3

Might Increased Rainfall Cause Flooding of the Proposed Repository?

INTRODUCTION

Although available geological and geochemical evidence does not support the contention that the water table has risen to the proposed repository level in the past 100 ka (see Chapter 2 of this report), the possibility that it may do so in the future must be assessed, because the most likely mode for release of significant radioactivity to the outside environment is ground-water transport. It is, therefore, of utmost importance to understand the ground-water system and the various mechanisms that may cause the ground water to rise to the repository level over the next 10 ka. This assessment requires the use of mathematical models, based on known physical principles, that can simulate what might happen in the future given certain known or assumed conditions, and expert judgement to determine the input and to evaluate the results. The uncertainty in the results of these simulations depends in part on the current understanding of processes and rates that can affect the mechanisms.

One mechanism that might cause a rise in water level is increased recharge to the ground-water system as a result of an increase in precipitation. The ability of scientists to predict the response of the water table to possible increased recharge in the future must rely to a large extent on mathematical modeling. The computed rise in the water table strongly depends on (1) the assumed increase in precipitation, (2) the relationship of precipitation to recharge, and (3) the specifics of the particular mathematical (ground-water) model used



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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? 3 Might Increased Rainfall Cause Flooding of the Proposed Repository? INTRODUCTION Although available geological and geochemical evidence does not support the contention that the water table has risen to the proposed repository level in the past 100 ka (see Chapter 2 of this report), the possibility that it may do so in the future must be assessed, because the most likely mode for release of significant radioactivity to the outside environment is ground-water transport. It is, therefore, of utmost importance to understand the ground-water system and the various mechanisms that may cause the ground water to rise to the repository level over the next 10 ka. This assessment requires the use of mathematical models, based on known physical principles, that can simulate what might happen in the future given certain known or assumed conditions, and expert judgement to determine the input and to evaluate the results. The uncertainty in the results of these simulations depends in part on the current understanding of processes and rates that can affect the mechanisms. One mechanism that might cause a rise in water level is increased recharge to the ground-water system as a result of an increase in precipitation. The ability of scientists to predict the response of the water table to possible increased recharge in the future must rely to a large extent on mathematical modeling. The computed rise in the water table strongly depends on (1) the assumed increase in precipitation, (2) the relationship of precipitation to recharge, and (3) the specifics of the particular mathematical (ground-water) model used

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? in the computations for the Yucca Mountain area. These issues are addressed in this chapter. HYDROGEOLOGICAL SETTING Yucca Mountain lies within the Alkali Flat/Furnace Creek subdivision of the Death Valley ground-water system (see Figure 3.1). The regional ground-water system also includes the Ash Meadows and Oasis Valley subbasins. The ground-water flow within all three subbasins is generally in a north-south direction. The principal aquifers in the Alkali Flat/Furnace Creek subdivision are in Cenozoic tuff and alluvium formations. Although a regional Paleozoic carbonate aquifer underlies a large part of southern Nevada and is thought to underlie the alluvium/tuff aquifers, only one borehole (UE-25p#1), southeast of Yucca Mountain, was drilled deep enough to encounter the carbonates (see Figure 3.2 for borehole locations). Its presence under Yucca Mountain, therefore, is still problematic. Discharge, or outflow of groundwater, from the Alkali Flat/Furnace Creek subbasin occurs by springs near Furnace Creek Ranch in Death Valley, and by evapotranspiration, a surface process of removing water by plant activity and surface evaporation, at Franklin Lake Playa (Figure 3.3). Discharge rates in the Franklin Lake Playa are poorly known. It is also possible that part of the ground water bypasses the Franklin Lake Playa and discharges at lower elevations elsewhere. No estimates of such a discharge rate at lower elevations are available. Czarnecki (1985) assumes that the major modern recharge areas, which supply the ground water for the Furnace Creek/ Alkali Flat subsystem, are the Pahute Mesa area to the north of Yucca Mountain and the Fortymile Wash area (Figure 3.3) east of Yucca Mountain. The amount of present-day recharge in other recharge areas (Jackass Flats, Crater Flat, and the Amargosa Desert (Figure 3.3)) is negligible compared to the recharge from the higher elevations of Pahute Mesa and Fortymile Wash. Carbon isotope age data imply that the water present in the deeper parts of the Alkali Flat/ Furnace Creek subbasin was recharged about 10-15 ka (Dudley, 1990a). This recharge presumably occurred under conditions that were cooler and possibly wetter during the last 5 ka of Wisconsin glaciation than those prevailing now. These ground-water age data raise the interesting possibility that the Alkali Flat/Furnace Creek subbasin is still draining, and is presumably not in a steady state (see also Czarnecki, 1990). Sufficient data on ground water age in the subbasin are not yet available to describe the evolution of the ground-water system over the past 10-20 ka.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.1 Regional ground-water systems and heat flow in the south-central Great Basin. (From Dudley, 1990a.)

