Mapping A Paleochannel System Controlling Contaminant Migration At A Wood-Treating Facility Using Electromagnetics

A wood-treating facility, located near Sheridan, Oregon, has been the focus of a groundwater<br>contamination investigation. A geophysical survey was conducted in April, 2000. Objectives of this<br>geophysical survey included detection and delineation of coarse-grained channel-type deposits in the<br>unconsolidated section above bedrock. These deposits constitute preferential pathways for groundwater<br>flow, and hence contaminant migration through the subsurface. Contaminants consist of petroleumbased<br>creosote and pentachlorophenol (PCP) solutions. Dissolved phases of these contaminants<br>comprise the groundwater contamination expected, and the delineation of the extent of this<br>contamination was the overall objective of the investigation. In addition, dense non-aqueous phase<br>liquid (DNAPL) contamination was expected in accumulations in topographic lows of the bedrock<br>surface near the source area. Therefore, another objective of the geophysical survey was to investigate<br>any topography on the bedrock surface.<br>In this preliminary phase of the investigation, geophysical methods used consisted of transient<br>electromagnetic (TEM) soundings, a resistivity sounding, and an extensive terrain conductivity (EM-31)<br>survey. Due to the extensive sources of cultural interference at the site (buildings, fences, railroad<br>tracks, etc.), the geophysical survey was limited. Three profiles of 20-m central-loop TEM soundings<br>were obtained, along with some isolated soundings where it was thought that cultural interference could<br>be avoided. TEM approximate depth sections clearly identify the paleochannel system in cross-section.<br>Correlation with terrain conductivity response provides confidence in the terrain conductivity<br>interpretation of the paleochannel system in areas where only that type of data were obtained. One<br>resistivity sounding, and an adjacent TEM sounding, were modeled simultaneously to the same layeredearth<br>in order to test the idea of improving the resolution of the interpretation. This proved valuable in<br>delineating an electrical equivalent of the working model of the hydrogeological section based on nearby<br>drilling information.