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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology 3 Recharge Through The Unsaturated Zone Issue 11 Potential Transfer of Contaminants Through The Unsaturated Zone To The Ground Water THE WILSHIRE GROUP POSITION The Wilshire group questioned the adequacy of the evaluation in the license application and supporting documents of the nature of water movement in the thick unsaturated zone beneath the Ward Valley site (Wilshire et al, 1993a, 1993b). They divided this issue into five subissues. These are: The soil properties measured and used in the modeling of water movement in the unsaturated zone do not adequately represent the variability and complexity of the materials present. No consideration was given to rapid migration of water along preferential pathways. The reporting of tritium at depths of up to 30 m indicates vertical water transport at rates much faster than that used in the performance-assessment models. Electrical sounding data from along Homer Wash suggest the possible existence of higher water content that may indicate ground-water recharge from the wash. Interpretation of both stable and radioactive tracers found in the ground water in the license application was incorrect and could be interpreted as evidence of recent recharge through the unsaturated zone. THE DHS/U.S. ECOLOGY POSITION The DHS concluded that, based on the general aridity of the site, hydraulic data collected from boreholes, the interpreted age of the ground water, and a simplified tritium diffusion model, the site shows no evidence for high water flux or rapid pathways. 1 In this report, the Wilshire group Issues 1 and 2 have been reversed. Issue 1 in this report is actually Issue 2 of the Wilshire group and vice versa. The committee found it more useful to address the Wilshire group's Issue 2 on the potential for recharge through the unsaturated zone first because it requires extensive discussion of the properties and characteristics of the unsaturated zone. Issue I of the Wilshire group, the potential for later flow in the shallow subsurface (Issue 2 of this report), requires much less discussion of unsaturated zone characteristics and therefore draws upon the information in the preceding chapter.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology THE COMMITTEE'S APPROACH The committee grouped the subissues raised by Wilshire et al. (1993b) into three broad questions relevant to the behavior of water in the unsaturated zone: What is the nature, direction, and magnitude of soil-water movement beneath the Ward Valley site, including the influence of geologic heterogeneity and the relative importance of piston and preferential flow (Wilshire et al., 1993a, b; subissues 2, 3, and 4)? Are the data and models used to quantify the rate and direction of water and contaminant movement through the unsaturated zone adequate to define the general performance characteristics (Wilshire et al., 1993b, subissue 1)? Does ground water below the Ward Valley site show evidence of recent recharge (Wilshire et al., 1993a, b; subissue 5)? In (1), the discussion of the broad issue of soil-water movement will address the three subissues. For each of these questions, the committee has looked closely at the data available from both the site license and supporting documents, at the relevant scientific literature, as well as at documents supplied by interested individuals and organizations at the open meetings and subsequently. To answer each of these questions, a brief review of the nature of soil-water movement and its implications for waste disposal will put the issues in perspective. Data from previous studies of water movement in the unsaturated zone with emphasis on add regions are instructive. These previous studies show that the science of unsaturated-zone hydrology is young and that the techniques employed continue to have significant uncertainties. As a result, the committee considered it prudent to base its conclusions on the results of multiple and independent lines of evidence, not only of the most likely estimate of water velocity, but also to assess the performance impacts of other, less likely, estimates. THE NATURE OF WATER MOVEMENT IN THE UNSATURATED ZONE In a typical add setting, most precipitation infiltrates the soil, except under conditions of intense precipitation. Some of this water evaporates and some is transpired by plants. Together these two processes, which return rain water to the atmosphere, are termed evapotranspiration. If the volume infiltrated is greater than that evapotranspired, it can result in deep downward or lateral percolation. If the unsaturated zone is thin or the percolation rate high, the recharge rate at the water table will approach the rate of percolation over a long time. If, however, the unsaturated zone is very' deep and the percolation rate is small, then the recharge rate may be out of time-phase with the current near-surface conditions. In such cases, it is important to differentiate between measurements of deep percolation in the unsaturated zone and estimates of recharge made at the water table.