The application of electromagnetic and electrical methods to groundwater problems in urban environments

Abstract

The use of electromagnetic (EM) methods for the mapping and monitoring of groundwater contamination in urban environments presents difficulties because of the presence of cultural noise and impediments to surveys from manmade structures. This paper shows the successful application of EM and dc methods for the detection and monitoring of groundwater conductivity changes caused by leachate contamination of groundwater, and for monitoring saline intrusions in these difficult environments. This paper discusses results obtained at four sites in the Canning Vale, Morley, Mindarie and Quinns Rocks suburbs of Perth, where any contamination of the groundwater poses a major threat because the city and suburbs rely on groundwater resources for up to 60% of their total water supply. Geophysical measurements were made by the transient electromagnetic (TEM) method using SIROTEM, dc sounding using a Schlumberger array and the down-hole frequency-domain electromagnetic (FEM) method using a Geonics EM-39 instrument. At Morley, where waste disposal began about 12 years ago, and TEM and dc measurements commenced 5 years later, inversion of the data assuming a layered earth showed that a region of low resistivity was developing in 1987 in the saturated zone to the southwest of the waste pit. Data obtained since then have shown a continuing development of this resistivity low further southwest. At the Tamala Park site in Mindarie, waste disposal commenced at the beginning of 1991 and, by November 1991, borehole sampling near the waste pit showed no significant change in groundwater conductivity. From background data collected at Tamala Park, it is possible to quantify seasonal variations in the apparent resistivity derived from the TEM and dc response and to predict that changes caused by leachate contamination should readily be detected above the seasonal variations. The data obtained from one TEM and one dc survey at the Quinns Rocks-Mindarie site appear to indicate that sea water could be affecting the conductivity of the aquifer at a distance up to 180 m inland. This interpretation of the surface TEM data has been difficult to verify with down-hole FEM measurements in the area because the boreholes do not penetrate to the base of the aquifer and measurements have not been made at sufficient depth to detect a significant increase in conductivity below the watertable.