Simulation of changes in stormwater quality at four potential flow-attenuation sites in the Irondequoit Creek watershed, Monroe County, New York

The potential for improving stormwater quality in the Irondequoit Creek watershed, New York, using flow attenuation basins was analyzed by deterministic simulations of sediment retention. Simulations are based on ' plug-flow ' concepts and settling of particulate matter according to Stoke 's law in (1) temporary storage basins, which retain storm runoff but allow normal runoff to pass unimpeded, and (2) maximum storage basins, which maintain a permanent pool of water. The rate of suspended-sediment retention varies with particle size and detention time. The detention time is a function of basin size, rate of inflow, and type of basin simulated and is greater in maximum storage basins, which produce an 8 to 84% greater suspended-sediment load reduction than temporary storage basins. The predicted annual suspended-sediment-load retention ranges from 28 to 32% in temporary storage basins and from 33 to 60% in maximum storage basins. Total annual loads of phosphorous, lead, and zinc, constitutents that are often absorbed to sediment particles, would be reduced by 22 to 59%, 20 to 47% and 16 to 38 %, respectively, according to suspended-sediment simulations. Actual sediment-retention and chemical-retention rates may be less than predicted, however, because (1) the sediment may be resuspended mechanically, (2) anaerobic conditions may cause release of adsorbed constitutents into the dissolved phase, and (3) metals may become remobilized in the presence of high concentrations of chlorides from road salts. (USGS)