Tracing soil and groundwater pollution with electromagnetic profiling and geo-electrical investigations

Abstract

Geophysical investigation is used to differentiate lithological units. In addition to this, geophysics is also useful for the detection, definition and monitoring of pollution in the ground, on the condition that the pollution, due to spills, leakage or illegal discharges causes a significant difference in conductivity/resistivity. The methodology of geophysical investigation in environmental issues will be illustrated. The first presented method is the electromagnetic profiling method, which measures the lateral variation in ground conductivity using a transmitter and a receiver. At the transmitter coil, a time-varying electromagnetic field is induced by an alternating current. This field interacts with the ground, proportional to ground conductivity. The resulting field is measured and recorded by the receiver. The background conductivity needs to be defined along a profile in a non-polluted zone. Subsequently, all the collected data can be used to plot the lateral variation in conductivity. Areas with higher conductivity reflect in most cases pollution from which the source can be traced. A second method is geo-electrical tomography, which is a combination of resistivity profiling and sounding where a large number of electrodes are placed at a constant distance along a line. During each measurement, the electrical potential caused by a current sent into the soil by two current electrodes is measured between the two potential electrodes. By automatically addressing a combination of four electrodes, and increasing the distance between the electrodes, the depth of penetration increases. Considering the resistivity of the corresponding lithology, pollution along the profile can be delimited vertically and horizontally. Finally, the investigation of the conductivity (resistivity) carried out with borehole loggings delivers information on the vertical distribution of the conductivity in the groundwater reservoir, resulting in the vertical delimitation of pollution close to the borehole. A great advantage for these methods applied to soil pollution consists in the avoidance of direct contact with the pollution, resulting in a reduction of health risks. These methods are non-destructive and fairly fast investigations are possible. They will be illustrated by case studies where the results are validated based on the analyses of soil and groundwater samples.