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Water Content Water content is a standard soil physics parameter useful for evaluating water movement in the unsaturated zone is water content. Water content is either measured in the laboratory using soil samples collected in the field or monitored in the field using a neutron probe or time-domain reflectometry to determine the soil dryness. Water-content monitoring can be used to evaluate the movement of water pulses through the unsaturated zone. In general, errors associated with the typical water-content measurement techniques (neutron probe or time-domain reflectometry) are generally ±1 percent volumetric water content. This may not be sufficiently accurate to detect small fluxes that could move through the deep unsaturated zone of desert soils. Under conditions of steady flow, water content will not vary; therefore, the absence of temporal variations in water content does not necessarily preclude downward flow. In such a case, the flow is controlled by the hydraulic conductivity of the soil. Water-content monitoring is most applicable for measuring large subsurface water fluxes under transient conditions. In addition, water content is discontinuous across different soil types; therefore, variations in water content with depth in heterogeneous soils do not necessarily indicate the direction of water movement. Water content data are available for many sites from interstream settings and show that water contents in desert soils are generally low. Long-term monitoring of soil water has been used to evaluate infiltration and deep percolation in several arid sites. Some of these studies were carded out at a site in the Chihuahuan Desert of New Mexico. Rainfall at the New Mexico site is approximately twice that at Ward Valley. The seasonal distribution of rainfall at the New Mexico site also differs from that at Ward Valley, with more summer precipitation at the New Mexico site, which is typical of the Chihuahuan Desert, and more winter precipitation at the Ward Valley site, typical of the Mojave Desert. Winter precipitation is more effective at infiltrating the soil than summer precipitation because of lower evapotranspiration in the winter. Long term precipitation records (1953-1989) at the Jornada Experiment Range in New Mexico indicate that only 23 percent of the rain falls between November and March whereas 50 percent of the rain falls between November and March in Needles, California (period of record 1948 to 1989). Although the seasonal distribution of rainfall differs between the two sites, the amount of winter rainfall is similar (5.6 cm Needles; 5.9 cm Jornada Experiment Station); therefore, results of studies at the New Mexico site are applicable to the Ward Valley rite. Long-term studies in the Chihuahuan Desert measured soil moisture along a transect from an ephemeral lake through the piedmont and up a steep rock slope (Wierenga et al., 1987). Large spatial variability in water content was recorded from a mean of 0.3 m3/m3 (± 0.04 m3/m 3) at 130 cm depth in the playa to 0.04 m3/m3 (± 0.04 m3/m3) at the upper rocky end of the transect (Figure 3.1). This variability reflects in part the textural variations from the clayey soils in the playa to more gravelly soils at the rocky slope. Temporal variations in water content were measured for a site near the center of the transect. This site is located on an alluvial fan with a deep loamy soil and is vegetated with creosote bush similar
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 3.1 Variation in water content on an alluvial fan along a 3 km transect in the Jornada Range in southern New Mexico. Water contents presented for depths of 30, 90, 130 cm during the week of 30 April - 6 May 1992. Symbols: ——, 30 cm;-- -- -,90 cm; -•- 130 cm. Vegetation is creosote bush and annual rainfall is 20 cm. to the Ward Valley site. Although the soils at the Las Cruces site in New Mexico and at Ward Valley are similar in terms of their soil hydraulic properties, we recognize that the ecological systems at the two sites are different. The data at the Las Cruces site display significant variations in water content at 30 cm depth during six of the nine years studied. At a depth of 130 cm, fluctuations in water content occurred only in response to the relatively wet fall and winter of 1985. Thus during eight of the nine years, rainfall that infiltrated the soil was taken up by the plant roots in the upper 130 cm of the root zone and evapotranspired back into the atmosphere. Plant roots were observed in a 6 m deep trench near the trench down to 4 m depth. Most likely, water that passed the 130 cm depth was taken up by plant roots below that, as observed during one of the nine years studied. Water-content monitoring at the Beatty site from 1978 to 1980 measured infiltration and redistribution of water down to approximately 2-m depth in 1979 to 1980 (Nichols, 1987). The deep penetration of water was attributed to high precipitation in 1978 (23.35 cm
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology at the site, twice the long-term average) and precipitation in January 1979 (4.62 cm) followed by additional rainfall in March 1979 (2.52 cm). This emphasizes the importance of antecedent water content in the soil and the sequence of precipitation events in controlling percolation. Monitoring at Beatty from 1984 to 1988 showed water movement, was restricted to the upper one meter during this period (Fischer, 1992). Water content was monitored in a small scale ephemeral stream setting (maximum topographic relief of 0.65 m) in the Hueco Bolson of Texas (Scanlon, 1994a). Approximately monthly monitoring of water content from July 1988 to October 1990 showed that within the detection limit of the neutron probe (± 0.01 m3/m3) water content remained constant from 0.3 to 41 m depth. Although precipitation during 1989 was 50 percent of the long term mean annual precipitation, precipitation during 1990 was similar to the long term mean. Data from these desert basins indicate that penetration of water is often restricted to the shallow subsurface. The reason for the shallow penetration of water in desert soils is the large storage capacity of the surficial sediments. For example, soils similar to those at Ward Valley with an initial average volumetric soil-water content of 10 percent and a water content at saturation of 35 percent have a maximum additional storage capacity of 25 percent by volume, which is equivalent to 25 cm of water storage per meter of soil profile. To