concentrations in meq/l were obtained from a direct reading flame

photometer. The concentration in mg/l is calculated by the relationship:

36

Sodium (mg/liter) = meq/ l x 23 mg/meq(eguiv.weight of Na)

The potassium composition as a percentage of the original soil extract

concentration is calculated by the following equation:

Sodium (%) = ^ Concentration |m 9_/1) X Vol^ .e ..Water..(L.)(10oo/„)Weight sample (g) X 1000 mg /g ’

Table 12. Sodium (Na) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 325B - Flame Photometer

SAMPLE

(#)

CONCEN

(meq/l)

CONCEN

(mg/l)

SODIUM COMPOSITION

(%)

SOIL A 10 70 1610 0.805

SOILL 11 78 1794 0.897

SOILM 12 2.51 57.5 0.029

SOIL 0 38 16.7 384.1 0.192

SOILD 13 3.6 82.80 0.0166

Metals - Standard Method 3120B Inductively Coupled Plasma (ICP)

The Plasma 40 Emission Spectrometer computer control provides a

direct read out of the concentrations measured for each metal selected for

analysis. The average, standard deviation and percentage of standard

deviation for the measured concentration for each sample replicate. Tables

13-15 list the concentrations of the four soluble metals measured for soil

samples A, L and M. Table 16 list the metal concentrations for the test

37

blank. Table 17 shows the relative emission intensity in arbitrary units for

the calibration standards and blank.

Table 13. Soil A Cation Concentrations of 5:1 Extract Test Method - 3120B Inductively Coupled Plasma (ICP)

CONCENTRATIONS(mg/l)

Mg Ca Na KSAMPLE

#Replicate

1 | 2Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

1 94 84 633 612 2066 2141 562 542Average 89 622.5 2103.5 552Stan Dev 7.07 14.85 53.03 14.14

% Stan Dev 7.95 2.39 2.52 2.56

2 110 103 810 789 2602 2509 557 602Average 106.5 799.5 2555.5 579.5Stan Dev 4.95 14.85 65.76 31.82

% Stan Dev 4.65 1.86 2.57 5.49

3 90 100 740 760 2292 2319 533 589Average 95 750 2305.5 561Stan Dev 7.07 14.14 19.09 39.60

% Stan Dev 7.44 1.89 0.83 7.06

38

Table 14. Soil L Cation Concentrations of 5:1 Extract Test Method - 3120B Inductively Coupled Plasma (ICP)

CONCENTRATIONS(mg/l)

Mg Ca Na KSAMPLE

#Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

Replicate

1 I 2

4 267 321 887 877 2385 2648 146 133Average 294 882 2516.5 139.5Stan Dev 38.18 7.07 185.97 9.19

% Stan Dev 12.99 0.80 7.39 6.59

5 280 187 853 449 2564 1758 156 127Average 233.5 651 2161 141.5Stan Dev 65.76 285.67 569.93 20.51

% Stan Dev 28.16 43.88 26.37 14.49

6 314 294 873 837 2831 2524 132 113Average 304 855 2677.5 122.5Stan Dev 14.14 25.46 217.08 13.44

% Stan Dev 4.65 2.98 8.11 10.97

39

Table 15. Soil M Cation Concentrations of 5:1 Extract Test Method - 3120B Inductively Coupled Plasma (ICP)

CONCENTRATIONS(mg/l)

Mg Ca Na KSAMPLE

#Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

7 29 25 671 630 127 117 31 22Average 27 650.5 122 26.5Stan Dev 2.83 28.99 7.07 6.36

% Stan Dev 10.48 4.46 5.80 24.01

8 30 26 944 733 139 139 20 26Average 28 838.5 139 23Stan Dev 2.83 149.20 0.00 4.24

% Stan Dev 10.10 17.79 0.00 18.45

9 28 26 864 934 185 221 24 34Average 27 899 203 29Stan Dev 1.41 49.50 25.46 7.07

% Stan Dev 5.24 5.51 12.54 24.38

Table 16. Test Blank Cation Concentrations Test Method - 3120B Inductively Coupled Plasma (ICP)

CONCENTRATIONS(mg/l)

Mg Ca Na KSAMPLE Replicate

1 | 2Replicate

1 | 2Replicate 1 | 2

Replicate 1 | 2

Blank 0 0 1 1 87 83 9 8Average 0.0 1.0 83.0 9.0Stan Dev 0.0 0.3 5.5 0.4

% Stan Dev 6.0 34.9 6.6 4.1

40

Table 17. Calibration Standards & Blank (Test Method - 3120B Inductively Coupled Plasma ICP)

Relative Emission Intensity (arbitrary units)

Mg Ca Na KSAMPLE

#Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

Replicate 1 | 2

STANDARD #1 29415 28912 6975 7385 800 814 3742 3654Average 29163.5 7180.0 807.0 3698.0Stan Dev 355.7 289.9 9.9 62.2

% Stan Dev 1.2 4.0 1.2 1.7Concentration 10 mg/l 10 mg/l 20 mg/l 20 mg/l

STANDARD #2 17445 15206 37587 37723 1059 1032 9567 9287Average 163259.0 37655.0 1045.0 9427.0Stan Dev 15827.9 96.2 19.8 198.0

% Stan Dev 9.7 0.3 1.8 2.1Concentration 60 mg/l 60 mg/l 100 mg/l 100 mg/l

STANDARD #3 none none 4276 4159 noneAverage 4217.5Stan Dev 82.7

% Stan Dev 2.0Concentration 1000 mg/l

BLANK 835 674 1105 903 741 694 2268 2208Average 754.5 1004.0 717.5 2238.0Stan Dev 113.8 142.8 33.2 42.4

% Stan Dev 15.1 14.2 4.6 1.9Concentration 0 mg/l 0 mg/l 0 mg/l 0 mg/l

Metals - Standard Method 3111B - Atomic Absorption Spectrometry

The results of test method 3111B is presented in Tables 18 - 21. The

metal concentrations in mg/l were obtained from a direct reading Perkin

Elmer HGA 500 direct air acetylene spectrometer. The samples were

41

diluted to insure the measured concentrations were within the linear range

of the calibration standards used. The metal concentrations are obtained

from the relationship:

Metal (mg/liter) = mg/ lx Dilution Factor of sample

The metal composition as a percentage of the original soil extract

concentration is calculated by the following equation:

Metal (%) = Metal Concentration (mg / 1) X Volume Water (L) ^ Q0%xWeight sample (g) X 1000 mg I g

Table 18. Magnesium (M g) Composition of 5:1 Extracts APHA-AWWA Method 3111B Atomic Absorption Spectrometry

SAMPLE

#

DILUTECONC(mg/l)

DILUTION

FACTOR

CONC

(mg/l)

MAGNESIUMCOMPOSITION

(%)SOIL A 1 5.3 20 106 .053SOILL 4 3.2 100 320 .16SOILM 7 3.0 10 30 .015SOIL 0 23 2.3 20 46 .023SOIL D 15 1.8 20 36 .0072

Table 19. Calcium (Ca) Composition of 5:1 Extracts APHA-AWWA Method 3111B Atomic Absorption Spectrometry

SAMPLE

#

DILUTECONC(mg/l)

DILUTION

FACTOR

CONC

(mg/l)

CALCIUMCOMPOSITION*

(%)SOIL A 1 2.8 200 560 .28SOILL 4 2.8 200 560 .28SOILM 7 2.7 200 540 .27SOIL O 23 3.0 200 600 .30SOIL D 15 2.4 200 480 .096

* Ca compositions measured for the 5:1 extract may represent a solution saturated with Ca and should not be considered the total calcium content of the soil.

