Estimating nitrate load reductions from placing constructed wetlands in a HUC-12 watershed using LiDAR data

Mitigating Gulf of Mexico hypoxia will require practices to reduce nitrate losses from tile drainage throughout the upper Mississippi River basin. Wetlands are a key practice to accomplish this via denitrification, but locations of feasible wetland sites will need to be determined on a watershed specific basis. This study's objective was to demonstrate that LiDAR topographic data can be used to identify feasible wetland locations in a 6500 ha watershed in northern Illinois, and then estimate the impact on watershed nitrate loads from wetlands hypothetically constructed at those locations. The evaluation resulted in the identification of eleven sites where wetlands could intercept tile drainage from 30% of the watershed. The USDA AnnAGNPS model was used to estimate nitrate loads delivered to candidate wetlands during a 30-year period of simulated climate. The model results were consistent with discharge and nitrate loading regimes in the region. Nitrate reduction by the wetlands was estimated using a published regression model. The wetlands could reduce average watershed nitrate loads by 11–13%, a significant reduction from treating just 30% of the watershed with a single practice. Results showed a wide variation in N-removal performance among wetlands, due to varying contributing-to-wetland area ratios, and varying land uses that affected discharge and N loads among locations. A large wetland installed at the watershed outlet could achieve an estimated N-load reduction of 35% under relaxed siting criteria, but would require the added expense of roadway reconstruction. These conclusions are relevant for planning watershed conservation efforts and establishment of nutrient trading schemes.