Nutrient Leaching from Green Waste Compost Addition to Stormwater Submerged Gravel Wetland Mesocosms

Abstract

Submerged gravel wetlands (SGWs) are subsurface-flow wetlands that can act as effective stormwater control measures (SCMs). To investigate SGW nitrogen (N) and phosphorus (P) performance and impacts from green waste compost (derived from leaves and grass) addition, indoor mesocosm studies were conducted using bioretention soil media (BSM) alone and mixed with 15% and 30% compost, by volume. The unamended mesocosm demonstrated effective and consistent N and P treatment. Compost-amended mesocosms were found to leach N and P above that of the system without compost. Maximum total N concentrations of 16 and 6.4 mg-N/L were reached after 1.7 and 3.0 cm of equivalent rainfall for 30% and 15% compost, respectively. Maximum total P concentrations of 2.9 and 0.52 mg-P/L were both reached after 2.5 cm of equivalent rainfall for 30% and 15%, respectively. Particulate P was more prevalent than dissolved P in effluent samples from compost systems, while N species were mixed among ammonium, nitrate, and organic N. Initial leaching of N and P from compost was counterbalanced by treatment of N and P in synthetic stormwater influent, with the 15% compost mesocosm reaching cumulative net-zero export of P after the equivalent of 6.1 cm of rainfall and of N after 12.7 cm. The 30% compost mesocosm did not achieve net-zero cumulative export of N or P after 22 cm of applied water. N treatment was attributed to denitrification and plant and microbial uptake. P treatment was attributed primarily to adsorption.