42

Table 20. Sodium (Na) Composition of 5:1 Extracts APHA-AWWA Method 3111B Atomic Absorption Spectrometry

SAMPLE

#

DILUTECONC(mg/l)

DILUTION

FACTOR

CONC

(mg/l)

SODIUMCOMPOSITION

(%)SOIL A 1 1.1 1000 1100 .55SOILL 4 1.9 1000 1900 .95SOILM 7 .3 100 30 .015SOIL 0 23 .7 500 350 .175SOILD 15 .4 200 80 .016

Table 21. Potassium (K ) Composition of 5:1 Extracts APHA-AWWA Method 3111B Atomic Absorption Spectrometry

SAMPLE

#

DILUTECONC(mg/l)

DILUTION

FACTOR

CONC

(mg/l)

POTASSIUMCOMPOSITION

(%)SOIL A 1 2.3 200 460 .23SOIL L 4 1.7 50 85 .0425SOILM 7 .5 20 10 .005SOIL O 23 .8 50 40 .02SOILD 15 .7 50 35 .007

Nonmetallic Inorganic Constituents (Anions)

Chloride - Standard Method 407A - Argentometric

The results of test method 407A is presented in Table 22. Chloride

concentrations were calculated from the amount of .0141 Normal solution

of AgN0 3 that was titrated in 10ml of the 5:1 extract with 1 ml of K2 Cr0 4

indicator from the following equation:

43

Chloride (mg/liter) = (" 'A gN O .) ,(0.0141 N AgNO.EDTA)(35450)ml of sample

The chloride composition as a percentage of the original soil extract

concentration is calculated by the following equation:

Chloride (%1 = C C oncentration (mg /l) x Volume W ater (L)(100%)Weight sample (g) x 1000 mg /g v

Table 22. Chloride (Cl) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 407A - Argentometric

SAMPLE#

AgN03(ml)

CONCEN(mg/l)

CHLORIDE COMPOSITION

(%)

SOIL A1 43 2149.33 1.0742 42.5 2124.34 1.0623 40.8 2039.37 1.019

SOIL L4 52 2599.19 1.3005 53.1 2654.18 1.3276 55.7 2789.14 1.392

SOILM7 1.1 54.98 .02758 1.0 49.98 .02499 .9 44.99 .0226

SOIL 0 38 2.9 144.96 .0725SOIL D 13 1.1 54.98 .0110

Sulfate- Standard Method 426C - Turbidimetric

The results of test method 436C are presented in Table 23. Sulfate

concentrations in mg/l were obtained from a comparing the direct reading

of the dilute extract sample with barium chloride from the photometer with

a standard curve. The concentration in mg/l is calculated by the

relationship:

44

Sulfate (mg/liter) = mg/ lx Dilution Factor of sample

The sulfate composition as a percentage of the original soil extract

concentration is calculated by the following equation:

0 . .... SO / 2 Concentration (mg/l) X Volume Water (L)Sulfate (%) = — ------------------------—-— -------------------------—Weight sample (g) X 1000 mg / g

Table 23. Sulfate (S O /2) Composition of 5:1 Extracts APHA-AWWA 15th Edition Method 426C - Turbidimetric

SAMPLE

#

CONC

(mg/l)

DILUTION

FACTOR

CONC

(mg/l)

SULFATECOMPOSITION

(%)

SOIL A1 12.5 121 1512.5 .7562 13.0 121 1573.0 .7863 12.5 121 1512.5 .756

SOILL4 23 121 2783.0 1.395 23.5 121 2843.5 1.426 24.5 121 2964.5 1.48

SOILM7 55.6 25 1390.0 .6958 54.8 25 1370.0 .6859 57.7 25 1442.5 .721

SOIL O 38 17.0 121 2057.0 1.029SOILD 13 14.5 121 1754.5 .351

Alkalinity - Method 403 H2S 0 4 Titration

The results of test method 403 are presented in Table 24. The total

alkalinity is determined from titration of H2 S 0 4 for two end points; first with

phenolphthalein indicator and then with a bromocresol green - methyl red

indicator in 10 ml extract sample. The volume of titrated sulfuric acid is

recorded for each endpoint. The total alkalinity is equal to the carbonate

45

and bicarbonate concentrations. The proportion of the concentrations to

the total alkalinity are determined from an alkalinity relationship table. The

concentrations in mg/l are calculated by the relationship:

C aC 0 3 (mg/liter) = ml H2 S 0 4 x .02 N H2 S 0 4 x 50,000ml sample

The carbonate or bicarbonate composition as a percentage of the original

soil extract concentration is calculated by the following equation:

. _ Concentration (mg / 1) X Volume Water (L) , >.Weight sample (g) X 1000 mg / g

Table 24. Measured Alkalinity of 5:1 Extracts APHA-AWWA 15th Edition Method 403 - H2S 0 4 Titration

SAMPLE#

CARBONATE ( CO3* ) BICARBONATE ( HCO3 '1)CONCEN.

(mg/l)COMPOSITION

%CONCEN.

(mg/l)COMPOSITION

%

SOIL A1 20.0 0.01 20.0 0.012 20.0 0.01 20.0 0.013 20.0 0.01 20.0 0.01

SOIL L4 20.0 0.01 20.0 0.015 20.0 0.01 20.0 0.016 20.0 0.01 20.0 0.01

SOILM7 0.00 0.00 30.0 0.0158 0.00 0.00 40.0 0.0209 0.00 0.00 30.0 0.015

SOIL O 38 0.00 0.00 20.0 0.01SOILD 13 0.00 0.00 40.0 0.01

46

Physical Properties of Soluble Constituents

pH Value - Method 423

The pH value for each 5:1 soil extract was measured with a Beckman

pH meter. The meter was calibrated to standard pH buffers of 7 and 10 at

25°C. The results are presented in Table 25.

Table 25. pH Values of 5:1 Extracts APHA-AWWA 15th Edition Method 423

SAMPLE#

pH

1 9.02SOIL A 2 9.18

3 9.234 9.03

SOILL 5 9.066 9.057 8.87

SOILM 8 8.609 8.43

SOIL 0 38 7.74SOILD 13 7.78

Conductivity - Method 2510 B

The conductivity of extracts of 2:1, 3:1, 5:1, 10:1 and 20:1 were

measured with conductivity meter that automatically corrects and reports

the specific conductance at 25°C. The meter was calibrated by immersing

47

the electrode in a .005 N KCI solution that has a conductivity of 707

pmhos/cm. The conductivities are reported in Table 26.

Table 26. Measured Conductivity of Various ExtractsAPHA-AWWA 15th Edition Method 2510B - Conductance

Conductivity (pmhos/cm)Extract Ratio Soil A Soil L Soil M SoilO Soil D

2 : 1 28000 27500 2850 3350 28203:1 19200 21500 2520 2420 25205:1 10683 11547 2323 2300 2480

1 0 : 1 6500 5960 2290 865 13902 0 : 1 3350 3640 2 0 1 0 489 61150:1 1320 1570 962 310 465

1 0 0 : 1 855 910 880 140 2 1 0

Total Dissolved Solids Dried at 180°C - Method 2540 C

The results of test method 2540C are presented in Tables 27-31 for the

2:1, 3:1, 5:1, 10:1, 20:1, 50:1 and 100:1 extracts tested for each soil. The

dry residue of the dissolved soluble minerals was weighed on a five point

gram balance. The concentrations of the dissolved solids in mg/l are

calculated by the relationship:

TDS(mg/liter) = Wei9ht of Dried Solids (9) * 1000 mg / gVolume of sample dried (ml)

The total dissolved solid composition expressed as a percentage of the

original dry soil weight was calculated by the following equation:

48

T n c ,„/UA, . . . , , .. TDS(mg/l) X Volume Water (L) / A n n n ,^TDS (%) Weight of dry soil = - ■ ■ ■ ■ —--------------------- — (100%)Weight sample (g) X 1000 mg/ g v '

Table 27. SOIL A - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Water:Soil Ratio (ml:g)

DryResidue(grams)

Sample Vol. Dried (ml)

Total Dissolved Solids (mg/l)

Dissolved Solids

(% Weight Soil)100:1 .0652 150 435 4.3550:1 .1185 150 790 3.9520:1 .2074 95.6 2169 4.3410:1 .3551 90.8 3911 3.915:1 .42688 50 8538 4.273:1 .9393 75 12524 3.762:1 1.2675 69 18370 3.67

Table 28. SOIL L - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Water:Soil Ratio(ml:g)

DryResidue(grams)

Sample Vol. Dried (ml)

Total Dissolved Solids (mg/l)

Dissolved Solids

(% Weight Soil)100:1 .0721 150 481 4.8150:1 1240 150 826 4.1320:1 .1977 93.8 2108 4.2210:1 .3565 95 3753 3.755:1 .43324 50 8665 4.333:1 1.1539 87 13263 3.982:1 1.3452 75 17936 3.59

Table 29. SOIL M - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Water:Soil Ratio (ml:g)

DryResidue(grams)

Sample Vol. Dried (ml)

Total Dissolved Solids (mg/l)

Dissolved Solids

(% Weight Soil)100:1 .1069 150 713 7.1350:1 .1177 150 785 3.9220:1 .1841 94 1959 3.9210:1 .1950 93.6 2046 2.055:1 .11385 50 2277 1.143:1 .1974 85 2322 .692:1 .1951 76 2567 .51

49

Table 30. SOIL O - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Water:Soil Ratio (ml:g)

DryResidue(grams)

Sample Vol. Dried (ml)

Total Dissolved Solids (mg/l)

Dissolved Solids

(% Weight Soil)100:1 .0110 150 73 .7350:1 .0244 150 163 .8120:1 .0208 75 277 .5510:1 .0397 75 529 .525:1 .1171 75 1561 .783:1 .1247 75 1163 .502:1 .1123 50 _ 2246 .45

Table 31. SOIL D - Total Dissolved Solids Dried at 180°C of Various Extracts APHA-AWWA 15th Edition Method 2540C - TDS

Water:Soil Ratio (ml:g)

DryResidue(grams)

Sample Vol. Dried (ml)

Total Dissolved Solids (mg/l)

Dissolved Solids

(% Weight Soil)100:1 .0140 150 93 .9350:1 .0463 150 309 1.5420:1 .0451 96.5 467.36 .9410:1 .0778 92.7 839.27 .845:1 .1582 91 1738 .873:1 .2056 86 2391 .722:1 .1929 76 2538 .51

Summary of Results

The results of the cation, anion, and physical properties test on the 5:1

extracts for each soil sample are presented in the following tables.

