Aquifer Pathway Features

AQUIFER An Aquifer pathway is intended to represent a feature that essentially behaves as a fluid conduit. It provides a way to simulate processes such as vertical transport through an unsaturated (vadose) zone, and horizontal transport in aquifers, rivers, channels and pipelines.

Aquifer and Pipe pathways can be used for many of the same purposes. Depending on the system to be simulated, one is typically preferred over the other..

Unlike Cell pathways, Aquifer pathways can contain only a single fluid medium. They can, however, contain a solid medium which can impact transport (e.g., by modifying the porosity of the pathway and/or acting to sorb and hence retard species).

An Aquifer pathway actually performs its computations by creating a temporary set of linked Cell elements during the simulation, which are subsequently removed at the end of the simulation. In most cases, there is no need for you to be aware of how this is done, or to view the temporary Cell pathways that are created.

Because Aquifers internally use Cells to carry out their calculations, they can represent most of the same processes that can be represented by a Cell network. In particular, Aquifers can represent the following:

• Partitioning. Within each internal Cell contaminants are partitioned between the the Reference Fluid and the Infill Medium based on the partition coefficients and masses/volumes of the various media present.

• Solubility constraints. You can assign solubility limits for the contaminants in the Reference Fluid (typically water) present in the pathway. The dissolved concentrations of the contaminants in the pathway cannot exceed the solubility limits. If you are simulating isotopes of the same element, GoldSim ensures that the sum of the concentrations of all isotopes does not exceed the solubility limit.

• Mass transport. Advective, dispersive and diffusive transport mechanisms are explicitly represented within the pathway (using advective and diffusive mass flux links between the internal Cells).

• Suspended Solids can be specified to be present in the Aquifer. These Solids are assumed to be advected and dispersed along the Aquifer. Species which partition onto the suspended Solids are transported with them as they move through the Aquifer.

Because multiple Cells are linked together via advective and diffusive mechanisms, the behavior of the Cell network is mathematically described using a coupled system of differential equations. In effect, the network of Cells created within an Aquifer pathway is mathematically equivalent to a finite difference network. GoldSim numerically solves the coupled system of equations to compute the contaminant mass present in the pathway (and the mass fluxes leaving the pathway) as a function of time.

The geometry of the pathway is defined by specifying a length and a cross-sectional area. Conceptually, the pathway may be straight or curved. It simply represents a stream tube for the advecting medium.

Aquifer pathways are ideal for simulating processes such as vertical transport through an unsaturated (vadose) zone and horizontal transport in aquifers, rivers, channels and pipelines.