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.2 Preliminary composite potentiometric-surface (water table elevations) map of the saturated zone. Yucca Mountain. (From Dudley, 1990b.)

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.3 Location of subregional area modeled by Czarnecki and Waddell (1984).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? The ground-water system in the Alkali Flat/Furnace Creek subdivision has been modeled by Czarnecki and Waddell (1984). These authors used a two-dimensional (areal) finite element model to simulate the steady-state ground-water flow occurring principally in the tuffs. The model was calibrated using available measurements of the elevation of ground-water levels. The transmissivities, or parameters describing how readily rocks will transmit ground water, and the amount of recharge in the Fortymile Wash area were adjusted until the model-calculated values of ground-water head were close to measured values. Only a few hundred meters north of Yucca Mountain, the water table level rises northward from ~730-750 meters above mean sea level (m AMSL), measured in wells G-1 and H-1, to ∼1030 m AMSL (in well G-2) over a maximum distance of approximately 2.5 km (Figure 3.2). The actual gradient may be steeper, but there are too few data at present to define it adequately. Understanding the nature and source of this steep hydraulic gradient is of fundamental importance in evaluating the long-term safety of the site for high level radioactive waste storage. In fact, it is the rapid decline of the water table level north of Yucca Mountain that allows for the “unsaturated” condition 300 m below the surface that was considered so important in the selection of the repository depth. The position of the gradient does not appear to correlate with presently known stratigraphic or structural features in the upper kilometer of the mountain (C. Fridrich, written communication, 1991). Currently, the reasons for this large lateral increase in hydraulic head are a matter of speculation. Three conceptual models have been considered by scientists associated with the project to explain the occurrence of the steep potentiometric gradient: (1) a hydrologic dam or barrier—a narrow vertical zone (1.5 km wide) of greatly decreased transmissivity; (2) a hydrologic drain—a highly transmissive vertical zone diverting most flow from the high water table region into the lower carbonate aquifer; and (3) a low transmissivity zone north of Yucca Mountain caused by tectonically controlled stress fields. Clearly, the fundamental differences between the models involve the geometry and characteristics of the causative feature. Both the dam and the drain models require a local, east-west-trending near-vertical zone coinciding in location with the steep gradient. In the dam model this vertical zone has very low transmissivity and acts as a barrier; in contrast, the drain model requires the vertical zone to have significant vertical permeability. The third model evokes high compressive horizontal stresses north of the gradient to cause the low transmissivity. Each of these models has some supporting

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? data and none can be eliminated with the currently available data (C. Fridrich, pers. comm.). The model of Czarnecki and Wadell (1984) treats only the dam hypothesis. The results of this model are discussed briefly further on in this chapter. A MODEL OF GROUND-WATER FLOW AT YUCCA MOUNTAIN The area of the steady-state ground-water model of Czarnecki and Waddell (1984) covered the region extending from Timber Mountain in the north to Alkali Flat and Franklin Lake Playa in the south (see Figure 3.3). Recharge from the Pahute Mesa area was simulated by prescribing a constant pressure boundary. Some minor fluxes were also applied to account for recharge from Jackass Flats and the Amargosa Desert, based on the earlier modeling work of Waddell (1982). However, all these fluxes amount to less than 2.5 percent of the total recharge or discharge. The recharge along Fortymile Canyon (zone 8, Figure 3.4) was obtained from the parameter estimation procedure. For the calculations the recharge in the Fortymile Wash area, estimated at 2.214 × 104 cubic meters per day (m3/d), was assumed to account for 40.3 percent of the total recharge. Most of the rest of the recharge (about 57.3 percent) was modeled to enter the subbasin through the constant head boundary along the northernmost end of the modeled region. Discharge from the subbasin was represented as: (1) a line sink east of Furnace Creek Ranch and (2) an areal discharge out of Alkali Flat. The discharge from the Alkali Flat area is 2.214 × 104 m3/d. It equals 64.8 percent of the total discharge from the subbasin. The remainder of the discharge was assumed to take place in the Furnace Creek Ranch area. All other boundaries were assumed to be no-flow boundaries. Czarnecki and Waddell (1984) divided the subbasin into 13 regions (Figure 3.4); transmissivities were assumed to be uniform in each of these regions. As part of a parametric estimation procedure, the recharge in the Fortymile Wash area and the transmissivities for zones 1 through 9 were varied until a satisfactory match was obtained between the computed and measured hydraulic heads. Final transmissivity values computed by Czarnecki and Waddell are given in Table 3.1. Considering the uncertainties in head measurements, the agreement between computed and measured head values is good. Model residuals for simulated versus measured heads range from −28.6 to 21.4 meters; most are less than ±7 meters. The simulated hydraulic heads are shown in Figure 3.5. Despite the impressive match between the measured and computed heads, the model results must be used with caution. The computed