Additionally, the sum value, in mg/l, of the anions and cations

concentrations are provided.

50

Table 32. Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil A Soluble Constituents

Soil ASample #1 Sample #2 Sample #3

Cone.mg/l

Comp%

Cone.mg/l

Comp.%

Cone.mg/l

Comp.%

CationsMg+/ 89 .0445 106.5 .0533 95 .0475

Ca+" 621.5 .311 798.5 .400 749 .374

Na+ 2020.5 1.01 2472.5 1.24 2222.5 1.11

K+ 543 .272 570.5 .285 552.0 .276

AnionsS 0 4 z 1512.5 .756 1573.0 .786 1512.5 .756

1 CO

oo

20.0 0.01 20.0 .01 20.0 .01

HCOa' 20.0 0.01 20.0 0.01 20.0 0.01

cr 2149.33 1.074 2124.34 1.062 2039.37 1.019

Totals 6975.83 3.45 7685.34 3.84 7210.37 3.60

TDS 8538 4.27

pH 9.02 9.18 9.23

Conductivity(|.imhos/cm)

10,500 10,660 10,089

**Reported cation concentrations from Test Method 3120B - Inductively Coupled PlasmaCation concentrations minus cation concentrations measured for the test blank

51

Table 33. Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil L Soluble Constituents

Soil LSample # 4 Sample #5 Sample # 6

Cone.mg/l

Comp%

Cone.mg/l

Comp.%

Cone.mg/l

Comp.%

Cations **Mg+* 294 .147 233.5 .117 304 .152

C a^ 881 .440 650 .325 854 .427

Na+ 2433.5 1.22 2078 1.04 2594.5 1.30

K+ 130.5 .065 132.5 .066 113.5 .057

AnionsS 0 4* 2783.0 1.39 2843.5 1.42 2964.5 1.48

C 0 3* 20.0 .01 20.0 .01 20.0 .01

HCOs' 20.0 .01 20.0 .01 20.0 .01

c r 2599.19 1.30 2654.18 1.327 2789.14 1.392

Totals 9161.19 4.582 8631.68 4.315 9659.64 4.828

TDS 8665 4.33

pH 9.03 9.06 9.05

Conductivity(pmhos/cm)

11,450 11,340 11,850

**Reported cation concentrations from Test Method 3120B - Inductively Coupled PlasmaCation concentrations minus cation concentrations measured for the test blank

52

Table 34. Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil M Soluble Constituents of 5:1 Extract

Soil MSample #7 Sample # 8 Sample #9

Cone.mg/l

Comp%

Cone.mg/l

Comp.%

Cone.mg/l

Comp.%

Cations **Mg+* 27.0 .0135 28.0 .0140 27.0 .0135

C a +Z 649.5 .324 837.5 .418 898.0 .449

Na+ 39.0 .020 56.0 .028 120.0 .060

K+ 17.5 .0088 14.0 .007 20.0 .010

AnionsS 0 4̂ 1390.0 .695 1370.0 .685 1442.5 .721

C 0 3‘* 0.0 0.0 0.0 0.0 0.0 0.0

HC03" 30.0 .015 40.0 .020 30.0 .015

c r 54.98 .0275 49.98 .0249 44.99 .0226

Totals 2207.98 1.10 2395.48 1.19 2582.49 1.29

TDS 2277 1.14

pH 8.87 8.60 8.43

Conductivity(pmhos/cm)

2,330 2,310 2,330

**Reported cation concentrations from Test Method 3120B - Inductively Coupled PlasmaCation concentrations minus cation concentrations measured for the test blank

53

Table 35. Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil O Soluble Constituents of 5:1 Extract

SoilOSample #38

Cone.mg/l

Comp%

Cations*Mg+" 46 .023

Ca** 600 .30

Na+ 350 .175

K+ 40 .02

AnionsS O ^ 2057 1.029

C0 3̂ 0 0

HC03' 20 .01

Cl" 144.96 .0725

Totals 3258 1.63

TDS 3217 1.61

pH 7.74

Conductivity(pmhos/cm)

3390

* Cation Concentrations from Method 3111B Atomic Absorption

54

Table 36. Metals, Inorganic Nonmetallic Constituents & Physical Properties of Soil D Soluble Constituents of 2:1 Extract

Soil DSample # 13

Cone.mg/l

Comp%

Cations*Mg+̂ 36 .0072

C a +Z 480 .096

Na+ 80 .016

K+ 35 .007

AnionsS(V 1754.5 .351

C 0 3 ' 2 0.0 0.0

HC03' 40 .01

cr 54.98 .0110

Totals 2480 .50

TDS 2538 .51

pH 7.78

Conductivity(|.imhos/cm)

2820

* Cation Concentrations from Method 3111B Atomic Absorption

CHAPTER 5

DISCUSSION OF RESULTS

Accuracy Of Test Methods

Charge Neutrality

Most salt compounds, when dissolved in a relatively large volume of

water, are practically completely dissociated into ions. A neutral charge

balance must exist between the dissociated cations and anions. Likewise

the ratio between the cation and anion concentrations in solution is unity.

For this study the ratio was calculated to verify the accuracy of the

measurements of the cation and anion concentrations. A ratio significantly

greater or less than unity would indicate an error in identifying the

concentration of any one of the various cations or anions.

To determine the ratio the concentrations were converted to equivalent

concentrations to express them in terms of their ionic charge. An

equivalent is equal to the charge of the ion. An equivalent weight of an

ion is its atomic weight per equivalent. The equivalent concentrations

expressed in milliequivalents per liter were computed from the following

55

56

Relationship:

r- . . .. , ,,x Concentration (mg/l)Equivalent Concentration (meq / 1) =Equivalent Weight (mg / meq)

The equivalent concentrations of each anion and cation, their sums, and

the cation to anion ratio for each soil sample extract are presented in

Tables 37-41. The ratios determined for Soils A, L &M were determined

from the cation concentrations measured by the Inductively Coupled

Plasma Method. The ratios for Soils O and D were determined from the

cation concentrations measured by the Atomic Absorption Spectrometry

Method.

As shown in Tables 37, 38 and 39, there was a variance in the ratios

for Soils A, L and M however the variation for Soils O and D were low.

The variance ranged from a high of 1.82 for Soil A to a low of 1.07 for

Soil L. The inaccuracy is a result of the reported cation concentrations. A

review of the data shows there is better repeatability of results for anion

test than for the cation test. The sum of the anion concentrations, in

meq/l, do not vary more than five percent, where as the variation in the

sum of the cations is as much as forty percent. Also from inspection of the

results there is very close agreement between cation concentrations

measured by the Flame Photometer and Atomic absorption Methods.

In most cases the cation concentrations determined from Test Method -

3120B Inductively Coupled Plasma (ICP) measured for each soil extract

57

exceeded the concentrations of the calibration standards used for this test.

The ICP method m easures wavelengths against those calibrated to known

standards. Measured concentrations encountered beyond the standard

concentrations by the computer controlling the ICP test apparatus allows

for an inaccurate extrapolation. In addition high concentrations of mixed

solutions with Na and Cl cause interference in measuring wavelengths.

As an alternative; the ratios for Soils A, L and M were computed using

the cation concentrations determined from Standard Test Method 325B -

Flame Photometric. The results of the new cation to anion ratios presented

in Table 42 indicate that all measured concentrations are within reason.

Table 37. Soil A Ratio of Cation to Anion of 5:1 Extracts

SOIL A EquivalentWeight

Sample #1 Concen.

Sample #2 Concen.

Sample #3 Concen.

(mg/meq) (meq/litre) (meq/litre) (meq/litre)Cations*

Mg** 12.156 7.32 8.76 7.815Ca*2 20.04 31.01 39.84 37.38Na+ 22.99 87.89 107.55 96.67K+ 39.102 13.89 14.59 14.12

Total 140.11 170.74 155.985

AnionsS O ^ 48.032 31.49 32.75 31.49co3* 30.005 .667 .667 .667HC03' 61.018 .328 .328 .328cr 35.453 60.62 59.92 57.52

Total 93.105 93.665 90.005Cation/Anion 1.50 1.82 1.73

* Reported cation concentrations from Test Method 3120B - Inductively Coupled Plasma

58

Table 38. Soil L Ratio of Cation to Anion of 5:1 Extracts

Soil L EquivalentWeight

Sample #4 Concen.

