STABLE ISOTOPE STUDIES OF SURFACE WATER-GROUNDWATER INTERACTIONS IN THE UPPER YAKIMA RIVER DRAINAGE, WASHINGTON

The Yakima River is one of the largest rivers in Washington draining 15,760 square kilometers of forested, range, and agricultural land. Intensive irrigation in parts of the Yakima River basin provide a setting to study the effects of humans on arid region hydrology. This research project uses stable isotopes to examine the effects of irrigation on groundwater recharge and discharge, evaporative water loss, and nitrate contamination. Groundwater samples are being collected monthly from private and municipal wells along a transect across the Yakima River near Ellensburg, Washington. H and O isotopic compositions for these groundwater samples, can be compared to local stream water, irrigation water and precipitation data and interpreted in terms of mass balance models to determine the contributions of geochemically distinct source waters (e.g. snowmelt, irrigation waters) to the groundwater and how these contributions change seasonally. Preliminary results indicate that local precipitation and surface waters form a local meteoric water line on a δD- δ¹⁸O plot with equation: δD = 6.6 δ¹⁸O – 10.7. Irrigation waters and their source, the Yakima River, generally fall above this local meteoric water line with relatively heavy isotopic compositions. Groundwaters from shallower wells also tend to be isotopically heavier and plot near the local surface and irrigation water, particularly when sampled during the irrigation season. Groundwaters from deeper wells are isotopically lighter and have a narrower range of values, comparable in composition to spring snowmelt. The stable isotope data will also be compared with major ion and trace element geochemical data to further constrain the seasonal variations in surface water-groundwater interaction.