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

Hexachlorocyclohexanes (HCHs) are the most abundant pesticides in arctic ai r and water. We have calculated in situ microbial degradation rates of alph a- and gamma-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 alpha-HCH, which provide the first direct evidence of microbial degradation, allow rates to be estimated. The half-lives due to microbial activity for (+)-alpha-HCH, (-)-alpha-HCH and gamma-HCH (respectively 5.9 /- 1.2, 22.8 +/- 4.7 and 18.8 +/- 10.1 years) are 3-10 times faster than re moval 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), alpha-HCH and 23 t y(-1), gamma-HCH. These tonnages represent 29-37% of the annual HCH removal from the Arctic Ocean.