HYDROGEOLOGY AND POTENTIAL FOR GROUNDWATER RECHARGE AND STORAGE ON ALLUVIAL FANS IN THE YAKIMA BASIN

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 is evaluating two large alluvial fan aquifers in the Ahtanum and Manastash watersheds within the Yakima Basin. We are combining hydrogeologic characterization of existing well logs, geologic mapping, and geochemical analysis to identify optimal locations for MAR. Preliminary review of well logs and geological maps have characterized physical and hydrogeologic features of the sedimentary aquifers. Due to varying stratigraphy and project requirements, independent well selection methods are applied on the Ahtanum and Manashtash fans. For Ahtanum, existing well logs are cataloged within the boundaries of the alluvial fan based on two established reaches identifying surface water recharge and groundwater transition zones. Manastash wells are selected based on fan morphology, creating proximal, midfan, and distal transects. Well log analysis and cataloging will proceed by selecting quadrants along determined transects to identify well location, proposed use, construction details, water levels, and surrounding stratigraphy. Well log observations effectively characterize surface water/groundwater interactions, through identification of unique lithologies and hydraulic properties. Properties highlighted from the well log analysis will aid in identifying shallow productive aquifers suitable for further investigation of water-level variations and sampling for geochemical analysis in the summer of 2024. Results will supplement previous and ongoing investigations for the YBIP into the most efficient and feasible locations for MAR.