ROLE OF ORGANIC CARBON FROM SURFICIAL AQUITARDS IN LIMITING DEGRADATION OF WATER QUALITY IN THE MIDWEST CORN BELT

Row-crop and animal agricultural practices continue to cause significant degradation of water quality in the Midwest Corn Belt.  More than 1200 stream segments and lakes appear on the EPA list of impaired waters in the Upper Mississippi River Basin.  Nutrients exported from the region have been implicated in the hypoxic zone in the Gulf of Mexico.  In Iowa, 18 percent of private wells contains NO₃-N above the 10 mg/L MCL.  Despite this gloomy picture, the problem could potentially be worse.  Research in Iowa during the past 15 years has shown that the surficial aquitard of northern and central Iowa, composed of late Wisconsin till (Dows Formation) and loess of the Peoria and Pisgah Formations, as well the underlying Mississippian aquifer, are “avenues” on the “redox-reaction highway.”  The aquitard contains up to 8 g C/kg and DOC concentrations up to 54 mg/L, causing depletion of electron acceptors to facilitate C oxidation.  As a result, large concentrations of methane (up to 2600 μmol/L) and dissolved Fe (up to 5 mg/L) begin at depths of about 4 m in the aquitard; dissolved O₂, NO₃-N, and SO₄  are absent below that depth.  Above-ambient concentrations of N₂O in groundwater (~ 0.5 μmol/L) at 4 m suggest that denitrification is removing NO₃-N.  In the Walnut Creek watershed in central Iowa, NO₃-N disappears below 3.5 m and coincides with concentration spikes in N₂O and CH₄.  In north-central Iowa, denitrification removes NO₃-N from groundwater before it discharges into Clear Lake. Concentrations of CH₄ and N₂O reach 1600 and 0.2 μmol/L, respectively.  Where the aquitard is fractured, radial diffusion experiments suggest that NO₃-N diffuses into an organic C-rich matrix and denitrifies.  In riparian buffer settings in alluvium, organic C from the aquitard may help drive denitrification in groundwater moving laterally to surface water.  Finally, both isotopic evidence and groundwater modeling suggest that organic C translocated from loess promotes identical redox conditions in the underlying Mississippian aquifer.  We conclude that even though widespread “leakage” of nitrogen occurs into surface water and groundwater as a result of agricultural activities, NO₃-N contamination in the Midwest Corn Belt might be worse if not for the redox reactions driven by organic C derived from surficial aquitards.