Sample #5 Concen.

Sample # 6

Concen.(mg/meq) (meq/litre) (meq/litre) (meq/litre)

Cations*Mg+" 12.156 24.18 19.21 25.01

C a +Z 20.04 43.96 32.44 42.61

Na+1 22.99 105.85 90.39 112.85

K+1 39.102 3.34 3.39 2.90

Total 177.33 145.43 183.37

AnionsS 0 4‘" 48.032 57.94 59.20 61.72

N 1 CO

Oo!

30.005 .667 .667 .667

HCO3 '1 61.018 .328 .328 .328

c r 1 35.453 73.32 74.86 78.67

Total 132.25 135.06 141.39

Cation/Anion 1.34 1.07 1.29

* Reported cation concentrations from Test Method 3120B - Inductively Coupled Plasma

59

Table 39. Soil M Ratio of Cation to Anion of 5:1 Extracts

Soil M EquivalentWeight

Sample #7 Concen.

Sample # 8

Concen.Sample #9

Concen.(mg/meq) (meq/l) (meq/l) (meq/l)

Cations*Mg+" 12.156 2.22 2.303 2.22

Ca~ 20.04 32.41 41.79 44.81

Na+ 22.99 1.69 2.44 5.22

K+ 39.102 .448 .358 .511

Total 36.768 46.891 52.761

Anionss c v ^ 48.032 28.93 28.52 30.03

COS 30.005 0.0 0.0 0.0

HCCV 61.018 .492 .656 .492

c r 35.453 1.55 1.41 1.27

Total 30.97 30.59 31.79

Cation/Anion 1.19 1.53 1.66

* Reported cation concentrations from Test Method 3120B - Inductively Coupled Plasma

60

Table 40. Soil O Ratio of Cation to Anion of 5:1 Extracts

SoilOSample #38

EquivalentWeight

Concentration

(mg/meq) (meq/l)CationsMg+/! 12.156 3.78

C a^ 20.04 29.94

Na+ 22.99 15.22

K+ 39.102 1.02

Total 49.96

AnionsS O ^ 48.032 42.83

C O ^ 30.005 0

HC03' 61.018 .33

c r 35.453 4.09

Total 47.25

Cation/Anion 1.06

61

Table 41. Soil D Ratio of Cation to Anion of 2:1 Extracts

Soil DSample #13

EquivalentWeight

Concentration

(mg/meq) (meq/l)CationsMg^ 12.156 2.96

Ca+" 20.04 23.95

Na+ 22.99 3.48

K+ 39.102 .89

Total 31.28

AnionsS 0 4̂ 48.032 36.53

C 0 3‘2 30.005 0

HCOs' 61.018 .65

cr 35.453 1.55

Total 38.73

Cation/Anion .81

62

Table 42. Soil A, L & M Ratio of Cation to Anion of 5:1 Extracts

EquivalentWeight

Soil A Concen.

Soil L Concen.

Soil M Concen.

(mg/meq) (meq/l) (meq/l) (meq/l)

CationsMg+*' 12.156 7.96 22.80 2.25

Ca+*" 20.04 28.80 26.60 26.60

Na+ 22.99 70.03 78.03 2.50

K+ 39.102 10.17 1.70 .30

Total 116.96 129.13 31.65

Anions ***S 0 4'/ 48.032 31.91 59.62 29.16

CO3* 30.005 .667 .667 0.0

HCCV 61.018 .328 .328 .547

cr 35.453 59.35 75.62 1.41

Total 92.26 136.24 31.12

Cation/Anion 1.27 0.9478 1.02

* Concentration reported is an average value of measurements from ICP method.

** Concentrations reported are from single sample measurements from flame photometric test.

*** Concentrations reported are average values of measurements from various methods.

63

Identification of Soluble Compounds

A method to precisely determine the form and concentration of the

electrolyte that individual ions dissociate from in a mixed solution does not

exist. However estimation methods exist. For this study gravimetric

analysis was used to estimate the type and concentration of the soluble

mineral compounds in the soil extracts. Gravimetric analysis is based on

the law of definite proportions, which states that in any pure compound the

proportions by weight of the constituent elements are always the same and

upon the law of constancy of composition, that states the mass of the

elements taking part in any given chemical change exist in definite and

invariable ratio to each other ( Hamilton, et. al., 1969).

Gravimetric factors were used to estimate the predominate minerals

most likely formed from the proportion of the individual anions and cations

measured as a percentage of the soil extract. A gravimetric factor is

defined as the weight of a desired substance equivalent to a unit weight of

a given substance (Hamilton, et. al., 1969). For instance the gravimetric

factors for Na and Cl in the form of NaCI are:

~ Formula Weight of NaCIG.F. of Na = ----------------- -------------- = 3.65Molecular Weight of Na

G.F. of Cl = F°™ ula Weight of NaCI = 1g5 Molecular Weight of Cl

64

The compounds expressed as a percentage of the original soil extract

were estimated from the measured anion and cation percentages reported

in Tables 32-36. As an example; if the measured Na and Cl compositions

as percentage of the original extract concentration were .753 % and .860

% respectively, then the percentage of the NaCI of the extract is estimated

as follows:

(1) The amount of NaCI is the less of the amount of NaCI that can exist

from the available amounts of either Na and Cl:

NaCI from Na = .753% x 3.65 (G.F of Na) = 2.79 %

NaCI from Cl = .860% x 1.65 (G.F of Cl) = 1.42 %

(2) Therefore all the Cl exist as NaCI and the amount of Na as NaCI is:

142% (% ^N aCI)3.7 (G FofN a)

(3) The amount of remaining Na available to form other compounds is:

Na = .753% - .383% = .370 %

This process is repeated until all the available Na is accounted for in the

forms of other compounds. Other anions and cations are paired to

estimate other predominate compounds. The results of the gravimetric

analysis and of the soils tested are presented in Tables 43-47. Also

presented in each table is a comparison between the total measured

concentrations of the individual ions and the totals calculated for the

estimated compounds. From an inspection of the results the values

reported for the compounds are very close to those measured for the

individual ions.

Further inspection of the tables indicate that one or two predominate

soluble minerals exist for each soil. For instance the predominate soluble

mineral in Soil A is readily soluble NaCI and the moderately soluble

C aS 04. Soil L is predominately readily soluble NaCI and Na2S 0 4 minerals.

The predominate mineral in Soil M is moderately soluble C aS 04. Soil O

and D contain relatively small amounts of C aS 04 and Na2S 0 4.

Table 43. Soil A Soluble Minerals Composition by Gravimetric Analysis of 5:1 Extract

Compound Composition%

Concentration(mg/l)

NaCI 1.42 2840MgCI2 .00897 17.94

KCI .378 756Na2S 0 4 .160 320C aS 0 4 .935 1870CaC03 .0167 33.40

CaHC03 .0166 33.20Totals

Estimated 2.93 5870Measured 3.13 6358

66

Table 44. Soil L Soluble Minerals Composition by Gravimetric Analysis of 5:1 Extract

Compound Composition%

Concentration(mg/l)

NaCI 1.84 3680MgCb .269 538

KCI .126 252Na2S 0 4 1.68 3360C aS 0 4 .39 780C aC 03 .017 34

CaHCOs .0167 33.4Totals

Estimated 4.34 8677.4Measured 4.32 8865

Table 45. Soil M Soluble Minerals Composition by Gravimetric Analysis of 5:1 Extract

Compound Composition Concentration(mg/l)%

NaCI .032 64KCI .0168 33.6

Na2S 0 4 .017 34M gS04 .037 74C aS 04 .937 1874

NaHC03 .02.0 40Total

Estimated 1.06 2119.6Measured 1.14 2277

67

Table 46. Soil O Soluble Minerals Composition by Gravimetric Analysis of 5:1 Extract

Compound Composition%

Concentration(mg/l)

NaCI .089 178KCI .0382 76.40

Na2S04 .428 856MgSC>4 .0633 126.6CaSC>4 .996 1992

CaHC03 .0166 33.2Total

Estimated 1.63 3262Measured 1.61 3217

Table 47. Soil D Soluble Minerals Composition by Gravimetric Analysis of 5:1 Extract

Compound Composition%

Concentration(mg/l)

NaCI .0076 38KCI .0134 67

Na2 S04 .0431 215MgSC>4 .0198 99C aS 04 .304 1520

CaHCOa .0166 83Total

Estimated .41 2022Measured .51 2538

68

Comparison of Specific and Equivalent Conductance

The Standard Test Method 251 OB - Conductance measures the

specific conductance of an electrolyte solution. Specific conductance k is

the reciprocal of resistance in ohms of a 1 - cm cube of liquid at a specified

temperature. Specific conductance is determined from the following

relationship:

0K = —

R

where © = cell constant in cm'1 and R = resistance in ohms. The cell

constant, the ratio of the area of the meter’s electrodes to the distance

between them, for the conductivity meter used in this study was 1 cm'1 .

