hydrogeological studies for the in-situ leach field test ... bromide tracer test ... figure 7....

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Hydrologic Field Test 1

Hydrogeological Studies For the In-Situ Leach Field Test at Florence, Arizona

Table of Contents

1. ABSTRACT ......................................................................................................................................... 2

2. INTRODUCTION............................................................................................................................... 3

3. GEOLOGY .......................................................................................................................................... 7

4. REGIONAL HYDROGEOLOGY .................................................................................................... 9

5. WELLFIELD DESRIPTION AND WELL CONSTRUCTION ....................................................10

6. HYDROGEOLOGIC TESTS ...........................................................................................................13

6.1 INTERFERENCE PUMPING TESTS ........................................................................................................13

6.2 TRACER TESTS .................................................................................................................................14

6.2.1 Water Injection Test ...............................................................................................................14

6.2.2 Bromide Tracer Test ..............................................................................................................15

7. INTERPRETATION OF HYDROGEOLOGIC TESTS ...............................................................20

8. NUMERICAL PREDICTION PRIOR TO THE LEACH TEST ..................................................26

9. FIELD LEACH TEST .......................................................................................................................28

9.1 FIELD TEST RESULTS ........................................................................................................................28

9.2 HYDRAULIC CONTROL .....................................................................................................................31

9.3 POND EVAPORATION ........................................................................................................................31

9.4 ELECTRIC RESISTANCE TOMOGRAPHY .............................................................................................35

9.5 RECLAMATION TESTS ......................................................................................................................35

10. CONCLUSIONS ................................................................................................................................37

11. RECOMMENDATIONS FOR FUTURE STUDIES ......................................................................38

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LIST OF FIGURES

Figure 1. Schematic draw of in situ leaching system. ........................................................ 4

Figure 2. Location of Florence In Situ Project. .................................................................. 5

Figure 3 Well layout in the field leach test. ....................................................................... 6

Figure 4. Geologic east-west cross section at 745700N. ................................................... 8

Figure 5. Location of the pre-leaching test. ..................................................................... 11

Figure 6. The design of wells. .......................................................................................... 12

Figure 7. Theis-type curve analysis for BHP-1 in BHP-5 pumping test. ........................ 14

Figure 8 Wellfield setup for the water injection test........................................................ 15

Figure 9. Relative concentrations of sulfate in recovery wells in water injection tests. .. 16

Figure 10. Relative Br concentration vs. time curves ...................................................... 17

Figure 11. Diagram representation of Br percentage reaching pumping wells ............... 19

Figure 12. Field drawdown curves and the calibrated drawdown curves for the BHP-9

pumping test. ............................................................................................................. 22

Figure 13. Calibrated results of hydraulic conductivity ................................................... 24

Figure 14. Sulfate concentrations of field data and calibration results. ........................... 24

Figure 15. Simulated and measured Bromide concentrations in BHP-6, BHP-7, BHP-8,

and BHP-9 ................................................................................................................. 25

Figure 16. Plan view of concentration spreading within the first mining block after 10

and 365 days. ............................................................................................................ 27

Figure 17. Vertical concentration profile from a cross section between injection wells

BHP-6 and BHP-8, and production wells BHP-1, BHP-10, and BHP-12 after one

year. Each layer represents 20 feet. ......................................................................... 27

Figure 18. Net positive pumping rate. ............................................................................. 29

Figure 19. pH of pumping wells during leaching phase. ................................................. 30

Figure 20. Evaporation rate of the evaporation pond. ..................................................... 31

Figure 21. Copper grade in pregnant solution from BHP-1. ........................................... 32

Figure. 22. Copper grade in pregnant solution from recovery wells. .............................. 33

Figure 23. Water levels in four pair of observations. ...................................................... 34

Figure 24. Conductivity difference from background after one month of acid injection. 36

Figure 25. Sulfate recovery during the leaching and rinsing phases. .............................. 37

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1. ABSTRACT

BHP Copper conducted a field test for the feasibility study of an in situ copper

leaching project near Florence, Arizona. Twenty wells were drilled in five-spot pattern at

a spacing of 50 feet. Hydrologic tests and field measurements were conducted to

understand the hydrologic system. Conceptual and numerical groundwater models were

built. Inverse modeling was used to determine the spatial distributions of hydrologic

parameters. The calibrated model was then used to predict the flow of injected solution

in the field test

The injection of dilute sulfuric acid solution started on Oct. 31, 1997 and stopped

on Feb. 9, 1998. The reclamation started upon the cessation of acid injection. During the

acid injection and reclamation phases, chemical samples were taken on a regular basis.

The water levels and flow rates were recorded to meet environmental requirements.

Electrical Resistance Tomography was used to monitor the solution flow. All the

information collected was used to understand the hydrogeologic system at the Florence in

situ leach site.

2. INTRODUCTION

BHP Copper is conducting a feasibility study for an in situ copper leaching

project near Florence, Arizona. The design of the in-situ mining process is surface well-

to-well leaching. Dilute sulfuric acid solution is injected into the oxide ore deposit,

which is 400 feet below the ground surface, via injection wells constructed to protect the

regional aquifers, as shown in Figure 1. The solution travels through fractures and

porous spaces in the ore body and reacts with the minerals, mobilizing and transporting

copper in solution. The pregnant solution is then recovered by a series of pumping wells

surrounding the injection wells and sent to the processing plant for copper extraction. The

economic and environmental criteria require the understanding the site hydrologic system

to address the concerns not only of BHP Copper but also of the regulatory agencies.

Specifically, the well injection capacities, flow paths, and solution control need to be

understood.

