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-point source pollution in urban areas, and storm water management basins are widely used on constuction sites to control down-stream pollution. These basins are designed to trap sediment, but very little information is available about the effectiveness of these basins in controlling potential chemical pollutants such as heavy metals and nutrients. These chemicals exist in both particulate and dissolved forms in storm water runoff and can change form during transport. Consequently, runoff leaving a storm water management basin with little sediment could still contain significant amounts of chemical pollution as colloids and dissolved 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 components of pollutants associated with finer sediments were observed to be less than total pollutant TEs. Dissolved load TEs were also relatively high. The bulk of the outflow chemical load was found to be associated with finer particulate matter (<2 micron) which can remain suspended in the basin for a long time. Distinct differences in inflow and outflow chemical load distributions, coupled with flow volume calculations, 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 undergoes changes in chemical load distribution, and then is displaced out of the basin by inflow of the next storm event. Because very little inflow actually leaves the basin during or just after a storm, existing methods of TE evaluation do not measure actual reduction of the chemical 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 system of pollutants is essential before practical methods can be developed to improve the chemical TE of storm water management basins.