Ward Valley An Examination of Seven Issues in Earth Sciences and Ecology (1995) / Chapter Skim
Currently Skimming:
3 RECHARGE THROUGH THE UNSATURATED ZONE
3 RECHARGE THROUGH THE UNSATURATED ZONE
Pages 63-122
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 63... ...
5. 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.
|
From page 64... ...
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)
|
From page 65... ...
The seasonal distribution of rainfall at the New Mexico site also differs Tom that at Ward Valley, troth 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.
|
From page 66... ...
to the Ward Valley site. Although the soils at the Las Choices 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.
|
From page 67... ...
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.S cm of soil.
|
From page 68... ...
I) where H is the hydraulic head, equal to the sum of the manic potential head and the gravitational potential head, and z is the vertical space coordinate taken as positive upward (see Box 3.
|
From page 69... ...
Therefore, the very low water contents found in the subsoils of vegetated areas at interstream positions of warm deserts suggest very low hydraulic conductiv~ties and low recharge rates. In typical interstream settings in arid regions where the soils are extremely dry and water fluxes are low, much of the water movement may occur in the vapor phase.
|
From page 70... ...
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.
|
From page 71... ...
Because water win enter fractures (even m~crofissures and relatively small vertical cracks) only when soils approach saturation, preferential flow has been documented mostly in more hu=d 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.
|
From page 72... ...
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.
|
From page 73... ...
TINE 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.
|
From page 74... ...
The shallow penetration of the wetting Dont is attributed by USE to the large storage capacity of the surficial sediments. Conclusions Regarding Water-Content Data from the Ward Valley Site Water contents from X2 soil samples collected near the surface to a depth of 27 m from 6 boreholes were low (94 percent of the samples had water contents less than 10 percent, and 6 percent of the samples had water contents between 10 and 15 percent)
|
From page 75... ...
Direction of Water Flow Rather than a direct calculation of the gradient between water potentials measured by the psychrometers at different depths, an estimate of the direction of water flow can be made by comparing the range of calculated metric potentials in the upper 3 0 m to that which would be expected if no liquid water were moving. As discussed earlier, under the conditions of no water movement, the metric potential at any point in the soil would be equivalent to the height above the water table (when expressed in the correct units when z is taken as positive upward and zero at the water table)
|
From page 76... ...
Because of the dry condition of the sediments at Ward Valley, as indicated by low measured water contents and water potentials, much of the water movement may occur in the vapor phase rather than in the liquid phase. If vapor-phase movement dominates, temperature gradients may be important in controlling the direction and rate of water movement.
|
From page 77... ...
The committee concludes that the water contents and water potentials are low, which indicates that the sediments are dry and that the subsurface water fluxes in the upper 30 m are extremely low. The license application states that the water-potential gradient is upward, but, in the committee's view, the Ward Valley data are not of sufficient quality to justify this conclusion.
|
From page 78... ...
The absence of chloride in soil water indicates either that water fluxes are sufficiently high to minimize chloride accumulation or that high water fluxes flushed out accumulated chloride. Because chloride is readily flushed out of the soil if subsurface water fluxes are high, the occurrence of high chloride concentrations is very good evidence of low water fluxes for very long time periods.
|
From page 79... ...
Extraction of water by plant roots followed by either diffusive loss of chloride to the water table or preferred flow of water through and below the root zone; (b) Piston flow of water with extraction of water through the root zone; (c)
|
From page 80... ...
The transient recharge conditions previously described would not be an unreasonable mode! for the Ward Valley site.
|
From page 81... ...
controlled by the distance to the ocean, its concentration at the Ward Valley site is unlikely to vary significantly with time. Under the current climatic regime, the precipitation at the site is approximately 12 cm/yr.
|
From page 82... ...
The chloride profiles are generally bulge-shaped with low chloride concentrations o us c: cn z ~ 15 ~ U.l O a 1 _ LO m I C) 10 20 25 30 35 or..~ ., - i 'I ·\ · Act' \ ,' ~ GB-2 · · · GB-4 · GB-1 1 1 1 1 1 1 0 2 4 6 8 10 12 14 16 CHLORIDE CONCENTRATION, IN GRAMS PER LITER Figure 3.4 Chloride concentrations in pore water of unsaturated sediments from three boreholes at the Ward Valley site.
|
From page 83... ...
The likelihood of relatively high recharge during the Pleistocene with little or none in the Holocene is not incompatible with ages of ground water below Ward Valley that are considerably younger than the '~chior~de accumulation age" because the saturated-zone ground-water recharge zone was probably not in the immediate vicinity of the site. Conclusions Regarding Chloride Datafrom the Ward Valley Site The committee regards the chloride data analyzed from the three boreholes as evidence that percolation at the site is very small through the unsaturated zone.
|
From page 84... ...
When water fluxes are very low, much of the water containing recent terbium may still be found in the root zone. In the root zone, velocities can be much higher than in the underwing sediments.
|
From page 85... ...
