Nutrient-Based Ecological Considerations for Stormwater Management Basins: Ponds and Wetlands

Abstract

The effects of stormwater pond and wetland best management practice (BMP) designs on phosphorus and nitrogen concentrations in effluent was considered using extant data and experimental observations from pond and wetland mesocosms. Relative difference between BMP types were evaluated with respect to several nutrient forms and in relation to untreated stormwater runoff. The mesocosms allowed for direct measurement of time integrated nutrient influent and effluent mass loading. Based on an extended effluent monitoring protocol, these BMP treatments tended to have little effect on total nutrient mass differences between influent and effluent loads. However, significant nitrate removal from BMP influent was observed and appeared to be controlled by influent concentration. The observed nutrient concentrations in BMP effluent could be explained by the reservoir of exchangeable nutrient in basin pore water and the nature of primary production within a BMP type; phytoplankton vs. macrophyte dominance. For example, wetlands tended toward lower effluent phosphorus loading than ponds, which coincided with lower pore water soluble phosphorus pools that were driven by sequestration in macrophyte tissues. Ponds showed a greater potential for effluent nutrient loading than wetlands, which was partly a function of the lower runoff volume to permanent pool volume ratios (Vr:Vb) in pond design. Overall the data suggest little potential for sustained total nutrient removal in these basins at influent loads typical of urban stormwater runoff. Due to the nature of nutrient cycling in mature pond and wetland systems, we suggest a refocusing toward temporallyextended BMP effluent monitoring for better estimation of their effects on nutrient load quantity and quality for TMDLs.