Scientific Investigations Report 2008–5066
This report describes a modeling approach for studying how redox conditions evolve under the influence of a complex ground-water flow field. The distribution of redox conditions within a flow system is of interest because of the intrinsic susceptibility of an aquifer to redox-sensitive, naturally occurring contaminantssuch as arsenicas well as anthropogenic contaminantssuch as chlorinated solvents. The MODFLOW-MT3D-RT3D suite of code was applied to a glacial valley-fill aquifer to demonstrate a method for testing the interaction of flow patterns, sources of reactive organic carbon, and availability of electron acceptors in controlling redox conditions. Modeling results show how three hypothetical distributions of organic carbon influence the development of redox conditions in a water-supply aquifer. The distribution of strongly reduced water depends on the balance between the rate of redox reactions and the capability of different parts of the flow system to transmit oxygenated water. The method can take account of changes in the flow system induced by pumping that result in a new distribution of reduced water.
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Feinstein, D.T., and Thomas, M.A., 2009, Hypothetical modeling of redox conditions for a complex ground-water flow field in a glacial setting: U.S. Geological Survey Scientific Investigations Report 2008–5066, 27 p.
Methods and Assumptions
Equations of the Geochemical Model
Initial Geochemical Conditions
Definition of Redox Zones
Hypothetical Scenarios—A Single Ground-Water Flow Field with Varied Distributions of Organic Carbon
Ground-Water Flow System
Organic Carbon Source Areas
Random Source Area
Lacustrine Source Area
Wetland Source Area
Results of Hypothetical Scenarios
Equilibrium Organic Carbon Concentrations
Equilibrium Redox Water-Chemistry Conditions
Sensitivity to Solver Strategy
Sensitivity to Attenuation Parameters
Sensitivity to Concentrations of Electron Acceptors
Sensitivity to Electron-Donor Behavior
Influence of Pumping on Distribution of Reduced Waters
Limitations of the Study