Mapping the transition between sewage sludge and water bottom

A lagoon in the Manchester area has been filled with sewage sludge from a nearby water treatment plant for the past 50 years. To enable a good planning of the dredging and soil improvement activities, information about the exact depth of the lagoon was needed. T&A was commissioned to map the transition between the sewage sludge and the actual water bottom, consisting of glacial clay.

ERT measurements

The technology that T&A has used to determine the transition between the different soil layers is called ERT. With ERT it is possible to map the electrical resistance of different soil layers. The estimated survey depth was 13 meters below the water surface. The survey was conducted from a vessel with a special marine measurement system. Because the water depth of the lagoon was between 0 and 0.3 meters, the boat sometimes had to be pulled over the sludge.

Results

The geophysical measurements showed that the sewage sludge had a very good electrical permeability, probably because it was discharged poorly into the lagoon. The clay had a much lower electrical permeability, making it possible to map the depth of the sewage sludge properly.