Analyzing Mono Basin Surface Waters for Stable Isotopes ( and ) and Major Cations to Assess Basin-Wide Evaporation and Geochemical Inputs

Mono Lake is a hypersaline lake located in Mono County, California at the base of the eastern central Sierra Nevada mountains. Four of the five freshwater streams in the Mono Basin that drain into Mono Lake are diverted to Los Angeles to help meet the needs of at least four million people. Even with strict water management regulations, Mono Lake is not rebounding to a healthy lake level as fast as expected. This indicates that there must be some process reducing the available water, such as evaporation, not correctly taken accounted for. Evaporation may be the primary reason for this uncertainty, but the evaporation rate of the basin is still unknown. This study aims to determine evaporation throughout the basin by using stable water isotopes (18O and 2H) to help with water management policies as well as assess hydrologic inputs through major cation measurements in Mono Basin. An analysis of 18O and 2H in water samples throughout the basin shows that the basin as a whole experiences higher evaporation than the average Global Meteoric Water Line (GMWL). Additionally, there are evaporative hotspots (June Lake and Lower Rush Creek) within the basin that experience higher evaporation than the local MWL. June Lake shows enriched isotopic samples, indicating high evaporation potentially from warmer and shallow waters and lack of shading from sun exposure. Rush Creek is the second evaporative hotspot in the basin, likely because of being the longest stream in the basin and maybe spending more time in the warmer and lower elevations. Another area of potential above average water loss concern is the June Lake loop area, which includes the Grant Lake Reservoir. Although the isotopic values do not show enrichment because of the many water inputs, caution should be used when storing water in and near basin evaporative hotspot locations. Mono Lake surface samples show heterogeneity, with eastern side more concentrated in cations, such as sodium. Higher cation concentrations, especially sodium, may indicate evaporative hotspots in Mono Lake. The eastern side of the lake may have high evaporation due to exposure to sunlight due to lack of mountain shadows, or from shallower and warmer water. Additionally, the freshwater stream inputs appear to be creating a slightly diluted layer on top of Mono Lake compared with the higher concentrated depth samples. This freshwater surface layer which may lead to higher evaporation because of the more available freshwater compared to even more saline water. This study hopes to point out evaporative hotspots in the basin so that water management can best allocate resources with this information