Toxicity to aquatic organisms of off‐target deposition of endosulfan applied by aircraft

Aerial drift from pesticide spray applications can result in contamination of nontarget environments such as aquatic systems. In this study, the off-target deposit of aerially applied endosulfan was collected on flat plate samplers and in containers of water placed at various distances downwind of the treated area. The water contaminated by that deposit was subsequently used to conduct 24-h lethal bioassays in a nearby temporary laboratory, using threespine stickleback (Gasterosteus aculeatus), water boatmen (Sigara alternata), caddisfly larvae (Limnephilus spp.), bivalve molluscs (Pisidium spp.), bloodworms (Chironomiidae spp.), and water fleas (Daphnia magna). Three different spray events were sampled, all of which occurred at times when meteorological conditions were within regulatory constraints. Chemical analysis of deposit collectors and exposed water indicated that deposit decreased in an exponential manner with distance downwind and ranged from 82 mg/m3 and 1.7 mg/L for plates and waters within the target site to 0.051 mg/m3 and 0.004 mg/L for plates and waters at 200 m downwind. After 24-h exposure to spray-drift contaminated water, no mortalities that could be attributed to the pesticide were observed for bivalve molluscs, bloodworms, or water fleas, even within the treated area. Water boatmen, caddisfly larvae, and threespine stickleback, on the other hand, suffered levels of mortality that varied directly with distance downwind from the treated area. The distances at which 50% mortality was measured in water boatmen and caddisfly larvae tests were 50 m and 10 m, respectively. Threespine stickleback were the most sensitive species tested, having a mortality of up to 90% at a distance of 200 m downwind. These results demonstrate the extreme risk to aquatic organisms from drift deposit of aerially applied endosulfan and indicate the need for similar assessments of other popular pesticides.