Joel Murray

The principal water supply for the Palouse Basin of eastern Washington and northern Idaho is groundwater. Groundwater levels in the Basin have been steadily declining for the past century. A Geographic Information System (GIS) database was developed to identify areas of the Basin where soil and geologic features are likely to impact groundwater recharge. Using GeoProcessing operations in ArcView, data layers of 1:24,000-scale soil survey data and surficial geology were combined. The resulting ArcView-based GIS coverage was used to delineate recharge mechanisms and classify recharge potential in the Palouse Basin. Based on hydrological studies of the Basin, four main mechanisms of recharge have been identified: 1) vertical and horizontal percolation through loess, 2) stream loss, 3) infiltration along Basin margins, and 4) infiltration through localized fracture systems within basement rock. Areas of the Basin where these mechanisms are operative were identified from the surficial geology data layer and the areal extent of each was calculated. A database was developed using binary weighting and index overlay modeling methods to assign values to soil map units based on selected soil characteristics including permeability rates, depth to bedrock, and the presence of perched water tables. These data were then linked to Basin recharge mechanisms to produce maps indicating the potential for deep percolation through soil and subsequent recharge to the local aquifer system. Results indicate that recharge through loess is the most spatially extensive recharge mechanism, operating over 71% of the total study area. Of this, approximately 2,300 ha have high potential for recharge. Recharge through stream loss operates over 16% of the total study area while infiltration through localized fractures in bedrock is the least extensive recharge mechanism, operating over just 13% of the total study area. Although stream loss and infiltration through localized fractures in bedrock are not extensive they have more area rated as 'high' potential for recharge. In general, there is lower recharge potential in the eastern portion of the Basin due to the presence of extensive hydraulically restrictive subsoil horizons. Future uses for the database might include identifying potential locations for artificial recharge systems and areas in the Basin where agrochemical contamination may occur