Diffusion of Particulates to/from Cells

If one or more Solids are suspended in a Fluid, the Solid particulate matter itself can diffuse, driven by the concentration gradient of the Solid. Any 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). mass associated with (partitioned onto) the suspended Solid(s) will be transported with the diffusing Solid.

With regard to the transport of species mass on particulates from Cells, the following rules are followed:

• If the diffusive mass flux link A mass flux link in which species diffuse between pathways according to a concentration gradient. is between two Cells, then the particulates are only diffused into the Influx Cell if the Solid (or a Solid with the same name) exists in that Cell. (The Solid does not have to be specified as being suspended in the Influx Cell, but it must exist in that Cell.) If it does not, GoldSim assumes that the Solid particulates are "filtered" out before entering the Influx Cell (and remain in the Outflux Cell).
• If the diffusive mass flux link An interconnnection between two transport pathways that defines the rate at which species move between the pathways. is between a Cell and any other type of pathway, the particulates always diffuse into the Influx pathway.
• For computational reasons, the mass in a Cell in excess of the saturation capacity (the "precipitated" mass) is assumed to be precipitated only onto those Solids which are not suspended. Hence, although species mass can be partitioned onto (and transported with) particulate Solids, precipitated species mass is never associated with suspended Solids.

If the Solid is not suspended in the Influx Cell (or the Influx pathway is not a Cell), the suspended Solid concentration in that pathway is simply assumed to be zero when computing the diffusive gradient (and, hence, the diffusive flux becomes uni-directional). Conceptually, the particulates are assumed to immediately disintegrate upon entering the Influx pathway, with the species mass which was associated with them instantaneously partitioning among the available 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 the pathway.

The diffusivity of particulate Solids is specified when defining the properties of the Solid.

As is the case for the advecting Fluids and particulates, GoldSim uses the diffusive flux of particulates only to transport species mass and does not keep track of the movement of the suspended Solid media through the pathway network. That is, the diffusive flux of particulates has no effect on the quantity of the suspended Solid present in either the Outflux or Influx pathway.