Interaquifer Leakage Through Uncased Boreholes Penetrations Yucca Valley, Nevada Test Site

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C. DESERT RESEARCH INSTITUTE <11 UNIVERSITY OF NEVADA SYSTEM INTERAQUIFER LEAKAGE THROUGH UNCASED BOREHOLES PENETRATIONS YUCCA VALLEY, NEVADA TEST SITE WATER RESOURCES CENTER

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INTEKAQUIFER LEAKAGE THROUGH UNCASED YUCCA VALLEY, NEVADA TEST SITE DRI Publication No. 45013 Water Resources Center Desert Research Institute University of Nevada System Prepared for the U.S. Department of Energy, Nevada Operations Office, under contract EY-76-C-08-1253

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This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accura cy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately-owned rights.

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DEVELOPMENT OF LEAKAGE EQUATION .................... 2 ESTIMATED NATURAL LEAKAGE RATE THROUGH LEAKAGE RATE THROUGH ONE UNPLUGGED 1. Diagrammatic Representation of Multiple

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INTERAQUIFER LEAKAGE THROUGH UNCASED BOREHOLE PENETRATIONS YUCCA VALLEY, NEVADA TEST SITE A method of calculating groundwater leakage rate into the carbonate aquifer through uncased boreholes is presented. An estimate of leakage rate over a long time interval rather than early transient calculations is required, and the appropriate basic equations are the straight line solutions of the radial flow equations. Aquifer parameters were estimated on the basis of a few measured values. The leakage rate through one unplugged uncased borehole into the Carbonate Aquifer is of the same order as the total natural leakage through the Tuff Aquitard. The effect of enhanced leakage rate will be to increase the transport rate of tritium if an uncased unplugged borehole is near an ex pended nuclear test in the Alluvial Aquifer or Tuff Aquitard.

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Localized enhanced leakage from the alluvium and tuff into the Paleozoic carbonates would probably be of little significance unless the Tuff Aquitard penetration is in the vicinity of an expended underground nuclear test. Then it is possible that water moving through the test region could transport radionuclides more rapidly into the Carbonate Aquifer where there is potential for more rapid transport of radio nuclides through the Carbonate Aquifer to discharge points off the Nevada Test Site. In this paper the equation is developed for estimating leakage rates from the Alluvial Aquifer and Tuff Aquitard into the Carbonate Aquifer through uncased and inadequately plugged borehole penetrations. The leakage rate is estimated using reasonable values for the parameters. An estimate is also made of the maximum natural leakage through the Tuff Aquitard into the Carbonate Aquifer. DEVELOPMENT OF LEAKAGE EQUATION The equation upon which this analysis is based is: This equation is the straight line approximation to both the constant discharge varying drawdown radial flow equation (Cooper and Jacob, 1946) and the constant drawdown varying discharge radial flow equation (Jacob and Lohman, 1952) . That is, the ratio of drawdown to discharge is specified by the equation, and both drawdown and discharge can vary within

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The aquifers and aquitard are assumed to be isolated by impermeable boundaries. In fact, however, a small amount of downward leakage does occur and is estimated later. The multiple aquifer analysis is carried out as shown in Figure 1. Each aquifer is considered to be discharging into the well. This convention will show discharge into the well as positive and recharge of any aquifer as negative discharge. By using an analytical method analogous to Kirchoff's laws, the following set of linear equations is derived from Figure 1 Discharge for the Carbonate Aquifer is obtained- by substituting the appropriately subscripted drawdown-discharge relationship from equation one into equation set three and then into equation (2) to obtain:

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rw= radius of well FIGURE I. Diagrammatic Representation of Multiple Aquifer Model

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Subscripts are: a = Alluvial Aquifer All other values of drawdown and discharge are then readily obtained from equations (1) and (3) . Average values for the aquifer parameters were obtained from Winograd and Thordarson (1975) . The Alluvial Aquifer was estimated to have a transmissivity of 12,000 gpd per ft (1600 ft /day), the Carbonate Aquifer was estimated to have a transAquitard was estimated to have a transmissivity of 50 gpd per ft (7 ft /day). The transmissivity assigned the Carbonate Aquifer was weighted toward transmissivities of the upper part of the aquifer because most uncased unplugged boreholes probably did not penetrate deeply into the Carbonate Aquifer. Values for the specific yield of the Alluvial Aquifer and storage coefficients for the Tuff Aquitard and Carbonate Aqui fer are not available. For the purpose of this first approx imation of leakage, the Alluvial Aquifer is assigned a specific yield of .25, the Tuff Aquitard is assigned a storage coeffi cient of .05, and the Carbonate Aquifer is assigned a storage Other parameters required for the approximate calculation are well radius and hydraulic potential for each aquifer. The well radius is arbitrarily placed at .7 feet. The head differ entials, (Ha-Hc) and (Hv-Hc), are assumed to be equal. That is, vertical head losses between the water table and the Car bonate Aquifer are assumed to be concentrated in the Tuff Aqui tard, which has orders of magnitude of lower hydraulic con ductivity than the Alluvial Aquifer. Measured head differen

