Tuesday, November 14, 2006 - 3:00 PM

The Role of Midwestern Wetlands in a Water Quality Trading Program.

Christopher Craft, Indiana Univ, School of Public and Environmental Affairs, 1315 E 10th St, Bloomington, IN 47405

Wetland restoration and creation has been suggested as one means to generate pollutant reduction credits in a water quality trading program.  However, different wetlands retain different quantities of nutrients depending on geomorphic (landscape) position, connectivity to aquatic ecosystems, loading rate, wetland age and other factors.  We surveyed the literature to identify relationships between wetland nitrogen (N) and phosphorus (P) removal and the aforementioned factors.  We also evaluated the relative importance of the different mechanisms, sediment deposition, organic matter accumulation and denitrification, responsible for effective N and P removal.  Nitrogen and phosphorus removal is two to three times greater in connected (floodplain, fringe) wetlands than in isolated (depressional) wetlands.  In floodplain wetlands, 8-15 MT N/km2 of wetland and 1-3 MT P/km2 are sequestered annually in soil as compared to 3 MT N/km2/yr and 0.5 MT P/km2/yr for depressional wetlands.  Denitrification may remove an additional 3 to 15 MT of N/km2/yr under low to moderate nitrate loadings.  Nitrogen removal is stimulated by increased nutrient loading, mostly through increased denitrification, and, in highly loaded wetlands, N removal may exceed 10-50 MT/km2 wetland/yr.  Increased nutrient loading also boosts P removal though P removal is about an order of magnitude less (1-5 MT/km2/yr) than N.  Phosphorus is retained by sediment deposition and sorption to Al, Ca, and Fe minerals.  With time, P removal declines as sediment accumulates and fills the wetland and sorption sites become saturated.  This is problematic for constructed and restored wetlands where, initially, P removal is high, but declines with time.  Use of wetlands in a water quality trading program must be cognizant that (1) not all wetlands are equal when it comes to nutrient removal, (2) N removal is greater than P removal and (3) effective N removal is sustainable over time but P removal declines as wetlands age.