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.