MAPPING CARBON DIOXIDE HAZARDS IN VOLCANIC AREAS -- LESSONS LEARNED AT MAMMOTH MOUNTAIN, CALIFORNIA

Mammoth Mountain, a Pleistocene volcano in eastern California, is a popular recreational area in the Sierra Nevada. Although considered dormant, long-period earthquakes beneath the mountain, a seismic swarm and associated deformation in 1989, and persistent diffuse emissions of carbon dioxide (CO₂) at high rates since 1990, suggest Mammoth Mountain is underlain by an actively degassing magmatic system. The CO₂ emitted from soils can collect in lethal concentrations in topographic depressions, depressions in snow, and in below-ground utility vaults. Four people have died from CO₂ asphyxiation since 1996, all during times of deep snowpack.

Mammoth Mountain Ski Area (MMSA) occupies a large area of the mountain and is operated under a lease agreement with the U.S. Forest Service (USFS). Four of the five main high-CO₂ emission areas are within the ski area, below treeline where ~40 ha of coniferous forest have been killed by high CO₂ concentrations (up to 95 vol-%) in the root zone.

The U.S. Geological Survey (USGS) has measured soil-gas [CO₂] and CO₂ emissions over large parts of the mountain and produced maps delineating CO₂ anomalies. Lessons have been learned from the application of three methodologies for mapping CO₂ anomalies. Emission rates have been measured mostly using infrared gas analyzers (IRGA) connected to closed chambers placed on the land surface. This method is slow because it requires a large number of closely spaced samples to produce accurate measurements of the total mass of CO₂ emitted even from small areas. Steep topography and thick forest cover render eddy covariance methodology inapplicable. Aerial surveys using IRGAs, have been made with limited success but are not capable of delineating specific emission areas.

The USFS and MMSA have used data and maps provided by USGS to post signs and alert visitors of the potential hazard from CO₂. Data collection however, has focused on anomalous tree-kill areas because they are easily recognized; the large part of the mountain above tree line has not been thoroughly investigated. Even in the known CO₂ emission areas, only annual measurements generally are made. The development of inexpensive CO₂ monitors with alarms and remote spectrographic equipment for detecting CO₂ emissions over broad areas could help improve public safety in recreational areas.