bring this into perspective, if an annual rainfall of 12.7 cm could fall in one day, the 12.7 cm of water could hypothetically be stored in only 50.8 cm of soil. In reality, the soil would not come to full saturation, but water would move deeper into the soil profile. Even if the soil-water content were to increase to 17.5 percent (which is 50 percent of saturation), the 12.7 cm annual precipitation would wet the soil down to only 170 cm. This demonstrates the great storage capacity of dry desert soils in general and of Ward Valley in particular. Potential Energy of Soil Water In contrast to water content, water potential energy is continuous across different soil types and is typically used to infer the flow direction. Water flows from regions of higher total potential energy to regions of lower total potential energy. In areas with moderate to high subsurface water flux, gravity and matric potential are the dominant driving forces. Matric forces result from the interactions of the water and the soil matrix and include capillary and adsorptive forces. An example of such behavior is demonstrated when a sponge is placed in water. Water can move upward, against gravity, into the sponge until some equilibrium is reached. Matric potential is expressed in meters of water, bars or megapascals (MPa). For comparison, 1 Mpa = 10 bars = 102 m of water. Because water is tightly held by unsaturated soil, the matric potential has a negative value. If soil becomes wetter, its matric potential becomes less negative until at saturation it becomes zero. Matric potential is related to soil-water content through the soil water retention curve.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Hydraulic Conductivity The rate of water flow q through soil is proportional to its hydraulic gradient dH/dz with the proportionality constant being the hydraulic conductivity, K. This is expressed through Darcy's law: where H is the hydraulic head, equal to the sum of the matric potential head and the gravitational potential head, and z is the vertical space coordinate taken as positive upward (see Box 3.1). BOX 3.1 DRIVING FORCES FOR FLOW IN THE UNSATURATED ZONE The soil-water potential energy is typically used to infer the water flow direction because water will flow from regions of high to low potential. In the unsaturated zone, many gradients may be important, as is indicated by the generalized flux law (modified from de Marsily ): where q is the flux, L1, L2, and L3 are proportionality constants, Δ is the gradient operator, H is the hydraulic head (sum of matric and gravitational potential heads), T is the temperature and C is the solute concentration. This equation holds for systems in which water flow occurs in liquid and vapor phases. The direction of liquid flux is controlled by gradients in hydraulic head whereas the direction of isothermal and thermal vapor flux is controlled by hydraulic head and temperature gradients, respectively. In some flow systems, temperature and osmotic potential gradients are negligible and the flux can be simplified to the Buckingham-Darcy law (the first term to the right of the equal sign in the above equation, the means of calculating downward liquid flow). To quantify the water flux, information on the proportionality constants is also required. Under isothermal conditions, which implies absence of a thermal gradient, and when liquid flow dominates, the relationship between hydranlic conductivity and water content or matric potential is used to calculate the flux. Hydraulic conductivity decreases exponentially with decreased water content or matric potential. In arid systems, where the matric potential gradient is large, the reduction in hydraulic conductivity with decreased matric potential will outweigh the effect of the increased gradient and will result in negligible flow. In cases where vapor flux is important, isothermal and thermal vapor diffusivities should be measured or estimated to quantify vapor flux.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The ability of unsaturated soil to conduct water, i.e. the hydraulic conductivity, is directly related to its water content. At saturation, soils have their highest hydraulic conductivity. As a soil dries, the larger pores empty first, causing the hydraulic conductivity to decrease. In fact, the hydraulic conductivity of many soils decreases exponentially with decreasing water content or soil-water potential (Gardner, 1958). Thus a decrease in water content by only a few percent can produce a 10-fold or a 100-fold decrease in hydraulic conductivity. Therefore, the very low water contents found in the subsoils of vegetated areas at interstream positions of warm deserts suggest very low hydraulic conductivities and low recharge rates. In typical interstream settings in add regions where the soils are extremely dry and water fluxes are low, much of the water movement may occur in the vapor phase. Because the fluxes are so small, the direction and magnitude of the fluxes may be difficult to determine. In addition to liquid flow, as a result of matric and gravitational potential gradients, vapor flow induced by temperature gradients may also be important. Potential Energy of Soil Water Darcy's law also requires a knowledge of the gradient in potential energy, dH/dz. Instruments used to measure potential gradients include tensiometers and heat dissipation probes (HDP's), which measure matric potential, and thermocouple psychrometers, which measure temperature and water potential (sum of matric and osmotic potential). Osmotic potentials can be calculated from soil-water chloride concentration data (Campbell, 1985) and can be subtracted from water potential to estimate matric potential. Except in the shallow subsurface after rainfall, levels of water potential measured in add and semiarid regions