Entering Media Properties for Isotopes of the Same Element

There are four 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. properties that must be specified for each 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). in the model:

• Solubilities in 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 optionally, for other fluids);
• 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. for other Fluids and Solids;
• Relative Diffusivities for Fluids; and
• Available Porosities for Solids.

If you define multiple isotopes of the same element, GoldSim treats these four properties in a special way: isotopes of the same element are assumed to have equivalent properties. If the properties are defined by element (the default), this is straightforward (as isotopes of the same element are represented by a single entry). However, you can optionally define these properties individually for each species. In this case, it is possible that they could be entered inconsistently (i.e., isotopes of the same element could be assigned different values).

In this case, within a Fluid or Solid element An element in GoldSim for defining the properties of solid media (such as sand, clay and rock)., GoldSim forces the properties for isotopes of the same element to be equivalent. In particular, all isotopes of the same element take on the properties of the first of the element's isotopes in the species array label set A special set of array labels that can only be populated and edited via the Species element.. For example, if the species array A collection of variables that share common output attributes and can be manipulated in GoldSim elements or input expressions. label set included U234, U235, and U238 (in that order), all three species would use the properties assigned to U234, regardless of what was specified for the other two species.

Note: When specifying solubilities, GoldSim requires that the molar solubilities of all isotopes of an element are identical. By default, solubilities are specified as molar solubilities. Note, however, that solubilities can optionally be specified on a mass basis (e.g., mg/l). In this case, the mass-based solubilities of isotopes of an element should be specified as being proportional to the atomic weights of the isotopes. For example, if you have two isotopes of an element with atomic weights 238 and 234, and the solubility of the first isotope is specified as 1 mg/l, then the solubility of the second isotope should be specified as (234/238) mg/l. If it is not, GoldSim will automatically correct the problem (using the molar solubility defined by the first isotope), and will write a warning message to the Run Log Text that is stored with a GoldSim model once it has been run. It contains basic information regarding the simulation, and any warning or error messages that were generated..

An important effect of defining multiple isotopes of the same element is related to how solubilities are computed in Cell pathways (which are the only type of 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. where solubility constraints can be applied). In particular, when computing solubility constraints in Cell pathways, the solubility is "shared" by (split among) multiple isotopes of the same element.

Note: Because solubilities are "shared" between isotopes of the same element, it is of critical importance that you include all isotopes that may be present to a significant degree in the system. This includes any stable isotopes (even if you are not interested in their concentrations due to the lack of radiological impacts). This is because if the stable isotope is present in significant quantities it could "claim" a portion of the solubility. Ignoring the stable isotope in such a case could significantly overestimate the amount of radioisotopes of that element in solution.