Factors affecting the occurrence and enantiomeric degradation of hexachlorocyclohexane isomers in northern and temperate aquatic systems

Concentrations of alpha- and gamma -hexachlorocyclohexane (HCH), alpha/gamma -HCH ratios, and enantiomer ratios (ER) of alpha -HCH were measured in lakes in the arctic, subarctic, Great Lakes, Canada, and temperate regions, and temperate and arctic wetlands and streams. The highest concentrations of alpha -HCH were found in cold, large, and oligotrophic lakes such as those in the arctic, subarctic, and the upper Great Lakes, which is attributed to greater inputs from atmospheric deposition and slower loss rates relative to warmer, temperate lakes. High alpha/gamma -HCH ratios in northern systems indicate aged HCH that has undergone long-range transport to high latitude areas, whereas low ratios in the lower Great Lakes and small temperate systems indicate recent gamma -HCH usage and residual alpha -HCH concentrations. Enantioselective degradation (ERs ranged from 0.31 to 0.7) was greatest in small. high arctic lakes and streams and in large lakes in the subarctic in which alpha -HCH concentrations and contact time between chemical and sediments are highest and nutrient concentrations are lowest. Low ERs were found in wetlands and streams in which contact between chemical and sediments was greatest. Conversely. minimal enantioselective degradation occurred in temperate small lakes and wetlands (ERs ranging from 0.77 to 1.06), despite the warmer temperatures, greater microbial populations, and nutrient availability. The results suggest that enantioselective degradation is optimized by maximal contact between chemical and sediment substrates in nutrient-poor waters in which, it is hypothesized, oligotrophic bacteria may act as biofilms.