Degradation and sorption of selected organophosphate and carbamate insecticides in urban stream sediments

Monitoring studies show that urban surface streams in the United States are commonly contaminated with pesticides, and contamination by organophosphates and carbamates is of particular concern because of their aquatic toxicity. The degradation and sorption of four common organophosphate and carbamate insecticides were studied in urban creek sediments from southern California, USA. In sediment, malathion was quickly degraded under either aerobic or anaerobic conditions, with a half-life (t(1/2)) <3 d. Diazinon and chlorpyrifos were moderately persistent under aerobic conditions (t(1/2) = 14-24 d). However, persistence of chlorpyrifos increased significantly under anaerobic conditions, and t(1/2) was prolonged to 58 to 223 d. The greatest effect of redox potential was found with carbaryl. Although rapid dissipation occurred under aerobic conditions (t(1/2) = 1.8-4.9 d), carbaryl became virtually nondegradable under anaerobic conditions (t(1/2), = 125-746 d). The sorption coefficient consistently increased with time for all pesticides, and chlorpyrifos displayed greater sorption potential than the other pesticides. This study indicates that pesticides in sediment may become less available with time because of increased sorption, and pesticide persistence in sediment may vary greatly among compounds and with redox conditions. Under anaerobic conditions, long persistence may occur even for nonpersistent compounds.