CHEMICAL LOAD FRACTIONATION AND TRAP EFFICIENCY OF A CONSTRUCTION SITE STORM WATER MANAGEMENT BASIN

Storm water runoff from construction sites is a major source of non-po int source pollution in urban areas, and storm water management basins are widely used on construction sites to control downstream pollution . These basins are designed to trap sediment, but very little informat ion is available about the effectiveness of these basins in controllin g potential chemical pollutants such as heavy metals and nutrients, Th ese chemicals exist in both particulate and dissolved forms in storm w ater runoff and can change form during transport. Consequently, runoff leaving a storm water management basin with little sediment could sti ll contain significant amounts of chemical pollution as colloids and d issolved fraction. Data collected during storm events for a management basin in northeastern Ohio show that sediment, metal, and phosphorus trap efficiencies (TEs) are highly variable. Generally, TEs for the co mponents of pollutants associated with finer sediments were observed t o be less than total pollutant TEs, Dissolved load TEs were also relat ively high. The bulk of the outflow chemical load was found to be asso ciated with finer particulate matter (< 2 micron) which can remain sus pended in the basin for a long time. Distinct differences in inflow an d outflow chemical load distributions, coupled with flow volume calcul ations, suggest that the outflow is not dominantly water that entered the basin earlier in the same storm, but rather is water that entered the basin during prior storms. Between storms, water in the basin unde rgoes changes in chemical load distribution, and then is displaced out of the basin by inflow of the next storm event. Because very little i nflow actually leaves the basin during or just after a storm, existing methods of TE evaluation do not measure actual reduction of the chemi cal load that flows into the basins during a single storm event. These results suggest that long term monitoring and better understanding of the complex chemical properties and processes controlling such a syst em of pollutants is essential before practical methods can be develope d to improve the chemical TE of storm water management basins.