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Figure 1. Schematic draw of in situ leaching system.

Florence copper deposit is located two miles northwest of Florence, Arizona

(Figure 2). The evaluation of the deposit began in 1969 when regional reconnaissance of

Arizona by the Continental Oil Company (Conoco) led to an examination of potential

copper resource at Florence. Conoco started drilling in March 1970, and by August

1970, core samples from drilling indicated that a potential ore body had been discovered.

A pilot mine and a plant were developed later. However, the project did not go forward

presumably due to a low copper price at the time and the relatively large capital

investment.

Magma Copper Company (now part of the BHP Copper) acquired the property in

July 1992 from Conoco and initiated a pre-feasibility study in January 1993. The study

focused on determining the most appropriate mining technology. The two methods

evaluated were Open Pit mining followed by heap leaching and in situ solution mining,

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Figure 2. Location of Florence In Situ Project.

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either method followed by Solvent Extraction and Electrowinning (SX/EW) to

produce a copper cathode product. The outcome of the study is that stand alone in situ

mining followed by solvent extraction and electrowinning is the technically preferred

method of development for this deposit (RDTG, 1995).

A field leach test was conducted from Oct. 31, 1997 to Feb. 9, 1998 as a part of

the pre-feasibility study. The objectives of the test were three-folds. First, the test was

conducted to verify the ability to operate the wellfield in a manner that met the

environmental requirements for the APP and UIC permits. Second, the test was used to

understand the in situ leach process, primarily the copper recovery curve as a function of

time. The test was also conducted to prove the ability to reclame. Twenty wells were

drilled in a spacing of 71 feet (Figure 3). Prior to the leach test, interference pumping

tests and tracer tests were conducted. Conceptual and numerical groundwater models of

the Florence mine site were built. The inverse modeling was used to determine the

spatial distributions of some hydrologic parameters.

Northowb 2

Observation well

Pumping well

Injection well

owb 5

bhp 12 bhp 10

bhp 13

owb 1owb 3

bhp 2

bhp 3bhp 4

bhp 5

bhp 6

bhp 7

bhp8

bhp 9

bhp 11

bhp 1

ch 2ch1

owb 5

71 ft

50 ft

Figure 3 Well layout in the field leach test.

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The reclamation started upon the cessation of acid injection and is planned to be

completed in the late 1999. During the acid injection and reclamation phases, chemical

samples were taken on a regular basis. Electrical Resistance Tomography was used to

monitor the solution flow. All the information collected was used to understand the

hydrogeologic system at the Florence in situ leach site.

3. GEOLOGY

The Florence deposit is in the Basin and Range Province of southern Arizona, in

the Eloy sub-basin of the Pinal Active Management Area (AMA). The geologic section

in the immediate Florence area is relatively simple in terms of rock types present, but

complicated in terms of structure. The oldest known rock type in the Florence area is a

Precambrian quartz monzonite prophyry of botholithic dimensions. Paleozoic

sedimentary rocks are considered to have overlain this rock originally, but these have

been removed by subsequent erosion. The Precambrian quartz monzonite is intruded by

a granodiorite prophyry of Laramide age. Figure 4 shows a cross section. This intrusive

event was accomplished by the development of extensive fractures which guided the flow

of hydrothermal fluids responsible for formation of the primiary sulfide mineralization at

Florence (see Florence Project Final Pre-Feasibility Report, Section II for detail).

Copper mineralization occurs in strongly faulted and fractured rocks of

Precambrian and Tertiary ages. Depending on copper mineralization, the bedrock is

subdivided into an upper oxide zone and a lower sulfide. The resource classification in

the oxidized portion of the mineralized system yields a total of 321 million tons with

average grades of 0.25% ASCu and 0.37% TCu, which is the leaching target. The sulfide

zone beneath the oxide has more than 1 billion tons of materials averaging 0.287% TCu.

The most structural features in the bedrock are dominated by a series post-mineral

faults. The thickness of these fault zones ranges from a few feet to as much as 50 feet.

Clayey, gaugy, and sheared zones on the fault walls are commonly associated with these

large-scale structural features.

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Figure 4. Geologic east-west cross section at 745700N.

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The rock units in the deposit area are intensely fractured and shattered. The

fracture in diamond core has been geologically logged in great detail. Statistical analysis

shows that there are an average of 11 to 15 fractures per foot compared 6 to 10 fractures

per foot in sulfide zone. The average spacing between fractures in oxide is about 1 inch

and 80% of the spacing less than 2 inch. Fractures in all directions have been found to

have copper mineralization.

4. REGIONAL HYDROGEOLOGY

The Florence oxide deposit is buried under more than 300 ft of post-mineral

cover. The sedimentary overburden material, possibly correlative to the Gila

conglomerate, is of Quaternary age and consists mainly of unconsolidated silt, sand,

gravel, and boulders. The 300 ft basin-fill overburden is divided into three major units:

the Upper Basin-fill Unit (UBFU), Middle Fine-Grained Unit (MFGU), and Lower

Basin-Fill Unit (LBFU). The UBFU is composed of unconsolidated to weakly cemented,

interbedded clay, silt, sand, gravel and boulders. Its thickness ranges from 200 ft to about

500 ft in the mine area. The UBFU is the principal source of groundwater in the area.

