Elevated carbon dioxide flux at the Dixie Valley geothermal field, Nevada;relations between surface phenomena and the geothermal reservoir

In the later part of the 1990s, a large die-off of desert shrubs occurred o ver an approximately 1 km(2) area in the northwestern section of the Dixie Valley (DV) geothermal field. This paper reports results from accumulation- chamber measurements of soil CO2 flux from locations in the dead zone and s table isotope and chemical data on fluids from fumaroles, shallow wells, an d geothermal production wells within and adjacent to the dead zone. A cumul ative probability plot shows three types of flux sites within the dead zone : locations with a normal background CO2 flux (7 g m(-2) day(-1)); moderate flux sites displaying "excess" geothermal flux: and high flux sites near y oung vents and fumaroles. A maximum CO2 flux of 570 g m(-2) day(-1) was mea sured at a location adjacent to a fumarole. Using statistical methods appro priate for lognormally distributed populations of data. estimates of the ge othermal flux range from 7.5 t day(-1) from a 0.14-km(2) site near the Stil lwater Fault to 0.1 t day(-1) from a 0.01-km(2) location of steaming ground on the valley floor. Anomalous CO2 flux is positively correlated with shal low temperature anomalies. The anomalous flux associated with the entire de ad zone area declined about 35% over a 6-month period. The decline was most notable at a hot zone located on an alluvial fan and in the SG located on the valley floor. Gas geochemistry indicates that older established fumaroles along the Still water Fault and a 2-year-old vent in the lower section of the dead zone dis charge a mixture of geothermal gases and air or gases from air-saturated me teoric water (ASMW). Stable isotope data indicate that steam from the small er fumaroles is produced by approximate to 100 degreesC boiling of these mi xed fluids and reservoir fluid, Steam from the Senator fumarole (SF) and fr om shallow wells penetrating the dead zone are probably derived by 140 degr eesC to 160 degreesC boiling of reservoir fluid. Carbon-13 isotope data sug gest that the reservoir CO2 is produced mainly by thermal decarbonation of hydrothermal calcite in veins that cut reservoir rocks. Formation of the dead zone is linked to the reservoir pressure decline caus ed by continuous reservoir drawdown from 1986 to present. These reservoir c hanges have restricted flow and induced boiling in a subsurface hydrotherma l outflow plume extending from the Stillwater Fault southeast toward the DV floor. We estimate that maximum CO2 flux in the upflow zone along the Stil lwater Fault in 1998 was roughly seven to eight times greater than the pre- production flux in 1986. The eventual decline in CO2 flux reflects the dryi ng out of the outflow plume. Published by Elsevier Science B.V.