Features and Capabilities of Aquifers

An Aquifer pathway A transport pathway element that is intended to represent a feature that essentially behaves as a fluid conduit. Internally, an Aquifer pathway actually performs its computations by creating a temporary set of linked Cell elements during the simulation. 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.

The geometry of the pathway is defined by specifying an Aquifer Length and an Aquifer Area. You must also define the Number of Cells to be used to discretize the system. Based on this, GoldSim internally creates a series of linked Cell pathways. The specified Infill Medium and Fluid Saturation are used to determine the quantities of media Materials (such as water, sand, clay, air) that constitute (are contained within) transport pathways. GoldSim provides two types of elements for defining media: Fluids and Solids. in each Cell. A pathway Dispersivity is also specified. Advective and diffusive flux links are automatically created by GoldSim to appropriately model advection, dispersion and diffusion through the linked Cells.

Mass enters at one end of an Aquifer, advects through (and disperses and diffuses within) the Aquifer, and then exits at the other end. Aquifer pathways (and hence the Cells that are internally created to represent them) contain only a single fluid medium (which is always, by definition, the Reference Fluid A special type of Fluid element that provides a basis for defining partition coefficients between media for the various species in the model (i.e., the ratio of the species’ concentration in the medium to its concentration in the Reference Fluid at equilibrium).), and an optional single solid medium which can impact transport (e.g., by modifying the porosity of the pathway and/or acting to sorb and hence retard species The chemical (or non-chemical, such as bacterial or viral) constituents that are stored and transported through an environmental system in a contaminant transport model. In GoldSim, the Species element defines all of the contaminant species being simulated (and their properties).).

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 Reference Fluid and Infill Medium based on the partition coefficients The ratio of the species’ concentration in a medium to its concentration in the Reference Fluid at equilibrium. Partition coefficients are inputs to Solid and Fluid elements. 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 the 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.

Note: If the Cell network created by an Aquifer pathway is too simple, and you need to represent a more complex Cell network (e.g., two-dimensional and/or homogeneous), and/or you do not want the Cell network to be temporary, you can use a CellNet Generator to automatically and rapidly create Cell networks.

The mathematical and computational details of how Cell pathways (and hence Aquifer pathways) are implemented within GoldSim are provided in Appendix B of the Contaminant Transport Module User’s Guide.