# Lesson 10 - Modeling Multiple Fluids and Solids

In the example we have been discussing in this Unit, we considered a single fluid (the Reference Fluid Water) and a single solid (Sand).  However, a Cell pathway can contain multiple Fluids and Solids (and mass can be partitioned among all of these media).

To illustrate this, start with a new model, and let’s insert a new Fluid: When you do so, the Fluid dialog will appear: Recall from our discussion in Lesson 6 that all models contain a special type of Fluid called a Reference Fluid.  The Reference Fluid provides a basis for defining partition coefficients between media for the various species in the model.  All models must have a Reference Fluid, and it cannot be deleted. The dialog for a new Fluid is similar to that for the Reference Fluid with one exception: whereas for the Reference Fluid we had the option to define Solubilities, for the Fluid dialog, we define Partition Coefficients (relative to the Reference Fluid). This is analogous to what we do for Solids.  This determines how species partition between the fluids.

Partition coefficients are defined as the concentration in the media divided by the concentration in the Reference Fluid (at equilibrium). Since solid concentrations are expressed in units of mass of species divided by mass of Solid, partition coefficients for Solids have dimensions of volume of Reference Fluid divided by mass of Solid (e.g., units of m3/kg). However, since fluid concentrations are expressed in units of mass of species divided by volume of Fluid, partition coefficients for Fluids have dimensions of volume of Reference Fluid divided by volume of Fluid (and hence are effectively unitless).

Note: Rather than entering partition coefficients for a Fluid, you can alternatively choose to enter solubilities (the drop-list allows you to switch between the two). If you choose to do so, GoldSim internally converts these to partition coefficients. By definition, the partition coefficient is equal to the ratio of the solubility in the Fluid to the solubility in the Reference Fluid.

If you do have multiple Fluids and/or Solids, the equations used internally by GoldSim to describe partitioning are a bit more involved than that described in the previous Lesson, but still straightforward.  In particular, the concentration of a species in medium m is as follows: In this equation,

Cm = the concentration in medium m;

K= partition coefficient between medium m and Reference Fluid (equal to 1 if m is Reference Fluid);

K= partition coefficient between medium n and Reference Fluid (equal to 1 if n is Reference Fluid);

N = number of media in Cell;

Gn = quantity (volume or mass) of medium n; and

M = mass in Cell.

One of the most common Fluids to include in a model is Air.  For example, you might have soil that contains Water, Air and Soil.  Species could partition between the Water and Solid, and volatile species could also partition into Air.  As noted in Unit 3, the partition coefficient for volatile organic compounds is referred to as Henry’s law coefficient.

One final item should be noted about specifying Fluids and Solids for a Cell. In the example we discussed in this Unit, we assumed that all of the Solid was suspended in the Water. Of course, in other cases, the Solid may not be suspended at all (in which case we would not check the S box next to the Media in the Cell dialog). However, what if part of the Solid is suspended and part is not?  In this case, we want to specify that some of the Solid is immobile, while some is suspended in the water. If the water were moving (i.e., contaminants were being advected), it would transport contaminant mass not only in the dissolved phase, but also on the suspended particulate matter. We will wait until Unit 7 to discuss how we would model this, when we discuss modeling the movement of suspended particular matter in more detail.