Soil Carbon Dioxide Flow Associated With The San Andreas And Calaveras Faults, California

Open Access
Author:
Lewicki, Jennifer Lynn
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 18, 2002
Committee Members:
  • Katherine Haines Freeman, Committee Member
  • Lee Kump, Committee Member
  • Susan Louise Brantley, Committee Chair
  • Michael Allan Arthur, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • unsaturated zone gas transport
  • faults
  • Soil gas
  • Carbon dioxide
Abstract:
Seismic activity may create highly fractured fault zones, providing permeable pathways for the transport of gases from depth to the atmosphere. Also, the flow of deeply derived fluids within fault zones may generate high pore fluid pressures and contribute to fault zone weakness. The spatial and temporal variability, origin, and transport of carbon dioxide 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 carbon dioxide study conducted (July-August, 1998) along the Parkfield segment of the SAF, single or double-peak carbon dioxide flux anomalies were observed along fault-crossing transects at five sites. Values of delta C-13 (-23.7 to -21.6 permil) and Delta C-14 (98.4 to 112.4 permil) for soil carbon dioxide were characteristic of carbon dioxide of biogenic origin. These observations suggest that anomalously high carbon dioxide fluxes are due to enhanced biogenic carbon dioxide flow along fault-related fractures. Soil carbon dioxide surveys were conducted (February-May, 2000) along the SAF in Parkfield and the Calaveras fault (CF) in Hollister, CA. Carbon dioxide flux was measured within grids with portable instrumentation, and continuously with meteorological parameters at a fixed station, in both faulted and unfaulted areas. Areal trends observed within grids suggest that zones of elevated carbon dioxide flux may be related to subsurface fracturing on small spatial scales. Greater spatial and temporal variability of surface carbon dioxide fluxes was observed at SAF and CF sites, relative to corresponding background areas. Values of delta C-13 (-23.3 to -16.4 permil) and Delta C-14 (75.5 to 94.4 permil) for soil carbon dioxide are indicative of biogenic carbon dioxide. SAF carbon dioxide flux and meteorological parameter time series indicate that effects of temperature variations on soil carbon dioxide respiration and wind speed variations on atmospheric airflow though fractures modulate surface carbon dioxide flux. Profiles of soil carbon dioxide concentration ([carbon dioxide]) as a function of depth were measured at multiple sites within SAF and CF grids and indicate that advective carbon dioxide flow accounts for up to 85% of the surface flux. Decrease in [carbon dioxide] with depth observed in some profiles suggests atmospheric air flow through soil fractures. The relatively high spatial and temporal variability of surface carbon dioxide fluxes observed at SAF and CF sites is therefore interpreted to be due to wind-driven atmospheric air flow through more highly fractured soils at these sites, relative to corresponding background areas. The response of soil gas transport processes and resulting soil gas concentration profiles to changing soil physical properties, biological respiration rates, and boundary conditions was tested using one-dimensional finite difference models of diffusive carbon dioxide flow and advective-diffusive carbon dioxide and air flow. When transport is purely diffusive, the shape of [carbon dioxide] profiles is sensitive to soil carbon dioxide production rates, carbon dioxide flux at the base of the soil column, and soil diffusivity. When advective and diffusive transport of carbon dioxide and air 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 carbon dioxide production rate. The time required for conditions in [carbon dioxide] and [air] profiles to approximate steady state decreases as the relative advective contribution to flow increases. Results suggest that small magnitude basal gas fluxes can produce total pressure gradients sufficient to drive advective gas flow through soil columns. Therefore, interpretations of soil gas data collected in faulted/fractured terrain should consider the effects of wind-driven air flow through soils on gas transport and resulting soil gas concentration profiles and surface fluxes.