Electrical Resistivity Tomography At The Doe Hanford Site

Recent work at the DOE Hanford site has established the potential of applying Electrical Resistivity Tomography<br>(ERT) for early leak detection under hazardous waste storage facilities. Several studies have been concluded to test<br>the capabilities and limitations of ERT for two different applications. First, field experiments have been conducted<br>to determine the utility of ERT to detect and map leaks from underground storage tanks during waste removal<br>processes. Second, the use of ERT for long term vadose zone monitoring has been tested under different field<br>conditions of depth, installation design, acquisition mode/equipment and infiltration chemistry. This work involves<br>transferring the technology from Lawrence Livermore National Laboratory (LLNL) to the Resource Conservation<br>and Recovery Act (RCRA) program at the DOE Hanford Site. This paper covers field training studies relevant to<br>the second application for long term vadose zone monitoring.<br>Electrical resistivity tomography is a cross-borehole, imaging technique for mapping subsurface resistivity<br>variations. Electrodes are placed at predetermined depths in an array of boreholes. Electrical current is introduced<br>into one electrode pair located in one borehole while the resulting voltage change is detected between electrode<br>pairs in other boreholes similar to a surface dipole-dipole array. These data are tomographically inverted to image<br>temporal resistivity contrasts associated with an infiltration event. Thus a dynamic plume is spatially mapped as a<br>function of time.<br>As a long-term vadose zone monitoring method, different field conditions and performance requirements exist than<br>those for short term tank leak detection. To test ERT under these conditions, two vertical electrode arrays were<br>constructed to a depth of 160 feet with a linear surface array between boreholes. The fielding was used to facilitate<br>the technology transfer from LLNL to the Hanford RCRA program. Installation methods, commercial equipment<br>and acquisition mode were evaluated to determine economic and technical feasibility to assist design of long-term<br>monitoring networks. Preliminary results of the training test are presented.<br>Until recently, vadose zone monitoring techniques could provide only local point or linear coverage for leak<br>detection and thus, are used primarily under liquid collection systems at land disposal units. As developed by<br>LLNL, ERT can provide area1 coverage under waste treatment and storage facilities given the right conditions.<br>Advantages of ERT to groundwater protection programs are explored along with suggestions for future uses where<br>ERT can be employed today.