Soil Gas Sampling for Identification of CAHS Sources in Groundwater: A Case Study

Groundwater contamination studies involving possible multiple sources of contaminants poses a considerable challenge. When the contaminants of interest are chlorinated aliphatic compounds (CAHs), soil gas sampling can be precious for focusing site investigations. Soil gas monitoring at a former industrial site in Milan in an area of diffuse groundwater contamination demonstrates the usefulness of soil gas data together with groundwater and soil data for the correct reconstruction of the conceptual site model. Repeated gas sampling at different locations and depths provided data on the spatial and vertical concentration gradients of contaminants, which were useful to distinguish between background and local sources. In all the soil gas monitoring wells, CAH concentrations increased with depth, clearly indicating groundwater as the source of vapors at the site. Moreover soil gas and groundwater concentrations showed a similar pattern, with the highest soil gas concentrations recorded in the probes located in the area overlying highly contaminated groundwater. The data suggested groundwater contamination at the site was to be traced to the diffuse plume, but a discernible plume of PCE appeared to originate from a local off-site cross gradient source, which was responsible for the high PCE concentrations in the eastern portions of the site.