GEOCHEMICAL ANALYSIS OF SURFACE WATER AND GROUNDWATER IN UPPER KITTITAS COUNTY, WA: IMPLICATIONS FOR RECHARGE AND FLOW PATH

During below-average water years, surface water supply in upper Kittitas County (UKC) often cannot meet the currently permitted withdrawals. Surface water is therefore considered over-allocated leaving the supply for new water rights to come from groundwater. A better understanding of the groundwater system is needed for effective water management. In order to assess groundwater recharge and flow based on the geochemistry of water throughout UKC, surface and groundwater samples were collected and analyzed for pH, conductivity, temperature, major ions, trace elements, and stable isotopes ratios. Surface water and shallow groundwater were predominantly Ca²⁺-type waters with high δD. Groundwater withdrawn from alluvium were largely Ca²⁺-Mg²⁺-type waters with intermediate δD. Groundwater collected from wells north of the Yakima River in the sedimentary rock were predominantly Na⁺-HCO₃^--type waters with low δD. These three major water types indicate that there are three major flow regimes in UKC. At the surface, water mainly flows across the landscape. At certain locations surface water enters the groundwater system but it has limited interaction with the subsurface before it is withdrawn through wells, due to fast flow rates or short flow paths. In the alluvium aquifer, groundwater is recharged in part by surface water, which spends an intermediate amount of time in the subsurface, either due to intermediate flow rates or intermediate length flow paths. South of the Yakima River, some of the groundwater in the upper part of the sedimentary rock aquifer is likely part of this intermediate flow regime. The sedimentary rock north of the Yakima River is recharged by isotopically light water that originates as precipitation which fell either during the winter, or during past climate conditions. Groundwater here likely spends a relatively long amount of time in the subsurface either because of low flow rates or long, regional flow paths.