Monitoring CO2 emissions in tree kill areas near the resurgent dome at Long Valley Caldera, California

We report results of yearly measurements of the diffuse CO₂ flux and shallow soil temperatures collected since 2006 across two sets of tree-kill areas at Long Valley Caldera, California. These data provide background information about CO₂ discharge during a period with moderate seismicity, but little to no deformation. The tree kills are located at long-recognized areas of weak thermal fluid upflow, but have expanded in recent years, possibly in response to geothermal fluid production at Casa Diablo. The amount of CO₂ discharged from the older kill area at Basalt Canyon is fairly constant and is around 3-5 tonnes of CO₂ per day from an area of about 15,000 m². The presence of isobutane in gas samples from sites in and around Basalt Canyon suggests that geothermal fluid production directly effects fluid upflow in the region close to the power plant. The average fluxes at Shady Rest are lower than average fluxes at Basalt Canyon, but the area affected by fluid upflow is larger. Total CO₂ discharged from the central portion of the kill area at Shady Rest has been variable, ranging from 6 to11 tonnes per day across 61,000 m². Gas collected at Shady Rest contains no detectable isobutane to link emissions chemically to geothermal fluid production, but two samples from 2009-10 have detectable H₂S and suggest an increasing geothermal character of emitted gas. The appearance of this gas at the surface may signal increased drawdown of water levels near the geothermal productions wells.