The mapping of groundwater contamination with TEM and DC methods

Abstract

Geophysical methods provide the capability to map the lateral and vertical distribution of pollution in areas between boreholes drilled to determine the spread of contamination from a waste disposal site. In particular, when contamination causes an increase in dissolved salts in the groundwater, the consequent increase in its conductivity may be detected and monitored by electromagnetic (EM) methods. Transient electromagnetic (TEM) and Schlumberger DC sounding measurements have been made at a liquid waste disposal site and a landfill site. At both these sites, significant changes in conductivity near the sources of contamination have been detected by the TEM and DC methods, and the results generally correlate with the conductivity of groundwater sampled in the boreholes. The TEM and DC data are inverted assuming a layered earth model. Measurements at the landfill site are being made at approximately six-monthly intervals to test the application of the methods to the monitoring of the spread of the pollution plume. Data are also being collected over the same time intervals at another site prior to its being used as a landfill site. This will enable the discrimination of any seasonal change in the sub-surface conductivity from a change in conductivity caused by groundwater contamination once disposal of wastes begins.