Stormwater retention ponds are constructed to decrease peak flows of runoff from urban development. In the process of retaining water, the quality of water may be enhanced. To determine the effects of retention ponds on water quality, I measured inputs and outputs of water quality during two summer storms in James City County, VA. Water was collected using automated ISCO samplers and analyzed for particulate phosphorus and dissolved forms of nitrogen (N) and phosphorus (P). Nutrient concentrations were combined with measurements of water flow volumes during storms to calculate nutrient budgets and removal efficiences. For the basin located at the Mulberry Place development, nutrient removal efficiency during storms ranged from 28% to 98% and was variable both by storm and by species (N or P). For the basin located at the Pointe at Jamestown development, nutrient removal efficiency ranged from 8% to 93%. For both basins, amounts of dissolved phosphorus input and output were larger than the amounts of particulate phosphorus. Similarly, amounts of dissolved nitrate+nitrite were larger than that amounts of dissolved ammonium. Although some nutrient removal was observed in both ponds for both storms, the efficiency of nutrient retention is influenced by watershed characteristics, basin shape, and the intensity and size of storms, among other environmental variables. As a result, the ability of stormwater retention basins to meet any specified water quality standard cannot be calculated without actual measurement, i.e., engineering solutions do not predict the observed field variation in basin performance.
For additional documentation Emily Hathaway provided a PowerPoint Presentation entitled "The Effects of Retention Basins on Stormwater Quality" provided here in PDF form.