Performance of a constructed wetlands in treating urban stormwater runoff.

An investigation was conducted on the pollutant removal performance of a constructed wetlands treating stormwater runoff from a residential townhome complex in northern Virginia. Constituent event mean concentrations for 33 runoff events between April 1996 and May 1997 were measured based on flow-weighted composite samples collected at the facility's inlet and outlet. With the results from a limited number of grab samples representing ungauged overland drainage from an adjacent wooded area, estimated removals were positive for most constituents and typically exceeded those obtained at a nearby companion wetland study site, consistent with expectations based on the relative ratios of wetland area to drainage area at the two sites. Median load removals of all constituents were greater for a subset of 22 storms that had inflow volumes less than the maximum volume of the marsh. Orthophosphate phosphorus and ammonia removals were significantly better during spring of 1996 than spring of 1997. Lysimeter data suggest a possible explanation for this, which is development of anaerobic conditions in the shallow sediments in 1997. Outlet concentrations of oxidized nitrogen were consistently lower in base flow than in storm samples, suggesting that removal of this constituent occurred primarily between, rather than during, storm events. Water Environ. Res., 72, 295 (2000).