Publication Summary

Diquat concentrations were monitored in the water column of Steilacoom and Gravelly Lakes following treatment with Reward® to control aquatic weeds. In Steilacoom Lake, diquat persisted at all four sampling sites for at least 12 days, with half-lives of 2.1 to 3.4 days. In Gravelly Lake, diquat persisted for 3 to 6 days following treatment. Diquat concentrations ranged from less than 0.5 to 91.2 mg/L in Steilacoom Lake, and from less than 0.5 to 60 mg/L in Gravelly Lake. Diquat was dispersed or drifted to untreated areas of both lakes, including the Steilacoom Lake outlet (Chambers Creek), within 24 hours after treatment.

Diquat concentrations were monitored in the water column of Steilacoom and Gravelly Lakes following treatment with Reward® to control aquatic weeds. In Steilacoom Lake, diquat persisted at all four sampling sites for at least 12 days, with half-lives of 2.1 to 3.4 days. In Gravelly Lake, diquat persisted for 3 to 6 days following treatment. Diquat concentrations ranged from less than 0.5 to 91.2 mg/L in Steilacoom Lake, and from less than 0.5 to 60 mg/L in Gravelly Lake. Diquat was dispersed or drifted to untreated areas of both lakes, including the Steilacoom Lake outlet (Chambers Creek), within 24 hours after treatment.

Concentrations of diquat resulting from the nearly whole-lake treatment of Steilacoom Lake did not comply with the current Reward® label restriction for drinking water (3 days) based on a maximum allowable level of 10 mg diquat/L. In Gravelly Lake, which was spot-treated, resulting concentrations did comply with the 3-day restriction. Neither lake showed marked changes in water quality, including nutrient and dissolved oxygen concentrations, following treatment with diquat.