Cross-hole complex-resistivity imaging of the vadose zone was performed at the A-014<br>Outfall at the Savannah River Site, Aiken, SC. Five vertical electrode arrays (VEAs) were<br>installed with ~15-ft separations in and around a suspected DNAPL source zone to depths of 72<br>feet. Amplitude and phase data were edited for quality and then inverted to form threedimensional<br>(3D) images of the target volume. The comparatively small magnitude of the<br>nonlinear resistivity Hilbert distortion allowed approximate linearized imaging of the 3D<br>distribution of these effects. Laboratory analysis of nearby soil contaminated in situ indicated<br>that the CR response to the PCE-clay reaction was maximized near 50 mHz. PCE occurrence<br>was predicted to track phase and Hilbert-distortion indicators. PCE concentrations were<br>measured at three drilling locations and were compared to predictions a receiver-operating<br>characteristic analysis. The optimum performance at 1000 mg/kg in situ PCE was >80%<br>detection (true positives) with <30% false alarms (false positives) at an effective resolution of 4<br>ft, ~1/3 of the interwell separation. The CR survey successfully predicted the general distribution<br>of PCE at parts-per-thousand concentrations, specifically widespread near-surface contamination<br>and a zone of discontinuous pods or stringers immediately below the source.


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