The ability of an ion to conduct an electrical current is defined as ionic

conductance X. At infinite dilutions ions are theoretically independent of

each other and each contributes its part to the total or equivalent

conductance of a solution A, and is expressed as:

A = I ( X . ) + ( /U ) where X is in units ohms'1 cm2

Specific and equivalent conductance are related by the following

expression:

k = ---- [Z(CA +) + E(CA )11000© *- V s - / j

where Cs is the equivalent concentration of an ion.

69

For each soil the total equivalent conductance was calculated from the

measured equivalent concentrations of each ion in the soil solution. The

total specific conductance, k , was calculated from this value from the

above relationship. The comparison between measured specific

conductance and the calculated specific conductance is presented in

Table 48 and in Figure 6. Additionally, the contribution of each ion to the

calculated specific conductance is presented in tables in the Appendix.

Table 48. Calculated Equivalent and Specific Conductance

SOILTotal

EquivalentConductance

(A)

CalculatedSpecific

Conductance(k)

MeasuredSpecific

Conductance(K)

(mhos/liter) ((.imhos/ cm) (l_imhos/ cm)

A 13.649 13,649 10,500

L 17.5343 17,534 11,547

M 4.5334 4,533 2330

0 6.7097 6709.7 3390

D 5.0824 5082.4 2820

As shown in Table 48 the measured specific conductance is less than

the calculated specific conductance as derived from the equivalent

conductances of the total ion content in solution. Figure 6 shows the plot of

Tota

l Di

ssol

ved

Solid

s (m

g/l)

70

9000

8000

7000

6000

5000

4000

3000

2000

1000

0100 1000 10000 100000

Conductivity (micromhos/cm)

Figure 6. Specific Conductance vs. Total Dissolved Solids of 5:1 Soil Extracts

->®h b

-

. ■ Calculated

- • Measured

-

■

-

•

m e ■

-• n

• H

■ ------------ 1-------1— i i i 1 11 ----- 1------ 1— 1— 1—1 I I I ------------ 1-------1— i l 1 1 11

71

the measured specific conductance and the calculated specific

conductance as a function of the total dissolved solids measured for each

5:1 soil extract. This figure shows the potential to obtain different

relationships between conductivity and total dissolved solids for different

soil to water extract ratios. For instance a more dilute soil to water ratio

may produce a measured specific conductance equivalent or closer to the

specific conductance of the actual ions in solution. The converse is true for

more concentrated solution. This can be explained by the behavior of

electrolytes in solution. As a solution becomes more dilute, its equivalent

conductance becomes greater due to the fact that in a more dilute solution

ionic interference between electrolytes is lessened, which gives the effect

of increasing the degree of ionization of the dissolved substance

(Hamilton, 1969).

Conductivity And Soluble Mineral Content Analysis

The correlation between conductivity and the soluble mineral content

was investigated by comparison of the physical measurements of the

soluble constituents of soil extracts. Measurements compared were total

dissolved solids and conductance from Methods 2540C and 251 OB

respectively. As discussed in the Experimental Approach Section, the

tests were performed on aqueous soil extracts of 2:1, 3:1, 5:1, 10:1, 20:1,

72

50:1 and 100:1 to determine an optimum ratio for an unsaturated solution

with the greatest dissolved mineral content.

Conductivity as Function of Total Dissolved Solids

Figures 7 and 8 show the plots of the specific conductance as a

function of the total dissolved solids presented in Tables 2 7 - 3 1 and Table

26 respectively. The plots for soils A and L are shown in Figure 7. The plot

for soil samples M, O and D are shown in Figure 8. Also shown in each

figure is the predominate soluble mineral content for each soil sample

represented by the plot. Each data plot for the soil represents the

measured total dissolved solids and conductivity for each water to soil

ratio.

All figures show as expected an increase in conductivity and total

dissolved solids with an increase in the soil solution concentration

(decrease in the water to soil ratio). However there are noticeable trends.

Soils with readily soluble minerals such NaCI or N aS04 as in soils A and L

have higher dissolved solids and conductivities than the soils like M, O and

D where the predominate mineral is moderately soluble C aS 04. They also

exhibit a wider range of solubility at the various dilutions since they are

more soluble. As shown in Figure 8, soil M with .937 % C aS 0 4 has a very

small range of total dissolved solids, increasing from only 1958 mg/l to

2567 mg/l. Soils composed of a moderately and readily soluble mineral in

TOTA

L DI

SSO

LVED

SO

LIDS

(m

g/l)

73

Increasing Concentration 100, 50, 20, 10, 5, 3, & 2:1 -------- >20000

SOIL A: CL-ML; 1.4% NaCI & .9% CaSOd18000-

SOIL L: ML; 1.9% NaCI & 1.7 % Na,SOd16000

14000

12000

10000

8000

6000

4000

2000

0 5000 10000 15000 20000 25000 30000CONDUCTIVITY (micromhos/cm)

Figure 7. Total Dissolved Solids as Function of Conductivity (Soils A & L)

TOTA

L DI

SSO

LVED

SO

LIDS

(m

g/l)

74

Increasing Concentration 100, 50, 20, 10, 5, 3 & 2:1 -------- >3000

SOILM: SM; .94% CaS04 &. 032% NaCI

SOIL O: CL; 1% CaS04 & .43%

N a,S042500

SOIL D: SW-SM; .3% CaSO,

® & .04% N a,S04

2000

1500

1000

500

0 500 1000 1500 2000 2500 3000 3500

CONDUCTIVITY (micromhos/cm)

Figure 8. Total Dissolved Solids as Function of Conductivty (Soils M, O & D)

75

similar proportions exhibit the solubility of the readily soluble mineral. As

an example Soil A has 1.42% of readily soluble NaCI and .935% of

moderately soluble C aS 0 4 concentrations, however as shown in Figure 7

it has almost the identical data points of Soil L which has 1.42 % of readily

soluble NaCI.

The most noticeable and important trend is that Soils A and L also

exhibit a more consistent linear increase in total dissolved solids with

increase in conductivity than Soils O, M and D. Soils O, M and D have

limited increase in total dissolved solids, or a leveling off of the plot line,

between increases in soil solution concentration. This is an indication that

at various water to soil ratios certain minerals become saturated or reach

their maximum solubility while other minerals have not reached saturation.

Figure 9 illustrates this effect for a soil extract solution with a single salt

compound, soluble mineral, as opposed to the mixed soil solutions shown

in Figures 7 and 8. If the salt is not saturated in the soil extract solution,

then there is an increase in total dissolved solids with an increase in

conductivity due to a proportional increase in salt from an increase in soil

weight. As the soil solution concentration increases, there is a point were

the amount of salt is sufficient enough to reach its limiting solubility or is

saturated in the volume of water used for the extract. Beyond this

saturation point an increase in soil m ass does not result in an increase

Tota

l Di

ssol

ved

Solid

s (m

g/l)

76

5000

Increasing Concentration

SaturatedUnsaturated

3000-

Point ofMaximumSolubility

2000 -

— Single Salt Solution

o 200 400 600 800 1000 1200 1400Conductivity (micromhos/cm)

Figure 9. Total Dissolved Solids as Function of Conductivity for Extract with Single Salt Compound

77

concentration of the salt in solution. The total dissolved solids remain at

the salt’s maximum solubility.

The noted trends are generally a function of the amount of water that

was available to dissolve the soluble minerals from the soil and the

proportions in which they exist in the soil. Choosing an optimum soil to

water ratio that dissolves the greatest amount of minerals from the soil

without reaching the maximum solubility, for the selected volume of water,

of any one the many minerals in the soil is difficult. The type, amount and

proportion of soluble minerals found in soils is highly variable. However;

Figures 7 and 8 show that soil to water ratios greater than 50:1 were

sufficient to prevent the saturation of any one the soluble minerals present

in the soils for this study. For instances, Table 8 shows a constant slope

of the lines representing total dissolved solids as a function of conductivity

for soils A and L between plots for water to soil extracts of 100:1 to 50:1.

The slope of the lines change, steeper slope, for greater concentrations

beginning with the range from 20:1 to 10:1 as a result of one of the soluble

minerals reaching its maximum solubility or saturation in this concentration

range. Other changes in the slope of the line between other concentration

ranges indicate saturation of other minerals. From Figure 8 similar

changes in the slope of the plot line for total dissolved solids as a function

of conductivity between concentration ranges can be identified. The effect

that water to soil ratio has on dissolving the maximum amount of soluble

78

minerals is again illustrated by expressing the soluble mineral content as

percentage of the dry soil weight as a function of conductivity.

