NITROGEN DYNAMICS IN RIVERINE WETLANDS: A CASE STUDY OF THE UPPER MISSISSIPPI RIVER

Biogeochemical processes in riverine wetlands have been shown capable of transforming excess nitrate concentrations into more innocuous and immobile forms. From 2013 to 2016, wetlands of the Upper Mississippi River (UMR) were found to reduce river nitrate concentrations at five times greater efficiency per area than any other landuse strategies. These findings advocate the potential benefit of wetland ecosystems along river corridors to improve in-channel water quality.

The wetland processes driving this phenomenon in the UMR are not clearly defined, nor have similar investigations been pursued downstream between the Missouri and Ohio River confluences. This study seeks to fill this knowledge gap by investigating two floodplain wetlands with contrasting hydrologic connectivity to the river; the levee protected Southern Illinois University Research Field Station (RFS), and the unprotected Wilkinson Island Management Unit (WIMU).

Across these study areas monthly groundwater, surface water, and rain water samples are being analyzed with ion selective electrodes, and ion chromatography. Measurements will continue through 2021, and thanks to 2019 GSA support, the project will soon be enhanced with nitrate stable isotope analyses of water samples.

So far, three important observations have surfaced. [1] The average rain water nitrate concentration was higher than expected at 25.0 ± 1.6 μM/L NO₃^—N. [2] Shallow and deep groundwater collected at both sites showed lower dissolved oxygen concentration (5.0-15.0 ± 0.3 μM/L O₂) than neighboring surface waters (400.0-650.0 ± 0.3 μM/L O₂), where near anoxic conditions indicate the potential for denitrification and anammox reactions. [3] During Spring 2019, near-record UMR flooding induced possible biogeochemical transformation of nitrogen species from surface water to groundwater at WIMU, where concentrations of nitrate dropped from 156.0 to 0.0 ± 1.6 μM/L NO₃^—N , and nitrite increased from 0.0 to as much as 40.0 ± 2.2 μM/L NO₂^—N.

These preliminary findings indicate complex hydrologic and biogeochemical processes are occurring throughout the study areas. Understanding these processes and their capacity, if any, to reduce nitrate concentrations are the next phases of this research.