Large river floodplain surface water-groundwater interactions: Insights from the 2019 Midwest Flood

Dynamic surface water-groundwater interactions drive extensive transport and transformation of nitrogen in large river floodplains and their wetlands. Detailed assessment of the transformation rates and products of nitrogen pollutants (i.e., nitrite [NO₂^-], nitrate [NO₃^-], and ammonium [NH₄⁺]) are not well quantified along most of the Upper Mississippi River Floodplain. Starting in autumn of 2018, we sampled the physiochemical properties of the surface water, soil, and shallow groundwater (< 5 m) at two sites located between Chester, IL and Cape Girardeau, MO to quantify the nitrogen reduction potential of floodplain wetlands. During the near-record 2019 Midwest Flood event, which extended from March through July, we also sampled six levee pressure relief wells located adjacent to our sites. These wells extend >20 m into the semi-confined alluvial aquifer and transmitted deep groundwater to the surface under artesian pressure during the flood. Our measurements of shallow and deep (>20 m) groundwater showed substantially lower DO concentrations than adjacent surface waters (<15.0 vs >400.0 μM/L O_2, respectively). Comparison of surface- and ground-water concentrations of NO₂^-, NO₃^-, and NH₄⁺ showed NO₂^- was negligible in surface water (<0.1 μM/L NO₂^-) and at appreciable levels in groundwater (up to 40.0 μM/L NO₂^-). Substantial concentration of NO₃^- was observed in the surface waters (41.9 to 156.0 μM/L NO₃^-), yet little to no NO₃^- in groundwater (<11.2 μM/L NO₃^-). For both surface water and groundwater, the concentrations of NH₄⁺ and NO₃^- displayed an inverse relationship. These findings suggest widespread nitrogen transformation is taking place at the surface water-groundwater interface within these floodplain wetlands. We continue to collect physicochemical data and are currently developing groundwater flow and transport models to improve our understanding of nitrogen transport and transformation in the floodplain wetlands along the Upper Mississippi River.