STORMWATER CONTAMINANT REMOVAL BY CHEMICALS. SUMMARY REPORT

The Washington State Department of Transportation designs, operates, and maintains stormwater detention basins for flood control. Initiatives by the Washington State Department of Ecology have prioritized the consideration of stormwater quality as well as quantity for stormwater treatment. Modification of existing stormwater detention basins could afford an efficient mechanism for addressing water quality by enhancing contaminant removal. A scale model of a typical basin was constructed to investigate contaminant removal capabilities of existing detention basins. A simulated highway stormwater runoff was formulated and applied in scale model detention basin testing over a range of flow rates. Four coagulants were evaluated for their ability to enhance removal of sediment and metals. Coagulant addition resulted in significant increases in metal removal over the range of stormwater flow rates studied. The greatest improvement was observed at the higher flow rates. Enhanced metals removal resulted from increased removal of small particles and their sorbed metals. Further improvement in contaminant removal was observed following the addition of an influent baffle, resulting in improved basin hydraulic characteristics which increased solids detention and minimized short circuiting.