Hydrogeochemical Characterization of the Ahtanum Alluvial Fan for Managed Aquifer Recharge, Yakima Basin, Washington
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Date published: July 12, 2025
Authors: Brook M. Bauer
Reference:
Bauer, Brook M., "HYDROGEOCHEMICAL CHARACTERIZATION OF THE AHTANUM ALLUVIAL FAN FOR MANAGED AQUIFER RECHARGE, YAKIMA BASIN, WASHINGTON" (2025). All Master's Theses. 2103. https://digitalcommons.cwu.edu/etd/2103
Abstract:
Overallocation of water resources in the Yakima River Basin, central Washington, has prompted investigation into facilitated aquifer recharge to enhance water supply during drought as part of the Yakima Basin Integrated Plan (YBIP). This study evaluates and characterizes the recharge potential of the Ahtanum Creek alluvial fan within the lower Ahtanum Sub-Basin by integrating geologic, hydraulic, and geochemical data.
Four main hydrogeologic units (unconsolidated, semi-consolidated, consolidated basin-fill deposits, and Columbia River Basalt aquifers) were identified from stratigraphic analysis of well logs across seven transects spanning the active fan and abandoned alluvial terrace portions of the alluvial fan study area. Stable isotope (δ¹⁸O, δD) and major ion analyses of 25 groundwater and 8 surface-water samples defined six hydrochemical groups exhibiting varying degrees of surface-water and groundwater interactions.
Groups A, B, D, and E show heavy to moderate isotopic and nitrate (NO₃⁻) signatures indicative of surface-water recharge, while Groups C and F display isotopically lighter and chemically evolved chemistries, suggesting minimal recharge. Group A, located in the active floodplain, is associated with highly transmissive unconsolidated to semi-consolidated units and well-drained soils, supporting both rapid infiltration and long-term storage. Groups B and D, in semi-consolidated and consolidated units along the well-drained soils along the anticlinal ridges, and offer moderate infiltration and extended retention potential. Group E, near the fan apex in shallow fractured basalt units overlain by excessively drained soils, presents characteristics suitable for distribution of flow and long term storage.
Group C, located in the mid-to-downstream sections of the active alluvial fan, includes consolidated aquifer units overlain by poorly drained soils, with minimal recharge from surface waters. Group F includes wells in two deep, sedimentary interbeds within the Columbia River Basalt flows in the upstream reaches of the abandoned alluvial terrace. Both C and E present opportunities for long-term storage through alternative recharge applications.
These findings highlight the upstream reaches of the active fan and the mid-fan aquifer units along the northern and southern slopes of the anticlinal ridges as optimal regions for surface-water infiltration recharge, while deeper consolidated units are more appropriate for long-term groundwater storage.
Contact person/agency: Central Washington University Geological Sciences Department
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