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.4 Model zone numbers, parameter groupings, and model boundary fluxes employed by Czarnecki and Waddell (1984).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Table 3.1 Values of model transmissivities and standard errors. (excerpted from Czarnecki and Waddell, 1984). [T, transmissivity, in meters squared per day; number following letter T in model variable column is zone number; dashes indicate that value was held constant] Model Variable Parameter Number Value Standard Error Coefficient of Variation   T1, T2 1 1.336 × 103 31.92 0.024 Alluvium T3, T4a 2 1.282 × 102 2.421 0.019 Volcanic Rocks T4b 2 1.197 × 102 2.260 0.019 Carbonate Rocks T5 1 1.169 × 104 2.792 × 102 0.024 Carbonate Rocks T6, T7, T8 1 3.340 × 103 79.79 0.024 Tuff T9 3 95.90 0.2711 0.003 Tuff T10 – 78.62 – – Tuff T11 – 3.888 – – Tuff T12 – 8.64 × 10−3 – – Lakebeds transmissivities in zones 5, 6, 7, and 8 are extremely high. The available permeability measurements do not provide support for these high values (see Appendix B for details of the measured permeabilities of the area). An extremely small transmissivity value was assumed for zone 11 to simulate the large hydraulic gradient north of the Yucca Mountain. As indicated above, the cause of this large hydraulic gradient is not understood. The uncertainty in the amount of total discharge from and recharge to the subbasin produces a corresponding uncertainty in the computed transmissivity values. The data currently available on discharge/recharge and transmissivity distribution in the subbasin provide inadequate constraints for the model. Czarnecki and Waddell (1984) used a very small transmissivity in zone 11 (see Figure 3.4) to simulate the large hydraulic gradient north of Yucca Mountain. The trend of this barrier (zone 11) is east-west, normal to the known faults in the area. Recently, Czarnecki modeled a sudden removal of this postulated permeability barrier. Such a removal of the barrier could occur due to faulting associated with an earthquake. This “dam break” causes a maximum rise of about 40 meters in the computed water-level at the repository site (J. Czarnecki, pers. comm., 1992).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.5 Simulated hydraulic heads. (From Czarnecki and Waddell, 1984.)

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Czarnecki (1985) presented a slightly modified form of the subbasin model. The constant head boundary condition along the northernmost boundary was replaced by a constant flux boundary. The prescribed flux was the same as that computed by Czarnecki and Waddell (1984). The prescribed flow conditions along the Furnace Creek and Alkali Flat were changed to constant head conditions. Hydraulic head values in these areas were estimated from values of land surface altitudes. This model also produced satisfactory agreement with observed heads. To the extent that the mathematical model reflects reality, it can be used to predict how ground-water levels may change in response to changes in precipitation. Before considering speculative modeling of this type, however, it is instructive to consider the paleoclimatic, paleoecological, and paleohydrological data that offer guidance on the likely magnitude of precipitation changes over the next 10,000 years. EVIDENCE FOR PAST VARIABILITY IN RAINFALL Evidence of former, wetter climatic conditions is widespread throughout the semi-arid southwestern United States. Among the most striking examples are the wave-cut terraces on the margins of closed valleys, evidence that these basins once supported vast lakes. In the late nineteenth century Gilbert (1890) was among the first to correlate high stands (or levels) of these lakes, and the “pluvial” (or wet) climates that they indicated, with glacial ages. The correlation between pluvial climatic episodes and glacial ages (or stades) has provided a basic time scale for major climatic fluctuations in North American deserts. Although the correlation does not hold true in other of the world 's great deserts (the last pluvial climatic episode in North Africa, for example, is broadly correlated with the beginning of the present interglacial 10 ka (Ritchie et al., 1985; Spaulding, 1991a), it applies well in the western U.S. Other important issues relating to climate change in the southwestern desert regions include what constitutes a pluvial climatic regime in this region, and how much of an impact pluvial climatic episodes have in terms of increased recharge to the aquifer. These are issues of special interest because the answers to these questions affect projections concerning the magnitude of the impact of pluvial climates on the water table. Chronological Framework Regulations mandate that calculation of the potential for release of radionuclides from the proposed Yucca Mountain repository be per-

