Crumpton, W.G., G.A. Stenback, B.A. Miller, and M.J. Helmers. 2006. Potential benefits of wetland filters for tile drainage systems: impact of nitrate loads to Mississippi River subbasins. U.S. Department of Agriculture, Project Report IOW06682. 34 pgs.
Nitrate concentration and stream discharge data from United States Geological Survey (USGS) National Stream Quality Accounting Network (NASQAN) monitoring stations in the upper Mississippi River (UMR) and Ohio River basins were used to calculate stream nitrate loading and annual flow-weighted average (FWA) nitrate concentrations. A model estimating FWA nitrate concentration was developed on the basis of nitrate data from selected NASQAN stations and their associated land use data. The model accounts for 90% of the variation among stations in long term FWA nitrate concentrations and was used to estimate FWA nitrate concentrations for a 100 ha grid across the UMR and Ohio River basins. Annual water yield for grid cells was estimated by interpolating over selected USGS monitoring station water yields across the UMR and Ohio River basins. For 1990 to 1999, mass nitrate export from each grid area was estimated as the product of the FWA nitrate concentration, water yield and grid area.
To estimate potential nitrate removal by wetlands across the same grid area, mass balance simulations were used to estimate percent nitrate reduction for hypothetical wetland sites distributed across the UMR and Ohio River basins. Nitrate reduction was estimated using a temperature dependent, first-order model using a daily time step over a ten year period. Model inputs included local temperature from the National Climatic Data Center and water yield estimated from USGS stream flow data. The simulation results were used to develop a non-linear model for percent nitrate removal as a function of hydraulic loading rate (HLR) and temperature. Mass nitrate removal for potential wetland restorations distributed across the UMR and Ohio River basin was estimated based on the expected mass load and the predicted percent removal. Similar functions explained most of the variability in percent and mass removal reported for field scale experimental wetlands in the UMR and Ohio River basins. Modeling results suggest that a 30% reduction in nitrate load from the UMR and Ohio River basins could be achieved using 210,000-450,000 ha of wetlands targeted on the highest nitrate contributing areas.