The GoldSim Contaminant Transport Module is a program extension to the GoldSim simulation framework that allows you to dynamically model mass transport within complex engineered and/or natural environmental systems. A mass transport model is a mathematical representation of an actual system (e.g., the subsurface environment near a waste disposal site) which can be used to simulate (and hence predict) the release, transport (movement) and ultimate fate of mass within the system. The "mass" that is typically simulated is that of chemical contaminants that have been accidentally released or intentionally disposed of within the system. As a result, such models are often referred to as contaminant transport models.
It is important to understand that unlike the basic GoldSim framework itself, which is quite generic, the Contaminant Transport Module was specifically designed to a be applied to a particular class of problems: those associated with modeling the transport (movement) of mass (typically contaminants) through a system (e.g., soils, surface water bodies, groundwater). As such, the fundamental output produced by the GoldSim Contaminant Transport Module consists of predicted masses and mass transfer rates at specified locations within the system, and predicted concentrations within environmental media (e.g., water, soil, air) throughout the system. If desired, concentrations in environmental media can be converted to doses and/or health risks to receptors by assigning appropriate conversion factors.
More specifically, the Contaminant Transport Module can be used to represent the following processes:
•Release of mass (e.g., contaminants) from specified sources, taking into account 1) the failure of engineered barriers and containers (if any) in which the contaminants are disposed; and 2) degradation of any materials in which the contaminants are bound (e.g., grout, metal, glass).
•Transport of mass through multiple transport pathways within an environmental system (e.g., aquifers, streams, lakes, soil atmosphere).
The transport pathways can consist of any number of transport and storage media (e.g., water, air, soil), and a variety of transport mechanisms can be directly simulated, including 1) advection via fluids (e.g., movement of dissolved constituents in water); 2) advection via solids (e.g., movement of constituents adsorbed to and/or mixed with solids via erosion and transport of contaminated soil); 3) diffusion through fluids; 4) advection and diffusion of contaminated particulates suspended in fluids; and 5) diffusion across boundary layers associated with adjacent fluids (e.g., transport across the air-water interface). Transport processes can incorporate solubility constraints and partitioning of contaminants between the media present in the system, and can include the effects of chemical reactions and decay processes.
In broad terms, you could use these capabilities to apply GoldSim and the Contaminant Transport Module to a wide variety of environmental problems, such as:
•investigation of the transport and fate of contaminants (or natural components) in aquifers, wetlands, lakes and other complex ecosystems;
•evaluation of the performance of existing or proposed disposal facilities and hazardous waste sites;
•investigation of the potential impact of engineered facilities such mines, power plants, and processing facilities on the environment; and
•simulation of the transport and fate of pharmaceuticals and other compounds within biological systems (e.g., physiologically-based pharmacokinetic modeling).
Having described the types of problems that it can be applied to, it is also important to highlight the manner in which GoldSim can represent these kinds of systems (and the limitations that imposes).
When environmental systems are modeled numerically, it is necessary to discretize space into finite volumes. For example, if you were modeling a lake, you could discretize the lake vertically (into layers) and horizontally (into areal sections), forming a three-dimensional grid of finite volumes. The question then becomes: how much discretization is appropriate? At one extreme, the lake could be represented using tens of layers and hundreds of areal sections (resulting in thousands of finite volumes). At the other extreme, the entire lake could be represented using a single finite volume.
The level of spatial discretization that is appropriate in such a case is a function of a number of things, the most important being the level of “mixing” that can be assumed. Representing the lake as a single finite volume is equivalent to assuming that the entire lake is always (instantaneously) well-mixed. Representing the lake as four vertical layers (four finite volumes) is equivalent to assuming that each layer is always (instantaneously) well-mixed.
When building a model, it is critical to give considered thought to the appropriate level of spatial discretization. For a case like a lake, it is often easy to do so (e.g., it is often quite appropriate to assume well-mixed conditions over a layer or even the entire water body). For other problems it is more difficult. For example, for systems involving advection and/or diffusion along a path (e.g, flow through an aquifer), a course level of discretization (e.g., representing an entire aquifer using a small number of finite volumes) can result in unrealistic spreading of the mass (a phenomenon known as numerical dispersion; as will be discussed, GoldSim has special features to deal with this specific issue.)
As a general rule, GoldSim is designed to represent systems at a relatively low level of discretization (tens or hundreds of finite volumes). That is, referring to the example above, a lake might be represented in GoldSim as two well-mixed layers (finite volumes), rather than many hundreds or thousands of finite volumes. This is not to say that GoldSim is not capable of representing systems at a higher level of spatial discretization. But it is not designed to be used in this way.
One other point is also important to understand regarding the Contaminant Transport Module. The Contaminant Transport Module itself is used to model the movement of mass through the system. This movement is typically the result of movement of media (e.g., water) through the system. This results in the advection of mass through the system (e.g., dissolved in the moving water). The key point here is that the Contaminant Transport Module requires the media flows through the system as input. That is, it solves equations based on specified media flow rates. It does not itself solve for the media flow rates. This means that you are required to create a flow model (using the basic GoldSim framework) that produces the media flow rates that can subsequently be used by the Contaminant Transport Module.