SPATIAL AND SEASONAL VARIATIONS OF STABLE ISOTOPES (18O AND 2H) IN PRECIPITATION, SPRINGS, AND SURFACE WATER IN THE OCHOCO MOUNTAINS, OREGON, USA

Managing water resources in many semi-arid regions of the American West continues to be challenged by limited knowledge of recharge areas, groundwater flow pathways, and the influence of a changing climate on watershed hydrology. Surface water in semiarid regions is sourced from a dynamic mixture of precipitation runoff and groundwater discharge. Meteoric water from precipitation, springs and streams was sampled throughout the Ochoco Mountains in Central Oregon once a month from June 2020 to June 2021. Samples were analyzed for their stable isotope compositions (δ¹⁸O and δ²H) to better understand spatial and seasonal variation in the hydrology of this semi-arid region. This research constrains groundwater recharge elevations, residence time, and flow pathways which are interpreted in terms of underlying geology and seasonal variation in surface water isotopes. The local meteoric water line (LMWL) based on surface water is defined as δ²H = 4.21 δ¹⁸O + 48.2 (R² = 0.66). The slope is relatively low compared to the global meteoric water line (8.13), consistent with the Ochoco Mountains being a semi-arid environment where evaporative enrichment of isotopes is significant.