Fredrickson, JK; Bolton Jr., H; Brockman, FJ
To be successful, bioremediation demands contributions from microbiology, hydrology, engineering, geochemistry, soil physics, and geology. The broad range of disciplines represented at the symposium prove this point. Multidisciplinary groups are developing innovative approaches such as bioventing, biosparging, use of anaerobic processes, and use of novel-design reactors on site to overcome many of the obstacles to bioremediation. Despite many successes, there continue to be as many or more situations in which the desired result of bioremediation, the decrease of contaminants below regulatory concentrations, is not achieved. There are often many reasons for failure, including: the presence of co-toxicants such as heavy metals that inhibit biodegradation, physical constraints on electron acceptor-nutrient delivery, slow reaction rates caused by physical constraints (e.g., low temperature), biologically unavailable contaminants, conversion of contaminants to toxic metabolites, heterogeneous distribution of contaminants, and lack of microorganisms with the necessary biochemistry to degrade target contaminants. Researchers are taking increasingly sophisticated approaches to overcome these constraints. For many synthetic organic compounds, biodegradation is hampered by low activity of catabolic enzymes or the production of toxic intermediates. Dick Janssen described the approach that he and his colleagues at the University of Groningen, The Netherlands, used to obtain bacteria with improved catabolic ability. By mutating the wild type of bacteria with genetic insertion elements or transposons they obtained higher levels of expression of 1,2-dichloroethane dehalogenase, resulting in the ability of the host organisms to withstand higher concentrations of this toxic organic. By applying these and other approaches of genetic engineering (such as combining cloned genes from different catabolic pathways), researchers might improve the substrate ranges and catabolic effectiveness of microorganisms. In situ applications for biological treatment of subsurface contaminants are also plagued by heterogeneities in the physical, chemical, and biological properties at a given site and by difficulties in characterizing the site and monitoring the reactions once bioremediation has been initiated. Substantial information has been gained from field experiments such as those at Moffett Field Naval Air Station in California, where researchers from Stanford University have investigated the in situ biodegradation of chlorinated solvents. However, most contaminated sites are less uniform and have less site characterization information; thus in situ reactions cannot be thoroughly monitored. Innovative approaches for site characterization and process monitoring, specifically soil gas measurements and in situ respiration, have been used to differentiate volatilization and biodegradation. The 1993 symposium attracted more than 1100 delegates from 26 countries; it is expected that the third symposium, being planned for 1995, will attract even more attention. Natural attenuation, air sparging, bioventing, and treatment of inorganics are likely to be major topics. It is anticipated that there will be an increase in field applications and implementation of both aerobic and anaerobic treatment processes.