• Christiansen, Anders Vest
• Allaart, Lis
• Meldgaard Madsen, Line
• Upadhyay, Akanksha
• Damgaard, Lars

Abstract

Groundwater contamination associated with biodegradation of organic compounds may result in negative electric Self-Potential (SP) anomalies at the ground surface. This negative SP anomaly is associated with a gradient in the redox potential caused by degradation of organic matter by cable bacteria. Cable bacteria transport electrons along their filaments as a function of their metabolism at the oxic/anoxic (water and oil/hydrocarbon) interface. The cable bacteria filaments can be several centimeters long and generate a considerable current density that can be measured on the surface. To delineate polluted zones, a SP survey was planned near an old gas station in Aarhus, Denmark. Since most of the area was covered with asphalt, it was not practical to dig holes in the ground and take measurements, thus, a new approach was established for measuring surface SP data. In this approach, small holes (1.5 cm diameter) were made in the asphalt surface using an electrical dill and wooden pegs soaked in saline water were inserted in these holes. Measurements were made by placing measuring and reference electrodes in small buckets filled with wet sand covered by saline-water drenched cloth making electrical contact with the wooden pegs in the ground. Using this approach, more than 400 surface SP data points referenced with differential GPS were measured. The results indicate a large negative anomaly over a region with known contamination (Figure 1a). At the same location, three borehole SP measurements revealed the depth to the interface of probable contamination, which is no more than 1 m below the surface. SP signals measured in the study area were also affected due to the presence of different conductivity structures in the area (asphalt, grass, buried metallic tanks). To support the SP measurements, four ERT measurements were also done in the same area showing depth of the interface lying between 1-2 m additionally showing effects of different conductivity structures in the measured profile. Results of the study propose that SP method could be a fast and non-intrusive technique to delineate contaminated interfaces associated with microbial activities in a polluted area.

Topics

• density
• impedance spectroscopy
• surface
• compound
• organic compound
• current density