Two sets of bioretentions cells are being studied as part of a larger monitoring project of a Wal-Mart in Nashville that has installed several low impact development (LID) stormwater practices. Bioretention, permeable pavement, and constructed wetlands have been installed at this site to compare how a commercial shopping center with LID stormwater practices compares to one with traditional stormwater practices and to one with no stormwater practices.

Bioretention cells were installed to treat the front half of the Wal-Mart parking lot. There are two sets of cells with varying media depth at this site to evaluate the influence of media depth on performance. Nutrient removal, runoff reduction, and runoff temperature reduction are being studied at this site. Construction was completed in February 2008, and monitoring began in April 2008. The project is funded by a 319(h) grant provided by NC DENR.

The cells were designed to capture and treat the 1-inch event. However, the surface storage was severely undersized that a 0.4-inch event generated overflow. During the first study period, the surface was also clogged from granite fines present in the "crush and run," and this resulted in slow surface drawdown. In March 2009, the fines layer was removed and the surface storage was increased to be closer to the design storage. This study site will analyze the impacts of maintenance and properly sizing bioretention cells.

Monitoring Period: Period #1 (undersized surface storage) April 2008 - March 2009 & Period #2 (adequate surface storage) March 2009 - Present

Media Depth: one set: 2 ft & one set: 3 ft

Media Characteristics: 86-89% sand & 11-14% fines; low P-Index (11-18)

Drainage Configuration: conventional drainage

Vegetative Cover: mulch/tree/shrub/perennials