Pollutant removal by a stormwater dry detention pond

Construction of a demonstration urban stormwater detention pond in Greenville, N.C., in 1992 provided an opportunity to assess the effectiveness of the pond in removing total suspended solids (TSS), nitrogen, phosphorus, organic carbon, and selected metals. The normally dry detention pond can hold, for ≥72 hr, the first 1.3 cm of runoff from its 81-ha watershed, which is 31% impervious and 93% residential. Excess runoff from large storms bypasses the pond, flowing over a spillway near the inlet, whereas water detained in the pond, flows out through a perforated riser. The study included eight storms encompassing wide ranges in rainfall amounts (1.2-23.6 cm) and duration (7.8-115 hr). One of the storms was unusually large, resulting in 70% of the runoff bypassing detention. Pollutant concentrations in the untreated runoff were comparable with those for other study sites with similar land uses. Median event mean concentrations (EMCs) were 98 mg/L for TSS, 1.0 mg/L for total nitrogen (TN), and 0.35 mg/L for total phosphorus (TP). Lead, zinc, and other metals concentrations were also within ranges found elsewhere. Pond treatment efficiencies (PTEs) were calculated by comparing pollutant loads leaving the pond through the perforated riser with loads entering the pond (minus spillway bypass). Median PTEs were 71% for TSS, ∼45% for particulate organic carbon (POC) and particulate nitrogen (PN), 33% for particulate phosphorus (PP), and 26-55% for metals. Dissolved pollutant loads leaving the pond were about the same as the runoff loads, except for phosphate phosphorus (PO 4 -P), which had an average PTE of ∼25%. Differences between PTEs and storm treatment efficiencies (STEs). which take into account pond bypassing that occurs in large storms, were roughly proportional to the volumes of runoff that bypassed detention. At current TSS retention rates, only 0.16% of the pond storage volume would be lost per year, but trash accumulation and woody vegetation growth in the pond may reduce the storage volume much more rapidly than sedimentation. It is difficult to predict how the pond's performance will be affected by these changes.