"Exclude sorbed species" settings and "bulk" vs. "free"

[Tom Meuzelaar]

[the following two questions are posed in GWB workshops with enough frequency that it merits a forum topic- TM]

 

• What is GWB doing when "Exclude sorbed species" is checked (or not). Can you define what GWB is doing, with respect to the math, when this option is on or off?
  

 

The “Exclude sorbed species” option is by default turned on (ie. checked), and literally means “sorbed species are not included in initial/Basis aqueous concentrations”. What this means is that when you configure your Basis, the concentrations of components that you specify are concentrations in the fluid phase only, not in the sorbed mineral phase. Thus, if you define 10 mg/kg As3+, and “Exclude sorbed species” is checked, GWB assumes the 10 mg/kg As3+ is referring to arsenic in the fluid only, and will add sorbed arsenic to your sorbing mineral surface to reflect the sorbing mineral surface reaching equilibrium with the 10 mg/kg As3+ in the fluid.

 

The other option, unchecking “Exclude sorbed species”, literally means “include sorbed species in initial aqueous concentrations”. In this case, your initial 10 mg/kg As3+ includes arsenic in the fluid phase and arsenic sorbed to the mineral surface. In this case, GWB, will partition the 10 mg/kg As3+ between fluid and sorbed species. In the latter case, you are fixing the total arsenic component available to both the fluid phase and sorbed phase. In the former (and default) scenario, you are quantifying arsenic component for the fluid phase only.

 

 

• A similar question regarding "free" mass. Does "free" mass mean that the mass does not change during a calculation?
  

 

Specifying a “free mass” or “free concentration” is conceptually similar to having the “Exclude sorbed species” option turned on. When you designate a mineral or aqueous species as “free”, you are defining the amount for that mineral or aqueous species only, not the amount of the entire component in the system. In other words, if you swap Quartz in for SiO2(aq) in the basis, and set its amount to 1 gram, GWB assumes that this amount refers to silica component in the mineral mass only, and will add additional silica component to the fluid during the initial speciation step. Similarly, if you add Na+ to your Basis, set its concentration at 10 mg/kg, and designate it as “free”, GWB assumes that this concentration refers to the concentration of free sodium ion only, not the total sodium component in the system, and will add additional sodium component to the system for other sodium aqueous species (NaSO4-, NaCl, NaHCO3 etc.) in order to reach equilibrium.

 

The other option, designating minerals and aqueous species as “not free” (or “bulk”) means that you are defining the total amount of the component within your system. GWB will partition this component among other aqueous species without adding additional component mass to your system. In the Quartz example, if we set our 1 gram of Quartz as “bulk” (uncheck the “free” box in the concentration drop-down menu), GWB will partition this 1 gram among Quartz mineral, and all dissolved aqueous species containing the SiO2 component. This means we’ll have less than 1 gram of Quartz mineral after speciation. Similarly, if we define 10/mg of Na+ as “bulk”, GWB assumes this is total sodium component, and will partition this amount among all dissolved aqueous species containing the sodium component.

 

By default, mineral masses are specified as “free” whereas aqueous species are specified as “bulk” because most modelers quantify dissolved species as total component concentrations (since this is what most lab analyses report), but choose fixed mineral masses when defining minerals in their initial system.

 

 

Feel free to respond to this thread with any questions or comments.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.