Pollution Removal Efficiency in a Restored Anabranching Wetland

Improving water quality is a serious concern of many state and local governments across the U.S. Nutrients, including nitrogen and phosphorus, can significantly impair water quality when found in high concentrations. Constructed and restored wetlands are known to capture pollutants and improve water quality. Anabranching is a new design for restored wetlands in which a primary stream is diverted into smaller braided streams and wetland cells. Its effectiveness at capturing pollutants has not yet been evaluated. This study investigates the effectiveness of an anabranched wetland at removing total nitrogen and total phosphorus from urban run-off during several winter storm events. The analysis compares the mass balances of total nitrogen and total phosphorus flowing into and out of the restoration area to determine the amount of nutrient reduction. Flow regimes were manipulated to divert water out of or into a stream or wetland cells. Stream and wetland configurations were compared to determine the effectiveness of the wetland cells in nutrient removal compared to streams alone. Results show that for a given discharge, wetland cells have a residence time up to six times longer than streams. Results indicate that streams may be effective at capturing total phosphorus during low discharge events and ineffective at capturing total phosphorus during high discharge events; the stream configuration does not appear to capture total nitrogen on a consistent basis. Wetland cells appear to be ineffective at retaining total nitrogen over winter storm events; wetland cells may be able to more effectively retain total phosphorus, but this pattern is inconsistent. Results show no clear relationships between discharge and nutrient removal efficiency for the stream or wetland configuration, which is noted in this limited study.