Hydrologic Studies in Long Valley Caldera
The USGS monitors springs, streams, wells, fumaroles, and precipitation in Long Valley to study the natural hydrologic variations and the response of the hydrologic system to volcanic and tectonic processes. The locations of all monitoring sites are shown below. Hydrologic data from key monitoring sites are posted every day allowing us to quickly review recent data for real or (faulty) equipment induced anomolies.
This USGS Ground-water Monitoring Network in Long Valley is maintained by Chris Farrar (cdfarrar@usgs.gov) and Jim Howle (jfhowle@usgs.gov), Carnelian Bay Field Office, California District, Water Resources Division, USGS.
Hydrothermal System
Long Valley caldera hosts an active hydrothermal system that includes hot springs, fumaroles (steam vents), and mineral deposits. Hot springs exist primarily in the eastern half of the caldera where land-surface elevations are relatively low; fumaroles exist primarily in the western half where elevations are higher. Mineral deposits from thermal activity are found on the resurgent dome, at Little Hot Creek springs, Hot Creek Gorge, and other locations in the south and east moats of the caldera.
Hot springs discharge primarily in Hot Creek Gorge, along Little Hot Creek, and in the Alkalai Lakes area. The largest springs are in Hot Creek Gorge where about 250 liters per second of thermal water discharge and account for about 80% of the total thermal water discharge in the caldera. At the other extreme are springs at Hot Creek Fish Hatchery which contain a small component (2-5%) of thermal water that raises water temperatures about 5°C higher than background temperatures. Use of the warm spring water in the hatchery has increased fish production because trout growth-rates are faster in the warm water than in ambient stream temperatures in Long Valley.
In hydrothermal systems the circulation of ground-water is driven by a combination of topography and heat sources. In Long Valley Caldera, the system is recharged primarily from snow-melt in the highlands around the western and southern rims of the caldera. The meteoric water infiltrates to depths of a few kilometers where it is heated to at least 220°C by hot rock near geologically young intrusions. Upflow occurs in the west moat where the heated water with lower density rises along steeply inclined fractures to depths of 1-2 km. This hydrothermal fluid flows laterally, down the hydraulic gradient, from the west to the southeast around the resurgent dome and then eastward to discharge points along Hot Creek and around Crowley Lake. Reservoir temperatures in the volcanic fill decline from 220°C near the Inyo Craters to 50°C near Crowley Lake due to a combination of heat loss and mixing with cold water.
The recently completed Long Valley Exploratory well, which was drilled on the resurgent dome to a depth of 9,832 feet (2,997 meters), has provided new information on the history of hydrothermal activity in the caldera, and will be used to study heat and fluid circulation in the region.
Geothermal Power Development
Wells drilled on the southwest side of the resurgent dome at Casa Diablo tap into the caldera's hydrothermal system by pumping hot water (170°C) to supply three hydrothermal power plants that generate about 40 megawatts of electricity. Using a binary technology, a secondary fluid (isobutane) is heated by the pressurized geothermal water, vaporized, and then run through the generating turbines. Cooled geothermal water is reinjected underground.
The hydrologic monitoring program has detected changes in the hydrologic system caused by geothermal development and variations in precipitation and recharge. For example, we have delineated decreases in thermal-spring discharge at sites within about 5 km to the east of Casa Diablo that are caused by subsurface pressure declines at the geothermal well field. No changes have as yet been detected in the springs in Hot Creek Gorge. There has also been an increase in steam discharge at Casa Diablo and sites farther west due to increased boiling in the geothermal reservoir caused by geothermal production.
Causes of Variations in the Hydrologic System:
- Precipitation and ground-water recharge: Streamflow at HCF and ground-water levels in wells SC-1 and SC-2 show large-scale variations primarily related to precipitation and ground-water recharge. These processes also influence water levels in wells tapping deeper aquifers, but to a lesser degree.
- Geothermal fluid production: Water levels in geothermal well CW-3 (5 km east of Casa Diablo) show the effects of pressure reductions caused by the withdrawal of geothermal fluid at Casa Diablo; water levels declined significantly in 1991 when the production rate was increased to supply two new power plants. A similar water level decline has not yet occurred in geothermal observation well CH-10B, located 9 km east of Casa Diablo.
- Earthquakes: Water-levels in wells LKT, CH-10B, Santa Fe, and CW-3 show changes in response to relatively large local (>M~4) and regional earthquakes (>M~5) and to large distant earthquakes (>M6). Water levels typically drop rapidly for several days following an earthquake; water-level recovery takes weeks to months. For example, the hydrograph for well LKT shows responses to earthquakes on July 11, 1989 (M4.6 in Long Valley area), October 17, 1989 (M~7 Loma Prieta), October 24, 1990 (M5.7 north of Long Valley caldera), and June 28, 1992 (M7.3 Landers).
Crustal Deformation: The relatively rapid and large water-level declines seen in wells LKT, CH-10B, Santa Fe, and CW-3 in the fall of 1997 correlate with the increase in extension rate across the resurgent dome that began in October and continued for the rest of the year. Relatively high strain rates may be required to cause measurable responses in the hydrologic system because strain-induced water-level changes tend to be dissipated by the increase in ground-water flow caused by localized fluid-pressure changes. Quantitative analysis of the relationship between strain rate and water-level changes is in progress. To date, no correlation has been delineated between strain rates and hot-spring discharge in Hot Creek Gorge. - Barometric pressure and earth tides: Water-level data from the continuously monitored wells are filtered to remove the effects of barometric pressure and earth tides.