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? fossil records between ca. 12 ka and 8 ka (Spaulding, 1990). However, because conditions were changing rapidly, it is premature to offer specific climatic reconstructions for the terminal Wisconsin and early Holocene. Early Holocene records of wet-ground plant species (Table 3.2) do suggest that recharge was still sufficient to support expanded springs and water courses, despite the development of increasingly arid-climate vegetation on upland slopes. The contrasting lack of wet-ground plants from full-glacial middens is suggestive, but may be simply due to sample distribution (Figure 3.7). Climates of the Last 8,000 Years Essentially modern vegetation and climatic conditions were established in the Southwest between 7.8 ka and 7 ka. Within the last seven millennia it appears unlikely that ∆Ta exceeded 1.5°C or that Pa varied more than 20 percent from current long-term averages. However, there were marked variations within these limits. Much of the first half of the middle Holocene (from ca. 7.5 ka to 5.5 ka) appears to have been effectively more arid than the present (Hall, 1985; Spaulding, 1991). And much of the late Holocene (after 3.5 ka) appears to have been characterized by levels of effective moisture equal to or slightly exceeding those of the present (Cole and Webb, 1985; Spaulding, 1990). MODEL CALCULATIONS OF POTENTIAL RISE IN THE GROUND-WATER TABLE DUE TO INCREASED PRECIPITATION Czarnecki (1985) applied the two-dimensional model for ground-water flow at Yucca Mountain to estimate the magnitude of water level changes that might occur in response to a change to a pluvial climate. He used the empirical approach of Eakin et al. (1951) (see Czarnecki (1985) for a detailed discussion) to estimate the increase in ground-water recharge that would occur under an assumed 100 percent increase in modern-day precipitation. He calculated that the consequent increase in recharge would exceed current amounts by more than an order of magnitude (13.7 times greater than at present, rounded upward to 15 times present recharge). It is important to note in this context that the 100 percent value for an increase in precipitation is a speculative figure proposed by Spaulding et al. (1984) to account for poorly constrained evidence of “monsoonal pluvial” climatic conditions between 12 ka and 8 ka (Spaulding and Graumlich, 1986; Spaulding 1991a). Detailed analyses, as discussed earlier in

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? this report, indicate that a ca. 40 percent increase above current average annual precipitation, coupled with a decline in average annual temperature of more than 6°C, accounts for conditions during the last glacial maximum. Despite the present uncertainties about the climatic conditions obtaining during the terminal Wisconsin and early Holocene, it is very likely that the 100 percent increase in precipitation assumed by Czarnecki is overly conservative. The computed rise in the water table beneath the proposed repository area, based on the assumed 100 percent increase in precipitation is about 130 m (Figure 3.8). Interestingly, a modeled increase in precipitation acting in concert with the “dam break” of the steep hydraulic gradient does not cause a larger rise in computed ground-water level than that caused by climate change alone. The predicted rise in ground-water level is extremely sensitive to the recharge in the Fortymile Wash area. To the extent that the modern recharge (and its 15-fold increase due to a 100 percent increase in precipitation) in the Fortymile Wash area is poorly constrained, the computed rise in water level must be regarded as speculative. The magnitude of the calculated rise in water level would place the water level some 70 m below the proposed MGDS and suggests that further investigation of this scenario is required. The modeled effect of a 15-fold increase in recharge is a substantial increase in spring discharge north of the steep hydraulic gradient immediately north of the Yucca Mountain (Czarnecki, 1985). This area of spring discharge includes the middle reaches of Fortymile Canyon, where as previously mentioned, there is one Middle-Wisconsin (>47 ka) record of wet-ground plant species at the Fortymile Canyon-7 site. It is significant that no such records have been recovered from middens in the same area that date to the last glacial maximum. This supports the inference that, during the Late Wisconsin, precipitation and recharge amounts were well below the maximum values incorporated into Czarnecki's model (Czarnecki, 1985, 1990). Estimates of ground-water recharge must be viewed with caution. Ground-water recharge, which is the movement of surface water to the water table through the unsaturated zone, cannot be measured directly. Recharge rates are controlled by the amount of rainfall, by plant and soil factors that govern evapotranspiration, and by geologic characteristics that determine the rate of movement of water to the water table. In semi-arid and arid regions, according to a National Research Council (NRC) ground-water study on recharge, evapotranspiration is nearly equal to precipitation, so that little or no water is available for recharge to the ground-water system except after very large rainfall events, which are infrequent in such climates (NRC,

