Open AccessWater Table Dynamics and Biogeochemical Cycling in a Shallow, Variably-Saturated Floodplain
Author(s) -
Steven B. Yabusaki,
Michael J. Wilkins,
Yilin Fang,
Kenneth H. Williams,
Bhavna Arora,
John R. Bargar,
Harry R. Beller,
Nicholas Bouskill,
Eoin L. Brodie,
John N. Christensen,
Mark E. Conrad,
Robert Danczak,
Eric King,
Mohamad Reza Soltanian,
Nicolas Spycher,
Carl I. Steefel,
Tetsu K. Tokunaga,
Roelof Versteeg,
Scott R. Waichler,
Haruko M. Wainwright
Publication year - 2017
Publication title -
environmental science and technology
Language(s) - English
Resource type - Journals
eISSN - 1520-5851
pISSN - 0013-936X
DOI - 10.1021/acs.est.6b04873
Subject(s) - biogeochemical cycle , vadose zone , anoxic waters , water table , aquifer , environmental chemistry , nitrate , floodplain , denitrification , groundwater , dissolved organic carbon , hydrology (agriculture) , chemistry , environmental science , geology , nitrogen , ecology , geotechnical engineering , organic chemistry , biology
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 U(IV) rapidly consume DO and nitrate to maintain anoxic conditions, yielding Fe(II) from FeS oxidative dissolution, nitrite from denitrification, and U(VI) from nitrite-promoted U(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 Fe(II), S compounds, and ammonium) is linked to the presence of DO, which has higher concentrations near the water table.