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APPENDIX 2C

GROUNDWATER HYDROLOGYl

OF j,

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! OCONEE NUCLEAR STATION

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| DUKE POWER COMPANY;

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PREPARED BY [

BECHTEL CORPORATIONGAITHERSBURG, MARYIAND

NOVEMBER 1966

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TABLE OF CONTENTS

Page

1. INTRODUCTION 2C-1

1.1 GENERAL 2C-1

2. GROUNDWATER 2C-1

2.1 GENERAL AREA 2C-1

2.2 STATION SITE 2C-2

2.3 GROUNDWATER DEVELOPMENT 2C-2

2.4 WATER QUALITY 2C-3

2.5 ION EXCHANGE AND SOIL FILTRATION 2C-4

2.6 FUTURE GROUNDWATER CONDITIONS 2C-5

3. CONCLUSIONS 2C-5

O FIGURES

Figure 2C-1 Areal Groundwater Survey

Figure 2C-2 Groundwater Survey at Station Site

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v/ GROUNDWATER HYDROLOGY

OF

OCONEE NUCLEAR STATION

1. INTRODUCTION

1.1 GENERAL

The Oconee Nuclear Station site lies within the drainage area of theLittle and Keowee Rivers which flow southerly into the Seneca Riverand subsequently discharge into the main drainage course of the SavannahRiver. The average annual rainfall at the site area is approximately53 inches.

The deposits of the Little and Keowee drainage basin art generally oflow permeability which result in nearly total runoff to the two riversand their numerous tributary creeks. Runoff cccurs soon after precipi-tation, particularly during the spring and su=mer months when the soilpercolation rates are exceeded by the short term but higher yieldingrainfall periods. The area is characterized by youthful narrow streamsand creeks which discharge into the mature Little and Keowee Rivers.

Throughout the area, groundwater occurs at shallow depths within the

O. saprolite (residual soil which is a weathering product of the underlyingb parent rock) soil mantle overlying the metamorphic and igneous rock

complex.(1) Refer to Section 2.5, " Geology." This saprolite soil, whichranges in thickness from a few feet to over 100 feet, is the aquifer formost of the groundwater supply. Wells are shallow and few exceed a totaldepth of 100 feet. Depths to water commonly range from 5 to 40 feet belowthe land surface. Seasonal fluctuation is wholly dependent on the rainfalland the magnitude of change may vary considerably from well to well due tothe limited areas of available recharge. Average fluctuation is about 3 to5 feet. Both surface water and groundwater in this area are of low mineralcontcat and generally of good quality for all uses.

2. GROUNDWATER

2.1 GENERAL AREA

To determine the general groundwater environment surrounding the proposedsite, groundwater levels were established in numerous domestic wells andexploratory drill holes within a four-mile radius. Additional data wereobtained from interviews with local residents regarding specific wellsand discussions with State and Federal personnel. The results of thegroundwater level survey are shown on Figure 2C-1. The results demonstratethat local subsurface drainage generally travels down the topographic slopes

Geologic Notes. Division of Geology, State Development Board, Vol. 7,No. 5, September-October 1963.

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2512C-1_ ,

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within the morc permeable saprolite soil zones toward the nearby surfacecreek or stream. Gross drainage is southward to the Little and KeoweeRivers which act as a base for the gradient.

Because the topography and thickness of the residual soil overlying bedrockcontrol the hydraulic gradient throughout the area, and further, the reliefis highly variable within short distances, it is not possible to assign ameaningful average gradient for the 15 square mile area surveyed. In allsmall areas studied within the four-mile radius, the groundwater hydraulicgradient is steep and conforms to the topographic slope. Water releasedon the surface will percolate downward and move toward the main drainagechannels at an estimated rate of 150 to 250 feet per year.

The gradient throughout the area represents the upper surface of unconfinedgroundwater and therefore is subject to atmospheric conditions. Confinedgroundwater occurs only locally as evidenced by the existence of isolatedsprings and a few exploratory drill holes which encountered artesianconditions. These examples do not reflect general conditions coveringlarge areas but merely represent isolated local strata within the saprolitesoil which contain water under a semi-perched condition and/or permeablestrata overlain by impermeable clay lenses which have been breached byerosion at its exit and recharged short distances upslope by verticalpercolation.

