Groundwater levels and temperatures in well CH-10b near Hot Creek, Long Valley Caldera, eastern California

Hot Creek Gorge contains the most obvious surface expression of the hydrothermal system in Long Valley Caldera, California, discharging 200-300 L/s of thermal water according to USGS measurements made since 1988. Formerly, Hot Creek was a popular public swimming area, but it was closed in 2006 due to unpredictable temperature fluctuations and sporadic geysering of thermal water within the creek (Evans et al., 2018). The USGS has monitored the thermal regime in the area since the mid-1980s, including a series of long-term studies 0.6 km away at well CH-10b. Temperature measurements in the ~100 m deep well, which have been performed on an intermittent basis since it was drilled in 1983, reveal a complex temperature profile (see Farrar et al., 2010). Temperatures increase with depth to a maximum at about 43 meters (140 ft) below the ground surface, and then decrease steadily to the bottom of the well. The depth of the temperature maximum in the well (~43 m) corresponds to an elevation of ~2,120 m, roughly equivalent to the elevation of Hot Creek, and appears to sample the same hydrothermal flow system that supplies thermal features at the surface in the gorge. Starting in the early 1990s, the maximum temperature in CH-10b rose from 93.4 degrees C to its peak in 2007 at 101.0 degrees C. A cooling trend was observed beginning in 2009 and continues to present (99.3 degrees C in June 2015). As the input into CH-10b is at the elevation of the creek, it exhibits the potential for response to thermal events at Hot Creek, and could provide a useful tool for monitoring future hazards. In January 2011, thirteen platinum resistance thermometers (PRTs) sealed against moisture and corrosion were installed in CH-10b at varying depths from 90 ft to 260 ft and connected to a datalogger programmed to record the temperatures every five seconds (initially) or ten seconds (after November 2015). The intent was to monitor the groundwater temperature profile in the vicinity of the Hot Creek site. In October 2011, a bubbler system that consists of a nitrogen tank, a pressure regulator, and tubing (further described in Howle and Farrar, 1996) was added to measure water level at CH-10b, and in September 2012 a barometer was added to measure barometric pressure. Due to the extreme environmental conditions, some PRTs eventually began to fail, leading to spurious readings that were later removed from the dataset (detailed in the Data Quality section). On November 9, 2016, most PRTs were removed from the well, leaving only those at 140-ft (PRT6) and 160-ft (PRT8) in order to focus monitoring efforts on the measurements around the maximum point of the temperature-depth profile. Evans, W.C., Hurwitz, S., Bergfeld, D., and Howle, J.F., 2018, Hot water in the Long Valley Caldera - The benefits and hazards of this large natural resource: U.S. Geological Survey Fact Sheet 2018-3009, 4 p., https://doi.org/10.3133/fs20183009. Farrar, C., DeAngelo, J., Clor, L., Murphy, F., Williams, C., Grubb, F., and Hurwitz, S., 2010, Temperature data from wells in Long Valley Caldera, California (ver. 3.0, November 2016): U.S. Geological Survey Data Series 523, https://pubs.usgs.gov/ds/523/. Howle, J.F. and Farrar, C.D., 1996, Hydrologic data for Long Valley Caldera, Mono County, California, 1987-93: U.S. Geological Survey Open-File Report 96-382, https://doi.org/10.3133/ofr96382.