Intrinsic and enhanced bioremediation in aquifers contaminated with chlorinated and aromatic hydrocarbons in The Netherlands

The feasibility of intrinsic and enhanced bioremediation approaches for 16 contaminated sites in the Netherlands are discussed. At at least five out of 10 chlorinated solvent sites, natural attenuation can be used as one of the tools to prevent further dispersion of the plume. At two sites stimulation of the intrinsic dechlorination processes in a bioactived zone is required, and pilot field tests are currently under way. For three sulphatereducing/methanogenic aquifers contaminated with aromatic compounds, microcosm studies demonstrated that natural anaerobic degradation of the risk-determining benzene does not to occur spontaneously. Benzene biodegradation could be initiated by feeding small amounts of oxygen (in all samples) or nitrate (only in one sample), and these techniques are currently tested in field-pilot studies. Redox-microbiological characterisation of two locations contaminated with hexachlorocyclohexanes indicated significant intrinsic biodegradation of HCH's in methanogenic/sulphate-reducing parts of the contaminant plumes, and of the HCH-degradation products chlorophenol and benzene under sulphate/iron reducing conditons. Biostimulated zone's remain required to complete the degradation of HCH and the degradation product monochlorobenzene. Risk-based guideline's for assessing the feasibility of natural attenuation approaches for various contaminant situations are currently under development, making use of already existing protocols (USA) and taking European and National specific conditions, knowledge and regulatory constraints into account.