Control of non-point source pollution by a natural wetland

Wetland creation and restoration is a reliable and efficient technology for the remediation of contaminated water. Knowledge from the natural wetland systems would be necessary to enhance the operational efficiency of constructed wetlands. In this study, a mountainous wetland located in McDowell County, North Carolina, USA was selected to demonstrate the effects of the natural filtration and restoration system on the maintenance of surface water quality. The hydraulic retention time (HRT) for the wetland was 10.5 days based on the results from a dye release study. Water quality monitoring of the wetland was conducted from May to August 1997. One major storm event and baseline water quality samples were collected and analyzed. Analytical results indicate that this wetland removed a significant amount of non-point source (NPS) pollutants [more than 80% N removal, 91% of total suspended solid removal, 59% of total phosphorus removal, and 66% of chemical oxygen demand (COD) removal] caused by the studied storm event. Sediment accretion monitoring results indicate that the accretion rate in the wetland was only 4 mm/year. Therefore, the wetland would require 100 years to fill at the measured sediment accretion rate. The high organic content of sediments (16%) indicates that the wetland is building the characteristic organic layer on the bottom of the wetland. Results from this study would be very useful in the maintenance of natural wetlands and design of constructed wetlands for water treatment.