Bioretention gardens for improved nutrient removal

Bioretention gardens are stormwater management practices capable of providing numerous water quantity and quality benefits. However, previous studies have reported inconsistent removal of nitrogen and phosphorus in these systems. This study used ten, vegetated, mesoscale (0.20 m(3)), bioretention cells in a field setting to provide a comparison of the nutrient removal capabilities of five, alternative bioretention designs. Applying a synthetic stormwater to the bioretention cells demonstrated that a sandy soil mix can provide a 75.5 and 53.4% reduction in concentrations of total phosphorus and total nitrogen, respectively. Phosphorus removal was found to be only slightly enhanced in bioretention cells where soil was amended with alum-based drinking water treatment residuals, a commercially available oxide-coated media, or a commercially available lanthanum-modified bentonite product. However, improvements in phosphorus removal were observed in some cells when elevated phosphorus loads were applied to evaluate longer term performance. In cells incorporating a permanently saturated zone containing shredded newspaper to promote denitrification, effluent concentrations of nitrate were reduced by >99%, however total nitrogen concentrations increased.