Monitored Natural Attenuation (MNA) and Enhanced Natural Attenuation (ENA) represent alternative concepts to classical soil and groundwater remediation techniques such as pump-and-treat. In Germany, at present concepts and strategies for the assessment of natural attenuation are developed.
Our investigations focus on intrinsic biodegradation of aromatic hydrocarbons occurring in the plume downgradient a landfill in the urban area of Karlsruhe, Germany. From 1925 to 1956, an exploited pit was filled with municipal waste, gasworks residues, waste products of benzene production, and other unknown chemicals. In the polluted groundwater, BTEX and PAH are detected. The aquifer consists of porous sand and gravel with low adsorption capacity. The plume length is significantly shorter than calculated from groundwater flow and abiotic retardation processes, indicating microbial Natural Attenuation processes.
In the field, a three-dimensional network of multi-level-wells was installed to monitor the polluted aquifer at about 110 sampling points. The spatial distribution of pollutants, microorganisms, and electron acceptors (oxygen, nitrate, sulphate) or respiration products (sulphide, methane), respectively, was determined. Different dominating redox zones were detected in the plume. Oxygen and nitrate were available in the plume fringe, and a succession of sulphate and Fe(III)-reduction was observed near the landfill.
The relevance of the different redox zones for pollutant biodegradation was examined in microcosms with authentic field samples. Pollutant elimination was observed in aerobic samples, but also in samples taken from the anoxic core region of the plume. Anaerobic biodegradation was observed with the electron acceptors sulfate and ferric iron. Microcosm studies and field data indicate that in particular Fe(III) is an improtant anaerobic electron acceptor at this site. The results obtained in laboratory microcosms were consistent with the field data, showing significant changes in pollutant composition with increasing plume length.
