SOIL-GAS SIGNATURES FROM VOLATILE CHLORINATED SOLVENTS - BORDEN FIELD EXPERIMENTS

Field experiments have been conducted at the Borden research site to evaluate the widely applied soil-gas survey method. In particular, the ability of surveys to delineate DNAPL chlorinated solvent sources and associated ground-water plumes has been investigated. Field experiments indicated that dissolved-phase plumes from DNAPL pools or residual located about a meter or more below the water table are unlikely to be directly detected by soil-gas surveys. Soil-gas plumes observed at real sites are attributed to volatilization of source material in the vadose zone and consequent formation of very shallow interface zone ground-water contamination that is able to partition to the soil gas as it transports downgradient. The distribution of DNAPL sources and dissolved-phase plumes deeper in the ground-water zone may often bear little resemblance to the shallow interface zone ground-water plume above, and hence the location of this deeper contamination will remain highly difficult to identify by soil-gas surveys. Field observations are confirmed with simple analytical 1-D diffusion modeling and the implications of the results to the practice of soil-gas surveys at contaminated sites indicated.