Monitoring thermal springs to improve land management decision-making, Sierra Nevada, California

The Sierra National Forest administers Mono Hot Springs and other nearby geothermal features, a concentration of more than a dozen springs, pools, and seeps in the high Sierra Nevada, California. The Native American Mono Tribe traditionally uses Mono Hot Springs for spiritual purposes, while simultaneously, the Mono Hot Springs Resort holds a special use permit for some of the geothermal waters. To support environmental assessments for area management, the Sierra National Forest studied thermal spring chemistry and temperature, evaluating potential use conflicts. An initial multi-year monitoring of eleven representative thermal springs was followed a decade later by another multi-year sampling of the same springs, providing insight into the geothermal character of Mono Hot Springs. Measured water temperatures ranged from 44.5° C to 24.3° C and pH 8.0 to 7.03, depending upon the thermal spring, higher pH values correlating with lower temperatures. Thermal spring temperatures varied seasonally with higher temperatures in springtime and lower ones in autumn. pH did not exhibit a coherent seasonal variation. Mono Hot Spring temperature decreased and pH increased during the decade-long study period, with even greater longer-term temperature change evidenced at nearby Mono Crossing. Silica and cation geothermometry at Mono Hot Springs suggests that the geothermal waters reached equilibrium with 74-79° C rock at depth at estimated pH 5-6. The spatial distribution of neighboring thermal springs, regional seismicity, and mapped faults suggests that Mono Hot Springs rises along faults running nearly north-south, connecting to Mammoth Mountain and Long Valley, California, 30 km to the north.