2.2 STATION SITE

The site area is on a moderately sloping, northwest trending topographicridge which forms a drainage divide between the Little and Keowee Riverslocated approximately 0.5 mile to the west and east, respectively. Ground-water levels at the site, measured during the 1966 drilling program andsubsequently in four piezometer holes drilled for pre-construction monitoringpurposes, ranged from elevation 792 to 696. The slope of this apparently freewater surface is predominantly southeasterly toward the Keowee River and itstributary drainage channels. An average hydraulic gradient to the southeastof approximately 8.0 percent was plotted along a line of measured wells.This closely conforms to the existing topography and as expected. Refer toFigure 2C-2, for measured water levels and typical water table profile.

Field permeability tests conducted during the 1966 exploratory programwithin the saprolite soil yielded values ranging from 100 to 250 feet peryear. Refer to Appendix 2D. The permeability tests were performed inholes of varying depths to determine if the zoned typed weathering of thesaprolite soil affects vertical permeability. Based on the test results,inspection of nearby road cuts, and a study of the exploratory drill logs,it is tentatively concluded that the surficial saprolite possesses lowerpermeability values than that found in the deeper strata. This correlateswith the general profile of the saprolite in that the later stages ofweathering produce a soil having a higher clay content than the more coarse-grained silty sand sediments below. This natural process of weathering '

results in the formation of a partial barrier to downward movement of surface '

water..

2.3 GROUNDWATER DEVELOPMENT|

The completed field survey of approximately 30 wells determined that ground- |

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2C-2

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Q water usage is almost entirely frc m the permeable zones within thesaprolite with only minor amounts obtained from the underlyirg fracturedbedrock. Yields from these shallow wells are low, generally less than 5gpm, and are used to supply domestic water for the homes and for irri-gation of lawns, gardens, and limited amounts for livestock. With onlya few exceptions, the wells are , hand dug., equipped with bucket lift and/orjet pump, and 40 to 60 feet deep. At present, there is no industrialdemand for groundwater within the area. The only appreciable groundwaterdraft observed is being supplied by eight wells for Keowae High School,located four miles west of the site.

2.4 WATER OUALITY

The surface water and groundwater of the area is generally of good quality.(1)Of the wells surveyed, none were noted where water treatment is being con-ducted. Temperature of well water measured ranged from a low of 46 to ahigh of 59 degrees. The majority of readings were from 50 to 53 degreesfahrenheit.

,

Water contairrs different kinds and amounts of mineral constituents. Temperaturepressure and lenSth of time a water is in contact with various rock types andsoils determine the type and amount of mineral constituents present. Becauseground waters are in intimate contact with the host rocks for longer periods oftime, they have a more uniform and concentrated mineral content than surfacewaters. The mineral content of natural surface waters in the Piedmont Province

t ) is low due to the relative insolubility of the granitic, gneissic, and schistoseV host rocks and the reduced contact time caused by rapid runoff in the mountainous

areas.

Tabulated below are the surface water constituents reported in parts permillion from the Keowee River near Jocassee, South Carolina. The watersample was taken and analyzed by the U. S. Geological Survey, Water ResourcesDivision in June 1965.

Silica (SiO ) 7.8 Carbonate (CO ) 0.02 3Iron (Fe) 0.01 Bicarbonate (HCO3) 7.0Calcium (Ca) 1.0 Sulfate (SO ) 1.04Magnesium (Mg) 0.1 Chloride (Cl) 0.6Sodium (Na) 1.2 Fluoride (F) 0.1Potassium (K) 0.4 Nitrate (NO ) 0.13Dissolved Solids 15.0 Phosphate (PO ) 0.04Hardness as CACO 3.03pH 6.6Specific 1

Conductance 13.0

At present, no water quality data is available of groundwater within thearea surrounding the Oconee Nuclear Station site. Selected representative jgroundwater and surf ace water samples will be analyzed in the near future jwith continued periodic analysis during the environmental surveillanceprogram. Tests will include complete chemical analysis; and gross Beta

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CY (1) Chemical Character of Surface Waters of South Carolina, South Carolina,1

State Development Board, (Bulletin No. 16C) 1962..

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and Gamma content to establish radioactive background count. It isexpected that groundwater analyses will indicate slightly higher relativeion concentrations than that of nearby surface waters but still in therange to meet requirements for classification as good quality water.

2.5 ION EXCHANGE AND SOIL FILTRATION

Soil surveys conducted by the U. S. Department of Agriculture in cooperationwith the South Carolina Agricultural Expericant Station assign pH valuesof between 5.0 and 6.0 for the Hayesville and Cecil soil series which arepresent at the site area.(1) Surface water samples taken from the KeoweeRiver within one mile of the site have a pH of 6.5 to 7.0. It is expectedgroundwater at the site has a pH ranging between 5.5 and 6.0.

