CHARACTERIZATION OF HIGHWAY RUNOFF IN AUSTIN, TEXAS, AREA

Water quality of highway runoff in the Austin, Texas, area was determi ned by monitoring runoff at three locations on the MoPac Expressway. D aily traffic volumes, surrounding land uses, and highway drainage syst em types were different at each site. The concentrations of constituen ts in runoff at all sites were similar to median values compiled in a nationwide study of highway runoff quality. A grassy swale at one site was effective in reducing the concentrations of most constituents in runoff The lower runoff coefficient at this site is attributable to in filtration of runoff into the grassy swale. The pollutant loads discha rged from the pipe draining the swale were lower than those observed a t the sites where runoff drained directly from the pavement. A first f lush effect (i.e., higher pollutant concentrations at the beginning of an event) was evident during selected events, but was generally limit ed to a small volume. The overall effect was negligible when all monit ored events were considered.