Modeling Transport Pathways

The GoldSim Contaminant Transport Module allows you to simulate the transport of mass through an environmental system by providing a number of specialized GoldSim elements. The most important of these is the transport pathway Physical components or compartments through which contaminant species can move and/or be stored, such as aquifers, lakes, sediments, surface soil and the atmosphere. GoldSim provides four different elements for simulating pathways. (of which there are several types).

Transport pathways represent physical components through which contaminant 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). can move and/or be stored, such as aquifers, lakes, sediments, surface soil compartments, and the atmosphere. You define the properties of the pathways, such as their geometry and which environmental 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. (e.g., water, soil, air) they contain. All pathways contain one or more environmental media. You define the general properties of each medium (e.g., its density) as well as the properties of each species in each medium (e.g., solubilities and 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.).

You create an environmental system by defining a network of transport pathways, as shown below:

To create such a network, individual pathways are connected via mass flux links. A mass flux link An interconnnection between two transport pathways that defines the rate at which species move between the pathways. defines the mechanisms by which species move between pathways. It represents a vector A one-dimensional array. by species (i.e., it has one item for each species), since the flux will differ for each species being simulated, and has dimensions An output attribute for an element that defines the dimensionality (in terms of Length, Time and other fundamental dimensions) of the output. of mass/time.

Note: The Contaminant Transport Module relies heavily on GoldSim's ability to create and manipulate vectors. Therefore, in order to use the module, you must be comfortable with using vectors in GoldSim.

Two major types of mass flux links and three special purpose mass flux links can be defined in GoldSim. In an advective mass flux link A mass flux link in which a quantity of a medium is specified to flow from one pathway to another, carrying dissolved, sorbed, and/or suspended species with it., a quantity of a medium is specified to flow from one pathway to another, carrying dissolved, sorbed, and/or suspended species with it. In a diffusive mass flux link A mass flux link in which species diffuse between pathways according to a concentration gradient., species diffuse between pathways according to a concentration gradient. Three special purpose mass flux links allow you to model processes that cannot be represented as using advection or diffusion. In a direct transfer mass flux link, species are moved from one pathway to another based on a user-specified transfer rate. In a precipitate removal mass flux link, species present as precipitated mass are moved from one pathway to another based on a specified transfer rate. In a treatment mass flux link A mass flux link in which species are filtered (treated) as they are moved from one pathway to another based on a user-specified fraction (treatment efficiency)., species are treated or filtered and are moved to from one pathway to another based on a specified treatment efficiency (a fraction).

Note: The Contaminant Transport Module is a mass transport model, not a flow model. That is, it does not directly solve for the movement of media through the environmental system being modeled. Hence, you must directly enter the media flow rates associated with an advective flux link (or provide GoldSim with the equations for computing them).

Based on the properties of each pathway, the media in each pathway, the species, and the specified mass flux links, GoldSim computes the temporally varying concentrations in each pathway's media, as well as the mass fluxes between pathways. Hence, the fundamental output of a pathway element is a series of vectors:

• the mass (of each species) in the pathway;
• the concentration (of each species) within each environmental medium in the pathway; and
• the mass transport rate (of each species) to each of the pathways to which it is connected via mass flux links.

A typical pathway output is shown below: