An Investigation and Comparison of Stable Isotopes in Meteoric Waters and Groundwaters from Southern Washington

Smoot, Emily, Portland State University, esmoot@pdx.edu (Poster)

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.


Emily Smoot is a graduate of Portland Community College and Portland State University (PSU) with an inspired love for learning and curiosity of the physical world stemming from her untraditional educational background as an unschooler. She is currently a graduate student at Western Washington University working with Dr. Robert Mitchell and will be graduating in 2022. Emily graduated from PSU this summer from the University Honors College with a Bachelor of Science, majoring in Geology and minoring in Physics. She has an affinity for research and completed two independent undergraduate research projects--one for the University Honors College with Dr. Robert Perkins and one for the McNair Scholars Program with Dr. Kelly Gleason.

In her free time, Emily enjoys learning about aspects of the physical world and its inhabitants and has been a volunteer for the Cascade Pika Watch for the past two years. She also enjoys foraging for wild edibles in the forests of the Pacific Northwest. Emily hopes to one day pursue a doctoral degree in hydrogeology and use her education to improve and promote the understanding and use of our natural environment and the resources it provides for us.

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