Tritium Measurements at Ward Valley From Table 3.] it is apparent that the highest tr~tium values were from water-vapor samples collected Tom the near-surface air piezometers.
|
From page 86... ...
6/26/89 GB-1 21.5 1.39 ~ 0.57 6/26/89 GB-1 35 1.72 ~ 0.51 6/7/89 GB-1 60 0.74 ~ 0.41 6/6/89 GB-1 99.5 ~.01 ~ 0.58 5/6/89 GB~ 16.5 5.60 ~ 0.37 5/7/89 GOB 58 1.37 ~ 0.49 5/7/89 GB-4 99.7 1.02 ~ 0.33 6/2/89 GB~ 16.5 6.00 ~ 0.72 6/3/89 GOB 58 1.15 ~ 0.66 6/4/89 GOB 99.7 1.18 ~ 0.54 6/24/89 GOB 18.5 3.94 ~ 0.73 6/23/89 GOB 33 2.07 ~ 0.89 6/23/89 GOB 59 1.38 ~ 0.62 6/16/89 GB~ 99.7 1.66 ~ 0.39 Air Moisture Sample 6.91 ~ 1.03 ~ From LA Table 242o.s-lo 2 Note: ~ values are only laboratory uncertainties (lc;) associated with the tritium value 3 lfoot=0.3048m Second, this ~ figure from the laboratory, unless specifically stated otherwise, represents only the uncertainty of the laboratory measurement, and does not include additional uncertainties associated with the field collection procedure itself (see, for example, Long and Kalin, 1990)
|
From page 87... ...
~ W ~ ~ ~ , , ~ ~ conducted modeling calculations, wnlcn Included tne eIlects of llqula Interaction, and concluded that gaseous diffusion of tritium will be limited to only 1.5-3.0 m below the land surface at the Ward Valley site.
|
From page 88... ...
The two-phase modeling approach is conceptually correct, although the liquid diffusion coefficients used in simulations are much larger than would be expected in the dry soils at Ward Valley. Two diffusion coefficients were used, 1.49 cm2/day and 4.47 cm2/day.
|
From page 89... ...
This sphere of extraction, in the case of the soil air sampled for tr~tium analysis at Ward Valley, would have a radius ranging from about 2 to 2.5 m, depending on the volume collected, and assuming an average porosity of 27 percent and an average water content of 7 percent in homogeneous media. The Ward Valley alluvium consists of variable mixtures of poorly sorted sand, silt, grave} and clay, as described in the license application.
|
From page 90... ...
As discussed above, these results are also consistent with small amounts of recent infiltration to these shallow depths. Atmospheric contamination may have affected the trivium measured at the collection points nearest the land surface (at 5 or 6 m)
|
From page 91... ...
m are consistent with small amounts of infiltration to those depths, but are also consistent with the samples that have incorporated some atmospheric water vapor during the sampling procedure Conclusions Regarding Tritium Found in the Unsaturated Zone In the committee's judgement, the gaseous diffusion mode! presented in the license application as the most likely explanation for tritium at depth in the unsaturated zone is incorrect.
|
From page 92... ...
Sampling for 36C! can be accomplished much more readily than for tritium at the Ward Valley site because large quantities of water are not required for analysis and chloride concentrations in the soil water are extremely high.
|
From page 93... ...
The license application states that the direction of water Bow is upward; however, problems with the thermocouple psychrometers at the site made it difficult to determine the water potential gradient. In the committee's judgment, the most likely current direction of water movement at the Ward Valley site is upward.
|
From page 94... ...
Freon determinations were negative in all water samples Tom the monitoring wells at the Ward Valley site.
|
From page 95... ...
As the available data do not unequivocally reveal recharge areas or even flow directions for Ward Valley, we discuss the data more generally in terms of a likely hydrogeochem~cal scenario. A viable general mode} for ground-water flow and geochem~cal interaction for ground-water alluvial aquifers in arid arid semiarid basin-and-range valleys involves: (~)
|
From page 96... ...
, who modeled reaction paths in the Tucson Basin in Arizona using NETPATH, did not find CO2 exchange to be important in the Tucson Basin. In the case of the Tucson Basin, the samples were from wells used for water supply and were completed deeper below the water table than the monitoring wells at Ward Valley.
|
From page 97... ...
A Holocene or Pleistocene age of ground water below Ward Valley does not necessarily indicate local recharge, because groundwater flow paths are very long and much of the ground water presently beneath the Ward Valley site probably originated from recharge in higher elevation regions.
|
From page 98... ...
Normally, extensive water-rock interaction is restricted to geothermal waters. Ward Halley Stable Isotope Data The data in the license application come from five ground-water wells, each of which was sampled on two dates (January-February 1989 and May-June 1989~.
|
From page 99... ...