generally decrease toward the surface. This suggests an upward driving force for liquid and isothermal vapor flux (Fischer, 1992; Estrella et al., 1993; Scanlon, 1994). The osmotic component of water potentials, which refers to the energy required to remove dissolved salts from the water, is generally low at depth at these sites because chloride concentrations below the top 10 m are generally low (Phillips, 1994). Long-term monitoring records of soil-water potentials and water contents for add sites are limited. Monitoring data from the Beatty site displayed increased water potentials between depths of 1 to 2 m after precipitation in November 1987 (Nichols, 1987). Neutron moisture probe readings for the same period of record did not show changes in water content, probably because the changes in water content were within the standard error of the neutron probe readings. In the Chihuahuan Desert of Texas, at a silty loam site within a small-scale ephemeral stream setting, water potentials measured from summer 1989 to summer 1990 were out of range (<-8.0 MPa) of the thermocouple psychrometers in the upper 0.8 m, because the sediments were too dry (Scanlon, 1994). Water potential monitoring has continued since that time to 1995 and has shown that the wetting front has not penetrated below the upper 0.3 m at this site. In general, below the shallow zone of active circulation, matric potentials show seasonal fluctuations in response to seasonal temperature fluctuations
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology down to depths of 12 m (Fischer, 1992; Scanlon, 1994). Below the zone of seasonal fluctuations, water potentials remain constant over time. One can also estimate the matric potential that would exist in the unsaturated zone if it were in hydraulic equilibrium with the water table. Under equilibrium conditions, the total soil-water potential energy is everywhere constant. (In the sponge example discussed previously, the capillary forces drawing the water upward are in balance with the force of gravity tending to pull the water downward). Under these conditions, the soil matric potential energy is at equilibrium with gravitational forces. If z (the vertical space coordinate) is taken as positive upward and zero at the water table, then under these conditions the equilibrium soil matric potential can be described by a line starting at zero at the water table and equal to the negative of the height above the water table (Figure 3.2). Under steady flow conditions, matric potentials displaced to the fight of the equilibrium line indicate downward flow, whereas those plotted to the left indicate upward flow (Figure 3.2). At several arid western sites (Fischer, 1992; Estrella et al., 1993; Scanlon, 1994) matric potentials plot to the left of the equilibrium line indicating upward driving forces for liquid and isothermal vapor movement (Estrella et al., 1993; Scanlon, 1994). At the Nevada Test Site (NTS) this upward driving force is restricted to the upper 40 m. Below this depth, water potentials plot to the right of the equilibrium line, which suggests that water in this portion of the profile may be moving downward to the water table (Sully et al., 1994). Below the zone of seasonal temperature fluctuations, the upward geothermal gradient provides an additional upward driving force for thermal vapor movement. This is approximately 0.06°C/m at the Beatty site (Prudic, 1994a). Sully et al. (1994) suggest that this upward thermal vapor flux may be of the same order of magnitude as the downward liquid flux at depth at the NTS; such a condition would result in no net flux of water but a slow downward migration of water-soluble chemicals. Vegetation also plays a critical role in removing water from desert soils. Lysimeter data from Hanford and Las Cruces showed that deep percolation from bare sandy soils can range from 10 to >50 percent of the annual precipitation (Gee et al., 1994). Therefore, the use of data developed on undisturbed sites in Ward Valley to predict long-term performance through the operation and closure period is acceptable only if the conditions include reestablishment of the active plant communities after the waste is emplaced. There remains uncertainty of the effects of trench construction on the long-term water balance at any disposal site, and the goal of the closure and trench cover program outlined in the license application is to reduce these uncertainties. Nature of Water Movement: Piston Flow And Preferential Flow In general, two types of water movement occur in unsaturated zones, piston flow and preferential flow. Piston flow refers to uniform water movement downward through the soil matrix. Infiltrated water displaces initial water. In contrast, preferential flow refers to nonuniform downward water movement along preferred pathways, such as continuous vertical fractures, or pathways created by changes in soil characteristics.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 3.2 The relationship of the equilibrium matric potential to height above the water table. If the soil water is stagnant, the matric potential is exactly balanced by the gravitational potential represented by the height above the water table. A simple measure of the direction of flow can be made by plotting matric potential head (calculated from field measured water potentials by subtracting the osmotic potential) versus the height above the water table. A variety of factors are important in preferential flow, including continuity of preferred pathways and exposure of the pathways to ponded or perched water (Beven, 1991). Because water will enter fractures (even microfissures and relatively small vertical cracks) only when soils approach saturation, preferential flow has been documented mostly in more humid regions with higher rainfall. The Ward Valley alluvium consists of variable mixtures of poorly sorted sand, silt, gravel, and clay, with greater variability vertically than horizontally, as discussed later in this chapter. Thus the most significant pathways are likely to be along root tubules, cracks, and fracture planes (including faults). Although some have considered flow associated with major hydrologic features such as ephemeral streams and playas as preferential flow (Gee and Hillel, 1988), we will restrict our discussion to the kinds of pathways described above. In uniform soils and at low rates of soil-water movement, water will generally flow in a piston-like manner, particularly if the soils are dry initially. Under piston-like flow
