Meadowcreek Stormwater Basin Retrofit

Two stormwater detention basins in the Meadow Creek watershed, one in Charlottesville and one in Albemarle County, were selected to be retrofit for stormwater quality control. Monitoring was performed by UVA before and after retrofit installation.

Pre-retrofit monitoring was performed in 2000. Construction of both retrofits commenced in summer 2001. Modifications made to the City’s basin behind the Seminole Square shopping center included measures to reduce erosion, construction of cells to improve stormwater detention and therefore settling time, installation of a smaller outlet orifice which also increases detention time, and installation of a trash rack to prevent debris from going into the creek. At the County’s basin on Hillsdale Drive, a smaller orifice was also installed, a rock check dam was built to create a small permanent pool, and wetland plantings were made to increase pollutant uptake. Physical modifications were completed in the fall; plantings were not completed until late winter, and establishment of grass cover at the Hillsdale Drive basin was problematic due to the drought.

Post-retrofit monitoring began in April 2002. Four storm events were sampled at Hillsdale Drive, and three at Michie Drive. Results indicated improved pollutant mass removal efficiencies for the Hillsdale Drive basin, including an increase from 42% to 92% for total suspended solids, from 80% to 88% for total phosphorus, from 74% to 91% for orthophosphorus (the type of phosphorus most accessible to plants), from 73% to 90% for total nitrogen, and from 82% to 92% for zinc. For the Michie Drive basin, removal efficiencies were not calculated before the retrofit because the inflows were insufficiently accounted for. After the retrofit, TSS removal efficiencies varied wildly, from 77% to –25%. It was hypothesized that the negative removal efficiency may have occurred because the basin may not have stabilized and scouring (picking up of additional sediment from within the basin) may have taken place. Other pollutant removal efficiencies were more consistent and included 79% for both total and orthophosphorus, 89% for total nitrogen, and 91% for zinc. The smaller pond area relative to its drainage area of the Michie Drive basin was thought to play a role in its lower removal efficiencies; however, the pond area of the Michie Drive basin was intentionally kept small because of the many small children living in the adjacent apartments.