Role of denitrification in subirrigation wetlands receiving agricultural tile drainage

Nitrate pollution of Midwestern streams resulting from intensive row crop agriculture and use of anhydrous ammonia fertilizer application leads to degraded water quality for humans and wildlife.  In central Illinois, drainage tiles bypass traditional approaches to intercept farm runoff.  Wetlands represent an underutilized approach to intercept, retain, and remove excess nitrate.  We are studying the effectiveness of wetlands to reduce nitrate loss from watersheds and the role of denitrification.  Nitrate concentrations entering the wetland commonly exceed the 10 mg/L maximum.  The complexes are composed of 3 individual wetlands in a series with each representing 3,6, and 9% of the watershed cumulatively.  Initial nitrate concentrations at the wetland inflow range from 0.2– 9.7mg/L (summer) to16-21 mg/L (spring), and these concentrations have been found to be reduced by 56-100% at the outflow.  This study will allow us to determine the residence time of water in the wetlands and the size of wetlands needed to reduce nitrate loading into nearby streams.  We are examining the potential for subirrigation wetlands to also serve as habitat for waterfowl use throughout the spring migration.  This data will provide us with the necessary wetland area to effectively reduce incoming nitrates, and provide necessary habitat for waterfowl.