The average hydraulic conductivity is about 60 ft/day. The MFGU, underneath the

UBFU, is a layer composed of calcareous silt and clay that is about 20 to 40 feet thick

and has very low permeability. The thickness of the LBFU varies from less than 50 ft on

the east of the mine site to about 800 ft on the west. It consists of weakly to moderately

cemented boulder, gravel, sand, silt and clay. The pumping tests and groundwater

geochemical data indicate that the LBFU and UBFU are hydrologically separated by the

almost impermeable MFGU with conductivity less than 0.001 ft/day. The hydraulic

conductivity of the LBFU is about 5 ft/day, which is one order of magnitude smaller than

the UBFU.

The hydraulic conductivity of the oxide zone varies from 0.1 ft/day to 1.0 ft/day

with a geometric mean of 0.5 ft/day. The pumping tests showed the oxide zone and the

LBFU have direct hydrologic communication. This indicates that the injected solution

could move away from the oxide to the LBFU if solution flow were not controlled. The

sulfide zone is much less permeable compared with the oxide zone. The average

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hydraulic conductivity is about 0.003 ft/day. The sulfide zone has been considered as an

impermeable boundary condition for the oxide aquifer.

The oxide and sulfide zones exist under saturated conditions. Generally,

groundwater in the mine area occurs at an elevation ranging from 1,250 to 1,350 feet

above sea level, which is approximately 100 to 200 feet below land surface. The

groundwater level varies as a function of weather, rainfall, and irrigation use. Generally,

the water level declines in summer and rises in winter.

The measurement of water levels near the mine site suggests that the general

direction of groundwater flow is from southeast to northwest at an approximate gradient

of 0.5 percent. The direction and gradient are caused by groundwater recharge and

withdrawal, and therefore vary with time. The UBFU is recharged primarily by

subsurface flow from the Gila River channel, as well as by percolation from agricultural

water and a small amount of rainfall. Some recharge also occurs as percolation of surface

runoff from the mountains at the basin perimeter. The LBFU is recharged primarily by

subsurface flow originating from the Gila River. The oxide zone is recharged from

LBFU and from subsurface flow under the Gila River. The primary withdrawal in the

area is used for agriculture. Since the early 1980s, the recharge and withdrawal have

been about equal and the water level has been stabilized (Florence PreFeability Report).

5. WELLFIELD DESRIPTION AND WELL CONSTRUCTION

The field test area is located in the southeast corner of the proposed leach zone, as

shown in the Figure 5. The area is selected because it is representative of the leach zone.

Figure 3 shows the wellfield layout. BHP-6 to BHP-9 are the four injection wells

surrounding the center production well, BHP-1. The copper recovery rate obtained from

this test area is believed to simulate the repeated five-spots in the large scale production

and will be used in the final financial analysis. Each of the four injection wells is

surrounded by four production wells in a five-spot pattern. Further out, observation wells

(OWB wells) have been positioned to monitor lateral solution excursion in the Oxide

Unit. Wells OWB2 and was used to monitor the UBFU. Two metallurgical wells were

drilled in between BHP-1 and BHP-6 to sample the solution in between the central

production well and an injection well.

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A universal well (uni-well) design for both injection and production wells was

used. The uni-well design allows easy switching between injection and production wells

to enhance sweep efficiency and to reduce well clogging and precipitation of minerals in

fractures. Figure 6 shows a schematic well design. The well is cased and the annulus is

cemented from the surface to 40 to 60 feet below the top of the oxide. To demonstrate

vertical solution control, annular conductivity devices were installed in the Upper Basin

Fill Unit (UBFU) at injection and production wells. The diameter is six inches wide. A

40 foot buffer zone (or bedrock exclusion zone) is designed to reduce potential solution

excursion into the lower basin-fill unit. The lower portion of the well is an open hole

approximately six inches in diameter. Four-inch or 1 ½ inch PVC screen is installed

inside the open hole. Because solution flowing into wells through fracture rock is very

clean, gravel pack was not used. This design also reduces well clogging due to bacteria

growth in gravel packs. The reverse circulation (RC) drilling method was used. This

technique allows the flexibility of being able to log and sample the drill cuttings for ore

control and to assess qualitatively the hydraulic character of the rock.

P12-O

P13.2-

P15-O

P19.1-

P28.2-

P8.1-O

PW1-

PW2-

PW2-

PW3-

PW7-

OWB

Test area

Figure 5. Location of the pre-leaching test.

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Figure 6. The design of wells.

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6. HYDROGEOLOGIC TESTS

6.1 INTERFERENCE PUMPING TESTS

Pumping tests were conducted in BHP-1, BHP-2, BHP-4, BHP-5, BHP-6, BHP-7,

BHP-8, BHP-9, BHP-10, BHP-11, BHP-12, and BHP-13, respectively. No tests were

conducted in BHP-3 because of time restriction. In each pumping test, the drawdown in

the pumping well and surrounding wells are recorded using In Situ data loggers and PWI

data recorders. The pumping rate in a pumping test was kept constant using PWI

controllers. Each test lasted from one day to three days. The next test started only after

the water levels had been recovered from the previous test. In BHP-6 and BHP-9 tests,

two cycles of pumping and recovery were employed.

Figure 7 is the typical drawdown curves in semi-log plot. The drawdown in all

wells shows a linear relationship in the period of 10 minutes to 1,000 minutes. The

curves flatten in later time because of the inflow leaking from the upper basin-fill aquifer.

All the analytical solutions for leaky aquifers assume that the overlying aquifer is much

less permeable than the tested aquifer. At the Florence site, the overlying basin-fill

aquifer is more much permeable than the oxide. Therefore, the analytical solutions for

leaky aquifers are not applicable. Numerical inverse modeling (calibration) is used to

estimate hydraulic parameters by matching the field data with the simulated results. To

provide an initial estimation for the numerical simulations, analytical solutions for

confined aquifers are used to analyze the drawdown curves using the pre-flattened

portion.

