AN INVESTIGATION AND COMPARISON OF STABLE ISOTOPES IN METEORIC WATERS AND GROUNDWATERS FROM SOUTHERN WASHINGTON

A main source of freshwater in the Pacific Northwest is the Columbia River Basalt Group aquifer system. The semi-arid region of eastern Washington has undergone heavy groundwater depletion in recent decades due to increasing population and expanding agricultural use. Aggressive mining has resulted in drawdowns of up to 30 meters. By using stable isotopes oxygen-eighteen (¹⁸O) and deuterium (D), this study confines the timing of groundwater precipitation and proposes an explanation for why drawdowns are so significant. The isotopic composition of meteoric water is compared to groundwaters from southeast Washington. The two populations are statistically different (p < 0.001, alpha = 0.05). The isotopic composition of groundwaters from the deepest wells (< -150 m msl) are isotopically lighter than meteoric waters and not achievable by recharge under the current climate. These deep groundwaters are interpreted to have precipitated during the Last Glacial Maximum in the late Pleistocene and finished precipitating roughly 5 ka. Thus, replenishment of the aquifer cannot be expected to keep pace with extraction. Additionally, this research examines the elevation response in the stable isotope signature of meteoric water along the 47° N latitude by computing three lapse rates for ¹⁸O for the windward and leeward sides of the Cascade Range. The windward lapse rates are -6.4 ‰/km (R² = 0.72) for elevations below 1300 meters (amsl), and -2.5 ‰/km (R² = 0.88) for elevations above 1300 meters (amsl). The leeward lapse rate is 12.9 ‰/km (R² = 0.70), suggesting minimal air mass mixing over the Cascade Range during transport.