Application of geophysical techniques to monitor natural attenuation at contaminated sites

Hazardous waste site deposits, disposed in the past without special precautions or preparations of the geological underground, led in present to frequent soil and groundwater pollution by leaking of contaminant solutions. To avoid further damages of the environment, generally remediation measures can stop the leaking of more contaminants from the waste site. But for the surrounding already polluted soil and groundwater, the way of a so-called natural attenuation, by migration processes, adsorption or degradation of the contaminants is chosen frequently. For the control of these natural attenuation, adequate monitoring methods are necessary. In this paper an example is presented where the downstream contaminated environment of a former hazardous waste site in SW-Germany, was left for natural attenuation. Remediation measures, including the construction of a slurry wall encircling the whole site, a surface cover as well as a pump and treat system of the leachate. was installed to prevent further leaking of contaminants. To supervise and to monitor the evolution and the extend of the untreated contamination plume in the downstream area of the waste site, beside classical hydrogeological and hydrochemical techniques, geophysical methods were applied. Since 9 years, complex geoelectrical techniques, including resistivity, induced polarisation and mise-ala-masse measurements. were periodically applied, proving an ongoing but slow natural attenuation of the contaminated area. The applied techniques have proved, as fast and cost efficient tools, for monitoring of natural attenuation processes.