Subsurface water and nutrient processing in a large temperate river floodplain

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Keith Schilling, Peter Jacobson

Thursday 2 july 2015

14:20 - 14:35h at Oceania Foyer (level 0)

Themes: (T) Water resources and hydro informatics (WRHI), (ST) Surface and subsurface flow interactions

Parallel session: 12L. Water resources - Flow interactions

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Elevated nutrient export from large rivers draining agricultural regions in Europe and North America contribute to stream eutrophication and development of large-scale hypoxia but in many areas, nutrient losses from basins may be attenuated by floodplains. Studies at the floodplain of the Cedar River in Muscatine County, Iowa, USA, are revealing that the degree of water and nutrient processing in a large river floodplain is highly variable and affected by floodplain lithology, hydrology and land cover. At a floodplain savanna site, topography and lithology are dominated by a series of sandy ridges and fine-textured swales typical of a natural meandering river floodplain complex. We observed that groundwater quality varied systematically in the two settings, with aerobic, fresh groundwater located beneath sand-dominated ridges and anaerobic and mineralized groundwater beneath fine-textured swales. Variations in groundwater quality with texture and topography in the floodplain indicates hotspots of biogeochemical activity. During both non-flood and flood periods, water and nutrient processing were dominated by vegetation and subsurface controls. During non-flood periods, floodplain groundwater fluctuated in response to external forcing (stream stage variation and precipitation input) and internal forcing (evapotranspiration). During flood periods, groundwater monitoring with logging geochemical sondes indicated that groundwater biogeochemistry was very dynamic, changing rapidly in response to flood inundation. Greater biogeochemical activity occurred in low swales compared to sandy ridges. Groundwater showed little evidence of NO3-N loading despite flood inundation from high NO3-N water from the agricultural watershed, thus providing evidence that floodplain sediments are effective N sinks. Additional logging of dissolved oxygen dynamics in floodplain groundwater with an optical probe has revealed substantial fluctuation in DO concentrations at daily and hourly scales. Overall, our work is highlighting a biogeochemically active, spatially heterogeneous subsurface environment present beneath the floodplains of large temperate river systems.