Hydrologic and geochemical monitoring in Long Valley caldera, Mono County, California, 1986

The U.S. Geological Survey continued to monitor hydrologic and geochemical conditions in the Long Valley caldera during 1986. The monitoring is directed toward detecting changes in the hydrologic system caused by tectonic or magmatic processes. Data collected during 1986 include chemical and isotopic composition of water from selected streams sites, springs, and wells; pumpage from four geothermal wells; flow rates of selected springs and stream sites; mean daily water or gas temperatures at selected sites; mean daily atmospheric pressures and water level at selected wells, and precipitation records for two sites. Seismicity within the caldera persisted at a relatively low level compared with the more active periods of 1978-84. The most significant events of seismicity that affected hydrologic monitoring sites in Long Valley during 1986 occurred during July , in response to the Chalfant Valley earthquakes, centered about 20 miles southeast of the caldera. Water level records for three wells show distinct responses to the Chalfant Valley earthquakes. (USGS)