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

Abstract

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.

Enantiomer Fractions of Organic Chlorinated Pesticides in Arctic Marine Ice Fauna, Zooplankton, and Benthos

“…Law et al (12) reported a higher degree of enantiomeric degradation of R-HCH in aquatic systems with a high chemical-sediment contact and a high chemical concentration. In the present study, although the animals living closer to the sediments (benthos) show a higher degree of enantioselective bioaccumulation, no correlation was found between either R-HCH or trans-chlordane concentration and their respective EFs (Table 2b).…”

“…The R-HCH enantiomer elution order is (+), (-) on BDX-120, BDX-325, and Rt-DEXcst, and (-), (+) on BGB-172 (12,36). Elution orders of trans-chlordane (TC) and cis-chlordane (CC) enantiomers are (+)-TC, (-)-TC, (+)-CC, (-)-CC on BDX-120, and (+)-TC, (+)-CC, (-)-CC, (-)-TC on BGB-172 (37).…”

“…Enantioselectivity increased with depth in the Canada Basin and eastern Arctic Ocean, with greater proportions of (+)-R-HCH being depleted below the polar surface water (PSW) (6,7). Preferential loss of (+)-R-HCH was also reported in arctic lakes and wetlands (11,12).…”

Chiral Chemicals as Tracers of Atmospheric Sources and Fate Processes in a World of Changing Climate

“…Enantioselective degradation is favoured in oligotrophic systems with higher α-HCH concentrations, long water residence time and higher dissolved inorganic carbon, while more nearly racemic α-HCH is found in mesotrophic and eutrophic systems with higher particulate organic carbon and total phosphorus. 39) Oligotrophic systems may contain bacteria that survive under low nutrient conditions by inducing multiple enzymes, shifting metabolic pathways, and taking up and using carbon from mixed sources. 39) Other chiral OCPs in water have been less investigated.…”

“…39) Oligotrophic systems may contain bacteria that survive under low nutrient conditions by inducing multiple enzymes, shifting metabolic pathways, and taking up and using carbon from mixed sources. 39) Other chiral OCPs in water have been less investigated. TC and CC were racemic in arctic-subarctic waters between 1994-2001.…”