Figure 7 also shows the fit between field data and the Theis’ analytical solution.

The hydraulic conductivity generated was 0.8 ft/day and storativity was 0.0005. The

Theis solution assumes that the rock is homogeneous and isotropic. Analyses of all the

pumping tests were conducted. The mean hydraulic conductivity of the all pumping tests

was about 0.5 ft/day and storativity was 0.0005. The analytical results were quite

uniform because each value represents the mean value over a large volume. However,

the analytical results cannot reproduce the recovery curves in the tracer tests, because the

local heterogeneity, which is important in the transport, was smeared. This was the major

motivation of using inverse modeling to calibrate the hydraulic parameters.

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It was seen that when the pumping well and observation well were switched, the

hydraulic conductivities obtained were very close. This is an indication that the data are

reliable.

0.01

0.1

1

10

100

0.1 1 10 100 1000 10000

Time, Minutes

Dra

wdo

wn,

ft

BHP 1

Fitting

Figure 7. Theis-type curve analysis for BHP-1 in BHP-5 pumping test.

6.2 TRACER TESTS

6.2.1 Water Injection Test

Two tracer tests were conducted. The first tests involved BHP-1, BHP-2, BHP-3,

BHP-4, and BHP-5. The rest of the wells had not been drilled when this test was

conducted. Figure 3 shows the wellfield setup for this test. Upper basin water from well

M10-GU, which is 1500 feet away from BHP-1, was injected into the central well, BHP-

1, at a rate of 52 gpm. The injected water was recovered from four corner wells, BHP-2,

BHP-3, BHP-4, and BHP-5 at a rate of 13 gpm per well. The injection test lasted about

two and a half months from March 12 to May 27, 1997. The test is, therefore, called

water injection test. Sulfate in the injected water was used as a tracer. The concentration

of sulfate in the upper basin water is 230 mg/l compared to 65 mg/l in the oxide water.

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The chemical lab tests and numerical simulations of chemical reaction show that sulfate

can be treated as a conservative tracer in this water injection test.

BHP-5Pump52 GPM

Booster pump

Pump13 GPM

52 GPM

Tank 21,000 GBleed port

Well M10-GU

BHP-1

BHP-2

BHP-3BHP-4

600 ft

Figure 8 Wellfield setup for the water injection test.

Samples were taken daily from pumping and injection wells. All the major ions

were analyzed by the BHP Metallurgical Laboratory in San Manuel, Arizona. Figure 9

shows the sulfate concentrations in all the wells. It is seen that the earliest detectable

breakthrough of sulfate happened at in BHP-4 at day three. BHP-3 and BHP-5 had

breakthroughs at day 5 and 6 respectively. The breakthrough was not seen until ten to

fifteen days in BHP-2. The different arrival times are attributable to the heterogeneities in

the oxide rock and ambient groundwater gradient, which is in the northwesterly direction.

6.2.2 Bromide Tracer Test

Following the 90 days of leaching operation of the field test area, the reclamation

phase began. During reclamation, a tracer test using Bromide was conducted. Ground

water from a nearby well WW-4 (3,000 ft away) and Sodium Bromide solution were

mixed together with an inline mixer. The mixed solution, with Bromide concentration of

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54 ppm, was injected into BHP at 55 gallon per minute. The injected solution was

recovered from BHP-2, BHP-3, BHP-4, BHP-5, BHP-6, BHP-7, BHP-8, and BHP-9.

Table shows the average pumping and injection rates during the tracer test. The average

total pumping rate was 93 gpm. Bromide was injected for 45 hours. After this time, only

ground water was injected into BHP-1.

Figure 9. Relative concentrations of sulfate in recovery wells in water injection

tests.

The relative concentrations vs. time curves are presented in Figure 10. Figure 11

is a diagrammatic representation of how much solution reached each pumping well.

About 56% of the injected bromide had been recovered in one month period with forcast

of 90% in three months. Notice that the percentage of Bromide solution recovered at the

pumping wells is not necessarily proportional to the pumping rate or distance from the

injection well. A large difference in heterogeneity and communication exist. BHP-7

pumped at only 7 gpm and recovered 12% Bromide; BHP-6 being at an equal distance of

50 ft from the injection well, pumped at a higher rate of 11 gpm but received only 9%

Bromide. BHP-8 and BHP-9 are of equal distance from the injection well and had a

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 10 20 30 40 50 60

Day

Rel

ativ

e Su

lfate

Con

cent

ratio

n BHP1

BHP2

BHP3

BHP4

BHP5

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Figure 10. Relative Br concentration vs. time curves

Br Field TestRelative Concentration vs Time

Injected 50 ppm for 2 days into BHP1

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

05/13/98 05/18/98 05/23/98 05/28/98 06/02/98 06/07/98 06/12/98

Day

Rel

. Con

c. (p

pm)

BHP2

BHP3

BHP4

BHP5

BHP6

BHP7

BHP8

BHP9

BHP10

CH1

CH2

Br Field TestRelative Concentration vs Time

Injected 50 ppm for 2 days into BHP1

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

05/13/98 05/18/98 05/23/98 05/28/98 06/02/98 06/07/98 06/12/98

Day

Rel

. Con

c. (p

pm)

BHP2

BHP3

BHP4

BHP5

BHP6

BHP7

BHP8

BHP9

BHP10

CH1

CH2

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similar pumping rate of 12 gpm, but they had different results. Fifteen percent of

Bromide reached BHP-8 and only 6% Bromide reached BHP-9.

