Environmental concentrations of methoprene and its transformation products after the treatment of Altosid® XR Briquets in the city of Richmond, British Columbia, Canada.

Water runoff from catch basins treated with Altosid® XR Briquets for mosquito larvae control was sampled at 10 storm drainage pump stations along the outskirts of the city of Richmond, British Columbia, Canada after rainfall events in 2006 to determine the residual concentrations of methoprene and transformation products: citronellic acid, methoprene acid, and 7-methoxycitronellic acid. Runoff of prior-to-treatment, posttreatment, and 150-d-after-treatment was collected. No residues were detected in the prior-to-treatment samples. However, methoprene was detected in posttreatment, and citronellic acid was detected in posttreatment and one 150-d-after-treatment sample. The detected environmental concentrations of methoprene (0.04-0.14 µg/L) and methoprene acid (0.07 µg/L) at pump stations were below known/reported toxicity values to aquatic organisms. However, concentrations detected inside the storm drainage system in catch basins (methoprene 122 µg/L, methoprene acid 1.74 µg/L) and inspection chambers (methoprene 622 µg/L, methoprene acid 20 µg/L, citronellic acid 0.05 µg/L) are known to be toxic to invertebrates, have chronic early-life-stage fish effects, and exceeded the Draft Interim Ontario Water Quality Objective and the numerical benchmarks for protection of amphibians (1.6 µg/L), invertebrates (10 µg/L), and fish (80 µg/L). The limited detection in the present study may have resulted from significant absorption of methoprene to sample bottle walls, substance decay during sample storage before methoprene extraction, flushing of briquettes from catch basins following heavy rainfall, and the burial of briquettes under thick layers of debris.