ML19322A760

App 2C of Oconee 1,2 & # PSAR, Groundwater Hydrology.

ML19322A760
Person / Time
Site:	Oconee
Issue date:	12/01/1966
From:	DUKE POWER CO.
To:
References
NUDOCS 7911210784
Download: ML19322A760 (12)

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APPENDIX 2C GROUNDWATER HYDROLOGY l

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! OCONEE NUCLEAR STATION l

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BECHTEL CORPORATION GAITHERSBURG, MARYIAND NOVEMBER 1966 L

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l O 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 i

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I v/ GROUNDWATER HYDROLOGY OF OCONEE NUCLEAR STATION

1. INTRODUCTION 1.1 GENERAL The Oconee Nuclear Station site lies within the drainage area of the Little and Keowee Rivers which flow southerly into the Seneca River and subsequently discharge into the main drainage course of the Savannah River. The average annual rainfall at the site area is approximately 53 inches.

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

Throughout the area, groundwater occurs at shallow depths within the O.

b saprolite (residual soil which is a weathering product of the underlying parent rock) soil mantle overlying the metamorphic and igneous rock complex.(1) Refer to Section 2.5, " Geology." This saprolite soil, which ranges in thickness from a few feet to over 100 feet, is the aquifer for most of the groundwater supply. Wells are shallow and few exceed a total depth of 100 feet. Depths to water commonly range from 5 to 40 feet below the land surface. Seasonal fluctuation is wholly dependent on the rainfall and the magnitude of change may vary considerably from well to well due to the limited areas of available recharge. Average fluctuation is about 3 to 5 feet. Both surface water and groundwater in this area are of low mineral contcat and generally of good quality for all uses.

2. GROUNDWATER 2.1 GENERAL AREA To determine the general groundwater environment surrounding the proposed site, groundwater levels were established in numerous domestic wells and exploratory drill holes within a four-mile radius. Additional data were obtained from interviews with local residents regarding specific wells and discussions with State and Federal personnel. The results of the groundwater level survey are shown on Figure 2C-1. The results demonstrate that local subsurface drainage generally travels down the topographic slopes Geologic Notes. Division of Geology, State Development Board, Vol. 7, No. 5, September-October 1963.

v 251 2C-1 s

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

Because the topography and thickness of the residual soil overlying bedrock control the hydraulic gradient throughout the area, and further, the relief is highly variable within short distances, it is not possible to assign a meaningful average gradient for the 15 square mile area surveyed. In all small areas studied within the four-mile radius, the groundwater hydraulic gradient is steep and conforms to the topographic slope. Water released on the surface will percolate downward and move toward the main drainage channels at an estimated rate of 150 to 250 feet per year.

The gradient throughout the area represents the upper surface of unconfined groundwater and therefore is subject to atmospheric conditions. Confined groundwater occurs only locally as evidenced by the existence of isolated springs and a few exploratory drill holes which encountered artesian conditions. These examples do not reflect general conditions covering large areas but merely represent isolated local strata within the saprolite soil which contain water under a semi-perched condition and/or permeable strata overlain by impermeable clay lenses which have been breached by erosion at its exit and recharged short distances upslope by vertical percolation.

2.2 STATION SITE The site area is on a moderately sloping, northwest trending topographic ridge which forms a drainage divide between the Little and Keowee Rivers located approximately 0.5 mile to the west and east, respectively. Ground-water levels at the site, measured during the 1966 drilling program and subsequently in four piezometer holes drilled for pre-construction monitoring purposes, ranged from elevation 792 to 696. The slope of this apparently free water surface is predominantly southeasterly toward the Keowee River and its tributary drainage channels. An average hydraulic gradient to the southeast of approximately 8.0 percent was plotted along a line of measured wells.

This closely conforms to the existing topography and as expected. Refer to Figure 2C-2, for measured water levels and typical water table profile.

Field permeability tests conducted during the 1966 exploratory program within the saprolite soil yielded values ranging from 100 to 250 feet per year. Refer to Appendix 2D. The permeability tests were performed in holes of varying depths to determine if the zoned typed weathering of the saprolite 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 lower permeability values than that found in the deeper strata. This correlates with the general profile of the saprolite in that the later stages of weathering 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 l

The completed field survey of approximately 30 wells determined that ground- l 2C-2 252

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Q water usage is almost entirely frc m the permeable zones within the saprolite with only minor amounts obtained from the underlyirg fractured bedrock. Yields from these shallow wells are low, generally less than 5 gpm, and are used to supply domestic water for the homes and for irri-gation of lawns, gardens, and limited amounts for livestock. With only a few exceptions, the wells are , hand dug., equipped with bucket lift and/or jet pump, and 40 to 60 feet deep. At present, there is no industrial demand for groundwater within the area. The only appreciable groundwater draft 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 a high of 59 degrees. The majority of readings were from 50 to 53 degrees fahrenheit. ,

Water contairrs different kinds and amounts of mineral constituents. Temperature pressure and lenSth of time a water is in contact with various rock types and soils determine the type and amount of mineral constituents present. Because ground waters are in intimate contact with the host rocks for longer periods of time, they have a more uniform and concentrated mineral content than surface waters. The mineral content of natural surface waters in the Piedmont Province t ) is low due to the relative insolubility of the granitic, gneissic, and schistose V 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 per million from the Keowee River near Jocassee, South Carolina. The water sample was taken and analyzed by the U. S. Geological Survey, Water Resources Division in June 1965.