BHP-2, BHP-3, BHP-4, and BHP-5, which are farther (71 ft) from BHP-1

recovered negligible amount of Bromide, with the exception of BHP-4. BHP-4 pumping

at 12 gpm recovered 10% Bromide. BHP-10, even farther (113 ft) from BHP-1, pumped

only 6 gpm, and recovered 3% of Bromide.

Notice that most of the recovery was obtained from wells (BHP-7, BHP-8, BHP-

4) on the northwest quadrant of the wellfield. These results indicate that there is a very

good communication between BHP-1 and those wells. There is also a good connection

with wells east of BHP-1 (BHP-6 and BHP-10). The vectors in Figure 11 indicate the

direction and quantatity of Bromide recovery.

The test demonstrated that Bromide is an excellent non-reactive tracer.

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Figure 11. Diagram representation of Br percentage reaching pumping wells

North

6 % Br

BHP9

BHP6

BHP7

BHP8

BHP4

BHP2BHP5

BHP3

BHP10

(13 gpm) (12 gpm) (7 gpm)

(12 gpm) (7 gpm) (7 gpm)

(12 gpm) (11 gpm) (6 gpm)

BHP11

BHP12

BHP13

owb1

owb3

owb4owb5

12 % Br

15 % Br

10 % Br 2 % Br

0 % Br

9 % Br

0 % Br

3 % Br

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7. INTERPRETATION OF HYDROGEOLOGIC TESTS

Numerical inverse modelings were used to calibrate hydraulic parameters in the

oxide zone, based on the field tests. The flow distribution was simulated using

MODFLOW, a flow model developed by the U.S. Geological Survey (McDonald and

Harbaugh, 1988). The tracer movement was simulated using MT3D, a transport model

that considers advective and dispersive processes (Zheng, 1990). The inverse calculation

was conducted using a Parametric Estimation software, PEST.

To calibrate the parameters, an initial estimation of these parameters was put in

the models. The results of the simulations were then compared with field data. If the

match was not satisfied, the hydraulic parameters were adjusted by PEST and simulations

were repeated, until a satisfactory comparison of field data and simulation results were

achieved. The interference pumping test data were used to calibrate the distribution of

hydraulic conductivity. The water injection test data were used to calibrate the

dispersivity, and effective porosity. Because of the heterogeneous nature of the rocks, the

calibration is usually non-unique. However, the major features within the area

surrounded by the wells will be captured.

The model grid covers 7,000 ft by 7,000 ft of area within the test site and 640 ft of

depth. The grid design is characterized by non-uniform cell spacings of 10 ft (middle

cells), 20 ft, 40 ft, 60 ft, 135 ft, and 140 ft which encompasses the boundary of 7000 ft by

7000 ft. The wells were positioned in the inland block which is separated (discretized) in

10 ft x 10 ft cells. The grid’s perimeter cells were assigned constant head boundary

conditions. The vertical dimension of the model was discretized into 3 layers. The top

layer covers the lower basin fill unit. The second layer covers the top 40 feet of oxide.

The bottom layer covers the rest of the oxide. All the layers were considered to be

confined aquifers. The transmissivities were allowed to vary and were calculated from

the saturated thickness and hydraulic conductivity. The storage coefficients were

considered to be uniform.

The test area was divided into 37 distinct zones. A high conductive zone and two

low conductive zones were introduced along BHP-5 and BHP-9, based on the

examination of pumping data and tracer data as well as the geological features. Some

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zones were combined as one zone during the calibration process to reduce the number of

parameters, in order to reduce the computation time and enhance the certainty of

calibration.

Figure 12 shows the field drawdown curves and the calibrated curves for BHP-9

pumping tests. Other 17 pumping tests were calibrated simutaneously. There were more

than 350 plots. Figure 13 shows the calibration results of transmitivity. The results show

that the zone connecting BHP-5 and BHP-9 does have significant differences with the

surrounding rocks. The conductivity of the high conductive zone is 5 ft/day, sandwhched

by the low conductive zones of as low as 0.1 ft/day. This feature indicates that there is a

short circuit between BHP-5 and BHP-9. The wellfield is separated into two somehow

isolated areas.

The SO4 concentration curves of the calbration and field water injection test data

were shown in Figure 14. The dispersivity value obtained is 70 feet and the effective

porosity is 6%. This is consistent with the previous studies (Orr, 1997). The match is

surprisely good, considering that the dsipersivity value and the effective porosity were

treated uniform. It was found that introducing more zones of dispersity and effective

porosity only slightly improved the match.

The bromide test was not used to calibrate the model because the bromide test

was conducted after three months of leaching. The conditions of rock had been changed

since the pumping tests and water injection tests were conducted. However, the test was

used to validate the numerical simulations, as shown in Figure 15. The match is

qualitatively good. The slower arrival times in the test data indicates the porosity

increased due to the leaching of minerals.

In summary, the approach of equivalent porous medium embedded significant

discrete features matches the field data very well in such a highly fractured and

heterogeneous rock, as was found by Neuman (1982).

