Methodology for the evaluation of engineered in situ bioremediation:: lessons from a case study

Engineered in situ bioremediation is an economically and ecologically sound technology for the clean-up of contaminated soils and aquifers. However, a successful bioremediation requires solid evidence for the detoxification of the contaminants, preferably proven by complete mineralization. This paper discusses a stepwise evaluation leading to the demonstration of successful engineered in situ bioremediation. Five major evaluation steps assess whether: (1) the contaminants can be mineralized by the indigenous microbial population (2) the mineralization rates can be increased (3) the remediation concept can be simulated under continuous flow conditions (4) the increase of mineralization rates can be achieved at the field site (scale-up), and (5) complete mineralization to harmless end products is achieved at the field site. For these evaluations, the applicability of four experimental approaches (field investigations, laboratory aquifer columns, microcosms and microbial cultures) and the relevance of various microbiological or chemical monitoring parameters are discussed. The evaluations are illustrated using a specific engineered in situ bioremediation of a diesel fuel-contaminated aquifer in Menziken, Switzerland. The case study demonstrates that microbiological and chemical monitoring parameters as well as field tracer studies and stable carbon isotopes should be combined for the unequivocal evaluation of engineered in situ bioremediation. (C) 1998 Elsevier Science B.V.