Induced-polarization detection and mapping of contaminant plumes

Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical proper-ties of exposed soils/rocks and therefore produce measurable induced polarization IP signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A 2D time-domain IP method is developed for mapping the FS-12 contaminant plume at the Massachusetts Military Reserva-tion MMR located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately 265 m3 of fuel that erupted from a broken underground pipeline in the early 1970s. Ben-zene and ethylene dibromide EDB are the primary contami-nants at FS-12, with concentrations exceeding the allowed maximum concentration levels MCL, while other constituents of the plume did not exceed their MCL. Therefore, the contami-nants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concen-trations 2400 and 1000 g/L, respectively among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resis-tivity and chargeability plots to 100 m depth and a horizontal extent of 400 m. By separately inverting IP data derived from time windows located at short and long decay times, a time-domain gross spectral chargeability difference is produced. Both the chargeability and gross spectral chargeability differ-ence show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot other-wise be inferred from the sparse borehole distribution