Environmental context. Recognising that road flares and air bags may contribute perchlorate to road runoff, we analysed perchlorate in runoff from two accident-prone suburban highways during a period of five rain events. This runoff reaches recharge basins where it then infiltrates into the groundwater. The concentrations of perchlorate averaged ∼2 µg L−1, high enough to be of concern if the drinking water standard for perchlorate were in the low µg levels. Abstract. First flush samples of road runoff were sampled in April 2006 corresponding with five rain events at 12 catch basins and two recharge basins in Suffolk County, NY. Most runoff here is directed into recharge basins or sumps that drain directly into the sole source aquifer. Thus, contaminants from road runoff can contaminate groundwater. In addition to ClO4, we present data for Cl, Na, Br, Sr, and nitrogen as nitrate (N-NO3) as they are the dominant ions in the most probable sources of ClO4 to road runoff, presumably road flares, road salt, air bags and road-side fertilizers. Correlations between the ions indicate that road salt elevates concentrations of Na, Br and Cl in road runoff. The average ClO4 value is 2.18 ± 0.66 (standard error of the mean) µg L−1 for the catch basins and 2.98 ± 0.87 µg L−1 for the two recharge basins. Excess ClO4 in the present study appears mainly from road flares. On average, only 47 % of the ClO4 in the catch basins and 11 % in the recharge basins can be accounted for by bulk precipitation
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