Nutrient and Sediment Removal by a Restored Wetland Receiving Agricultural Runoff
Thomas E. Jordan, Dennis F. Whigham, Kirsten H. Hofmockel, and Mary A. Pittek
Journal of Environmental Quality, 2003, Volume 32, p. 1534-1547
Few studies have measured removal of pollutants by restored wetlands that receive highly variable inflows. We used automated flow-proportional sampling to monitor the removal of nutrients and suspended solids by a 1.3 ha restored wetland receiving unregulated inflows from a 14 ha agricultural watershed in Maryland , USA . Water entered the wetland mainly in brief pulses of runoff, which sometimes exceeded the 2500 m 3 water holding capacity of the wetland. Half of the total water inflow occurred in only 24 days scattered throughout the two-year study. Measured annual water gains were within 5% of balancing water losses. Annual removal of nutrients differed greatly between the two years of the study. The most removal occurred in the first year, which included a three-month period of decreasing water level in the wetland. In that year, the wetland removed 59% of the total P, 38% of the total N, and 41% of the total organic C it received. However, in the second year, which lacked a drying period, there was no significant (p>0.05) net removal of total N or P, although 30% of the total organic C input was removed. For the entire two-year period, the wetland removed 25% of the ammonium, 52% of the nitrate, and 34% of the organic C it received, but there was no significant net removal of TSS or other forms of N and P. Although the variability of inflow may have decreased the capacity of the wetland to remove materials, the wetland still reduced non-point source pollution.