A Simple Model of Soil-Gas Concentrations Sparged into an Unlined Unsaturated Zone |
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| Authors: | David W Ostendorf Erich S Hinlein Alan J Lutenegger Pierre S Tehrany |
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| Institution: | David W. Ostendorf;is a professor in the Civil and Environmental Engineering Department of the University of Massachusetts at Amherst. His research interests include subsurface contamination and environmental fluid mechanics. He is a P.E. with a B.S. degree in civil engineering from the University of Michigan, and S.M. and Sc.D. degrees in civil engineering from MIT. Erich S. Hinlein;is a research engineer in the Civil and Environmental Engineering Department of the University of Massachusetts at Amherst. His research interests include field sensor development and laboratory analysis of hazardous waste site investigations. Hinlein has a B.S. degree in electrical engineering, an M.S. degree in environmental engineering, and a Ph.D. in civil engineering from the University of Massachusetts at Amherst. J. Pierre Tehrany;is a research assistant in the Civil and Environmental Engineering Department of the University of Massachusetts at Amherst. His research interests include hazardous waste site remediation and mathematical modeling. Tehrany has a B.S. degree in electrical engineering and an M.S. degree in environmental engineering from the University of Massachusetts at Amherst. Alan J. Lutenegger;is a professor and head of the Civil and Environmental Engineering Department of the University of Massachusetts at Amherst. His research interests include geoenvironmental engineering and downhole instrumentation for hazardous waste site characterization. Lutenegger is a P.E. with a B.S. degree in construction engineering, and M.S. and Ph.D. degrees in civil engineering from Iowa State University. |
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| Abstract: | 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. |
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