The organochlorine pesticide Lindane is the gamma-isomer of hexachlorocyclohexane (HCH). Technical grade Lindane contains a mixture of HCH isomers which include not only gamma-HCH, but also large amounts of predominantly alpha-, beta- and delta-HCH. The physical properties and persistence of each isomer differ because of the different chlorine atom orientations on each molecule (axial or equatorial). However, all four isomers are considered toxic and recalcitrant worldwide pollutants. Biodegradation of HCH has been studied in soil, slurry and culture media but very little information exists on in situ bioremediation of the different isomers including Lindane itself, at full scale. Several soil microorganisms capable of degrading, and utilizing HCH as a carbon source, have been reported. In selected bacterial strains, the genes encoding the enzymes involved in the initial degradation of Lindane have been cloned, sequenced, expressed and the gene products characterized. HCH is biodegradable under both oxic and anoxic conditions, although mineralization is generally observed only in oxic systems. As is found for most organic compounds, HCH degradation in soil occurs at moderate temperatures and at near neutral pH. HCH biodegradation in soil has been reported at both low and high (saturated) moisture contents. Soil texture and organic matter appear to influence degradation presumably by sorption mechanisms and impact on moisture retention, bacterial growth and pH. Most studies report on the biodegradation of relatively low (< 500 mg/kg) concentrations of HCH in soil. Information on the effects of inorganic nutrients, organic carbon sources or other soil amendments is scattered and inconclusive. More in-depth assessments of amendment effects and evaluation of bioremediation protocols, on a large scale, using soil with high HCH concentrations, are needed.

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The denitrification potential in different size aggregates of a silt loam soil was studied to explain spatial variability of denitrification. Seven aggregate size ranges (<0.25, 0.25–0.5, 0.5–1, 1–2, 2–5, 5–10, and 10–20‐mm diam.) were obtained by gentle dry sieving of the sampled soil. The denitrification rate was determined by the acetylene block method following imposition of several different treatments. It was observed that the denitrification rate was much larger with the smaller aggregates and generally decreased as aggregate size increased. Crushing of the aggregates to <0.25‐mm diam. resulted in some diminishment of this relationship. The supply of NO−3 was not limiting in the different size aggregates but the addition of glucose suggested that C substrate supply in larger aggregates was limiting denitrifier activity. Carbon dioxide production during aerobic incubation was greatest in microaggregates (<0.25 mm) and decreased with increasing aggregate size, further substantiating the deficiency in C substrate in large macroaggregates. It was found also that biomass C was greatest in microaggregates and decreased with increasing aggregate size when determined for both aerobic and anaerobic incubation conditions. The results of this study may be interpreted to suggest that considerable variability of available substrate C occurs in association with different size aggregates and could partly explain the large spatial variability of denitrification that occurs over relatively short distances in field soil

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