). For many performance assessments and licensing requirements by the EPA and NRC, contaminant release rates and groundwater travel times must be determined. For the vadose zone, the most important factor for travel time is the recharge, defined as the amount of precipitation that passes the upper few meters of soil to enter the vadose zone.
Case Study: Recharge Mapping at a VOC Site
Measurements of hydraulic conductivity using the UFA Method allow detailed mapping of the subsurface flux or recharge. The UFA achieves hydraulic steady-state in any porous media in hours by using an adjustable, whole-body driving force together with precision fluid flow.(Conca, J. L. and J. V. Wright. 1992. Applied Hydrogeology 1:5-24.) The Plutonium Finishing Plant in the 200-West Area at Hanford in Washington State is the site of a mixed-waste contaminant plume containing carbon tetrachloride (CCl4) with plutonium (Pu) and americium (Am).(Last, G., and V. Rohay. 1993. Technical Report PNL-8597, Pacific Northwest Laboratory, Richland, WA.) The water table at this site is the top of an unconfined aquifer approximately 73 m (240 ft) below the surface. The overlying vadose zone consists of clastic sediments of poorly sorted glacio-fluvial gravel, sand, and silt, and semi-continuous layers of loess, paleosols, and low-permeability paleoplaya lake deposits that have developed extensive caliche. These sediments have a variety of water contents and a wide range of hydrologic properties.
The recharge at any point in the subsurface depends upon precipitation rate, type of surface cover, and any lateral recharge that may occur from perched saturated zones or artificial recharge within the vadose zone. For a normally vegetated fine-grained cover soil at this site, the recharge is less than 10-10 cm/s.(Gee, G. W., M. J. Fayer, M. L. Rockhold, and M. D. Campbell. 1992. "Variations in Recharge at the Hanford Site," Northwest Science, 66:237-250.) However, this has been locally increased by human influences. Under normal conditions, hydraulic steady state is usually achieved within a few meters of the surface, i.e., the recharge is everywhere the same, each material has reached its steady-state volumetric water content for that water flux, and the recharge is equal to the hydraulic conductivity of the materials at that water content. Because borehole samples were collected as undisturbed as possible, the in situ volumetric water content and the actual hydraulic conductivity behavior can be measured and used together to determine the recharge into each sample just prior to sampling. The UFA Method was used to measure the hydraulic conductivity of over sixty samples from boreholes drilled in an around these disposal facilities. Figure 1 is a plot of hydraulic conductivity curves for two samples from the same borehole. A line is drawn from the sample's field moisture content on the horizontal axis to the hydraulic conductivity curve. A second line is drawn from the point intersected on the curve to the vertical axis. This point on the vertical axis indicates the subsurface flux in the sample at the time of sampling. The sample at 55.3 ft has a flux that is less than 10-9 cm/s. Arid soils in the vadose zone in this region require tens to hundreds of years to achieve hydraulic steady state when fluxes are 10-9 cm/s or less; therefore, it can be assumed that this sample has not undergone artificially high flux from recent disposal of wastewater to the ground, and this flux is probably close to the actual recharge rate. For the sample at 140.8 ft below the surface, the flux is much higher, 3 x 10-7 cm/s. This result indicates that the sample has recently been subjected to increased flux, but does not provide any specific information about the timing and/or magnitude of the subsurface flux. The sample could have experienced a flux of 3 x 10-7 cm/s the day before sampling and never been subjected to any greater flux, or the sample could have experienced a flux of 3 x 10-5 cm/s 10 years ago and still be draining.
This type of determination was made on each sample and then used to assemble a subsurface flux or recharge distribution map for the subsurface beneath the site showing which samples have had
artificially high recharges, presumably from the disposal facilities. Figure 2 is the resultant recharge or subsurface flux distribution map projected onto a NE-SW trending vertical plane showing the disposal facilities at the surface, the eight boreholes and samples at various depths, and a possible plume geometry that is consistent with these observations and with the hydrostratigraphy of the site. It can be seen that most borehole samples investigated have natural recharges expected at the Hanford Site. Recharge through the Hanford formation is essentially vertical and channelized as evidenced by the lack of increased recharge into borehole samples from the Hanford formation adjacent to the cribs. The only Hanford samples showing increased recharge are those directly below the cribs. No extensive lateral sheet flow is occurring in this area.
On the other hand, lateral dispersion of the plume and increased recharge is definitely occurring within the Palouse/Plio-Pleistocene units and along or near their boundaries. These results show that the plume is heterogeneously distributed and which specific areas should be targeted for remediation. This method of subsurface flux or recharge mapping works in any arid to semi-arid region.
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