A geochemical study of the impact of irrigation and aquifer lithology on groundwater in the Upper Yakima River Basin, Washington, USA

The Yakima River, a major tributary of the Columbia River, is currently overallocated in its surface water usage in part because of large agricultural water use. As a result, groundwater availability and surface water/groundwater interactions have become an important issue in this area. In several sub-basins, the Yakima River water is diverted and applied liberally to fields in the summer creating artificial recharge of shallow groundwater. Major ion, trace element, and stable isotope geochemistry of samples from 26 groundwater wells from a transect across the Yakima River and 24 surface waters in the Kittitas sub-basin were used to delineate waters with similar geochemical signatures and to identify surface water influence on groundwater. Major ion chemistry and stable isotope signatures combined with principal component analysis revealed four major hydrochemical groups. One of these groups, collected from shallow wells within the sedimentary basin fill, displays temporal variations in NO₃ and SO₄ along with high δ¹⁸O and δD values, indicating significant contribution from Yakima River and/or irrigation water. Two other major hydrochemical groups reflect interaction with the main aquifer lithologies in the basin: the Columbia River basalts (high-Na groundwaters), and the volcaniclastic rocks of the Ellensburg Formation (Ca–Mg–HCO₃ type waters). The fourth major group has interacted with the volcaniclastic rocks and is influenced to a lesser degree by surface waters. The geochemical groupings constrain a conceptual model for groundwater flow that includes movement of water between underlying Columbia River basalt and deeper sedimentary basin fill and seasonal input of irrigation water.