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology conditions most, if not all, preexisting water (''old'' water) is displaced and moved ahead of the "new" infiltration water added from above. Under certain conditions, only a fraction of the "old" water is displaced and water moves through preferential pathways. Preferential flow is the process whereby water and solutes move along preferential pathways through a porous medium (Helling and Gish, 1991). During preferential flow, local wetting fronts may propagate to considerable depths in a soil profile, essentially by passing the matrix pore space. (Bevin, 1991; Steenhuis et al., 1994). Preferential flow has been demonstrated under laboratory conditions (Glass et al., 1991) and under simulated rainfall conditions (28-152 cm/day) in forested field soils (Turton et al., 1995). However, in dry desert soils, water that is moving through preferential pathways, such as roots and root tubules, is expected to be absorbed by the dry soil around the pathways. Also, layering caused by differences in texture will disrupt the continuity in the flow paths. Therefore preferential flow is expected to be damped out over a relatively short distance below the root zone. The rate and depth of infiltration along preferential pathways is often greater below areas of surface ponding or frequent flooding than below higher ground. Piston flow is generally found in add sites where sediments are not subjected to intermittent or continuous ponding. At study sites in both southern New Mexico and southern Nevada, where long-term monitoring of soil-water content has been conducted, measurements have confirmed that significant changes in water content are restricted to the upper 1-2 m. The water input to these systems that receive water only from precipitation may be too low to result in preferential flow. In addition, much of the water in these dry soils is adsorbed on the grain surfaces and therefore cannot move along preferred pathways, Piston flow is also indicated by single peaks on profiles of the chemical tracer, chlorine-36 (36Cl), as in the profile measured in a small-scale ephemeral stream setting in the Hueco Bolson in Texas (Scanlon, 1992a). The single 36Cl peak suggests that the bomb related 36Cl which fell on the land surface in the mid 1960's has moved downward uniformly and retains its bell-shaped profile. If non-piston flow had occurred, the 36Cl in the soil zone would show a much more erratic pattern, with alternating highs and lows corresponding to zones of fast and slow water movement. Large-scale infiltration experiments conducted at Las Cruces showed that the movement of dissolved solids, or solutes, lagged behind the water, or wetting front (Young et al., 1992). The lag between the solutes and wetting fronts increased with depth, which indicated piston displacement of initial soil water, rather than preferential flow. The lag also increased with increased initial water content. Previous studies specifically identifying preferential flow in alluvial deposits in desert regions are limited. Allison and Hughes (1983), in a study of recharge in south central Australia, observed tritium much deeper (12 m) in the soil profile than was predicted from other tracers and the general aridity of the region. They attributed the tritium transport to flow along channels occupied by living roots. No data on tritium transport below the root zone were presented (Allison and Hughes, 1983). Shrubs at Ward Valley, such as creosote bush have much shallower roofing depths (< 1-2 m) than the eucalyptus plants reported by Allison and Hughes (1983). Fracture flow has also been reported at Yucca Mountain, Nevada by Yang (1992) and Fabryka-Martin et al. (1993); however, the geologic setting at Yucca
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Mountain differs greatly from that at Ward Valley, as the Yucca Mountain unsaturated zone is in faulted and fractured volcanic rock. Preferential flow has also been found in fissured sediments in the Chihuahuan Desert of Texas (Baumgardner and Scanlon, 1992; Scanlon, 1992a). They conducted detailed studies of fissured sediments in the Hueco Bolson in Texas. These features consist of an alignment of discontinuous surface gullies (<140 m long) underlain by sediment-filled fractures (2 to 6 cm wide) that extend to a depth of >6 m. Similar features have not been found at Ward Valley. Fractures were observed in the Quaternary alluvium at the site and are prominently exposed in the old I-40 borrow pit. These could be either desiccation or tectonic fractures. Their nature and continuity at depth are unclear, but unless a systematic investigation is made of these fractures, it is not possible to assess their origin or continuity. The impact of fractures on fluid flow can be deduced from the distribution of water content and tracers found in the unsaturated zone. In summary, studies have shown that water movement in arid soils is controlled by several factors, such as energy gradients, topography, and the complexity of the soil materials. It is therefore critical that comprehensive and detailed studies be performed