Soil carbon dioxide flow associated with the San Andreas and Calaveras faults, California

The spatial and temporal variability, origin, and transport of CO ₂ in fractured terrain are evaluated from the perspective of field observations along the San Andreas fault (SAF) system, CA, and numerical modeling. In a preliminary soil CO₂ study conducted (July–August, 1998) along the Parkfield segment of the SAF, CO₂ efflux anomalies were observed along fault-crossing transects. Values of δ13C (−23.7 to −21.6‰) and Δ14C (98.4 to 112.4‰) for soil CO₂ were characteristic of CO₂ of biogenic origin. These observations suggest that anomalously high CO ₂ fluxes are due to enhanced biogenic CO₂ flow along fault-related fractures. Soil CO₂ surveys were conducted (February–May, 2000) along the SAF and the Calaveras fault (CF). CO₂ efflux was measured within grids with portable instrumentation, and continuously with meteorological parameters at a fixed station. Observed trends suggest that zones of elevated CO₂ efflux may be related to subsurface fracturing. δ 13C (−23.3 to −16.4‰) and Δ14C (75.5 to 94.4‰) values of soil CO₂ are indicative of biogenic CO₂. CO₂ efflux and meteorological parameter time series indicate that effects of temperature variations on CO₂ respiration and wind speed variations on atmospheric airflow though fractures modulate CO₂ efflux. Profiles of soil CO₂ concentration ([CO ₂]) as a function of depth were measured at multiple sites within SAF and CF grids and suggest that advective CO₂ flow accounts for up to 85% of the surface efflux. Response of soil gas transport processes and resulting soil gas concentration profiles to system parameters was tested using one-dimensional models of diffusive CO₂ flow and advective-diffusive CO₂ and air flow. When transport is purely diffusive, the shape of [CO₂] profiles is sensitive to soil CO₂ production rates, CO₂ flux at the base of the soil column, and soil diffusivity. When advective and diffusive transport are considered, transport processes operating through the soil column and the geometry of gas concentration profiles are most sensitive to the basal gas flux, followed by soil diffusivity, permeability, and CO₂ production rate. Results suggest that small magnitude basal gas fluxes can produce total pressure gradients sufficient to drive advective gas flow through soil columns.