An Evaluation of Stormwater Wetlands in Series

Retrofit BMPs are an important tool for achieving water quality goals in urban watersheds, providing treatment to areas not originally developed with stormwater management practices. One such BMP was installed and monitored in Mooresville, N.C. The system includes three wetlands built "in-series" to treat runoff from a highly impervious 30 acre watershed. The configuration of the wetlands allowed monitoring of water quality parameters at the outlet of each of the three stormwater wetlands. The results of this study indicate that water quality improvement for a number of pollutants experiences diminishing returns as it passes through the three wetland cells. For total suspended solids (TSS), approximately 95% of the total concentration reduction took place in the first wetland cell despite it only comprising 60% of the total surface area of the system. Analysis of the effluent concentrations of the first wetland cell indicates that undersized wetlands (roughly 1.5% of the contributing watershed in this case) may still contribute to water quality improvements in urban watersheds. Thus, retrofit BMPs may be desirable even in circumstances when they cannot be sized appropriately for the contributing watershed.