Simulation of Vapor Transport Through the Unsaturated Zone — Interpretation of Soil‐Gas Surveys

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Abstract
Soil‐gas surveys are becoming more widely accepted as a tool for the preliminary determination of the extent of soil and ground water contamination by volatile organic compounds (VOCs). The interpretation of the results of published soil‐gas surveys has been necessarily limited and qualitative due to a lack of adequate models. There has been considerable effort in the recent past to characterize the transport and fate of pesticides in soil. However, the behavior of pesticides generally differ substantially from those of VOCs.This paper presents a computer model developed to simulate the diffusive transport of VOC vapor through unsaturated soils using a two‐dimensional, finite‐difference, solution to Fick's second law of diffusion. An effective diffusion coefficient that incorporates the effects of tortuosity, moisture content, and soil organic carbon content is computed. Although the model has not been validated due to the unavailability of adequate field or laboratory data, nevertheless, sensitivity analyses demonstrate the importance of soil moisture and, secondarily, organic matter content in controlling the migration of VOC vapor through the unsaturated zone. The interpretation of soil‐gas surveys can be complicated by unknown spatial heterogeneities in soil moisture and organic carbon content, temporal variations in moisture content, extent of contaminant migration as a non‐aqueous phase liquid and by the unknown extent of VOC liquid and contaminated ground water.