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Figure 3.8 Differences in simulated hydraulic head between baseline simulation representing present-day conditions and the simulation involving a postulated 100 percent increase in precipitation. (From Czarnecki, 1985.)

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? 1990). While there are several methods for estimating recharge, the NRC study cautions that different methods result in different estimates even for the same locale and time period, and concludes that recharge estimation is difficult and characterized by large uncertainties. Moreover, it suggests that reliable estimates of recharge in dry climates may be beyond present-day technology (NRC, 1990). For this reason, hydrologic models that describe water table responses to changing climatic conditions using simplified recharge assumptions must be used with caution because of the large uncertainty associated with those assumptions. While these caveats are true, the method suggested by Eakin et al. (1951, P. 14-16)—often referred to as the Maxey/Eakin Method—has been widely utilized in Nevada for more than 40 years. The method is purely empirical but has been shown to provide reasonable estimates of recharge. However, the use of this method to predict recharge under climatic conditions that are quite different from the present is speculative. Thus, to the extent that an increase in the number of discharge points (indirectly measurable with the fossil record) indicates an increase in recharge, the sparse fossil record of wet-ground species allows a qualitative observation that recharge increase during the last glacial maximum was moderate (Table 3.2). Czarnecki's model of increased recharge predicts discharge in the central Amargosa Desert west of Ash Meadows in an area that is currently dry. Sediments in this area indicate the existence of a past discharge area (J. Czarnecki, written comm., 1992). CONCLUSIONS Much more information is needed on discharge, recharge and transmissivity to characterize the ground-water flow system. The panel concludes that identifying the cause of the steep hydrologic gradient north of Yucca Mountain, where the potentiometric surface descends sharply about 300 m southward, is the top priority in predicting future behavior of the Yucca Mountain flow system in general, and the water table in the vicinity of the proposed repository, in particular. There is virtually no evidence in the glacial-age fossil record for an increase in average annual precipitation exceeding 40 percent of modern amounts. The nature of full-glacial plant species assemblages can be attributed to a relatively cold and dry climatic regime with increased average annual precipitation approximately 40 percent above that of the present, coupled with mean annual temperatures approxi-

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? mately 7°C below those of the present. The only local record of perennial water where none exists now, other than the Fortymile Canyon-7 site, comes from Dead Man Canyon in the Sheep Range. The general dearth of fossils of wet-ground plant species in the Late Quaternary fossil record and their absence during the full glacial, suggest that an arid to semi-arid climate and low recharge conditions have prevailed over the last ca. 50,000 years. The panel concludes that pluvial climates in this region were much drier than that which would be inferred from the standard application of the word “pluvial.” Therefore, models of climate variability that call for 100 percent increase in precipitation are probably overly conservative. More refinements in the data and techniques for estimating recharge, together with the use of model scenarios that reflect more closely established paleoclimatic conditions, are necessary to obtain realistic models of the response of the water table to increases in precipitation. Nevertheless, according to the only model to date, an increase in precipitation due to a climate change has the potential to cause a rise of the water table on the order of 100 m. Until more complete hydrologic data of the area are obtained to constrain the model assumptions, the panel must regard climate change as a possible means of raising the water table significantly in the Yucca Mountain area. The one known record of local wet-ground vegetation in Fortymile Canyon, which is dated to the Middle Wisconsin, is consistent with modeled responses of ground-water to increased recharge north of the steep hydraulic gradient north of Yucca Mountain (Czarnecki, 1985). This record deserves further consideration, in part because it lies ca. 60 m above the present floor of Fortymile Canyon, and 100 m above the present water table. It points to the need for additional investigations of paleohydrologic conditions in the vicinity. However, it does not necessarily constitute evidence for a radical change in the elevation of the water table in the vicinity of the Yucca Mountain, south of the break in elevation of the potentiometric surface. As previously mentioned younger, full-glacial middens from the same area, within 0.5 km of that site, provide no evidence of wet-ground vegetation at elevations somewhat below and above the site at which the wet-ground evidence was found. RECOMMENDATIONS Because of the importance of understanding the steep hydrologic gradient, the panel recommends that a series of wells be drilled in the region of the gradient north of Yucca Mountain. These wells should be drilled both within and outside the gradient, and should

