Date of this Version
Boerner, A.R. 2013. Hydrochemical Investigation of a Transient Parafluvial Zone Under Drought Conditions, Platte River, Nebraska (MS Thesis). University of Nebraska, Lincoln, NE
Shallow groundwater (0.5 m -1.2 m deep) beneath a vegetated and non-vegetated fluvial island was observed in the lower Platte River, Nebraska, USA during exceptional summer drought. Over the course of three months, sub-hourly measurements of hydraulic head, and weekly measurements of redox indicators, δ2H, δ18O, and dissolved gases were analyzed together with nitrogen and carbon species from an array of shallow piezometers in the river bed and islands. These data were compared with the same parameters collected from a 15 m-deep riparian borehole. Vertical hydraulic gradients in the island piezometers indicated the vertical component to groundwater flow was downward over the observation period. Despite high nitrate concentrations in regional groundwater, shallow groundwater and river water contained very low nitrate. Sediment microcosm lab experiments indicated that the island sediment denitrification potential was high, but that carbon-limitation likely limited denitrification within the islands. Redox indicators and dissolved gas data suggest that denitrification of regional groundwater occurred outside of the shallow groundwater monitoring zone. Shallow groundwater also exhibited a positive correlation between water temperature and alkalinity. Analysis of conservative ions, stable isotopes, and carbonate mineral saturation indices indicate that the alkalinity increase is most likely attributed to soil PCO2 over the observation period. Thermodynamic equilibrium modeling indicated that shallow groundwater was supersaturated with respect to carbonate minerals and in equilibrium with soil PCO2 of 10-2.9 atm to 10-1.7 atm throughout the study period. The results suggest that the spring/summer seasonal transition and drought impacted shallow groundwater nitrogen and carbon chemistry: Low-flow conditions in the river decreased delivery of dissolved organic carbon to streambed and island pore water, reducing the denitrification potential. It is also plausible that low-flow conditions in the river facilitated upgradient denitrification by increasing groundwater residence times. Additionally, soil respiration likely increased as a result of increasing air temperatures and the drought-induced water table decline. This increase of soil CO2 led to increased mineral dissolution and dissociation of carbonic acid, evident from the carbonate saturation and temporal trends in alkalinity, Ca2+, and Mg2+.
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