AN ISOTOPE STUDY OF THE HYDROGEOLOGY OF THE UPPER YAKIMA RIVER BASIN, WASHINGTON

The object of this study is to identify recharge sources and describe ground water flow paths in the Ellensburg Basin, a sub–basin in the Upper Yakima River drainage.  The study basin lies in the rainshadow of the Cascade Mountains in a semi–arid region dominated by irrigated agriculture.  Oxygen and hydrogen isotope data from precipitation, surface and ground waters are compared and used to form a preliminary conceptual model of the ground water system.  Major ion chemistry, tritium and ¹⁴C analyses are used to aid ground water type classification, elucidate flow paths and indicate residence times.

Preliminary results show the d¹⁸O of precipitation ranges from –5.5 to –24‰ with lower d¹⁸O in colder months, a typical pattern for climates with large seasonal temperature differences.  The d¹⁸O of seven local tributaries range from –14.9 to –15.8‰ and decrease in ¹⁸O content from east to west––likely due to Rayleigh fractionation of the precipitation.  The Yakima River, the source of irrigation water, is isotopically heavier (d¹⁸O=  –13.8‰) than local surface waters. 

The d¹⁸O of 29 ground water samples ranges from –13.3 to –17.3‰ with the majority falling within –15.0 to –16.0‰.  The depth of the wells sampled ranges from 3 to 369 meters.  A few wells tap basalt aquifers, the remaining draw from the sandstones of the Ellensburg Formation.  Nitrate and sulfate concentrations are elevated in shallow well waters, but are also measurable in samples from deeper wells.  Half of the shallow ground water samples have relatively high nitrate concentrations as well as high d¹⁸O values, suggesting a significant contribution of irrigation water.  Tritium concentrations in samples from eight deep wells indicate that all are greater than 50 years old.

The correlation between d¹⁸O values of surface and ground waters suggests that a significant portion of the ground water is recharged by local runoff or local precipitation.  When viewed in relation to well depth, the d¹⁸O of ground water generally decreases with depth.  This trend may be explained by the mixing of irrigation water with natural recharge in shallower ground waters and may indicate the presence of Pleistocene paleowaters in lower portions of the aquifer.