Nitrogen Removal in Bioretention Systems with Hydraulic Outlet Controls

Abstract

Nitrogen pollution in stormwater runoff is among the most difficult to mitigate via bioretention, particularly for dissolved species like NO3−-NO2− (NOx). Bioretention soil mix (BSM) amended with compost often leaches nitrogen, making the system a nitrogen source rather than a sink. Several design modifications have been proposed to enhance nitrogen removal pathways, like microbial denitrification, by creating anoxic zones, but studies report varied removal rates. This study evaluated nitrogen removal in bioretention systems equipped with orifice-outlet controls. While these controls primarily reduce outflow rates, it is expected that nitrogen removal would increase due to extended hydraulic residence time. The study consisted of six field-scale bioretention mesocosms with two outlet configurations (orifice/standard) and three BSM types (NEW, mature, and alternative). Six synthetic storms measured total Kjeldahl nitrogen (TKN) and NOx removal performance. Continuous flow monitoring characterized the orifice-outlet performance in response to natural storm events, and salt-pulse tracer testing measured the mean residence time. Additionally, soil samples were characterized for TN, TOC, pH, C∶N, NH4+, and NO3− and analyzed with qPCR for nirK and nirS gene abundance, which are indicative of denitrification activity. Orifice-controlled outlets significantly increased residence times and decreased NOx effluent concentrations compared to standard outlets. However, they also increased TKN effluent concentrations, resulting in no net benefit from a TN reduction perspective. Nitrogen leaching was most significant in the new BSM, but this leaching gradually reduced even within this study of six storms. The alternative BSM had the lowest effluent concentrations and the best removal performance, but plant growth was severely limited. NirK genes were only detected in the alternative BSM; however, nirK was present in low abundances compared to other published studies, suggesting denitrification played a minor role in nitrogen removal from these mesocosms.