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Figure 12. Field drawdown curves and the calibrated drawdown curves for the

BHP-9 pumping test. BHP2

-2

3

8

13

18

1 10 100 1000 10000minutes

BHP3

-2

3

8

13

1 10 100 1000 10000minutes

BHP4

-2

3

8

13

1 10 100 1000 10000minutes

BHP5

-238

131823

1 10 100 1000 10000minutes

BHP6

-2

3

8

13

1 10 100 1000 10000minutes

BHP7

-2

3

8

13

1 10 100minutes

BHP8

-2

38

13

18

1 10 100 1000 10000minutes

BHP9

-28

182838

1 10 100 1000 10000

minutes

BHP10

-2

3

8

13

1 10 100 1000 10000minutes

BHP12

-2

3

8

13

18

1 10 100 1000 10000minutes

BHP13

-2

3

8

13

18

1 10 100 1000 10000minutes

OWB1

-2

3

8

13

1 10 100 1000 10000minutes

OWB3

-202468

1 10 100 1000 10000

minutes

OWB4

-2

3

8

13

18

1 10 100 1000 10000minutes

OWB5

-2

3

8

13

1 10 100 1000 10000minutes

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X

Y

Z Background K = 0.6

K= 0.03K= 0.01

K= 0.01

K=0.25

K= 0.08

K= 9.7

Figure 13. Calibrated results of hydraulic conductivity

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 10 20 30 40 50 60 70

day

rela

tive

conc

entr

atio

n

BHP2 meas

BHP2 calc

BHP3 meas

BHP3 calc

BHP4 meas

BHP4 calc

BHP5 meas

BHP5 calc

Figure 14. Sulfate concentrations of field data and calibration results.

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Figure 15. Simulated and measured Bromide concentrations in BHP-6, BHP-7, BHP-8, and BHP-9

BHP6 Br Test

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

Day

Rel

ativ

e C

once

ntra

tion

0

0.2

0.4

0.6

0.8

1

1.2

Wei

ght

Measured

Calculated

Residual

Weight

BHP7 Br Test

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

Day

Rel

ativ

e C

once

ntra

tion

0

0.2

0.4

0.6

0.8

1

1.2

Wei

ght

Measured

Calculated

Residual

Weight

BHP8 Br Test

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

Day

Rel

ativ

e C

once

ntra

tion

0

0.2

0.4

0.6

0.8

1

1.2W

eigh

t

Measured

Calculated

Residual

Weight

BHP9 Br Test

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35

Day

Rel

ativ

e C

once

ntra

tion

0

0.2

0.4

0.6

0.8

1

1.2

Wei

ght

Measured

Calculated

Residual

Weight

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Draft


8. NUMERICAL PREDICTION PRIOR TO THE LEACH TEST

The hydrologic parameters obtained in the water injection tests were used in the

simulations to predict the solution flow in the field leach test. Because of the lack of data

and model incapability, chemical reactions were not included in the model and studied

separately in a simplified model (See Florence Pre-Feasibility Report, Vol. IV). This

represents a worst-case scenario for the spreading of injected solution because the

neutralization processes should retard the solution movement.

Figure 16 presents the plan view of lateral concentration spreading after 10 days

and 365 days of leaching in the oxide zone. A five percent hydraulic gradient in the

northwest direction was imposed in the simulations to account for irrigation wells effect

on the natural hydraulic gradient of 0.5 percent. After ten days, the concentration plume

did not surpass the area bounded by BHP-10 to BHP-13. The inner production wells

(BHP-1 through BHP-5) start exhibiting about 22 percent of the leaching concentration.

BHP-2 is an exception; only 12 percent of the leaching concentration is achieved after the

first 10 days. At this time, the outer production wells (hydraulic control wells, BHP-10 to

BHP-13) display a 3 percent to 8 percent concentration, while the observation wells have

no sign of the solution. This figure shows that the outer production wells, besides

functioning to maintain hydraulic control, will also capture part of the leaching solution.

After one year, the model indicates that the 0.05 concentration contour could travel

between 50 ft to less than 100 feet beyond the area bounded by BHP-10 to BHP-13. The

injected solution will be detected in the observation wells (OWB1, OWB3, OWB4,

OWB5) located 50 feet from production wells. The model predicts that observation well

OWB6 (not shown in Figure 16), constructed about 200 feet northwest from the leaching

block, will not see the injected solution. The concentration of injected solution at BHP-1

is about 85 percent, representing a 15 percent dilution. The observation wells, which are

50 ft from the outer production wells, display 12 to 38 percent concentration. By the

year-end, most of the wells are on their way to reaching equilibrium. The simulations

represent the worst case scenario. The solution spreading shall be smaller due to solution

neutralization by the rock formation. If this phenomenon is confirmed during the leach

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Figure 16. Plan view of concentration spreading within the first mining block

after 10 and 365 days.

0.050.100.200.300.400.500.700.80

0.95

UBFU

OXIDE

LBFU

MFGU

MODFLOW BC Symbols

Well

Point Source/Sink

MT3D BC Symbols

Figure 17. Vertical concentration profile from a cross section between injection

wells BHP-6 and BHP-8, and production wells BHP-1, BHP-10, and BHP-12 after one

year. Each layer represents 20 feet.

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test, the magnitude of the spreading should be taken into account in future in-situ

leaching design around the permitted leaching boundary zone. The wells shall be placed

at a minimum of 150 feet from the permit boundary.

Figure 17 presents a vertical concentration profile from a cross section between

injection wells BHP-6 and BHP-8, and production wells BHP-1, BHP-10, and BHP-12.

The wells are placed 40 ft underneath the LBFU and oxide contact zone. The cross

section between the two other injection wells has similar concentration contours. It is

seen that the injected solution had moved 20 ft to 40 ft into the LBFU at the end of one

year. The simulations assume that the hydraulic conductivity in the LBFU does not

change. The laboratory test on cores from LBFU showed that the permeability of LBFU

could be reduced 50% during the leaching because of mineral precipitation. The actual

vertical excursion could be smaller.

9. FIELD LEACH TEST

9.1 FIELD TEST RESULTS

Injection of sulfuric acid solution started on October 31, 1997. The leaching

solution was made of ground water from Florence, concentrated sulfuric acid from San

Manuel, and San Manuel raffinate. The injected solution had a pH of 1.6.