at any site where the unsaturated zone is to be used as a primary barrier for waste isolation. In the next sections, the data for the Ward Valley site are reviewed with respect to this criterion. THE NATURE, DIRECTION, AND MAGNITUDE OF WATER FLUX BENEATH THE WARD VALLEY SITE The Wilshire group (Wilshire, 1993a, b, and 1994) questioned the assertion in the license application that percolation is negligible beneath the site. They based this on observations of preferential flow in other areas and on the reported tritium concentrations found at depths to 30 m below the surface. They also raised the question of the potential for recharge in Homer Wash and its implication for site performance. The techniques commonly used to estimate soil-water flux in the unsaturated zone fall into two broad categories: soil physics methods and chemical tracer techniques. Soil Physics Methods Applied To Ward Valley The two types of soil physics methods used to compute deep percolation at the Ward Valley site are: water-content monitoring and hydraulic gradients.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Chloride concentrations measured in three boreholes to a depth of 30 m were very high (up to 15 g/l). The time required to accumulate these large quantities of chloride down to 30 m depth was calculated to be approximately 50,000 yr (Prudic, 1994b). Estimated water fluxes based on chloride data were very low (0.03 to 0.05 mm/yr below 10 m depth). Measured tritium at 30 m depth, which could be interpreted as evidence for recent rapid downward migration of water, is not consistent with the foregoing soil-physics or chloride data, which indicate very dry conditions with very slow water movement and limited infiltration. The committee concludes that the most likely explanation for the measured tritium is contamination with atmospheric tritium owing to inappropriate sampling procedures. In addition, the committee has reviewed data from similar regions and cited results of field experiments and related literature from other add-region unsaturated zones to supplement the limited field evidence. We have resolved several inconsistencies in the data sets by attributing the source(s) of ambiguity to (a) inherent uncertainties in many of the measurements, (b) errors in procedural and collection methodologies, and (c) analysis and reporting errors. Limitations of Field Data The committee notes that monitoring hydraulic parameters in dry soils like those at the Ward Valley is very difficult, leading to several limitations in collecting field data. The restrictions imposed have three main causes: (1) the effects of low water fluxes; (2) the effects of instrument limitations in add soils; and (3) the results of unresolved inconsistencies in the data and/or project decisions on where and how deep to test. Effects of Low Water Fluxes The soil physics and chloride data indicate that subsurface water fluxes in the upper 30 m of the unsaturated zone are extremely low. Collection of water for tritium analysis in these dry soils is quite difficult because of the large quantities of water required for the analysis relative to the very low water content of the soils. In addition, because of these very low water fluxes, it is difficult to resolve easily the rate and direction of water movement with available equipment and sampling procedures. Based on the committee's experience and understanding of the unsaturated zone at Ward Valley, it is not currently possible to resolve definitely the exact magnitude and direction of the water flux. In the case of the Ward Valley site, qualitative terms such as very small and extremely slow can and should be used in place of more quantitative terms of water flux.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Effects of Instrument Limitations The committee also notes that monitoring hydraulic parameters in arid unsaturated zones is very difficult because of the lack of methods, procedures, and reliable instruments to measure precisely the hydraulic and hydrochemical parameters used to estimate water flux in dry desert soils. Some of the instruments, particularly the thermocouple psychrometers and heat dissipation probes, are not very robust and have a high failure rate. Although serious problems were encountered with instrumentation for water potential monitoring, all recorded water potential values were very low (-3 to -6 MPa) and are consistent with measured water contents. Matric potentials monitored by heat dissipation probes (-0.2 to -0.5 MPa) were much larger than the water potentials monitored by the thermocouple psychrometers (-3 to -6 MPa), indicating wetter conditions than would be expected from the water content values. The committee attributes this inconsistency between the high matric potential values and low measured water contents to the wet silica flour used for installation of the heat dissipation probes. In these dry soils, however, larger standard errors in the data can be tolerated, compared with much wetter soils, without significantly affecting the estimates of water flux because of the exponential decrease in hydraulic conductivity with decreased water content. Quantity and Quality of Data In part because of instrumentation and sampling problems, but also because of project decisions on where and how deep to test, the number and distribution of observations and the quantity of data collected for site characterization were very limited. In the committee's view, more confirmatory information on spatial variability of hydraulic and hydrochemical attributes is highly desired to provide further assurance that the limited data from which site characteristics were determined are representative of the entire site both areally and vertically. In several instances, additional data and/or sampling will be required to interpret data correctly. This particularly applies to the tritium measurements in the unsaturated zone and to the apparent vertical gradient in the ground water beneath the site. These are inconsistent with most other data. Similarly, uncertainties in the measurements of soil-water potential and uncertainties generated by the limited hydraulic data from the unsaturated zone below 30 m can be reduced only through additional characterization of this zone. If the site is developed as a LLRW facility, resolution of these uncertainties will be important to the development of reliable baseline data for the planned monitoring during site operations. These issues, which relate to the monitoring program plan during site operations and beyond, are discussed in detail in Chapter 6 of this report.