|Title||A simple model of soil-gas concentrations sparged into an unlined unsaturated zone|
|Publication Type||Journal Article|
|Year of Publication||2003|
|Authors||Ostendorf DW, Hinlein ES, Lutenegger AJ, Tehrany PS|
|Journal||Ground Water Monitoring and Remediation|
We derive an analytical model of soil-gas contamination sparged into an imlined unsaturated zone. A nonaqueous phase liquid (NAPL) source lies in the capillary fringe, with an exponential sparge constant within the radius of influence and a constant ambient evaporation rate beyond. Advection, diffusion, and dispersion govern the conservative soil-gas response, expressed as a quasi-steady series solution with radial Bessel and hyperbolic vertical dependence. Simulations suggest that sparged contamination initially spreads beyond the radius of influence down a negative gradient. This gradient eventually reverses, leading to a subsequent influx of ambient contamination. Soil-gas concentrations accordingly reflect slowly varying source conditions as well as slowly varying diffusive transport through the radius of influence. The two time scales are independent: One depends on NAPL, airflow, and capillary fringe characteristics, the other on soil moisture, gaseous diffusivity, and unsaturated zone thickness. The influx of ambient contamination generates an asymptotic soil-gas concentration much less than the initial source concentration. The simple model is applied to a pilot-scale sparging study at Plattsburgh Air Force Base in upstate New York, with physically plausible results.