Estimating Recharge Rates for a Groundwater Model Using a GIS

Some of the defense wastes at the Hanford Site in southeastern Washington State are stored in the vadose zone. It is possible that natural recharge could mobilize and transport the contaminants in these wastes to the groundwater, as well as influence groundwater velocities and directions. The objective of this study was to estimate the areal distribution of natural recharge for use as a boundary condition for a groundwater flow and transport model. A geographic information system (GIS) was used to identify all possible combinations of soil type and vegetation and assign to each an appropriate estimate of recharge. The strategy was to assign estimates based on field data and supplement with simulation results only when necessary. The estimated rates varied from 0.7 to 127.1 mm/yr. The order of preference for assigning estimates was lysimetry > water content measurements > tracers > modeling, based on qualitative estimates of the relative error of each method as applied.The GIS software was used to estimate the annual recharge volume attributable to specific soil-vegetation combinations. The total annual recharge volume was 8.47 x 10' L for the 765 km 2 portion of the site containing the major waste storage areas. This volume is from 2 to 10 times higher than estimates of runoff and groundwater flow from adjacent higher elevations and is equivalent to facility discharges in 1992. The recharge map showed the impact of a 1984 fire on increasing recharge ; it also illustrated the higher recharge rates associated with disturbed soils in the waste storage areas.