Acetochlor sorption and degradation in limestone subsurface and aquifers.

Acetochlor, introduced on the market in 1994, is used extensively worldwide, but sorption and degradation studies, including subsurface, are scarce, and there appear to be no such studies with aquifer sediment according to the present mini-review. Sorption, degradation and mineralisation of acetochlor were investigated in heterogeneous limestone down to 43 m below surface (mbs) in four European aquifers (1.7-59 mbs), both aerobic and anaerobic. Sorption revealed K(d) values of 3.39-4.96 L kg(-1) in topsoil, < 0.01-2.02 L kg(-1) in heterogeneous limestone, 0.06-0.72 L kg(-1) in aerobic aquifers and 1.03-4.60 L kg(-1) in microaerobic or anaerobic aquifers. The mineralisation half-lives in the samples from 0.0-0.6 mbs were 0.8-2.1 years and 4.7-95 years in the unsaturated limestone samples from 1-43 mbs. Out of 22 samples from four different European aquifers, acetochlor was mineralised in five samples (half-lives of 9-19 years), all from the same aquifer and core section (19.25-19.53 mbs). Sorption was weak in limestone and aerobic sandy aquifers, and strong in topsoils and in reduced sandy aquifers. Redox conditions controlled the extent of acetochlor sorption in aquifers, as reduced conditions induced increased sorption. Acetochlor was mineralised in deep subsurface, though slowly, and, as mineralisation is the only true removal mechanism in natural attenuation, even slow mineralisation in aquifers with long residence times can have a significant impact.