Pollutant mass flushing characterization of highway stormwater runoff from an ultra-urban area

Water quality of highway stormwater runoff from an ultra-urban area was characterized by determining the event mean concentration (EMC) for several pollutants and by evaluating pollutant flushing. Thirty-two storm events were monitored between June 2002 and October 2003. Mean EMCs in mg/L were 0.035, 0.11, 0.22, 1.18, 420, 3.4, 0.14, 1.0, and 0.56 for Cd, Cu, Ph, Zn, total suspended solids (TSS), total Kjeldahl nitrogen (TKN), NO2-N, NO3-N, and TR First flush as defined by flushing of 50% of the total pollutant mass load in the first 25% of the event runoff volume occurred in 33% of the storm events for NO2-, 27% for TP, 22% for NO3- and TKN, 21% for Cu, 17% for TSS, 14% for Zn, and 13% for Pb. Median values for the mass flushed in the first 25% of runoff volume were greater than the mass flushed in any 25% portion beyond the first for all pollutants. The mass in later 25% volume portions were greater than in the first 25% volume in at least 17% of the events for all pollutants, indicating that a significant amount of the pollutant load can be contained in later portions of the runoff volume. Nonetheless, management of the first 1.3 mm (1/2 in.) of runoff was able to capture 81-86% of the total pollutant mass.