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Subissue Conclusions With regard to the specific subissues raised (Wilshire et al., 1994): With respect to the measured soil properties of subissue 1, the committee concurs with the license application that water movement through the upper 30 m of the unsaturated zone is extremely slow. As explained previously, dry soils with low subsurface water fluxes, do not permit easy determination of the direction and rate of water movement; however, in the opinion of the committee, the most probable direction of water movement is upward, based on the low water potentials measured, as well as on what has been observed at other arid-zone sites with similar characteristics. The conclusions are also based on the observations of limited infiltration during the monitoring period, the low water contents and potentials found, and the significant accumulations of chloride in the upper 30 m of the unsaturated zone. With respect to the modeling of the water movement in subissue 1, the committee determined that the performance modeling is consistent with the resolved data to encompass the expected range of variability and complexity of transport through the unsaturated zone, with the exception of the failure scenario for the B/C trench cover. For the B/C trench cover failure scenario, the committee has not determined if a more conservative scenario, that includes enhanced percolation through the cover, would result in significant risk at the compliance boundary. The committee concludes that site characterization has found no evidence for rapid deep migration of water along preferred pathways. Although it is impossible to demonstrate its absence conclusively, the lack of major surface features indicative of rapid flux, the significant quantities of soluble chloride found in several of the boreholes, low water content, low water potentials, the lack of evidence for recent recharge at the water table or for deep penetration of natural infiltration, do not support the hypothesis of rapid pathways. The committee finds that the conclusion in the license application that gas diffusion is responsible for the tritium reported in the unsaturated zone is conceptually incorrect. The committee concludes that inappropriate sampling procedures most probably introduced atmospheric tritium into the samples. Except for three data points at depths of 5.1 m and 5.4 m, the tritium data are not distinguishable from zero owing to inadequate evaluation of the sample-collection blank. The three results from the uppermost sampling depths may represent atmospheric contamination, or they may indicate small amounts of shallow infiltration. Due to these uncertainties, the tritium data may not be used for evaluation of infiltration. The committee concurs with both the DHS and the Wilshire group that significantly higher rates of percolation and recharge are possible beneath Homer Wash. Given its location downgradient from the site, the committee concludes that such potential recharge is unlikely to affect the trenches designed for the disposal facility.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The committee concludes that the concentrations of isotopic tracers D, 18O, and 14C of ground water at Ward Valley do not support an interpretation of recent ground-water recharge by rapid movement of water through the unsaturated zone in the last 50-100 years, as proposed by the Wilshire group. The committee also concludes that the bulk 14C age of the ground water exceeds 4,500 years. RECOMMENDATIONS In the committee's opinion, thick unsaturated alluvial sediments in arid environments such as are beneath the Ward Valley site are favorable hydrologic environments for the isolation of low-level radioactive waste. Both theoretical understanding and a growing body of field evidence demonstrate that the amount of water and rate of water movement throughout most of these unsaturated zones is very small and very slow, respectively. This does not imply, however, that siting of disposal facilities can be made without careful study and analysis, as it has also been found that percolation can occur in certain locations in arid regions, particularly where water is concentrated at the surface. Such concentrations occur in ephemeral washes and natural or artificial depressions. For this reason, site characterization data must be of sufficient quantity and quality to address the areal variability of percolation at a potential site to provide reasonable assurance that rapid movement of water through the unsaturated zone is unlikely. The committee attributes some of the incomplete and/or unreliable data sets to the fact that hydrologic processes in arid regions are characterized by extreme events which do not follow a one-year calendar. For this reason, regulatory and/or budgetary guidelines that permit one-year or other short-duration time frames not suitable for arid-soil characterization can result in conflict between permissible regulatory timetables and the optimum time requirements for arid-region hydrologic investigations, which can easily lead to incomplete or ambiguous results. In the committee's opinion, characterization activities should receive priority over arbitrary regulatory timetables, or short time-frame budgetary constraints, particularly in arid regions. General Recommendation To guard against deficiencies in characterization and monitoring efforts, and as more emphasis is placed on arid regions for waste disposal, the committee recommends, as a general rule, that an independent scientific peer review committee be established for such sites to provide oversight early in the permitting process, to help resolve scheduling conflicts, and to assess and suggest improvements in the site characterization plans and investigations. In this way, conflicts in, and other concerns with, characterization data from the unsaturated zone can be resolved as they arise. In
