| Popis: |
Petroleum released to the subsurface may be held in capillary tension above the water table for years, serving as a source of groundwater and soil gas contamination. Soil venting can be used to attack this ongoing source, sometimes in conjunction with biodegradation to permanently destroy the released hydrocarbons vapors. These processes were explored using intact soil cores from the site of an aviation gasoline release. Hydrocarbon vapor concentration profiles were analyzed by gas chromatography and interpreted using mathematical models. In the venting experiment, an intact core was subjected to a horizontal sweep flow of nitrogen. Residual petroleum in the soil volatilized and hydrocarbon vapors diffused upward. Soil venting significantly increased the rate of contaminant removal relative to ambient field conditions. No correlation between hydrocarbon vapor exit flux and sweep flow rate was observed, indicating that flow rates in excess of a minimum value were no more effective. A steady state model balancing volatilization and diffusion successfully predicted the shape of the hydrocarbon concentration profiles. The volatilization source was construed as an LNAPL droplet surrounded by an air water aggregate surrounded by a free air pore, with the aggregate reducing the mass transfer of hydrocarbons from LNAPL to air. Source strength, estimated from a diffusive flux model, decreased with time as LNAPL droplets became smaller. The biodegradation experiment employed an intact core from mid-depth in the unsaturated zone which was subjected to a upward flow of nitrogen, oxygen, water vapor, and hydrocarbon vapors. Significant biodegradation was indicated by reductions in hydrocarbon concentrations with elevation in the core. First order biodegradation rate constants were estimated by calibrating the experimental data to a simple model balancing advection and biodegradation. Agreement with kinetic parameter estimates from microcosm and probe cluster studies was good. For comparison, hydrocarbon vapor profiles due to volatilization, diffusion, and biodegradation were obtained for unvented subsurface conditions by trapping vapor samples on adsorbents in the field, with subsequent laboratory desorption and analysis. The method was effective, and field profiles agreed well with uncalibrated model predictions. Concentrations exhibited considerable scatter at depth, and distillation of the residual was apparent. |
