Urban sprawl places watersheds and associated streams and rivers under increased stress in terms of impaired quality from stormwater runoff. Our objective was to determine the responses of one wet and two dry stormwater detention basins during storm events and rate their effectiveness as sediment and nutrient processors, for comparison with untreated stormwater runoff. We hypothesized wet detention basins would discharge lower concentrations of sediment and nutrients, relative to dry basins. The study was conducted in James City County, Virginia, a county presently experiencing rapid residential and commercial growth. Using automated (ISCO©) water samplers we collected samples before, during, and after three storm events of variable magnitude (0.02 inches, 0.19 inches, and 0.24 inches). The water samples were analyzed for the following: dissolved inorganic phosphorus, total particulate phosphorus, dissolved nitrate + nitrite, dissolved ammonium, total suspended sediment (TSS), and total dissolved sediment (TDS). We observed significant increases in concentration of dissolved nutrient and sediment discharging from both the untreated runoff and dry detention basins during the storm events of 0.19 inches and 0.24 inches. The two dry detention basins performed similar to or worse than untreated runoff in terms of total suspended sediment, total particulate phosphorus, and dissolved phosphorus. The dry basins also consistently discharged higher concentrations of nitrate + nitrite, ammonium, and phosphate relative to the wet basin. We concluded that dry basins discharge higher concentrations of sediment and dissolved nutrients than the wet basin during storm events. In James City County there has been a recent policy push to install wet detention basins rather than dry basins. Our data support such a change in stormwater management practices to maintain better water quality in developing watersheds.
For additional documentation Caroline Fortunato and Owen McDonough provided a PowerPoint Presentation entitled "The Effectiveness of Dry and Wet Stormwater Detention" provided here in PDF form.