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Scientific Investigations Report 2008–5066

National Water-Quality Assessment Program

Hypothetical Modeling of Redox Conditions Within a Complex Ground-Water Flow Field in a Glacial Setting

By Daniel T. Feinstein and Mary Ann Thomas

ABSTRACT

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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 contaminants—such as arsenic—as well as anthropogenic contaminants—such 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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Suggested citation:

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.



Contents

Abstract

Introduction

Methods and Assumptions

Flow Model

Geochemical Model

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 Analysis

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

Conclusions

Acknowledgments

References Cited



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