Wash-off potential of urban use insecticides on concrete surfaces. 
Contamination of surface aquatic systems by insecticides is an emerging concern in urban watersheds, but sources of contamination are poorly understood, hindering development of regulatory or mitigation strategies.  Hardscapes such as concrete surfaces are considered an important facilitator for pesticide runoff following applications around homes.  However, pesticide behavior on concrete has seldom been studied, and standardized evaluation methods are nonexistent.  In the present study, a simple batch method for measuring pesticide wash-off potential from concrete surfaces was developed, and the dependence of washable pesticide residues was evaluated on pesticide types, formulations, time exposed to outdoor conditions, and number of washing cycles.  After application to concrete, the washable fraction of four pyrethroids (bifenthrin, permethrin, cyfluthrin, and cyhalothrin) and fipronil rapidly decreased, with half-lives < or =3 d, likely due to irreversible retention in micropores below the concrete surface.  The initial fast decrease was followed by a much slower declining phase with half-lives ranging from one week to two months, and detectable residues were still found in the wash-off solution for most treatments after 112 d. The slow decrease may be attributed to a fraction of pesticides being isolated from degradation or volatilization after retention below the concrete surface.  Wash-off potential was consistently higher for solid formulations than for liquid formulations, implying an increased runoff contamination risk for granular and powder formulations.  Trace levels of pyrethroids were detected in the wash-off solution even after 14 washing-drying cycles over 42 d under outdoor conditions.  Results from the present study suggest that pesticide residues remain on concrete and are available for contaminating runoff for a prolonged time.  Mechanisms for the long persistence were not clearly known from the present study and merit further investigation.