IN-SITU MICROCOSMS IN AQUIFER BIOREMEDIATION STUDIES

The extent to which aquifer microbiota can be studied under laboratory or simulated conditions is limited by our inability to authentically duplicate natural conditions in the laboratory. Therefore, extrapolati on of laboratory results to real aquifer situations is often criticize d, unless validation of the data is performed in situ. Reliable data a cquisition is critical for the estimation of chemical and biological r eaction rates of biodegradation processes in groundwater and as input data for mathematical models. Typically, in situ geobiochemical studie s relied on the injection of groundwater spiked with compounds or bact eria of interest into the aquifer, followed by monitoring the changes over time and space. In situ microcosms provide a more confirmed study site for measurements of microbial reactions, yet closer to natural c onditions than laboratory microcosms. Two basic types of in situ aquif er microcosm have been described in recent years, and both originated from in situ instruments initially designed for geochemical measuremen ts. Gillham et al. [Ground Water 28 (1990) 858-862] constructed an ins trument that isolates a portion of an aquifer for in situ biochemical rate measurements. More recently Shati et al. [Environ. Sci. Technol. 30 (1996) 2646-2653] modified a multilayer sampler for studying the ac tivity of inoculated bacteria in a contaminated aquifer. Keeping in mi nd recent advances in environmental microbiology methodologies such as immunofluorescence direct counts, oligonucleotide and PCR probes, fat ty acid methyl esther analysis for the detection and characterization of bacterial communities, measurement of mRNA and expression of protei ns, it is evident that much new information can now be gained from in situ work. Using in situ microcosms to study bioremediation efficienci es, the fate of introduced microorganisms and general geobiochemical a quifer processes can shed more realistic light on the microbial underw orld. The aim of this paper is to emphasize the importance of in situ studies and to describe two different concepts of construction and app lication of in situ microcosms for studying microbial activity in aqui fers. The overall goal is to promote the development and utilization o f these valuable and largely unexplored tools.