Phosphorus Release From A Restored Farmed Wetland in Northeastern Illinois.

In northeastern Illinois, restored wetlands have been used to improve water quality in streams degraded by agriculture and urban development. The effectiveness of restored freshwater wetlands in reducing nitrogen in waterways is well documented; however, fewer studies address their effectiveness in removing phosphorous, despite the fact that phosphorus is frequently the limiting nutrient in these systems. Since 1998, we have conducted systematic water quality monitoring at Prairie Wolf Slough Wetland Demonstration Project (PWS), a restored palustrine emergent wetland located on abandoned farm fields adjacent to the Chicago River in Lake County, Illinois. Our objectives are to assess long-term spatial and temporal variations in soluble reactive (SRP) and total phosphorous (TP) and compute a mass balance and retention efficiency for these constituents. Water samples regularly are collected from five sites, including a swale carrying urban stormwater runoff into PWS, and the wetland's outlet to the Chicago River. Water quality and flow data indicate that the restored wetland acts as a point source of SRP and TP reaching the Chicago River.. Long-term mean SRP and TP concentrations increased 279% between the inlet and outlet. Soil testing and analysis of phosphorus release from decaying vegetation suggest that net phosphorous export was likely due to exposure of P-laden sediment to anoxic conditions during flood events. Weekly synoptic sampling in 2008-2009 confirm these results and demonstrate the need for including both soil and water quality testing into wetland restoration planning, design and monitoring protocols.