Abstract

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

Electrical resistivity and very low frequency electromagnetic induction data were collected at a 9-km² site of shallow hydrocarbon contamination in central Utah County, Utah. Electrical resistivity data were also collected over a two-year period. 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 map the contaminant plume.

Contour maps were constructed from both the apparent resistivity and interval resistivity data, the latter determined from the apparent resistivities by iterative modeling. These maps outline the hydrocarbon plume. The plume 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 concentration data collected during the same period of time. The results indicate that interval resistivity data is a reliable method for monitoring changes in hydrocarbon concentration over time.