The wellfield was operating at a total injection rate of 122 gpm and a total

recovery rate of 150 gpm.

Table 1 presents the average injection and pumping rates for the wells. The rates

declined with time due to the clogging of pumps. The pumping rates was kept low

thereafter. The problem was solved later by injecting raffnate around pumps to disolve

the precipitation on the pumps. Injection rates ranged between 21 to 35 gpm per well.

Pumping rates in the inner production wells ranged between 14 to 19 gpm per well.

Pumping rates in the perimeter pumping wells ranged between 10 to 13 gpm. The total

pumping rates have been kept larger than the total injection rates, based on permit

requirements. The net pumping and injection rates are shown in Figure 18. BHP1, which

is surrounded by the four injection wells, was the main production well. It pumped at an

average rate of 39 gpm.

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Draft


The leach solution had an average pH of 1.6. During the leaching phase, the pH

of most recovery wells gradually decreased as the leach solution traveled from the

injection point to the pumping wells. The main production well, BHP-1, reached a pH of

3 at the end of the leaching period. The rest of the pumping wells ranged between pH of

3 and 7. The pH decline was hardly noticeable at BHP-13 and BHP-2 (Figure 19).

Table 1. Average injection and pumping well rates during leaching phase

Figure 18. Net positive pumping rate.

Inflow vs. Outflow Rates

02040

6080

100

120140

160180

200

10/31/97 12/30/97 02/28/98 04/29/98 06/28/98 08/27/98 10/26/98 12/25/98 02/23/99 04/24/99

Rate

(gpm

)

Injectate to f ield

Total Flow from Field

average averageWell ID injection (gpm) Well ID pumping (gpm)BHP6 32 BHP1 39BHP7 35 BHP2 14BHP8 21 BHP3 17BHP9 33 BHP4 16sum 122 BHP5 19

BHP10 11BHP11 11BHP12 10BHP13 13

sum 150

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Figure 19. pH of pumping wells during leaching phase.

The copper production at the main pumping well, BHP-1 and other recovery wells

are shown in

Figure 21 and Figure 23. As is found in the hydrogeologic investigations, a short

circuit exists between injection well BHP-9 and pumping well BHP-5. Therefore, a

greater copper concentration is seen at BHP-5. The maximum concentration was about

250 ppm. BHP-10 reached about 200 ppm copper after 100 days of leaching. Wells

BHP-2, BHP-12, and BHP-13 showed low copper levels because of the low conductive

zones discussed in the previous section.

pH of Recovery Wells

1

2

3

4

5

6

7

8

11/1/97 12/1/97 12/31/97 1/30/98

Date

pH

BHP1

BHP2

BHP3

BHP4

BHP5

BHP10

BHP11

BHP12

BHP13

INJECTATE

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9.2 HYDRAULIC CONTROL

Environmental permits require a) the total injection be less than the total recovery,

and b) perimeter pumping wells have a deeper water level than the adjacent observation

wells. These have been achieved during the leaching and reclamation periods as shown

in Figure 18 and Figure 23.

9.3 POND EVAPORATION

A seven-arce pond was built to store and evaporate the preganant leach solution.

Three spary lines were installed to enhance the evaporation. The daily evaporation rate

was estimated by measuring the pond solution level as shown in Figure 20 The natural

evaporation rates vary from 2 gpm/arce in winter to about to 7 gpm/arce in summer. The

average is 3 gpm/arce. The total evaporation rates, including four spary lines, vary from

4 gpm/arce in winter to about to 12 gpm/arce in summer. Increase the number of spary

lines in the pond would increase the evaporation rate. But the number of lines should be

limited to 1 to 1.5 per arce. The rate provides the bases for the design of evaporation

ponds for the full production.

Figure 20. Evaporation rate of the evaporation pond.

0

4

8

12

16

15-Apr 24-Jul 1-Nov 9-Feb

Gal

lon

per m

inut

e natural evaTotal Evaporation

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bhp1 Cu

0

20

40

60

80

100

120

140

10/17/97 11/6/97 11/26/97 12/16/97 1/5/98 1/25/98 2/14/98Time, day

Cu,

ppm

Figure 21. Copper grade in pregnant solution from BHP-1.

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Figure. 22. Copper grade in pregnant solution from recovery wells.

Copper Concentration during Leaching Phase

0

50

100

150

200

250

300

10/30/97 11/29/97 12/29/97 1/28/98

Date

BHP1BHP10BHP11BHP12BHP13BHP2BHP3BHP4BHP5

SWVP-022588

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Figure 23. Water levels in four pair of observations.

Hydraulic ControlNE well pairs

140

150

160

170

180

190

200

210

220

10/01/97 12/30/97 03/30/98 06/28/98 09/26/98 12/25/98 03/25/99 06/23/99

pumping wellOBS well

Hydraulic ControlNW well pairs

100

150

200

250

300

350

10/01/97 12/30/97 03/30/98 06/28/98 09/26/98 12/25/98 03/25/99 06/23/99


Hydraulic ControlSW well pairs

140

150

160

170

180

190

200

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220

10/01/97 12/30/97 03/30/98 06/28/98 09/26/98 12/25/98 03/25/99 06/23/99


Hydraulic ControlSE well pairs

100

120

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180

200

220

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9.4 ELECTRIC RESISTANCE TOMOGRAPHY

To monitor the flow paths of solution from injection well to pumping wells,

Electric Resistance Tomography (ERT) were used during the injection test. Dr. Douglas

LaBreque from Steamtech Environmental Services in Bakersfield, California performed

these investigations. Electric Resistance Tomography is a new technology that is similar

to medical Magnetic Resistance Image (MRI) in theory but uses electrical resistance

instead of magnetic resistance as signals. Electrodes are placed in wells along wellbore.

