STABLE ISOTOPE STUDY OF WATER IN OREGON CAVES NATIONAL MONUMENT

Oregon Caves National Monument (ORCA), situated in the Klamath Mountains of Southern Oregon, is an important location for hydrological research, with nine endemic species and a large bat population. Geochemical research has provided valuable information about the hydrologic cycle in ORCA; however, the many aspects of the cave's hydrology remains poorly understood. Employing stable isotopes has significant potential to increase understanding of ORCA’s hydrology, offering a cost-effective alternative to conventional methods used in examining karst hydrology, such as geochemical tracers and dye tracing tests. Environmental stable isotopes in meteoric water (δ18O and δ2H and increasingly δ17O) are commonly used to study hydrologic cycle characteristics such as recharging patterns, recharge altitudes, residence times, and aquifer depth/flow type. Karst waters typically originate from local precipitation, aligning closely with the meteoric water line, and deviations from this line can reveal water sources and movements within and around the cave system. Stable isotope analyses in ORCA's chambers reveal effective preservation of rainwater isotopic signatures in cave drips suggesting that further studies in ORCA could offer valuable insights into the cave's hydrologic processes. To better understand water inputs and pathways, we examine the relationship between stable isotopes of water within the cave system and meteoric water in the surrounding drainage basin. Our findings revealed that surface and cave water generally plot at or above the global meteoric water line, with tributary streams exhibiting consistently higher δ18O and δ2H values than trunk streams. Despite being located at a higher elevation than trunk streams, cave water more closely resembled the signature of trunk streams. These isotopic variations in and around ORCA are interpreted as reflecting varying hydrologic signals from snow, liquid precipitation, and groundwater baseflow.