EFFECTIVENESS OF SATURATED RIPARIAN BUFFERS: A CASE STUDY IN CENTRAL ILLINOIS

In the U.S. Midwest, agriculture is a principal use of the land. Successful cultivation of corn and soybean depends upon a balance of nutrients and water. However, the application of excess nitrogen (N) fertilizers coupled with tile-drainage systems contribute to eutrophication and hypoxic conditions in surface water. To abate these conditions, the U.S. EPA established a goal to decrease N loading by 45% in the Mississippi River by 2035. One proposed best-management practice is the implementation of a saturated riparian buffer (SRB). To assess their utility, an SRB was installed in a restored prairie, diverting the tile-water into the vadose zone rather than directly discharging into a stream. The tile-system drains an upgradient field that grows corn and soybeans. This work summarizes nearly seven years of research and provides a conceptual model of the SRB. As the water table rises in response to precipitation, the diversion system directs water to the soil beginning late winter (late-February) until late spring (early June). The tile-waters are rich in nitrate, greater than 10 mg/L nitrate as nitrogen (NO₃-N), while NO₃-N concentrations of background water are below 1 mg/L. Travel times from the diversion system to the stream are variable but are greater than 30 days. Along the flow paths, NO₃-N concentrations decrease. While the mixing of tile-waters and groundwater will produce diluted water, a mixing-model indicates dilution is not the sole mechanism for the decrease in NO₃-N concentration. The mixing model reveals losses of NO₃-N exceeding 30%. Analysis of δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- highlight a denitrification signature among the waters traveling away from the diversion system. Measured rates of denitrification were low, less than 1.4 mg NO₃-N/h/g. During the growing season, plant uptake by riparian vegetation results in the minimum NO₃-N concentrations in groundwater occurring in the afternoon, between 1300 to 1800, and the maximum concentrations measured at night, from 0000 to 0600. Decomposition of the vegetation increases NO₃-N concentrations and generates organic carbon within the vadose zone. The vegetation is interpreted as a seasonal sink for NO₃-N. While a single system is not the basis of review, the reduction of NO₃-N by 30% indicates the use of SRBs may help to achieve the 45% reduction goal.