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? be deep enough to penetrate the pre-Tertiary carbonates underlying the tuffs. Hydraulic head and permeability measurements from both pumping and interference tests in these holes should lead to at least a qualitative improvement in understanding the hydrological regime in this important area, as well as the cause of the steep gradient. These wells would also provide data on the elastic properties of the Paleozoic rocks underlying the site, data that are much needed to understand the potential for rises in the water table due to seismic activity (see Chapter 5 of this report). The wells should be designed in close coordination with those responsible for geochemical studies (including isotopic, studies) because, as outlined in Chapter 2, an understanding of hydrogeological processes at Yucca Mountain will depend heavily on inferences based on geochemical signals. There is a need for a better characterization of the long-term variability of the hydrologic regime in the Yucca Mountain area. Additional chronological data are needed from isotopic analyses of ground waters, as well as of spring deposits and dry lake sediments. The panel recommends that samples of water present at various depths in the Alkali Flat/Franklin Lake subbasin be dated and further analyzed for the isotopic concentrations. If these studies confirm that the last recharge episode actually dates to 10-15 ka, then important inferences may be drawn regarding the coupling of climatic change and recharge events, for instance, that the full glacial was not wet enough to recharge the hydrologic system in the Yucca Mountain region. Furthermore the ground water history can be deduced from the isotopic content of the water. The results of mathematical modeling are also strongly conditioned by available hydraulic data (Appendix B). The panel has several specific recommendations on the collection and interpretation of hydraulic data, which should lead to improvements in the ability to estimate potential changes in water level, are listed below. Existing well data (drilling, stratigraphy, repeat temperature surveys, pumping tests) should be re-examined to determine if the major permeable horizons are associated with specific formations and/or formation interfaces. Permeability studies of the “slug test” variety in some Yucca Mountain area wells produced an anomalous fall-off response in the graphic representation of fluid behavior. This has been interpreted to indicate the state of the minimum horizontal stress in the crust (Szymanski, 1989). The panel recommends that the anomalous response in the slug tests be reanalyzed to determine the cause of the observed behavior. (See Appendix B for a fuller discussion).

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? The panel considers it worthwhile to attempt to remeasure hydraulic potential in isolated sections of existing boreholes. A knowledge of the three-dimensional hydraulic head distribution is essential for developing a detailed three-dimensional model of fluid flow in the Yucca Mountain area. One of the major sources of uncertainty in the present understanding of the hydrologic regime is the recharge in the Fortymile Canyon area, and the rate of evapotranspiration in the Franklin Lake Playa area. The panel therefore recommends that efforts be made to characterize more fully the recharge and discharge rates for the ground-water system in the vicinity of Yucca Mountain. Independent determination of permeability is necessary to constrain and guide computer modeling studies. Since much of the permeability is believed to be fracture controlled, laboratory measurements of permeability on small rock samples may not be representative of flow conditions in situ. Well tests in this case are invaluable. The panel recommends, therefore, a carefully designed set of pressure interference tests between wells to delineate the permeability structure in the Yucca Mountain area. The hydrologic models of the Yucca Mountain area have been restricted to the Tertiary tuff aquifer, which may be an oversimplification of the ground-water system. The panel recommends that a multi-layered model be constructed which includes both the shallow Tertiary aquifer and the Paleozoic carbonate rocks with currently available data. The data should also be used in a sensitivity analysis to test the coupling between the tuff aquifer and the Paleozoic carbonates. Current hydrologic information from the single hole penetrating the carbonate aquifer in the Yucca Mountain area is insufficient to characterize such a model. Additional drill hole data and tests in the carbonate aquifer are critically needed. Such a model should also be useful in assessing the “drain” concept as an explanation for the steep hydraulic gradient north of Yucca Mountain. Moreover, the panel recommends that geochemical data, as well as hydraulic data, be used to assess the validity of the modeling. The panel strongly urges that the use of geochemical interpretations become an integral part of the hydrogeological modeling at Yucca Mountain. The panel recommends that, as sufficient data become available, more definite three-dimensional modeling studies be carried out for both the transient and steady states. A transient three-dimensional model can provide new insights into the evolution of the ground-water system over the past 10-20 ka. Such modeling of the transient

