The phenylpyrazole insecticide fipronil has become a popular replacement pest management tool as organophosphorus insecticides have been phased out for residential use and pyrethroids have come under scrutiny as a surface water contaminant. There has been an increasing concern of offsite transport of fipronil to surrounding surface waters and a corresponding increase in potential toxicity to aquatic organisms. The California Department of Pesticide Regulation Environmental Monitoring Program has collected over 500 urban surface water samples throughout California since 2008 to determine the presence and concentrations of fipronil and five degradate products. Statewide, fipronil was detected at high frequency (49%), as were the sulfone (43%) and desulfinyl (33%) degradates. Data collected at long-term monitoring stations indicate higher concentrations in southern California, corresponding to a higher use pattern in the region. There is a clear pattern of increased transport of fipronil with higher flow associated with rain events. However, the lack of seasonality effects on degradates' concentrations suggest a constant source of fipronil with a corresponding lag time of transport to surface waters during the dry season. Receiving waters had a diluting effect on concentrations; however, a significant proportion (46%) of receiving water samples had associated fipronil concentrations above USEPA aquatic life chronic benchmark values. Total mass loading estimates from a long-term monitoring site suggest that the annual fipronil loading is greater in the dry season than during storm events. This could have implications for future mitigation efforts because most runoff during this period was generated from irrigation and outdoor residential use.
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