Identification of groundwater flowpaths and denitrification zones in a dynamic floodpain aquifier

The groundwater flow system in the thick sand and gravel aquifer of the wide forested floodplain along the Lower Wisconsin River was characterized using major ion and oxygen and deuterium stable isotope analyses to explain nitrate attenuation in the floodplain. Critical to the groundwater flow system are the 60-90 m bluffs, the Pleistocene river terrace, and the modern floodplain with ridge and swale microtopography. Upland nitrate concentrations reached 12 mg NO3-N/L, but were largely below the detection limit in the downgradient floodplain wetlands. A transect of monitoring wells and multilevel samplers, with depths of up to 6 m, were installed to characterize spatial and temporal variability in the floodplain groundwater flowpaths. Results showed that the topography induces deep, intermediate, and shallow flow systems and that temporal variability causes mixing between water from different sources. Nitrate attenuation in this system is likely a combination of dilution, plant uptake and denitrification. The absence of dissolved oxygen and nitrate and presence of high soluble iron concentrations in the lower floodplain indicate an ample supply of electron donors for oxygen consumption, denitrification and subsequent iron reduction. The data indicate that conditions created by the topography and temporal variability of the system are suitable for denitrification to occur at depth.