The application of mise-á-la-masse and resistivity surveys to the detection of pollution from leaking sewers

Leakage of sewage from underground sewerage pipes can pollute waterways, and is of environmental and community concern. Traditional methods of analysis using flow gauging within the pipes, water quality monitoring within the receiving waters, and visual inspection within the pipes using closed circuit television are inadequate to precisely locate the source of the exfiltration.

Tests were carried out to determine the effectiveness of the mise-á-la-masse method and two-dimensional resistivity imaging for locating the source of sewage leaks within sand- and clay-rich soils. The mise-á-la-masse method detects electrical continuity between the sewage and soil surrounding the pipe. An increase in electrical continuity, or a decrease in resistance, is caused by sewage leaking from within the electrically resistive sewerage pipes into the surrounding soils. Resistivity imaging measures the electrical resistivity in the subsurface as a function of depth below, and distance along the traverse. A decrease in resistivity, particularly at the depth of the sewerage pipe may be associated with the leakage of effluent.

The mise-a-la-masse method is able to detect the leakage of pollution given a sufficient conductivity contrast compared with variations in the subsurface geological conditions. The resistivity pseudosections provide confirmation of the existence of a leak as a resistivity contrast in the vicinity of the pipe. The use of parallel traverses with the mise-á-la-masse method helps separate laterally continuous anomalies indicative of exfiltration from localised inhomogeneities.

Tests at four sites in the Sydney area showed that electrical anomalies correlated with defects seen on within-pipe closed circuit television, and surface features, such as more prolific vegetation. This evidence strongly suggests leakage from the pipes, and indicates that surface geophysical methods are a viable method for detecting exfiltration.