Nutrient Leaching from Leaf-and-Grass Compost Addition to Stormwater Submerged Gravel Wetlands

Abstract

Submerged Gravel Wetlands (SGWs) are subsurface-flow wetlands, and are effective stormwater control measures (SCM). Compost addition has many properties beneficial to SGWs but may also lead to leaching of nitrogen (N) and phosphorus (P). To investigate nutrient leaching effects of leaf-and-grass compost addition in SGWs, mesocosm studies were conducted using bioretention soil media (BSM) mixed with 30% and 15% compost, by volume. Synthetic stormwater was applied to mesocosms and effluent analyzed for N and P. Compost-added mesocosms were found to leach N and P. Maximum N concentrations of 16 and 6.4 mg-N/L were reached after 1.7 and 3.0 cm of rainfall for 15% and 30%, respectively. Maximum P concentrations of 2.9 and 0.52 mg-P/L were both reached after 2.5 cm for 30% and 15%, respectively. Particulate P was the dominant P species found in effluent samples, while N species were mixed. Although compost addition led to leaching of N and P, treatment of both nutrients was achieved, with the 15%, reaching a net-zero export of P after the equivalent of 20 cm of rainfall. Nitrogen treatment was attributed to denitrification and plant and microbial uptake. Phosphorus treatment was attributed primarily to adsorption.