By measuring electrical resistance between two electrodes before and after the injection,

the changes in electrical resistance of the rock can be calculated by inverse modeling

(ref). Because the injection fluids have smaller resistance (10,000 µS/cm) than the

formation water (850 µS/cm), the zones with negative changes in resistance (positive

changes in conductance) represent the flow paths of injected solution. In this study,

electrodes were placed in BHP-1, BHP-2, BHP-3, BHP-4, and BHP-5 at three-meter

intervals. The data was collected before the injection of leach solution and after two

months of leach. Figure 24 shows the changes in conductivity due to the injection of

leach solution on the cross section of BHP-5 to BHP-1 and BHP-3. Although the results

are qualitative, it is clear that the solution sweep through most of the oxide rocks.

Although the screen of the well is 40 feet below the top of the oxide, the solution has

flow vertically into overburden for about 20 feet which is consistent with the numerical

simulations. The detailed results are described in Appendix I.

9.5 RECLAMATION TESTS

Upon the termination of the three-month leaching test on Feb. 9, 1998, the rinsing

test commenced. During the rinsing phase, the acid solution was replaced by water while

the recovery wells were kept pumping. Then, the fresh water was injected into BHP-1

and rest of wells were used as recovery well. This switch of recovery and injection

helped to extract acid solution faster. Following the period of groundwater injection, the

residual wellfield solutions were pumped with no injection.

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Figure 24. Conductivity difference from background after one month of acid

injection.

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Sulfate was used as an indicator to the recovery of the injected acid solution.

About 1.2 million pounds of sulfate were injected into the wellfield during the leaching

phase (Figure 25). In August, 1999, 94 percent of the sulfate (1.14 million pounds) was

recovered based on the flow and geochemical assay data. This validated the numerical

models which showed that the injected solution would be contained during the leach test.

Figure 25. Sulfate recovery during the leaching and rinsing phases.

10. CONCLUSIONS

Hydraulic tests were conducted in the wellfield before the leaching tests started.

Numerical inverse modeling was used to built hydrogeological model and calibrated

parameters. The calibrated model was used to simulate the solution movement in the

leaching test. The results showed that the injected solution will flow vertically into

Lower Basin Fill Unit for 20 feet. Although permits allow this, it is not desiable for the

leaching purposes. The vertical excursion can be reduced by increase the distance

between the top of the screen and the botton of the LBFU from 40 feet to 60 feet.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20 30 40 50 60 70 80 90 100Weeks After the Start of Injection

SO4,

lbs

in m

illio

ns

total SO4 INtotal SO4 out

92.7% Recoveredafter 99 weeks (Sep. 22, 99)94.5% will be recovered on Oct 31,1999

SWVP-022592

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Horizontally, the results showed that injected solution could be controled. These findings

were confirmed by the field test conducted later.

Field test demonstrated that the field operation can meet the environmental

regulations. For three months of test, there was no violation. The hydraulic control

systems worked as designed to prevent the excursion of leaching solutions in the

wellfield. This is verified by the fact that the injected solution was able to be recoveried

during the reclamation phase. More than 95 percent of the sulfate injected into the

ground was recovered during the rinsing phase by Oct. 1999.

The hydraulic tests and field test showed that the well design was satisfactory.

The well spacing of 75 feet that was used in the test met environmental and hydraulic

requirements. A well spacing of 50 feet can also meet these requirements but is more

difficult to drill due to the requirements on well derivation; however, the former will

result in a better financial .

11. RECOMMENDATIONS FOR FUTURE STUDIES

It is suggested that the post-leach tests be conducted. These tests will includes

pumping tests and tracer tests to determine the changes in hydraulic conductivity and

porosity due to leaching. The test results can help to understand the leach processes, such

as, the locations of mineral dissolution and precipitation. They can also be used to verify

the geochemical modeling which predicts the changes in porosity. Post-leach drilling

shall also be conducted to exam the completeness and the location of mineral dissolution,

as well as the excursion of injected solution.

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References

Florence Project Final Pre-Feasibility report, Vol, II, Geology, Growth and Technology Group, BHP, 1997.

Florence Project Final Pre-Feasibility report, Vol, IV, Hydrogeologic and Metallurgic Evaluations, Growth and Technology Group, BHP, 1997.

McDonald, M. G., and Harbaugh, A. W., 1988, A modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Techniques of Water Resources Investigations 06-A1.

Neuman, S. P., 1982, Statistical characterization of aquifer heterogeneities - An overview, in T. N. Narasimhan, ed., Recent trends in hydrology: Geol. Soc. Am. Spec. Paper 189, p. 81-102.

Orr, S., Porosity estimation review, Appendix IV, Vol. IV, in Florence Project Final Pre-Feasibility report, Growth and Technology Group, BHP, 1997.

Resource Development Technology Group (RDTG), 1994, Pre-feasibility study Florence Project: Tucson, Ariz., Magma Copper Co., unpub. report, Oct. 1994, 333 p., 5 pls.

Zheng, C., 1990, MT3D - A Modular Three-Dimensional Transport Model for Simulation of Advection, Dispersion and Chemical Reactions of Contaminants in Groundwater Systems: S.S. Papadopulus & Associates, Inc.

SWVP-022594

hydrogeological studies for the in-situ leach field test ... bromide tracer test ... figure 7....

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