A CONCEPTUAL GROUNDWATER SYSTEM MODEL FOR THE COLUMBIA RIVER BASALT GROUP (CRBG) IN THE COLUMBIA BASIN GROUND WATER MANAGEMENT AREA (GWMA) OF SOUTH-CENTRAL WASHINGTON

The CRBG hosts a regional aquifer system that is the primary, and in many cases the only, water supply for numerous communities, small water systems, individual homes, industry, and agriculture in south-central Washington. In the central portion of the semi-arid Columbia Plateau, primarily in Adams County and southern Lincoln County, the CRBG aquifer system has experienced significant water level decline, which is a significant concern to water resources managers in the region. The GWMA has conducted a Subsurface Geologic Mapping and Hydrogeologic Assessment project to map the geologic framework of the CRBG aquifer system in the region and develop a conceptual model of the groundwater flow system. Like previous studies, this project has concluded that the occurrence and movement of groundwater in the CRBG is governed by 1) its geologic features (lithology, folding, faulting, the areal extent of individual interflow zones [including lateral pinchouts], the presence of buried structures [granitic bedrock highs or vertical basalt dikes], the distribution of feeder dikes, and the presence of erosional features [coulees]); and 2) its exposure to surface water sources (lakes, rivers, streams, canals, irrigation). The subsurface geologic mapping, along with updated evaluations of long-term groundwater elevation trends and recent geochemical and age-dating sampling studies conducted for the project, have resulted in two fundamental conclusions regarding current groundwater use: 1) the common practice of deepening wells is only a temporary solution to mitigating declining groundwater levels and well yields, and 2) the gradually increased pumping from deeper CRBG aquifers (primarily the Grande Ronde Basalt [GRB]) is tapping ancient sources of water that receive little, if any, recharge near groundwater pumping centers or at the margins of GWMA. While the long-term outlook for deep GRB water supplies is not promising within the GWMA, restoration of historical groundwater supplies in shallower (predominantly Wanapum Basalt) aquifers via artificial recharge may be possible in certain areas because of the prevalence of Wanapum Basalt exposures in coulee floors and in the channeled scablands of the northern portion of the Columbia Basin.