Surface Water Quality Pollutant Removal Efficacy of Three Wet Detention Ponds Michael A. Mallin,* Scott H. Ensign, Tracey L. Wheeler, and David B. Mayes ABSTRACT conducted, Cahoon (1994) conducted a survey of 16 wet detention ponds. He found that approximately half of Monthly inflow and outflow data were collected from three wet the ponds surveyed had higher total phosphorus in the detention ponds in Wilmington, North Carolina, for a 29-mo period. Two ponds drained urban areas consisting primarily of residential, pond effluents than at sites within the ponds, and several mixed services, and retail usage, while the third mainly drained resi- of the ponds exported higher concentrations of chloro- dential and golf course areas. One of the urban ponds achieved signifi- phyll a as well. In the North Carolina piedmont, Borden cant reductions in total nitrogen, nitrate, ammonium, total phospho- et al. (1997) found that two large wet detention ponds rus, orthophosphate, and fecal coliform bacterial counts. This pond in close proximity to each other had very different pol- was characterized by a high length to width ratio, with most inputs lutant removal performances. Factors influencing this directed into the upper area, and extensive coverage by a diverse included detention time, incoming nutrient load and community of aquatic macrophyte vegetation. The second urban pond algal bloom formation in-pond, and incoming sediment achieved significant reductions in turbidity and fecal coliform bacterial size. Schueler (1994) concluded that two important fac- counts, but there were no significant differences between inflowing tors influencing wet pond performance were the nature and outflowing water nutrient concentrations. There were substantial suburban runoff inputs entering the mid- and lower-pond areas that of the techniques used for treatment and internal design short-circuited pollutant removal contact time. The golf course pond geometry of the system. showed significant increases in nitrate, ammonium, total phosphorus, As part of a large-scale water quality analysis of fresh- and orthophosphate in the outflow relative to the inflow, probably water and estuarine streams in New Hanover County as a result of course fertilization. However, nutrient concentrations (Mallin et al., 1999), three wet detention ponds, all in the outflow water were low compared with discharges from a within the boundary of the City of Wilmington, were selection of other area golf courses, possibly a result of the outflow analyzed for pollutant removal performance. Sampling passing through a wooded wetland following pond discharge. To was conducted on a pre-set schedule, with no attempt achieve good reduction in a variety of pollutants, wet pond design made to favor either rain events or dry periods. The should include maximizing the contact time of inflowing water with streams entering and exiting the ponds maintained suffi- rooted vegetation and organic sediments. This can be achieved through a physical pond design that provides a high length to width cient water depth for sample collection year-round. ratio, and planting of native macrophyte species. SITE DESCRIPTION Ann McCrary Pond, located in the Burnt Mill Creek water- W et detention ponds are commonly used as a shed, is a large (8.82 ha) regional wet detention pond approxi- means to reduce pollutant levels in urban and mately 700 m in length with a length to width ratio of 4.5 suburban stormwater. In general, these ponds are de- (Table 1; Fig. 1). It was built in 1990 and primarily drains signed primarily to reduce suspended sediments. For mixed retail and residential land use, including both single- family and multiple-family residences. The pond contains two example, in North Carolina the only pollutant removal islands within the basin (Fig. 1). It was sampled at three loca- criterion required is an 85% reduction in suspended tions: at the inflow to the pond (AP1), along shore at mid- solids (North Carolina Department of Environment, pond (AP2), and about 40 m downstream of the pond outfall Health, and Natural Resources, 1995). However, these (AP3) (Fig. 1). The pond itself usually maintains a thick growth ponds can effectively reduce concentrations of other of submersed aquatic vegetation, particularly Florida elodea pollutants such as nutrients, fecal bacteria, and heavy [Hydrilla verticillata (L. f.) Royle], Brazilian elodea (Egeria metals, depending on the situation. Schueler (1994) re- densa Planch.), alligatorweed [Alternanthera philoxeroides viewed a broad selection of detention pond performance (Mart.) Griseb.], coontail (Ceratophyllum demersum L.), and studies and found generally good removal of suspended tapegrass (Vallisneria americana Michx.), with lesser coverage solids, variable removal of phosphorus, and rather poor of parrotfeather [Myriophyllum aquaticum (Vell.) Verdc.], soft rush (Juncus effusus L.), and false water-pepper (Polygo- removal efficiencies for nitrogen. Within New Hanover num hydropiperoides Michx.). A survey in late summer 1998 County, North Carolina, where the present research was indicated that approximately 70% of the pond area was vege- tated. There have been efforts to control this growth, including Michael A. Mallin, Scott H. Ensign, and Tracey L. Wheeler, Center addition of triploid grass carp (Ctenopharyngodon idella Val.) for Marine Science, Univ. of North Carolina at Wilmington, 5001 Mas- as grazers. Varying amounts of waterfowl also use this pond, onboro Loop Rd., Wilmington, NC 28409. David B. Mayes, City of Wilmington Stormwater Services Dep., P.O. Box 1810, Wilmington, particularly the middle and lower reaches. During the early NC 28402. Received 21 June 2000. *Corresponding author (mallinm@ portion of the study active apartment construction was under- uncwil.edu). Abbreviations: TSS, total suspended solids. Published in J. Environ. Qual. 31:654–660 (2002). 654