D. D. Werkema

Investigations of geoelectrical signatures at a hydrocarbon contaminated site

Abstract This study provides an evaluation of the utility and resolution of different geoelectrical methods in mapping contaminant distribution in the subsurface, and provides a window into the processes that may control their response at a site in Central Michigan. In situ and 2D surface resistivity, ground penetrating radar (GPR), and electromagnetic methods (EM) constrained by soil boring data were used to investigate the electrical properties of a light nonaqueous phase liquid (LNAPL) contaminant plume that resulted from 50 years of leakage into a glacio-fluvial geologic setting. Overall, the electrical signature from the in situ resistivity measurements were best able to image the subsurface stratigraphy and the associated contamination zone. GPR also mapped the subsurface stratigraphy. In particular, the GPR recorded a reflector that is subparallel to the water table, and occurs a few meters above the current free product level, which is coincident with the top of an oil-stained, light-gray sand layer. Further, regions of attenuated GPR reflections (shadow zones) due to enhanced conductivities were found to be coincident with low apparent resistivities. 2D geoelectrical measurements successfully imaged the top of the saturated zone and the underlying clay layer, but was unable to resolve any anomalous region that could be attributed to the hydrocarbon contamination. Likewise, the EM results provided no evidence of the presence of the free product plume at depth. Throughout this investigation, geoelectric measurements consistently recorded low resistivities (high apparent conductivities) associated with zones containing the free/residual product plume instead of high resistivities as has been suggested by the simple intuitive model. From this, it is inferred that substantial modification of the geochemical characteristics of the plume, surrounding media, and associated groundwater has occurred as a result of biogeochemical reactions. It is evident from this study that in situ resistivity measurements combined with surface geoelectrical measurements can characterize the distribution of conductive zones that may be associated with the biodegradation of LNAPL in the subsurface. Thus, the application of these techniques to hydrogeologic, contaminant monitoring, and remediation studies are far reaching.

A generic automated/semiautomated digital multi-electrode instrument for field resistivity measurements

The objective of this work is to design and test a digital multi-electrode acquisition system for use in geophysical investigations utilizing direct current electrical (geoelectric) methods. The system is a 64-electrode modular system, which utilizes field electrodes with individual wires that lead back to a digital switch box and then to a data acquisition instrument (Iris Syscal R2). The electronic design allows for different arrays or geometrical switching configurations of the field electrodes in order to allow for rapid data reading and yield essentially real-time results. Additionally, the electronic switch box has the capability to be connected in series to similar switch boxes and, therefore, has the ability to switch between an unlimited number of electrodes. The acquisition control software, developed as an interface for the hardware, controls operation of both the digital switch box and the resistivity acquisition instrument.