Conductivity as Function of Mineral Content as % Dry Soil Weight

Figures 10 and 11 show the plots of the specific conductance as a

function of the mineral content expressed as a percentage of the original

weight of dry soil from data presented in Table 26 and Tables 2 7 - 3 1

respectively. The plots for soils A and L are shown in Figure 10. The plot

for soil samples M, O and D are shown in Figure 11. Also shown in each

figure is the predominate soluble mineral content measured from the 5:1

extracts for each soil sample represented by the plot. Each data plot for

the soil represents the measured total dissolved solids and conductivity for

each water to soil ratio.

For unsaturated extracts the mineral content as a percentage of the

dry soil weight remains constant with increase in soil concentration. The

percentage of the minerals remains the sam e in the soil regardless of the

amount of soil. If an increase in concentration results in the saturation of

any one mineral, then any additional increases in the soil solution

concentration results in a decrease of the soil expressed as a percentage.

This is illustrated in Figure 12 for a single salt soil solution. After the

limiting solubility is reached, an additional increase in concentration results

SOLU

BLE

MIN

ERAL

CO

NTEN

T AS

AP

PARE

NT

WEI

GHT

OF

SOIL

(%)

79

Increasing Concentration 100, 50, 20, 10, 5, 3 & 2:1-------->

4.8

4.6

4.4

4.2

3.8

3.6

3.4 SOIL A: CL-ML; 1.42% NaCI & .934% CaSO,

SOIL L: ML; 1.94% NaCI & 1.68% Na2SO,3.2

0 5000 10000 15000 20000 25000 30000CONDUCTIVITY (micromhos/cm)

Figure 10. Soluble Mineral Content as Function of Conductivity ( Soils A & L)

SOLU

BLE

MIN

ERAL

CO

NTEN

T AS

AP

PARE

NT

WEI

GHT

OF

SOIL

(%)

80

Increasing Concentration 100, 50, 20, 10, 5, 3 & 2 :1 --------->

3.5

SOIL M: SM; .937% CaS04 & .032% Na2S 0 4

SOIL O: CL; .996% CaS04 &

.428% Na,SO,2.5

SOIL D: SW:SM; .3% CaSO,

& .04% Na,SO,

—©

0.5

0 500 1000 1500 2000 2500 3000 3500CONDUCTIVITY (micromhos/cm)

Figure 11. Soluble Mineral Content as Function of Conductivity ( Soils M, O & D )

Solu

ble

Min

eral

Con

tent

as

App

aren

t W

eigh

t of

Soil

(%)

81

Increasing Concentration ----------------->6

5

4

3

2

1

00 500 1000 1500 2000 2500

Conductivity (micromhos/cm)

Figure 12. Soluble Mineral Content as Function ofConductivity for Extract with Single Salt Compound

Unsaturated Saturated

Single Salt Solution i i

ft

- i— i— i— I— f— i— i— i— i— I— I— g— i— i— t— i— i— i— i— |— i— o— i— r

82

in a weight of the soluble mineral at saturation expressed as percentage of

continuos increase in weight.

For the soils in this study there are various decreases as a result of the

saturation of some of the soluble minerals at different soil solution

concentrations. Soils A and L, which contain readily soluble salts, have a

relatively constant plot. The solubility of NaCI is 357 grams/liter. Assuming

that the NaCI was not saturated in the 5:1 solution, then the percentage of

the soluble mineral content would decrease to a constant 1.42% as all the

other soluble minerals reach their limiting solubility. Even at 2:1

concentration, the NaCI concentration based on the amount of soil weight

of 50 grams would be 7.1 grams/liter, far below its limiting solubility, The

plot for Soil M, containing predominately C aS 04, in Figure 11 most

resembles the relationship for a single salt solution. Figures 10 and 11

show that the optimum water to soil ratios where the soil solutions are not

saturated are for those greater than 50:1.

Conductivity as Function of TDS and Mineral Content

Figures 13 - 17 show the combined plots of the specific conductance as

function of the total dissolved solids and the soluble mineral content

expressed as a percentage of the original dry soil weight for soils A, L, M,

O, and D. Comparisons between these two functions of conductivity can

easily be made from the combined plots. For example, from Figure 15 the

Tota

l Di

ssol

ved

Solid

s (m

g/l)

83

Increasing Concentration--------------->2 0 0 0 0 -4.5

18000

160003.5

Soil A % Weight vs Conductivity14000

120002.5

10000

8000

6000

Soil A TDS vs Conductivity

4000

0.52000

5000 100000 15000 20000 25000 30000Conductivity (micromhos/cm)

Figure 13. Soil A Total Dissolved Solids and Mineral Content as % Weight of Soil as Function of

Conductivity

Min

eral

Con

tent

as

Wei

ght

of So

il (%

)

Tota

l Di

ssol

ved

Solid

s (m

g/l)

84

Increasing Concentration18000

4.516000

14000

3.5Soil L % Weight vs Conductivity12000

10000

2.5

8000

6000

4000

Soil L TDS vs Conductivity2000 0.5

50000 10000 15000 20000 25000 30000Conductivity (micromhos/cm)

Figure 14. Soil L Total Dissolved Solids and MineralContent as % Weight of Soil as Function of

Conductivity

Min

eral

Con

tent

as

Wei

ght

of So

il (%

)

Tota

l Di

ssol

ved

Solid

s (m

g/l)

8 5

Increasing Concentration3000

3.5Soil M % Weight vs Conductivity

2500

20002.5

1500

1000

5000.5Soil M TDS vs

Conductivity

0 500 1000 1500 2000 2500 3000

OV)

szO)o

(/>(0-*->cQ)•*->coOEa>c

Conductivity (micromhos/cm)

Figure 15. Soil M Total Dissolved Solids and MineralContent as % Weight of Soil as Function of

Conductivity

Tota

l Di

ssol

ved

Solid

s (m

g/l)

86

Increasing Concentration-------------- >2500 0.9

Soil O % Weight vs Conductivity

0.8

20000.7

0.6

15000.5

0.41000

0.3

0.2500

Soil 0 TDS vs Conductivity 0.1

5000 1000 1500 2000 2500 3000 3500Conductivity (micromhos/cm)

Figure 16. Soil O Total Dissolved Solids and Mineral Content as Weight of Soil as Function of

Conductivity

Min

eral

Con

tent

as

% W

eigh

t of

Soil

Tota

l Di

ssol

ved

Solid

s (m

g/l)

87

Increasing Concentration ------------->3000

Soil D % Weight vs Conductivity

2500

2000

1500 0.8

1000

500 Soil D TDS vs Conductivity

o 500 1000 1500 2000 2500 3000Conductivity (micromhos)

Figure 17. Soil D Total Dissolved Solids and Mineral Content as % Weight of Soil as Function of

Conductivity

iner

al C

onte

nt a

s W

eigh

t of

Soil

(%)

88

increase in concentration from 100:1 to 50:1 shows an increase in the total

dissolved solids with increase in conductivity up to 2000 micromhos/cm

represented by a constant slope of the plot line. For the sam e range, the

mineral content remains at a constant 3.9% indicating that none of the

minerals are saturated in solution. After the 50:1 concentration saturation

occurs for some of the soluble minerals. This is indicated by the decrease

in the soluble mineral content as indicated by the change in the slope of

the plot line. Additionally there is a corresponding change in the slope of

the total dissolved solids plot line, a flatter slope, indicating some minerals

are saturated in solution.

Proposed Correlation and Test Method

Conductivity And Total Dissolve Solids Correlation

Figure 18 shows the proposed correlation between conductivity and

total dissolved solids. The correlation is a best fit of the specific

conductance and total dissolved solids measured for each soil extract

ratio. Though the best fit is from a limited amount of soils, it is likely that

soils exist with soluble mineral compositions that would yield the measured

values obtained for the various extracts. The mineral content for each soil

as identified from the 5:1 extracts are also shown in Figure 18. It should be

noted that at the 5:1 water to soil extract some measured minerals, such

TOTA

L D

ISSO

LVED

SO

LIDS

(m

g/l)

89

20000

SOIL A: CL-ML; 1.4% NaCI & .9% CaS04

18000SOIL L: ML; 1.9% NaCI & 1.7 % Na,SO,

16000SOILM: SM; .94% CaSO.

