MONO HOT SPRINGS-MONITORING A GEOTHERMAL RESOURCE TO ENSURE PROPER STEWARDSHIP

Mono Hot Springs is a geothermal area consisting of springs, seeps, and semi-developed pools located adjacent to the South Fork of the San Joaquin River within the southern Sierra Nevada, California. To ensure meeting recreational demands and traditional Native American spiritual uses, Sierra National Forest decision-makers needed to better understand the geology of this resource. A multi-year monitoring program of eleven springs and pools found their annual temperature range to be as small as 0.4^oC for some springs and as great as 7.5^oC for others. The average temperature for all eleven springs in spring (May/June) is consistently 1.5^oC hotter than in fall (Oct.). A relationship of decreasing spring temperature with increasing pH (7.0 to 7.8) was found. Regression of temperature and pH shows this relationship is more consistent for measurements in the fall than for those in the spring. The silica and Na-K-Ca geothermometers yield temperatures for the heat source as being 109^oC and 149^oC, respectively. The difference between these calculated temperatures maybe due to mixing of hot water with cooler, shallow groundwater before discharge. A number of vertical to near-vertical joints trending north-northeast cross the Mono Hot Springs area and provide an opportunity for deep circulation of water and the introduction of shallow groundwater following snowmelt. Mixing of cold water would be lowest at the end of the water year (Sept/Oct) which would explain the more consistent relationship of temperature and pH found then. This understanding of the geothermal system indicates that management activities should be evaluated to avoid increasing shallow groundwater mixing and thus generally lowering the temperature of water issuing from springs and pools. This will ensure conditions expected by Native Americans seeking spiritual renewal and the general public looking to enjoy a unique recreational experience will be equally satisfied.