Compositions of first flush and composite storm water runoff in small urban and rural watersheds,north-central Texas

The objective of this study was to evaluate chemical compositions of first flush and composite storm water runoff in four small, mixed land use watersheds in north-central Texas. The watersheds range from urban to rural, all discharging to a local lake providing aquatic habitat, recreation, flood control and drinking water. Automated devices near watershed outlets collected samples from seven storm events, from April 2001 to September 2003. Samples were analyzed for suspended solids, pesticides (diazinon, triazine and chlorpyrifos), nutrients (nitrogen and phosphorus), fecal coliforms and metals. Observed concentrations of most parameters were low relative to drinking water maximum contaminant levels (MCLs) and aquatic life criteria. Fecal coliforms and lead were detected in most samples, with highest lead concentrations in the most developed watershed, likely influenced by building material and automobile traffic. Pesticide concentrations were higher in an initial spring storm event following applications to control weeds and insects. Typical of urban runoff, most ammonia and phosphorus observations exceeded freshwater criteria for streams draining to lakes. Treated wastewater effluent accounted for the highest nitrogen and phosphorus observations. For most constituents, composite concentrations exceeded first flush concentrations, though differences were significant only for fecal coliforms.