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Chapter 6 of this report, this recommendation is also discussed with reference to monitoring. The committee further recommends that such on-going peer review activities and recommendations be specifically included in the permitting applications to provide assurance to both the public and the scientific community of the expected performance of any waste-disposal site. Such independent review would also provide guidance to the regulatory community in assigning realistic and appropriate timetables for all phases of development. The committee hopes that other states and compacts consider these ideas and recommendations for future waste sites. Specific Recommendations As both water-content and water-potential monitoring, and tritium analyses, are proposed for site and post-closure monitoring, the committee recommends that attempts be made to resolve or improve these data. To accomplish this, the following are recommended to establish base levels for monitoring: additional sampling for tritium, water-content logging, and water-potential monitoring; continued characterization of the unsaturated zone using corrected methodologies and equipment to provide a more complete data set for monitoring during site operation, and for an effective closure plan for the facility. confirmatory data from a greater depth in the unsaturated zone below the present 30-m limit for confident monitoring of the site during operation. The committee recommends that to obtain more complete knowledge of the background levels of tritium in order to ensure that subsequent monitoring data can be adequately understood, and to develop action levels (see Chapter 6 for details), the following actions will be necessary: because of the difficulty of determining tritium blank values for the air piezometer collection method, alternate collection techniques, such as vacuum distillation of core samples, should be considered; sampling and analyzing for 36Cl as a check on the tritium data. (The chemical tracer, 36Cl, has been used successfully elsewhere to constrain the magnitude of liquid water movement in the unsaturated zone and should be considered. Sampling for 36Cl can be accomplished much more readily than for tritium at the Ward Valley site because (1) large quantities of water are not required for analysis and (2) chloride concentrations in the soil water are extremely high.) The committee recommends that an analysis be conducted of a more conservative failure scenario for the B/C trench cover which includes enhanced percolation through the cover.
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Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Vogel J. C. and D. Ehhalt. 1963. The use of the carbon isotopes in groundwater studies, in Radioisotopes in Hydrology. Proceedings of the Symposium on the Application of Radioisotopes in Hydrology. International Atomic Energy Agency, Tokyo, March 5-9. pp. 383. Vogel J. C., D. Ehhalt, and W. Roether. 1963. A survey of the natural isotopes of water in South Africa, in Radioisotopes in Hydrology. Proceedings of the Symposium on the Application of Radioisotopes in Hydrology. International Atomic Energy Agency, Tokyo, March 5-9. pp. 407. Weeks, E. P., D. E. Earp, and G. M. Thompson. 1982. Use of atmospheric fluorocarbons F-11 and F-12 to determine the diffusion parameters of the unsaturated zone in the southern high plains of Texas. Water Resources Research 18(5): 1365-1378. Wierenga, P. J., J. M. H. Hendricks, M. H. Nash, J. Ludwig, and L. Daugherty. 1987. Variation of soil and vegetation with distance along a transect in the Chihuahuan Desert. Journal of Arid Zone Environments, 13(1):53-64. Wierenga, P. J., R. G. Hills, and D. B. Hudson. 1991. The Las Cruces Trench Site: Characterization, experimental results, and one-dimensional flow predictions. Water Resources Research 27:2695-2705. Wilshire, H. G., K. A. Howard, and D. M. Miller. 1993a. Memorandum to Secretary Bruce Babbitt, dated June 2. ——— 1993b. Description of earth science concerns regarding the Ward Valley low-level radioactive waste site plan and evaluation. Released December 8. Wilshire, H. G., K. A. Howard, D. M. Miller, K. Berry, W. Bianchi, D. Cehrs, I. Friedman, D. Huntley, M. Liggett, and G. I. Smith. 1994. Ward Valley Proposed Low-Level Radioactive Waste Site: A Report to the National Academy of Sciences. Presentations made to the review committee on July 7-9 and August 30-September 1. Yang, I. C. 1992. Flow and transport through unsaturated rocks - data from two test holes, Yucca Mountain, Nevada. High Level Radioactive Waste Management: Proceedings of the Third International Conference. American Nuclear Society. pp. 732-737. Young, M. H., P. J. Wierenga, R. G. Hills, J. Vinson. 1992. Evidence for piston flow in two large scale field experiments, Las Cruces, New Mexico. EOS, Trans. AGU 73:156.
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Representative terms from entire chapter: