A CONCEPTUAL FRAMEWORK FOR MANAGED AQUIFER RECHARGE NEAR YAKIMA, WASHINGTON, USA

Increasing demands for water, overallocation of surface water, and a changing climate in the Yakima Basin in south-central Washington are leading to a loss of water storage and increasing demands in drought years. A warming climate has reduced snowpack in the Cascade mountains, a vital reservoir for the irrigated agricultural industry which serves as the backbone of the basin’s economy. Mitigating this impending water crisis is essential for the economic output of the region. Managed aquifer recharge (MAR) is a sustainable and cost-effective approach for securing water supply by storing water underground for recovery during drought. Diminishing groundwater levels in regional basalt aquifers over the last several decades suggest there is significant storage available for intentional groundwater recharge of these aquifers.

This study focuses on the ridge and valleys east of Yakima, WA, particularly the area served by the Roza Irrigation District. The region consists of east-west trending folds and faults of the Yakima Fold Belt (YFB). The bedrock is composed of the Grande Ronde, Wanapum, and Saddle Mountain formations of the Columbia River Basalt Group, as well as sedimentary interbeds of the Ellensburg Formation. There is greatest interest in utilizing the basalt aquifers for MAR due to the immense depths and vast spatial extent of the formations, the water-bearing vesicular flow tops and interbeds, and the structural controls of the YFB.

Informed by the 2011 USGS Columbia Plateau groundwater study, this research quantifies the groundwater storage capacity in various basalt aquifers to assess the viability of MAR east of Yakima. In order to investigate storage potential, we developed a conceptual hydrogeologic model depicting the relationship between geology and flow dynamics at depth. Well logs were used to construct cross sections of stratigraphy and water levels, and previous pump tests were used to decipher structural barriers to flow that maximize or minimize MAR efforts. This model is used to make preliminary quantitative estimates of available storage volume and to assist in identifying recharge sites within the study area. Applications of this methodology can be impactful for improving water security in other regions of the world, especially those with continental flood basalts or complex folds and faults.