MANAGED AQUIFER RECHARGE POTENTIAL OF AHTANUM ALLUVIAL FAN IN THE YAKIMA BASIN, WASHINGTON

Increasing population and climate change are expediting the need for water management solutions and storage to secure the future water supply in the Yakima Basin of central Washington. Under the coordinated efforts of the Yakima Basin Integrated Plan (YBIP), managed aquifer recharge (MAR) is a proposed strategy to balance water allocation for agriculture, domestic, instream flow, and habitat. MAR is a form of facilitated groundwater replenishment to store water underground for recovery during seasonal dry periods or drought. The stored groundwater would naturally discharge and contribute water to streamflow or be available for extraction through wells during the drier summer months.

This study evaluates the potential for MAR on the large alluvial fan of Ahtanum Creek, a tributary of the Yakima River. We combined hydrogeologic characterization, stratigraphy from well logs and surface exposures, geologic mapping, and geochemical analysis to identify optimal locations for MAR. A review of well logs and geological maps characterized the physical, stratigraphic and hydrogeologic features of the sedimentary aquifers that could influence aquifer properties, groundwater flow and surface-water recharge. The analysis of well properties aid in determining the local lithologies and hydraulic properties of the sedimentary and bedrock aquifers that are relevant to the groundwater sources and transport within the alluvial fan.

Field applications include groundwater and surface-water sampling for geochemical analysis and monitoring of seasonal changes in water levels from three deployed pressure transducers in wells within the fan. These samples and measurements will assist in determining and documenting current surface water and groundwater interactions throughout the fan. Analyses were conducted for oxygen and hydrogen stable isotopes and major ion geochemistry of surface waters and wells across the fan and in adjacent bedrock aquifers. The results enable us to trace the locations and depth of surface water recharge and degree of mixing with groundwater by identifying distinct geochemical concentrations and stable isotope signatures within the alluvial fan. The combined results will aid in selection of suitable locations for groundwater recharge and inform management as executed under the coordinated efforts of the YBIP.