Electromagnetic Induction and GPR Measurements for Creosote Contaminant Investigation

Multifrequency EM induction and GPR parallel dipole (co-pole) and orthogonal dipole (cross- pole) surveys were conducted to assist in the characterization of a former industrial site prior to it being remediated by the Ohio EPA and the US EPA. The site has been a major concern to both agencies for the past decade due to high concentrations of creosote present in clay-rich surficial soils, resulting from many years of wood treating at the site. Information provided on the approximate extent of contamination at the site and the locations of several contaminant-filled structures determined through the use of quadratu re phase EM data and cross-pole GPR data served as the basis for an efficient, comprehensive and cost- effective site remediation plan. Geophysical data interpretations were confirmed through exploratory trenching and soil sampling subsequent to the comple tion of this study. This study demonstrates the potential for mapping the extent and variation with depth of resistive compounds under circumstances where high levels of contamination are present with relatively conduc- tive background materials. The approximate extent and depth of creosote compounds within the near- surface materials at the site was mapped using 4 kHz and 9 kHz out of phase (quadrature) EM data, as contaminated areas exhibited anomalously low conductivity values relative to non -contaminated areas. Anomalies in the EM data that were attributed to high levels of contamination correlated with analytical soil sample test results that were obtained prior to geophysical surveys, and were also consistent with the actual extent of contamination determined through exploratory sampling subsequent to data interpreta- tions. Multi-component GPR measurements at the site complemented the EM data. Cross-pole GPR data were more useful than co-pole data at the site for imaging several structures containing creosote that would have remained undiscovered and acted as future sources of contamination. A general overview of the GPR surveys at the site is presented in this paper.