Age, Double Porosity, and Simple Reaction Modifications for the MOC3D Ground-Water Transport Model

Water-Resources Investigations Report 99-4041
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Abstract

This report documents modifications for the MOC3D ground-water transport model to simulate (a) ground-water age transport; (b) double-porosity exchange; and (c) simple but flexible retardation, decay, and zero-order growth reactions. These modifications are incorporated in MOC3D version 3.0. MOC3D simulates the transport of a single solute using the method-of-characteristics numerical procedure. The age of ground water, that is the time since recharge to the saturated zone, can be simulated using the transport model with an additional source term of unit strength, corresponding to the rate of aging. The output concentrations of the model are in this case the ages at all locations in the model. Double porosity generally refers to a separate immobile-water phase within the aquifer that does not contribute to ground-water flow but can affect solute transport through diffusive exchange. The solute mass exchange rate between the flowing water in the aquifer and the immobile-water phase is the product of the concentration difference between the two phases and a linear exchange coefficient. Conceptually, double porosity can approximate the effects of dead-end pores in a granular porous media, or matrix diffusion in a fractured-rock aquifer. Options are provided for decay and zero-order growth reactions within the immobile-water phase. The simple reaction terms here extend the original model, which included decay and retardation. With these extensions, (a) the retardation factor can vary spatially within each model layer, (b) the decay rate coefficient can vary spatially within each model layer and can be different for the dissolved and sorbed phases, and (c) a zero-order growth reaction is added that can vary spatially and can be different in the dissolved and sorbed phases. The decay and growth reaction terms also can change in time to account for changing geochemical conditions during transport. The report includes a description of the theoretical basis of the model, a detailed description of input requirements and output options, and the results of model testing and evaluation. The model tests illustrate use of these modifications and demonstrate that accurate solutions can be obtained for these simple cases. Two test cases have no dispersion, illustrating the suitability of this method-of-characteristics model for simulation of advection-dominated transport in ground water.

Suggested Citation

Goode, D.J., 1999, Age, double porosity, and simple reaction modifications for the MOC3D ground-water transport model: U.S. Geological Survey Water Resources Investigations Report 1999-4041, 34 p., https://doi.org/10.3133/wri994041.

Table of Contents

  • Abstract
  • Introduction
  • Theoretical Background and Governing Equations
  • Numerical Methods
  • Computer Program
  • Model Testing and Evaluation
  • Summary and Conclusions
  • References Cited
  • Appendix A - Data Input Instructions for Age, Double Porosity, and Simple Reactions in MOC3D
  • Appendix B - Annotated Example Input Dataset for Sample Problem
  • Appendix C - Selected Output for Sample Problem

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Age, double porosity, and simple reaction modifications for the MOC3D ground-water transport model
Series title Water-Resources Investigations Report
Series number 99-4041
DOI 10.3133/wri994041
Edition -
Year Published 1999
Language ENGLISH
Publisher U.S. Geological Survey ; Branch of Information Services [distributor],
Publisher location Reston, VA
Contributing office(s) Pennsylvania Water Science Center
Description vi, 34 p. :ill. ;28 cm.
Online Only (Y/N) Y
Additional Online Files (Y/N) N