INTRINSIC AND STIMULATED IN SITU BIODEGRADATION OF HEXACHLOROCYCLOHEXANE (HCH) A. A. M. LANGENHOFF 1∗ , J. J. M. STAPS 1 , C. PIJLS 2 , A. ALPHENAAR 2 , G. ZWIEP 3 and H. H. M. RIJNAARTS 1 1 TNO Environmental, Energy and Process Innovation, Department of Environmental Biotechnology, Apeldoorn, The Netherlands; 2 Tauw Milieu Consultancy, Deventer, The Netherlands; 3 Akzo Nobel Chemicals, Hengelo, The Netherlands ( ∗ author for correspondence, e-mail: a.a.m.langenhoff@mep.tno.nl, fax +31 55 5493523) Abstract. The feasibility of the biodegradation of HCH and its intermediates has been investig- ated. A recent characterisation of two sites in The Netherlands has shown intrinsic biodegradation of HCH. At one site, breakdown products (monochlorobenzene, benzene and chlorophenol) were found in the core of the HCH-plume, whereas the HCH-concentration decreased over time and space. Characterisation of a second, industrial site indicated less intrinsic biodegradation and the need to stimulate biodegradation. In the laboratory, enhanced HCH degradation was tested with soil and groundwater material from both sites, and the required conversion to the intermediates benzene and monochlorobenzene was demonstrated. Furthermore, the biodegradation of these in- termediates could be initiated by adding low amounts of oxygen (<5%). Adding nitrate enhanced this degradation. We hypothesise that this occurs through anaerobic nitrate reducing conversion of oxidised intermediates. At the non-industrial other site, intrinsic degradation took place, as shown in the laboratory experiments. Interpretation of the field data with computer codes Modflow and RT3D was performed. As a result of the modelling study, it has been proposed to monitor natural attenuation for several years before designing the final approach. At the industrial site, the results of the batch experiments are applied. Anaerobic HCH degradation to monochlorobenzene and benzene is stimulated via the addition of an electron donor. Infiltration facilities have been installed at the site to create an anaerobic infiltration zone in which HCH will be degraded, and these facilities are combined with the redevelopment of the site. Keywords: biodegradation, contaminated soil, groundwater treatment, hexachlorocyclohexane, pilot test 1. Introduction The pollution of soil and groundwater with hexachlorocyclohexane (HCH) has caused serious environmental problems. Lindane (γ -HCH) is the best known and effective insecticide component of HCH, and only 9–18% of technical HCH con- sists of this γ -isomer. The remaining part consists of α-, β - and δ -HCH, which do not have insecticide activity. These isomers were separated from γ -HCH, and dumped at waste sites resulting in polluted soils and groundwater. In contrast to α-, γ - and δ -HCH, β -HCH was for a long time known as recalcitrant towards biode- gradation under anaerobic and aerobic conditions (Bachmann et al., 1988). How- Water, Air, and Soil Pollution: Focus 2: 171–181, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.