U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of
the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3
of 3--Subsurface Contamination From Point Sources, Water-Resources
Investigations Report 99-4018C
Modifications to the Solute-Transport Model MOC3D for Simple Reactions, Double
Porosity, and Age, with Application at Mirror Lake, New Hampshire, and Other
Sites
By Daniel J. Goode
ABSTRACT
The U.S. Geological Survey (USGS) three-dimensional solute-transport model
MOC3D has been extended to increase the flexibility of first- and zero-order
reaction terms to approximate geochemical reactions and to simulate double-porosity
exchange and ground-water age. The flexibility of first- and zero-order reaction
modeling is increased by allowing the rate coefficients to vary from cell to
cell in space and to change in time in a step-wise manner. This flexibility
improves the ability of this single-solute model to approximate the effects
of multi-species solute interactions during reactive transport. Double-porosity
exchange accounts for diffusive exchange of solute mass between water flowing
in the aquifer and immobile water. This first-order formulation can approximate
the effects of dead-end pores or matrix diffusion. Ground-water age is simulated
by a zero-order source term of unit strength that accounts for the aging of
water during transport. Using this option, the output from the model is the
spatial distribution of age and the volume-weighted age of mixtures such as
water in discharging wells. These new capabilities are illustrated by applications
at Mirror Lake, N.H., and at other sites where research on transport in ground
water has been carried out through the USGS Toxic Substances Hydrology Program.
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