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

Concentrations of alpha- and gamma -hexachlorocyclohexane (HCH), alpha/gamm a -HCH ratios, and enantiomer ratios (ER) of alpha -HCH were measured in la kes in the arctic, subarctic, Great Lakes, Canada, and temperate regions, a nd 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 t o greater inputs from atmospheric deposition and slower loss rates relative to warmer, temperate lakes. High alpha/gamma -HCH ratios in northern syste ms indicate aged HCH that has undergone long-range transport to high latitu de areas, whereas low ratios in the lower Great Lakes and small temperate s ystems indicate recent gamma -HCH usage and residual alpha -HCH concentrati ons. Enantioselective degradation (ERs ranged from 0.31 to 0.7) was greates t in small. high arctic lakes and streams and in large lakes in the subarct ic 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 sedime nts was greatest. Conversely. minimal enantioselective degradation occurred in temperate small lakes and wetlands (ERs ranging from 0.77 to 1.06), des pite the warmer temperatures, greater microbial populations, and nutrient a vailability. The results suggest that enantioselective degradation is optim ized by maximal contact between chemical and sediment substrates in nutrien t-poor waters in which, it is hypothesized, oligotrophic bacteria may act a s biofilms.