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? state is routinely used in geothermal reservoir engineering to model the natural state. As discussed earlier in this Chapter, an understanding of the relationship between recharge and precipitation is still evolving. It is essential to consider methods to reduce uncertainty in estimates of ground water recharge under different climatic conditions. In particular, the panel recommends undertaking an assessment of the reliability of empirical methods and newly developing considerations in estimating recharge under present arid, as well as much wetter and cooler, conditions to evaluate the potential effects of climate change on the water table. The panel recommends that the assumptions and results of Czarnecki 's (1991) model of increased rainfall and recharge be critically reviewed considering paleoclimate reconstructions, the potential for increased precipitation, and methods of calculating recharge in arid regions. To reduce the present uncertainty level, it will be necessary to obtain additional hydrologic, paleoecologic, and recharge data to provide constraints on future modeling efforts. To resolve the apparent contradiction between what appears to be increased discharge (as a consequence of increased high-elevation recharge at Fortymile Wash) and evidence in the Yucca Mountain area for increased aridity in existing fossil records, the panel recommends a continued search for evidence of perennially moist conditions in currently dry water courses in the area, and for high-elevation (>2000 m) fossil records contemporaneous with a possible latest Wisconsin-early Holocene pluvial episode. The panel also recommends establishment of a data base relating species' ranges to measured climatic parameters, and its application to the macrofossil record. This would provide a great deal of new information on climatic stability of the Yucca Mountain vicinity, and would allow more sophisticated climatic interpretations of the fossil data. The known climatic affinities of plant species within a given fossil assemblage could be used for quantitative derivation of paleoclimatic parameters, if standardized data existed. REFERENCES Antevs, E. 1948. The Great Basin, with emphasis on glacial and postglacial times. University of Utah Bulletin 38: 1-24. Beatly, J.C. 1975. Climates and vegetation pattern across the Mojave/Great Basin Desert transition of southern Nevada American Midland Naturalist 93: 53-70. Beatly, J.C. 1976. Vascular plants of the Nevada Test Site, and central-southern Nevada National Technical Information Service Rpt. TID-26881. Benson, L. V., and R. S. Thompson. 1987. The physical record of lakes in the Great

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? Spaulding, W. G., S. W. Robinson, and F. L. Paillet. 1984. Preliminary assessment of climatic change during Late Wisconsin time, southern Great Basin and vicinity, Arizona, California, and Nevada U. S. Geological Survey Water-Resources Investigations Rpt. 84-4328 Spaulding, W. G., and L.J. Graumlich. 1986. The last pluvial climatic episodes in the deserts of southwestern North America. Nature 320: 441-444. Szymanski, J. S. 1989. Conceptual Considerations of the Yucca Mountain Ground-water System with Special Emphasis on the Adequacy of This System to Accommodate a High-Level Nuclear Waste Repository Unpublished DOE report. Van Devender, T. R., J. L. Betancourt, and R. S. Thompson. 1987. Vegetation history of the deserts of southwestern North America: The nature and timing of the Late Wisconsin-Holocene transition In North America and adjacent oceans during the last deglaciation W. F. Ruddiman and H. E. Wright, Jr., eds. Geological Society of America, Boulder, pp. 323-352. Waddell, R. K. 1982. Two-Dimensional, Steady-State Model of Ground-Water Flow, Nevada Test Site and Vicinity, Nevada-California Water Resources Investigations Report 82-4085. U.S. Geological Survey, Denver, Colorado. Winograd, I. J., and W. Thordarson. 1975. Hydrogeologic and hydrochemical framework, south-central Great Basin, with special reference to the Nevada Test Site U. S. Geological Survey Professional Paper 712-C.