Ward Valley An Examination of Seven Issues in Earth Sciences and Ecology (1995) / Chapter Skim
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4 INFILTRATION AND LATERAL FLOW
4 INFILTRATION AND LATERAL FLOW
Pages 123-132
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From page 123... ...
is that "there is little, if any, deep percolation of water through the thick, highly moisture-deficient sediments in the vadose zone underlying the Ward Valley site." According to DHS, 'the infiltrating precipitation is either removed by evapotranspiration or held in storage in the highly moisture-deficient soils near the ground surface until such time as it is removed by evapotranspiration." DHS further stated that their findings showed "the caliche layers at the site to be discontinuous and permeable such that one would not expect them to perch infiltration water in undisturbed areas where they are overlain by sediments. However, where exposed on the surface in the abandoned highway borrow pit, the shallow caliche layers act as a form of discontinuous pavement and the resulting rapid runoff of surface water has led to pending in the low end of the borrow pit" (Brandt, 1 994)
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From page 124... ...
Under rainfall conditions, large areas of soil would be wetted, but the slope of the calcic soil horizon would be an important factor in controlling the extent of lateral flow. Although lateral flow is more likely under ponded conditions because of the higher fluxes, the localized nature of the ponded conditions, the amount of water ponded, and the lithologic continuity of layers are important in determining the extent of lateral flow.
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From page 125... ...
These field experiments showed that only limited lateral water movement occurred under enhanced rainfall conditions in a layered soil system that contained calcic horizons. In addition to the possibility of soil layering and resultant changes in permeability causing lateral flow, another process called tension-dependent anisotropy can also promote lateral flow.
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From page 126... ...
Computer simulations with 5 inches of rain falling over a 2-hour period in a soil with similar hydraulic characteristics as the Ward Valley site and under a slope of 2 percent has shown no saturated conditions over the less permeable layer located at 50 cm below the soil surface (Pan and Wierenga, 1995)
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From page 127... ...
In order to constrain the maximum flow rate that could be expected into the trench by lateral flow, let us assume that a completely impermeable layer is located at some shallow depth below the land surface. Overlying this impermeable layer is a layer of soil with hydraulic conductivity similar to that found at the Ward Valley site.
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From page 128... ...
Under low water-content conditions, the very small downsiope gravity component of subsurface Bow is negligible compared to the diffusion component. LATERAL FLOW UNDER PONDED CONDITIONS AT ARID SITES Although the natural interstream setting at Ward Valley shows no evidence for ponded conditions, information Tom pending experiments provides conservative estimates of the degree of lateral flow if ponding occurs.
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From page 129... ...
Clay layers or native soil amended with benton~te, as proposed for the Ward Valley B/C trench covers, are used to me ze downward water movement. Clay soil bamers are generally wetted and compacted to achieve permeabilities of approximately 10-' cm/s.
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From page 130... ...
In summary, the committee concludes that the moister - deficient nature of the soils at Ward Valley, the absence of an effective slope factor, the very low subsurface water fluxes, and the limited ability of the calcareous horizons to impede vertical Bow, eliminate lateral flow as a significant issue at the Ward Valley site. The committee
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From page 131... ...
1994. Summary of Borrow Area Investigation, Low Level Radioactive Waste Disposal Facility, Ward Valley, California.
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From page 132... ...
1994. Ward Valley proposed low level radioactive waste site.
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