Storm Water Pollutant Removal by Two Wet Ponds in Bellevue, Washington

Two wet detention ponds were investigated for their ability to remove pollutants, primarily phosphorus, from storm water runoff. The two ponds lie within the Phantom Lake watershed, a subbasin of the Lake Sammamish watershed in Bellevue, Wash., which is developed as a commercial and residential area with impervious surface area as high as 57%. There are design differences between the two ponds, yet both are comparable to design recommendations set forth by local agencies. One pond was built for flow attenuation and water quality treatment; the other serves only to improve water quality. Fifteen storms and two baseflows were successfully sampled during the Northwest's wet season from October 1996 through March 1997. Pollutant removals varied between one-fifth and one-half for phosphorus, and greater than one-half for total suspended solids and most of the analyzed metals. Removal efficiencies were consistently better in the pond designed primarily for water quality.