Geochemical Studies of Groundwater Recharge in the Upper Yakima River Basin, Washington

The Yakima River, basin is one of the most intensively irrigated areas in the country. In the upper Yakima River basin, most irrigation is accomplished through diversion of waters from the Yakima River and its tributaries into a network of canals. These waters are applied to fields between the months of May and October, causing artificial recharge to groundwater during the summer months.

In this study, groundwater, surface water and precipitation samples were collected seasonally and analyzed geochemically in order to determine the impact of irrigation on groundwater recharge. Samples were collected from 25 domestic and municipal wells along a transect perpendicular to the Yakima River. Precipitation was sampled throughout the year; surface waters were collected from natural streams and irrigation canals. The stable isotope, major ion, and trace element geochemistry of these samples were compared and statistically analyzed to constrain the origin of the groundwater samples.

Our results distinguish three different groups of groundwater samples. On the southwestern side of the transect, wells supplied by an aquifer within the Columbia River basalts have higher proportions of Na and K. Their isotopic composition can be explained by recharge from spring snowmelt or surface water derived from rivers to the east of the basin. In the center of the basin, groundwaters from shallower domestic wells have major ion chemistry dominated by Mg, Ca, and HCO3. Stable isotope data suggest that these waters are a mixture of spring snowmelt and local surface waters. Several shallow wells have seasonally higher nitrate concentrations and stable isotope signatures indicative of large contributions of irrigation water. The third type of groundwater is found in deep municipal wells. These waters are isotopically distinct from the local meteoric water and may be derived from older precipitation or from local meteoric water that has undergone considerable evaporation.