& .032% NaCI14000 SOIL O: CL; 1% CaS04 & .43

% N a,S04

12000SOIL D: SW-SM; .3% CaSO,

& .04% N a,S04

10000

8000

6000

4000f(x)= .6618651

R2 = .9914422

x + 66.7 530

2000

0 5000 10000 15000 20000 25000 30000CONDUCTIVITY (micromhos)

Figure 18. Best Fit of Total Dissolved Solids as Functionof Conductivity for all Extracts of Soils A,L,M, 0 and D.

90

as calcium sulfate, were saturated and do not represent the total calcium

sulfate content of the soils. A proposed test method utilizing Figure 18 is

also proposed.

Proposed Test Method

The following steps are recommended to estimate the soluble mineral

content of a soil as percentage of its dry weight:

(1) Split the field sample to ASTM C 702-87 , Reducing Field Samples of

Aggregate to Testing Size.

(2) Dry approximately 1200 grams of the sample in an 60°C oven in

accordance with ASTM D421, Practice for Dry Preparation of Soil Samples

for Particle Size Analysis and Determination of Soil Constants.

(Temperatures greater than 60°C can dehydrate calcium sulfate in the

form of gypsum .)

(3) Crush any large gypsum crystals observed in the dry sample and then

sieve all through a No. 10 sieve. ( The smaller soil particle sizes have

more surface area than large particle sizes and therefore contain more of

the soluble minerals.)

(4) Mix the dry soil to distribute any visible minerals throughout the soil in

order to produce a more homogenous mixture. Split the mixed soil into

sample portions in accordance with step 1.

91

(5) Prepare a minimum of three soil solutions, using distilled or reverse

osmosis water, for water to soil ratio of 50:1 (250 ml: 5 gram).

(6) Agitate the solutions in closed containers in a mechanical shaker or by

hand for 15 minutes.

(7) Vacuum filter the solutions through a No. 1 Whatman filter or in the

absence of vacuum filter, do not disturb the samples and allow all visible

sediments to settle to the bottom of the sample containers.

(8) Measure the conductance of the samples in accordance with Standard

Method 2510-B. If there are large variances in the measured values for the

three samples then prepare separate samples until consistent

measurements are achieved.

(9) For an average of the three conductivity measurements made for the

50:1 samples obtain the total dissolved solids from Figure 18.

(10). Express the result as percent of dry soil weight from the following

relationship:

/o/m/w ■ u* i TDS(mg/l) X Volume Water (L) U n n a / ^(%) Weight of dry soil = ...............- ------------------------- — (100%)Weight sample (g) X 1000 mg / g

( Note: Various water to soil ratios could be chosen to find an optimum;

however the purpose of the test is to identify the potential amount of

minerals that will dissociate in the engineering environment; therefore this

ratio should be reasonable. If the goal were to determine all of the soluble

minerals, then an acid wash could be performed.)

92

Comparisons to Existing Correlations

Agricultural Handbook No. 60. - United States Salinity Laboratory

The United States Salinity Lab (Richards, 1954) correlation expresses

total salt concentration in terms of equivalent concentrations, meq/l. This

limits the comparison of the data from this study to the concentration

measurements made for the 5:1 extracts of each soil. These are the only

concentrations that can be expressed in equivalent concentrations. The

data from this study that was compared is from Tables 37 to 41 and Table

26 for equivalent concentrations and conductance respectively. The

comparison is made in tabular and graphical form as presented in Table 49

and Figure 19 respectively. Table 49 compares the measured values from

this study to the predicted values. The graphical comparison plots the

results of this study in relationship to the approximate average line of the

U.S. Salinity Laboratory Correlation.

Table 49 shows that the soluble mineral concentrations obtained from

the U.S. Salinity Laboratory Correlation are lower than the values

measured for the Las Vegas Valley Soils. The range of the differences

reported in Table 49 approximately equate to soluble mineral contents of

.5% to 1.2% by weight of the dry soil extracts by assuming the equivalents

were expressed in terms of sodium which has a lower equivalent weight

the majority of the soluble ions measured. From inspection of Figure 19 the

93

Las Vegas Soils plot in a trend parallel to the approximate average line of

the U.S. Salinity Lab Correlation, however the approximate average line

covers a higher range of conductivities than measured for the Las Vegas

Valley Soils. This is most likely due to fact that the Las Vegas 5:1 soil

extracts were from a specific area with more soluble minerals than the 5:1

extracts for the soils in the western United States used to develop the U.S.

Salinity Lab correlation.

Table 49. Comparison of Measured Concentrations to U.S. Salinity Laboratory Correlation Values

SOILMeasured

ConductanceMeasured

ConcentrationU.S. Salinity Laboratory

Difference

(^mhos/liter) (meq/l) (meq/l) (meq/l)

A 10683 209 130 70

L 11547 265 160 105

M 2323 63 24 39

0 3390 97.21 38 59

D 2480 70.01 27 43

CO

NC

ENTR

ATI

ON

(m

eq/l)

94

1000

♦ Las Vegas Soils

100

Approximate Average Line

100 1000 10000 100000SPECIFIC CONDUCTANCE (micromhos/cm)

Figure 19. Comparison of Las Vegas Valley Soils to U.S. Handbook No. 60 Approximate Average Line of

Soluble Soil Extracts as Related to Conductivity

95

Atlas Chemical Consultants Empirical Correlation

The data from this study that were compared is from Tables 37 to 41

and Table 26 for equivalent concentrations and conductance respectively.

The comparison is made in tabular form as presented in Table 50 for the

5:1 extracts. The predicted values are given by the Atlas (Summers, 1994)

empirical relationship:

TDS (mg/l) = Measured Conductance (pmhos/cm) x Empirical Factor

The Atlas empirical relationship compare reasonably well to the measured

values of the 5:1. This is expected since the empirical relationship was

developed from 5:1 extracts. Additionally, the empirical values are plotted

in Figure 20 along with the plot of the 5:1 measured values, and the best fit

correlation from Figure 18. The two correlation curves compare

reasonable for conductivity values less than 3000 micromhos/cm. Beyond

3000 the Atlas correlation yields higher total dissolved solids at equivalent

conductances. The empirical relationship is more valid for the moderately

soluble minerals, as the CaSC>4 in soil M, that have conductivities around

3,000 pmhos/cm. The greatest differences are for the conductivities

above 3,000 that are defined by a constant empirical factor of .95 by the

Atlas correlation.

96

Table 50. Comparison of Measured Concentrations to Atlas Chemical Consultant Empirical Correlation Values of 5:1 Extracts

Measured Values Atlas Empirical MethodConductivity TDS Empirical Factor TDS

((.tmhos/cm) (mg/l) (mg/l)SOIL A 10,683 8,538 .95 10,149SOILL 11,547 8,665 .95 10,970SOILM 2,323 2,277 .85 1,975SOIL 0 3390 3,217 .95 3,220SOILD 2480 1738 .85 2108

Tota

l Di

ssol

ved

Solid

s (m

g/l)

97

30000

Best Fit Correlation

Atlas Empirical Correlation

Measured 5:1 Values25000

20000

15000

10000

5000

25000 300000 5000 10000 15000 20000Conductivity (micromhos/cm)

Figure 20. Comparison of Atlas Empirical Values from 5:1 Extracts to Best Fit Correlation of Various Soil Extracts

CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

Several conclusions on the correlation between the soluble mineral

content of soils and conductivity of their aqueous extracts can be drawn

from this study. Additionally recommendations for application of the

proposed correlation developed from this study and for areas of further

study are made.

® Soluble soils exist primarily in arid regions. The soluble minerals are

encountered in various proportions. The mineral constituents are

predominately the cations; magnesium, sodium, calcium, and potassium;

and the cations; sulfate, carbonate, bicarbonate and chloride.

• Las Vegas Valley soluble soils contain a wide variety of soluble

minerals; however the dominate minerals are readily soluble NaCI, Na2S 0 4

and moderately soluble CaSCV

• In concentrated solutions the measured specific conductance is less

than the specific conductance calculated from the equivalent conductance

of the individual ion concentrations.

98

99

• Soil extracts with moderately soluble minerals such as C aS 0 4 have a

lower range of conductivity and total dissolved solids between dilute and

concentrated soil solutions than extracts with readily soluble minerals such

as NaCI.

• Total dissolved mineral content and conductivity increase

proportionally with an increase in soil solution concentration until any one

of the soluble minerals in the mixed system are saturated in the solution.

• The soluble mineral content when expressed as a percentage of the

dry soil weight remains constant with increase in conductivity for increases

in soil solution concentration until any one of the soluble minerals is

saturated in solution. When minerals are saturated there is a decrease in

the mineral content expressed as percentage of the dry soil weight and a

decrease in conductivity for additional increase in soil concentration.

• The soluble mineral content in soils is more heterogeneous than

homogenous.

• Soils solutions where NaCI is the dominate mineral will most likely be

unsaturated regardless of the water to soil extract were as C aS 04

solutions are more readily saturated at concentrated soil solutions since it

has a lower solubility than NaCI.

