Particles, metals, and water quality in runoff from large urban watershed

Water quality, metals concentration, and particle size distributions were characterized in urban runoff. The distribution of metals in the macrocolloidal (0.45-20 mu m) and dissolved (<0.45 mu m) size fractions was determined from samples taken under both storm and background conditions. Concentrations of particle number, organic carbon, suspended solids, iron, and zinc increased during storms. The presence of zinc was highly correlated with organic carbon, each displaying significant concentrations in both size fractions. Iron existed almost exclusively in the macrocolloidal fraction. Differences in iron and zinc behavior suggest that sedimentation is not always an effective technique for metals removal. Data from two storms followed throughout their duration show individual materials eluting at different stages during storms. These measurements also indicated potential relationships between the zinc/organic carbon and iron/macrocolloid pairs. In addition, elevated contaminant concentrations and increased flows during storms created loadings equating to weeks dr months of background flow. Data also showed no evidence of the ''first flush,'' which has been observed in many smaller watersheds. Results have implications for the design of large-scale storm-water management strategies.