Characterization of an old diesel fuel spill? Results of a multi-receiver OhmMapper survey

Summary Geoelectric data were acquired with the OhmMapper TR4 (OM-TR4) over a former tank farm at the Camp Parks Reserve Forces Training Area (RFTA) to map the near subsurface resistivity and possible association with a 30 to 40 year old hydrocarbon plume and locate any additional pipelines and subsurface infrastructure. The OM TR4 is a towed capacitively coupled resistivity system that comprised of one transmitter and four receivers in the dipole-dipole array configuration. Densely sampled data were acquired on both north-south and east-west profile lines. Apparent resistivity maps and two-dimensional inverse models were used in an attempt to correlate geophysical signatures with soil samples analyzed for hydrocarbons. We found some correspondence between conductive zones in the lower vadose zone-shallow saturated zone to concentrations of petroleum hydrocarbons in the diesel range (TPH-d) of 140 ppm at a depth of 2.4 m and 470-17000 ppm at 3.6 m. At a depth of 1 m concentrations of less than 100 ppm were detected and corresponding resistivity values were relatively high. Site and Survey Description