It is also the sample closest to the water table. The samples plotted in Figure 3.5 are very close to the meteoric water line, indicating the isotopic composition of non-evaporated waters in the region.
|
From page 100... ...
used ED and 6i8O values for monthly mean temperatures at a single mountain site in Colorado, where climatological conditions are vastly different from those at Ward ValleY. Compilations of ~D, SILO, and temperature data by the International Atomic ~ ~ , , ~ · ~~ ~ a_ ~~ ~ ~ ~ At- · ~ 1 1 .
|
From page 101... ...
Chemical Variations in Ward Valley Ground Water The license application and a letter report (Harding Lawson Associates, 1994a) conclude that the chemical variations are not the result of recharge events to the water table.
|
From page 102... ...
The geochemst~y of ground water from monitoring wells WV-MW-O] to -05 was reported in the original site license application and supplemented with data contained in a letter report (Harding I,awson Associates, 1994a)
|
From page 103... ...
Most, or all, of the reported temporal differences in the chemical composition of the Ward Valley site ground water are likely due to collection and laboratory procedural uncertainties and errors. The committee concludes, therefore, that the reported ground-water chemical fluctuations are not attributable to current recharge, but rather are the result of sampling, analytical, or reporting uncertainties and/or errors.
|
From page 104... ...
Fluctuations The committee concludes that the variations in water-level measurements reported for the Ward Valley site do not constitute evidence for local recharge and are most probably the result of measurement errors and uncertainties. Ground-Water Gradients In simple aquifer systems where flow is predominantly in a lateral direction, it is uncommon to encounter appreciable water-level differences in adjacent wells completed at different depths.
|
From page 105... ...
SUMMARY OF CONCLUSIONS OF SUBISSUES 3, 4, AND 5 I The tritium isotope compositional pattern of the ground water strongly supports the license application conclusion that significant recharge to the water table is not occurring directly beneath the Ward Valley site.
|
From page 106... ...
(1993b and 1994) concluded that the data collection efforts and modeling do not adequately represent the complexities of the processes of water movement in the unsaturated zone at the Ward Valley site.
|
From page 107... ...
Extensive use of the infiltration test data and calibration is also justified in light of these difficulties. These data appear to be consistent with literature values, although the departure of the calibrated hydraulic conductivity data at low water contents that was found in the infiltration experiment is not well explained in light of other literature results.
|
From page 108... ...
In the opinion of the committee, the assumptions used in the modeling are reasonable for the Ward Valley site. The use of non-site hydraulic data is less than optimal, but should have little impact on this scenario because of the strong control of the water budget by the vegetation.
|
From page 109... ...
No flux values were reported, but the increases in water content were small. The discrepancy between the amount of calculated recharge at Ward Valley and other studies may be caused by differences in climatic regime, soil properties, or evaporation modeling techniques.
|
From page 110... ...
Following this phase of transport, both water and solutes diffuse slowly to the water table. Very low concentrations of long-lived, non-sorbing radionuclides were calculated to reach the water table within 10,000 years.
|
From page 111... ...
SUMMARY OF CONCLUSIONS3 Major Conclusion The committee concludes from multiple lines of evidence that recharge or potential transfer of contaminants through the unsaturated zone to the water table at the Ward Valley site, as proposed by the Wilshire group, is highly unlikely. Basis for Committee Judgements and Conclusions The committee reviewed multiple lines of evidence to evaluate subsurface water flux at the Ward Valley site The committee based its conclusions conceding the unsaturated zone On the data, observations, and discussions in this chapter, including the following information · In 82 samples from near the surface to a depth of 27 m, water contents were generally very low (94 percent of the samples had water contents less than 10 percent, and 6 percent of the samples had water contents between 10 and 15 percent)
|
From page 112... ...
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.
|
From page 113... ...
The committee attributes this inconsistency between the high metric 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.
|
From page 114... ...
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.
|
From page 115... ...
The committee also concludes that the bulk i4C 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.
|
From page 116... ...
(The chemical tracer, 36CI, has been used successfully elsewhere to constrain the magnitude of liquid water movement in the unsaturated zone and should be considered. Sampling for 36C!
|
From page 117... ...
1983. The use of natural tracers as indicators of soil-water movement in a temperate sem~-ar~d region.
|
From page 118... ...
Ina Alterman; Summary of performance assessment modeling; Proposed low-level radioactive waste disposal facility. Ward Valley California.
|
From page 119... ...
Administrative Record, Ward Valley LowLJeve! Radioactive Waste Disposal Facility, Sections 2420.B and 2600.A; Appendices 2420.B, 2500.A.
|
From page 120... ...
1990. Supplemental Unsaturated Zone Data Report, Administrative record of the Ward Valley Proposed Low Level Radioactive Waste Disposal Site.
|
From page 121... ...
1994. Ward Valley Proposed Low-Level Radioactive Waste Site: A Report to the National Academy of Sciences.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.