Ecological Risk of Down-the-Drain Fiproles and Imidacloprid in California Wastewater-Receiving Streams

Abstract Fipronil and imidacloprid are widely used urban insecticides that enter municipal wastewater through down-the-drain pathways and can occur in treated effluent above aquatic toxicity thresholds, yet the capacity of receiving streams to buffer them remains poorly understood. We conducted a screening-level ecological risk assessment for 160 wastewater treatment plants discharging to California surface waters by integrating effluent monitoring data with modeled receiving-stream dilution across four flow regimes. Next, we estimated in-stream pesticide concentrations, calculated risk quotients (RQs) relative to the minimum U.S. Environmental Protection Agency aquatic life benchmark (BM) or biological evaluation (BE) threshold, and validated modeled concentrations against downstream monitoring within 10 km of discharge points. Under a conservative worst-case scenario (90th percentile statewide effluent concentrations and maximum effluent flows), RQs exceeded the level of concern at 32–43% of sites for fipronil and 46–58% for imidacloprid relative to BM, with 0.6–6% of sites for imidacloprid relative to BE. Exceedances were more prevalent under low-flow conditions and within smaller streams and may increase with climate-driven streamflow reductions. These findings highlight specific sites that merit further study. Because modeled treatment upgrades and discharge-timing controls only modestly reduced RQ exceedances, source reduction remains essential.