Silica (SiO2) 7.8 Carbonate (CO3 ) 0.0 Iron (Fe) 0.01 Bicarbonate (HCO3) 7.0 Calcium (Ca) 1.0 Sulfate (SO4 ) 1.0 Magnesium (Mg) 0.1 Chloride (Cl) 0.6 Sodium (Na) 1.2 Fluoride (F) 0.1 Potassium (K) 0.4 Nitrate (NO3 ) 0.1 Dissolved Solids 15.0 Phosphate (PO4 ) 0.0 Hardness as CACO 3 3.0 pH 6.6 Specific 1 Conductance 13.0 At present, no water quality data is available of groundwater within the area surrounding the Oconee Nuclear Station site. Selected representative j groundwater and surf ace water samples will be analyzed in the near future j with continued periodic analysis during the environmental surveillance program. Tests will include complete chemical analysis; and gross Beta 0

, 1 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 is expected that groundwater analyses will indicate slightly higher relative ion concentrations than that of nearby surface waters but still in the range 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 cooperation with the South Carolina Agricultural Expericant Station assign pH values of between 5.0 and 6.0 for the Hayesville and Cecil soil series which are present at the site area.(1) Surface water samples taken from the Keowee River within one mile of the site have a pH of 6.5 to 7.0. It is expected groundwater at the site has a pH ranging between 5.5 and 6.0.

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

Sample No. pH Saturation Extract Values SAR Milligram-equivalent per 100 grains of soil Cond. (mhos) Calcium Magnesium Sodium 1 5.8 5 0.015 0.000 0.0108 0.122 2 5.7 7 0.010 0.000 0.0166 0.235 Considering the amount of soil that is available is so great, it is evident that many times the amount of strontium and/or cesium contained in the waste could be adsorbed. Further, the distribution coefficient for ion exchange of radionuclides with the sediments is dependent on the pH of the water in the formation (2). The distribution coefficient is a ratio of the reaction of these radionuclides that are adsorbed on the soil and the fraction remaining in solution. It is expected that the soils surrounding the Oconee Nuclear Station have a ratio in the range of 80 to 150, and consequently a substantially lower average velocity for any radionuclide to that of natural water will result.

The estimated saximum rate of movement of water through the soils is about 0.75 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 weight of soil to volume of groundwater it indicates the radionuclide velocity will be about .0015 that of groundwater. Using a safety factor of five (1)

Soil Survev - Oconee County, South Carolina, United States Department of Agriculture, Series 1958, No. 25, February 1963.

(2) l Storage of Radioactive Wastes in Basement Rock Beneath the Savannah l l River Plant, DP-844 Waste Disposal and Processing (TID-4500, 28th Ed.), 1 March 1964 l

2C-4 254 l l

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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 ions to migrate a distance of one-half mile. In summary, the movement would be so extremely slow that the saprolite soil is an effective natural barrier to the migration of radionuclides.

2.6 FUTURE GROUNDWATER CONDITIONS As previously discussed, the present groundwater levels at the site range from elevation 792 to below elevation 696. The proposed Keowee Reservoir will operate with a maximum pool elevation of 800. This will result in raising the surface water elevation to that datum on the northern and western portions of land adjoining the Oconee Nuclear Site. It will also raise the existing groundwater table for those local areas bordering the reservoir where presently the groundwater surface is below elevation 800.

The reservoir will materially contribute in establishing a potentially larger recharge area and where it effects the groundwater will result in a more stable hydraulic gradient with less seasonal fluctuation than presently exists.

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

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

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

It is unknown if the saprolite soil existing beneath those topographic ridges provide a hydraulic connection between the nuclear plant and the proposed reservoir. However, it is probable that there will be avenues of slow seepage whereby percolating water may locally raise the groundwater surface at the plant to an elevation approaching elevation 800. A drainage system will be provided to control all seepage encountered.

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

3. CONCLUSIONS Within the general area, free groundwater occurs in the saprolite soil and migrates slowly at a gradient approaching the topographic slopes.

Water discharged on the ground surface at the nuclear site will percolate dcwnward 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 adverse 4

2C-5 255

groundwater conditions at the plant site.

Infiltration of domestic wells, located beyond the proposed one-mile exclusion radius, by surface water from the site should not be possible under the existing or future groundwater conditions imposed by Keowee Reservoir.

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