DISSIPATION AND PERSISTENCE OF CHLORPYRIFOS WITHIN LITTORAL ENCLOSURES

This study was performed to characterize the distribution, persistence, and mass balance of chlorpyrifos in pond water and sediment and to investigate the feasibility of an enclosure design for environmental chemistry studies. The organophosphorus insecticide chlorpyrifos was applied once to the surface of 12 littoral enclosures (5 x 10 m) constructed in a 2-ha pond near Duluth, MN. The insecticide was applied as Dursban 2E at nominal concentrations of 0.0, 0.5,5.0, and 20.0-mu-g/L active ingredient. Maximum concentrations in the water were measured 1 h after application, and the half-life ranged from 4.7 h in the 20.0-mu-g/L enclosure to 8.0 h in the 0.5-mu-g/L enclosure. In the 20.0-mu-g/L treatment, 95% had dissipated by 7 days. The bulk of the pesticide was initially in the aqueous phase but, after quickly dissipating from the water, trace amounts remaining were primarily associated with the sediment phase. The mass balance at 1 day was 22.5% aqueous and 3.2% sediment, by 64 days it was 0.17% and 0.95%, respectively, and by 420 days there was an estimated 0.04% and a measured 0.5%, respectively. The littoral enclosure design proved to be a feasible means of studying the environmental chemistry of chlorpyrifos.