GEOCHEMICAL AND HYDROLOGIC MONITORING OF THE HYDROTHERMAL SYSTEM IN LONG VALLEY CALDERA, EASTERN CALIFORNIA

Abstract

Long Valley Caldera in eastern California formed 767,000 years ago following the eruption of the Bishop Tuff. The area surrounding the caldera remains active, with Holocene-age eruptions of the Mono-Inyo craters between the western margin of the caldera and Mono Lake. Meltwater recharged along the western margin of the caldera is heated by residual cooling magma, then flows eastward into the caldera and is used for power production at the Casa Diablo geothermal plants, ~15 km east. Geothermal power generation began in 1985 and has increased to 40 megawatts electric (MWe); new production wells west of Casa Diablo are expected to increase the gross power generation capacity to approximately 70 MWe. Further east, hot groundwater discharges from springs in Hot Creek Gorge. Changes to subsurface water flow driven by pumping from geothermal production wells can impact surface hydrothermal features; however, volcanic activity can also drive changes. To better understand changes in hydrothermal activity, the U.S. Geological Survey (USGS) has implemented geochemical and hydrologic monitoring plans that track CO2 flux and soil temperature at tree-kill and bare-ground sites, chemistry and isotopic composition of gases emitted from fumaroles and springs, thermal water discharge in Hot Creek Gorge, and temperature and chemistry in shallow and deep wells. Airborne thermal infrared surveys were performed in 2014 and 2016. The monitoring network is being expanded to include new wells to provide better understanding of the flow path of the deep geothermal fluid and its interaction with the non-thermal groundwater that supplies the town of Mammoth Lakes. These efforts, combined with geophysical monitoring, can help understand changes in the hydrothermal system. The data also provides input for recommendations made by the multi-agency Long Valley Hydrologic Advisory Committee, which provides oversight to protect the thermal and non-thermal hydrologic resources of Long Valley Caldera.