Microbial degradation is a key elimination pathway of hexachlorocyclohexanes from the Arctic Ocean

Hexachlorocyclohexanes (HCHs) are the most abundant pesticides in arctic air and water. We have calculated in situ microbial degradation rates of α‐ and γ‐HCH for the Arctic Ocean using enantiomer ratio (ER) data and concentrations in water at the surface and after it has subducted to depth. The ERs of α‐HCH, which provide the first direct evidence of microbial degradation, allow rates to be estimated. The half‐lives due to microbial activity for (+)‐α‐HCH, (−)‐α‐HCH and γ‐HCH (respectively 5.9 ± 1.2, 22.8 ± 4.7 and 18.8 ± 10.1 years) are 3–10 times faster than removal by the only other known breakdown mechanism, hydrolysis ( Harner et al., 1999 ). Microbial degradation is highlighted as an important elimination process in a revised HCH loss budget for the Arctic Ocean, removing 204 t y −1 , α‐HCH and 23 t y −1 , γ‐HCH. These tonnages represent 29–37% of the annual HCH removal from the Arctic Ocean.