The cation exchange potential can be evaluated by knowing the SAR (SodiumAbsorption Ratio), saturation extract values, and the pH of the soil. Twosamples of saprolite soil were obtained from drill holes used in determiningfield perroability values and tested for Sodium Absorption Ratio (SAR). Theresults are tabulated as follows:

Sample No. pH Saturation Extract Values SARMilligram-equivalent per100 grains of soil

Cond. (mhos) Calcium Magnesium Sodium1 5.8 5 0.015 0.000 0.0108 0.1222 5.7 7 0.010 0.000 0.0166 0.235

Considering the amount of soil that is available is so great, it is evidentthat many times the amount of strontium and/or cesium contained in thewaste could be adsorbed. Further, the distribution coefficient for ion

exchange of radionuclides with the sediments is dependent on the pH ofthe water in the formation (2). The distribution coefficient is a ratioof the reaction of these radionuclides that are adsorbed on the soil andthe fraction remaining in solution. It is expected that the soils surroundingthe Oconee Nuclear Station have a ratio in the range of 80 to 150, andconsequently a substantially lower average velocity for any radionuclideto that of natural water will result.

The estimated saximum rate of movement of water through the soils is about0.75 feet per day. Using this rate in relation with the above distributt on

coefficient, bulk density and porosity of the soil, and ratio of the weightof soil to volume of groundwater it indicates the radionuclide velocity willbe about .0015 that of groundwater. Using a safety factor of five

(1)Soil Survev - Oconee County, South Carolina, United States Department ofAgriculture, Series 1958, No. 25, February 1963.

(2) |Storage of Radioactive Wastes in Basement Rock Beneath the Savannah |

| River Plant, DP-844 Waste Disposal and Processing (TID-4500, 28th Ed.),1

March 1964|

254 |2C-4

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x_,) for variance in flow and connetition for exchangesole sodium ions,it would require more than 1000 years for strontium or cesium ionsto migrate a distance of one-half mile. In summary, the movement wouldbe so extremely slow that the saprolite soil is an effective naturalbarrier to the migration of radionuclides.

2.6 FUTURE GROUNDWATER CONDITIONS

As previously discussed, the present groundwater levels at the site rangefrom elevation 792 to below elevation 696. The proposed Keowee Reservoirwill operate with a maximum pool elevation of 800. This will result inraising the surface water elevation to that datum on the northern andwestern portions of land adjoining the Oconee Nuclear Site. It will alsoraise the existing groundwater table for those local areas bordering thereservoir where presently the groundwater surface is below elevation 800.The reservoir will materially contribute in establishing a potentiallylarger recharge area and where it effects the groundwater will result ina more stable hydraulic gradient with less seasonal fluctuation thanpresently exists.

Preliminary studies indicate that Keowee Re:ervoir will create thefollowing groundwater conditions at the Oconee Nuclear Station:

Groundwater should continue to migrate downslope through the saprolite soilon a slightly steeper gradient in a southeasterly direction toward theKeowee River base datum.

There are two topographic divides which will separate the nuclear stationfrom the nearby reservoir: (1) a one-half mile wide north-south stretchof terrain west of the site, and (2) a narrow 500 foot wide ridge northof the site. Recent groundwater measurements in drill hole K-12, locatedatop the northern ridge, show water table conditions exist at aboutelevation 810.

It is unknown if the saprolite soil existing beneath those topographicridges provide a hydraulic connection between the nuclear plant and theproposed reservoir. However, it is probable that there will be avenuesof slow seepage whereby percolating water may locally raise the groundwatersurface at the plant to an elevation approaching elevation 800. A drainagesystem will be provided to control all seepage encountered.

There should be no reversal of groundwater movement at the site, and allwater will percolate downward and away from the plant area.

3. CONCLUSIONS

Within the general area, free groundwater occurs in the saprolite soil andmigrates slowly at a gradient approaching the topographic slopes.

Water discharged on the ground surface at the nuclear site will percolatedcwnward and mix with groundwater moving southeasterly toward the nearby

- Keowee River and its tributary creeks.

C~- The construction of Keowee Dam and Reservoir will not create adverse4

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2C-5255

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groundwater conditions at the plant site.

Infiltration of domestic wells, located beyond the proposed one-mileexclusion radius, by surface water from the site should not be possibleunder the existing or future groundwater conditions imposed by KeoweeReservoir.

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