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tials between the Alluvial Aquifer and the Carbonate Aquifer are about 100 feet and this number is used in the calculation. ESTIMATED NATURAL LEAKAGE RATE THROUGH TUFF AQUITARD The most direct method for estimating the natural leakage rate through the Tuff Aquitard is to use Darcy's Law and the measured hydraulic gradient, hydraulic conductivity, and area of the Tuff Aquitard. Another method is to assume that all of the water underflowing Yucca Valley in the Carbonate Aquifer has leaked through the Tuff Aquitard. No water has entered the Yucca Valley underflow through lateral flow in the Carbonate Aquifer. This later leakage estimate will prob ably represent a maximum value for leakage. The leakage estimate based upon Darcy's Law will probably be a minimum, since matrix values for the hydraulic conductivity are used, and the presence of fractures and faults, the main pathways for leakage, are neglected. In both methods vertical leakage Winograd and Thordarson (1975) estimated the leakage rate with Darcy's Law to be about 25 acre-feet per year or 15 gpm. Underflow for Yucca Valley is estimated to be 350 acre-feet per year indicating a total recharge of the Car bonate Aquifer of about 220 gpm. Some of this water, perhaps as much as half, probably enters the Carbonate Aquifer directly in areas where the top of the Carbonate Aquifer is above the water table. It would seem that a reasonable volume of natural leakage through the Tuff Aquitard would LEAKAGE RATE THROUGH ONE UNPLUGGED UNCASED BOREHOLE A small computer program, based on equation (4) was written to calculate the leakage rate from the Alluvial Aquifer and the Tuff Aquitard into the Carbonate Aquifer as

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a function of time as well as the associated drawdowns. The results of this calculation for the estimated aquifer and aquitard characteristics are shown in Table 1. After approx imately 275 years (1 x 105 days) the leakage into the Carbonate Aquifer from primarily the Alluvial Aquifer in one borehole still represents a significant portion of the esti mated natural leakage through the Tuff Aquitard. If there were as many as two uncased and unplugged penetrations into the Carbonate Aquifer from the Alluvial Aquifer, the leakage rate would exceed the natural leakage rate through the Tuff The ultimate effect of enhanced recharge to the Carbonate Aquifer through uncased unplugged boreholes is twofold: 1. If any one of the boreholes is in the near vicinity of an expended nuclear test in the alluvium and below the water table, radioactivity, probably tritium only, will be transported more rapidly into the Carbonate Aquifer and thence to offsite discharge points. 2. Enhanced recharge to the Carbonate Aquifer will cause a general increase in head and local hydraulic gradient in the Carbonate Aquifer, increasing the rate of groundwater movement and increasing the rate of The mathematical analysis derived for estimating the leakage rate through uncased unplugged boreholes penetrating into the Carbonate Aquifer considers the aquifers to be iso lated by impermeable boundaries. In reality, water leaks through the Tuff Aquitard into the Carbonate Aquifer. Since this leakage rate is probably less than 100 gpm for the total Tuff Aquitard area, not considering leakage will probably not lead to serious differences in the estimate. Parmeters

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Time is in days Drawdown is in feet Discharge is in gallons per minute

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used in the analysis were estimated from Nevada Test Site data. The estimated leakage rate down a single borehole is such that it would probably exceed the natural leakage recharge to the Carbonate Aquifer through the Tuff Aquitard. The ultimate effect of the leakage through the boreholes will be to move tritium more rapidly toward discharge areas if a borehole is in the vicinity of an expended nuclear test in the alluvium and below the water table. Site specific calculations for individual boreholes require values for transmissivities, s.torage coefficients and heads for the Alluvial Aquifer, Tuff Aquitard and Carbonate Aquifer. Although estimates of leakage rates down uncased unplugged boreholes penetrating the Tuff Aquitard could be improved by obtaining these better estimates of aquifer parameters, it is unlikely that the general conclusions in this report would

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graphical method for evaluating formation constants and summarizing well-field history. Am. Geophys. Union Jacob, C.E. and S.W. Lohman, 1952. Nonsteady flow to a well of constant drawdown in an extensive aquifer, Am. Geophys. Union Trans., Vol. 33, pp. 559-569. hydrochemical framework, South-Central Great Basin, Nevada-California, with special reference to the Nevada Test Site, U.S. Geol Survey Professional Paper

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U.S. Department of Energy, Nevada Operations Office/ Las U.S. Department of Energy, Nevada Test Site Support Office, Office of Public Affairs U.S. Department of Energy, Technical Information Center, Defense Nuclear Agency: Clifford Snow), Mercury, NV Los Alamos Scientific Laboratory, Las Alamos, NM:

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Lawrence Livermore Laboratory, Livermoref CA: Technical Information Library Division Lawrence Livermore Laboratory, Mercury, NV: Sandia Laboratories, Mercury, NV: Panel of Consultants: Carl Kisslinger, University of Colorado, Boulder, CO Joseph Lintz, University of Nevada, Reno, NV N.M. Newmark, University of Illinois, Urbana, IL Vincent Schultz, Washington State University, Pullman, WA J.T. Wilson, University of Michigan, Ann Arbor, MI Defense Advanced REsearch Projects Agency, Arlington, VA; Colorado School of Mines, GoIden, CO: Desert Research Institute, Reno, NV: Environmental Protection Agency, Seattle, WA; Attn: Water Division

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Environmental Protection Agency, National Environmental Research Center, Las Vegas, NV: Fugro National, Inc,f Long Beach, CA: National Oceanic and Atmospheric Administrationf Weather Service Nuclear Support Office, Las Vegas, NV: Grant Bruesch, Mercury, NV Holmes & Narver, Inc.: Resident Manager, Mercury, NV U.S. Army Corps of Engineers, Waterways Experiment Station, U.S. Bureau of Mines, Denver, CO: Geologic Data Center, Mercury, NV (15) Library, Denver, CO Library, Menlo Park, CA U.S. Geological Survey, Reston, VA: Military Geology Unit