1 research outputs found
Water Table Dynamics and Biogeochemical Cycling in a Shallow, Variably-Saturated Floodplain
Three-dimensional variably saturated
flow and multicomponent biogeochemical
reactive transport modeling, based on published and newly generated
data, is used to better understand the interplay of hydrology, geochemistry,
and biology controlling the cycling of carbon, nitrogen, oxygen, iron,
sulfur, and uranium in a shallow floodplain. In this system, aerobic
respiration generally maintains anoxic groundwater below an oxic vadose
zone until seasonal snowmelt-driven water table peaking transports
dissolved oxygen (DO) and nitrate from the vadose zone into the alluvial
aquifer. The response to this perturbation is localized due to distinct
physico-biogeochemical environments and relatively long time scales
for transport through the floodplain aquifer and vadose zone. Naturally
reduced zones (NRZs) containing sediments higher in organic matter,
iron sulfides, and non-crystalline UÂ(IV) rapidly consume DO and nitrate
to maintain anoxic conditions, yielding FeÂ(II) from FeS oxidative
dissolution, nitrite from denitrification, and UÂ(VI) from nitrite-promoted
UÂ(IV) oxidation. Redox cycling is a key factor for sustaining the
observed aquifer behaviors despite continuous oxygen influx and the
annual hydrologically induced oxidation event. Depth-dependent activity
of fermenters, aerobes, nitrate reducers, sulfate reducers, and chemolithoautotrophs
(e.g., oxidizing FeÂ(II), S compounds, and ammonium) is linked to the
presence of DO, which has higher concentrations near the water table