HR: 1340h
AN: H13G-1674    [Abstracts]
TI: Biogeochemical Cycling at Soil Interfaces in the Vadose Zone and its Impact on Hydraulic Conductivity
AU: * Hansen, D J
EM: dhansen@geo.tamu.edu
AF: Department of Geology and Geophysics, Texas A&M University, 3115 TAMU, College Station, TX 77843, United States
AU: McGuire, J T
EM: mcguire@geo.tamu.edu
AF: Department of Geology and Geophysics, Texas A&M University, 3115 TAMU, College Station, TX 77843, United States
AU: Mohanty, B P
EM: bmohanty@tamu.edu
AF: Department of Biological and Agricultural Engineering, Texas A&M University, 2117 TAMU, College Station, TX 77843, United States
AB: Much research has focused on understanding and predicting chemical fate and transport in subsurface systems to protect drinking water reserves and ecosystem health. However, chemical changes that occur in the unsaturated zone due to processes such as mineral-water interactions, desorption, or biogeochemical cycling have often been neglected. In particular, the effects of soil structure (i.e. layers, lenses, macropores, or fractures) on these processes remain poorly understood. This study focuses on characterizing the linkages between geochemical processes, hydrologic flow, and microbial activity in the vadose zone using packed soil columns. We constructed three laboratory soil columns: a homogenized medium-grained sand, homogenized organic-rich silty clay, and a sand-over-clay layered column. Both upward and downward infiltration of water was evaluated during experiments to simulate rising water table and rainfall events respectively. In situ collocated probes measured soil water content, matric potential, and Eh. Water samples extracted by lysimeter were analyzed for major cations and anions, ammonium, organic acids, alkalinity, Fe²⁺, and total sulfide. Enhanced biogeochemical cycling was observed in the layered column. For example, concentrations of the electron acceptor sulfate were two-fold greater in the layered column than in either of the homogeneous columns likely due to increased oxidation/reduction reactions. Rainfall events enhanced denitrification in the layered column through the addition of NO₃^- via enhanced ammonium oxidation. Biogeochemical cycling was directly linked to hydrologic flow and varied as a function of water infiltration direction (upward/downward). Enhanced biogeochemical activity produced mineral crusts and biofilms that decreased overall hydraulic conductivity. Preliminary results suggest that changes in the vadose zone occur too rapidly for the system to achieve redox equilibrium and suggest that a new conceptual framework to analyze and predict biogeochemical cycling in the vadose zone is needed.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0471 Oxidation/reduction reactions (4851)
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting