Abstract

Gasoline and other hydrocarbons leaking from underground storage tanks are common ground-water pollutants. The extent of the associated contaminant plumes is often assessed using only monitoring wells. However, surface geophysical surveys can also help map areas of contaminated soil and ground water. Geophysical methods are appealing because they are inexpensive, quick, can be performed at most locations, and are nondestructive to the environment.

Electrical resistivity and very low frequency electromagnetic induction data were collected at a site of shallow hydrocarbon contamination in Utah County, Utah. Electrical resistivity data were also collected during a period of over two years. Previously installed monitoring wells facilitated analysis of water chemistry to enhance interpretation of the geophysical data. The electrical resistivity and very low frequency electromagnetic data correlate well and were used to help map the contaminant plume.

Contour maps were constructed from the apparent resistivity data and from the interval resistivity data, which were determined from the apparent resistivities by iterative modelling. These maps outline the hydrocarbon-contaminant plumes as a function of location. The plume at this site was delineated as an area of low apparent and low interval resistivities. Apparent and interval resistivity data were plotted as a function of time against hydrocarbon-contaminant data collected over the same period of time. The results indicate that interval resistivity data may be a reliable way of monitoring changes in hydrocarbon-contaminant levels over time.