THE EFFECTIVENESS OF RETENTION PONDS FOR WATER QUALITY IMPROVEMENT IN SUBURBAN DEVELOPMENTS, JAMES CITY COUNTY, VIRGINIA

As impervious surface cover increases due to suburbanization, the potential for rapid delivery of non-point source pollution during storms is enhanced. In many areas, the primary strategy to improve water quality prior to release to streams is the retention pond. In James City County, Virginia, design standards for retention ponds include a requirement for a lag of 24 hrs between the inflow and outflow centroids during the one year, 24 hour storm. This standard is more rigorous than most municipalities, and is intended to provide both reduction in peak outflows and water quality improvement. This retention period is intended to allow particle settling and to facilitate biological uptake by vegetation. We monitored water quality in several retention ponds designed under this standard by measuring nutrient levels during storm events in summer 2006. ISCO automated sampling systems were deployed at pond inflow and outflow points to collect samples at set time intervals (generally 15-30 minutes) over several storm periods. The samples were then analyzed using standard colorimetric water quality techniques to determine concentrations of dissolved forms of nitrogen (N) and phosphorous (P). Concentrations of most species plotted over the period of the storm were found to generally rise and fall in concert with the hydrograph. Nutrient concentrations were combined with measurements of flow volumes during storms to calculate nutrient budgets and removal efficiencies. 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. While all ponds were found to generally improve water quality, removal efficiencies varied widely, in part due to storm size and pond design. Therefore, the ability of stormwater retention basins to meet any specified water quality standard cannot be calculated without actual measurement as engineering solutions do not predict observed field performance.