Bob Grimm - Complex Resistivity

DETECTION OF ORGANIC SOIL CONTAMINANTS

Identification of subsurface organic contamination, particularly dense nonaqueous phase liquids (DNAPLs) is one of the highest priorities - and among the most difficult - for environmental remediation of numerous sites. Complex resistivity (CR) is the only geophysical method that has been demonstrated in the laboratory to have high sensitivity to organic compounds, by detecting responses indicative of clay-organic electrochemistry. Direct detection of organics in the field has been elusive, in part due to the difficulty of obtaining robust measurements at very low contaminant levels in the presence of heterogeneous geological materials and cultural interference. In collaboration with Gary Olhoeft of the Colorado School of Mines, we performed a cross-hole CR experiment using several borings at the DOE Savannah River Site A-014 Outfall and imaged the three-dimensional distribution of perchlorethene (PCE). Soil sampling (performed double-blind with respect to the CR) confirmed the results: with a 4-m volume resolution element, the method formally achieved an 80% detection rate (true positives) for PCE at 30% false alarms (false positives), where PCE was present at mass fractions exceeding just a few parts per thousand. This is a site-specific result, depending on the general geology, types and abundances of clay minerals, types of organic contaminants, and experiment geometry. Nonetheless, the protocol that was established can now be applied to other sites.

Three-dimensional views of predicted PCE abundance at SRS A-014 Outfall. Upper 20 ft is a clay, which is relatively heavily contaminated. Isolated stringers and blobs of PCE exist in the intervening sand, which accumulate in the underlying silty mix at 50-65 ft. Drilling confirmed parts-per-thousand contaminant concentrations at 80% true positives and 30% false positives.