• Test methods to identify the concentrations of the various cations and

anions should have been performed on water to soil ratios other than 5:1

100

to allow further analysis of the effects of saturation and the relative

proportioning of the various soluble minerals encountered in Las Vegas

soils.

• The soluble mineral content of soils can be estimated by measuring the

conductivity of aqueous extracts from soil solutions. The optimum water to

soil ratio for the extract that insures the minerals are not saturated is 50:1.

The corresponding total dissolved mineral concentration in the solution

can be estimated from best fit correlations of total dissolved minerals as a

function of conductivity. The total dissolved solids can then be expressed

as a percentage of the original dry soil weight of the soil used to prepare

the soil solution.

• Comparisons with previous correlations show that the correlation from

this study provides higher estimates of the total dissolved solids for

equivalent conductivities estimated from the US Salinity Laboratory. The

difference is due to the use of Las Vegas soils to prepare the correlation

were US Salinity Laboratory was developed for the western region of the

United States. The Atlas correlation provides comparable estimates to this

study up to 3000 micromhos/cm for which beyond this the Atlas values are

greater than this study. This is due to the constant empirical value used to

relate total dissolved solids to conductivities greater than 3000

micromhos/cm. Also the correlation from this study was developed from a

limited data base.

101

• Engineering estimates that utilize correlations between conductivity and

total dissolved solids to estimate the soluble mineral content of soils

should be limited to residential, light commercial applications and as a

preliminary estimate for large scale projects were numerous samples shall

require testing.

• Further areas of study should include testing of a larger set of Las

Vegas soil samples to develop a more comprehensive and reliable

correlation curve based on measurements taken from unsaturated

extracts.

• Further study should include quantifying the effects of soluble minerals

on the strength characteristics of soils and the change in strength of soils

during the process of dissolution of the soluble minerals from the soil

structure.

• Further study should quantify the volume change that may be expected

based upon the types of minerals and their percentage of the dry soil

weight.

APPENDIX

102

103

Local Geotechnical Firms Test Procedures

Testing procedures to determine the soluble mineral content of soils

that are used by various local geotechnical firms are described below.

Firm A:

Dry approximately 150 grams of natural soil in a 60°C oven. Weigh the

sample then flush it four times with 500 ml of tap water for a total of 2000

ml of water. Redry and reweigh the soil. The difference between the soil

weights before and after flushing is the weight of the soluble minerals

which is expressed as a percent of the original weight.

Firm B:

Dry approximately 200 grams of natural soil in a 115°F oven. Weigh the

sample then flush with one gallon of distilled water. Redry and reweigh the

soil. The weight of the soluble minerals is expressed as a percent of the

original dry weight.

Firm C.

Dry approximately 300 grams of natural soil in a 60°C oven. Weigh

sample then flush it three times with about one-half gallon of deionized

water. The same water is used for all three flushings. Redry and reweigh

the soil. The weight of the soluble minerals is expressed as a percent of

the original dry weight.

104

Firm D

Dry soil at oven temperature not exceeding 140°F. Select between 150

and 200 grams of representative material. Large clumps may be broken

up, but care should be taken not to grind or change the grain size of

gypsum. Place sample in filter paper, Whatman No. 42 or equivalent, and

filtering apparatus such as a coffee filter or hand strainer. Inundate

sample with distilled or deionized water between 68°F and 75°F,

completely covering sample and allowing for some standing water above

the sample. Do not stir or adjust the sample. Repeat the inundation

process a minimum of nine times and a maximum of fourteen times. Total

water volume should be between 5000 +/- 500 ml. Redry and reweigh the

soil. The weight of the soluble minerals is expressed as a percent of the

original dry weight.

Table 51 presents the concentrations of the calibration standard

solutions used in Method 3111B to test the cation concentrations for the

5:1 soil extracts.

Table 51 Calibration Standards Method 3111B Atomic AbsorptionSpectrometry

Cation Mixed Standard Concentrations (mg/l) Wavelength1 2 3 4 X

Na .5 1.0 1.5 2.0 589K 1.0 1.5 2 2.5 766.5

Mg 2 4 8 10 202.6Ca 1 2 3 5 422.7

Tables 52 through 56 include the equivalent conductances the ionic

and equivalent conductances for the measured anion and cation

concentrations of the 5:1 soil extracts for soils A, L, M, O and D.

Table 52. Soil A Calculated Equivalent and Specific Conductancefor 5:1 Extract

SOIL A ConcentrationIonic

Conductance(X)

EquivalentConductance

(equiv/l) (mhos crrf̂ /equiv) (mhos/liter)

CationsMg+̂ .00796 53 .42188

C a w " .0288 60 1.728

Na+1" .07003 50 3.5015

K+1" .01017 74 .7525

Anions ***S 0 4" .03191 80 2.5528

C C ^ .000667 72 .048024

HCO3 '1 .000328 422 0.13842

cr1 .0593 76 4.506

Total equivalent conductance (mhos/l) 13.649

Calculated Specific Conductance (nmhos/cm) 13,649

Measured Specific Conductance (pmhos/cm) 10,500

I

107

Table 53. Soil L Calculated Equivalent and Specific Conductancefor 5:1 Extract

SOILL ConcentrationIonic

Conductance(A.)

EquivalentConductance

(equiv/l) (mhos/equiv) (mhos/liter)

CationsMg+/ .02280 53 1.208

Ca+* 'J .02660 60 1.596

Na+1" .07803 50 3.9015

K+1" .00170 74 .1258

Anions ***S 0 4v .05962 80 4.7696

C 0 3̂ .000667 72 .04802

HCO3 '1 .000328 422 .1384

c r 1 .07562 76 5.747

Total equivalent conductance (mhos/l) 17.5343

Calculated S pecific Conductance (pmhos/cm) 17534

Measured Specific Conductance (pmhos/cm) 11547

108

Table 54. Soil M Calculated Equivalent and Specific Conductancefor 5:1 Extract

SOILM ConcentrationIonic

ConductanceEquivalent

Conductance

(equiv/l) (mhos/equiv) (mhos/liter)

CationsMg+'" .00225 53 .11925

C a t * .02660 60 1.596

Na+1" .0025 50 .125

....... k ^ " ” " .0003 74 .0222

Anions ***S 0 4'" .02916 80 2.333

COs'^ 0 72 0

HCO3 '1 .000547 422 .2308

Cl'1 .00141 76 .1072

Total equivalent conductance (mhos/l) 4.5334

Calculated S pecific Conductance (pmhos/cm) 4533.4

Measured Specific Conductance (^mhos/cm) 2330

109

Table 55. Soil O Calculated Equivalent and Specific Conductancefor 5:1 Extract

SOIL 0 ConcentrationIonic

Conductance( X )

EquivalentConductance

(equiv/l) (mhos/equiv) (mhos/liter)

CationsMg+" ' .00378 53 .20034

C a +Z" .02994 60 1.7964

Na+1 .01522 50 .761

K+1 " .00102 74 .0755

Anions ***S 0 4* .04283 80 3.4264

C 0 3-z 0 72 0

HCOS1 .00033 422 .13926

cr1 .00409 76 .3108

Total equivalent conductance (mhos/l) 6.7097

Calculated Specific Conductance (pmhos/cm) 6709.7

Measured Specific Conductance (iamhos/cm) 3390

110

Table 56. Soil D Calculated Equivalent and Specific Conductancefor 5:1 Extract

SOILD ConcentrationIonic

Conductance(X )

EquivalentConductance

(equiv/l) (mhos/equiv) (mhos/liter)

CationsMg+̂ ' .00296 53 .1569

Ca+/ u .02395 60 1.437

Na+1 'u .00348 50 .174

K+1 " .00089 74 .06586

Anions ***\t-d*OC

O .03653 80 2.9224

cc>3 0 72 0

HCCV1 .00065 422 .2743

cr1 .00155 76 .1178

Total equivalent conductance (mhos/l) 5.0824

Calculated Specific Conductance (pmhos/cm) 5082.4

Measured Specific Conductance (pmhos/cm) 2820

Table 57 list the gravimetric factors used in the gravimetric analysis to

determine the soluble mineral compounds that the individual anions

dissociated from upon dissolution in water.

Table 57. Gravimetric Factors Of Various Soluble Minerals

Compound Formula Weight GravimetrCation

ic Factor Anion

NaCI 58.44 3.7 1.65MgCI2 125.84 2.3 1.77CaCI2 110.99 2.77 1.57

KCI 74.56 1.91 2.10Na2S04 142.02 3.1 1.48MgS04 150.98 2.75 1.57CaS04 136.12 3.4 1.42

NaHC03 83.99 3.65 1.38100.09 2.5 1.67

CaHC03 101.09 2.52 1.66

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