Issues with entering iron into the REACT basis for mineral saturation modeling

[Gregg W. Jones]

Hi Jia - I thought I submitted this question last week but I can't find it anywhere in this forum so maybe I did something wrong. Here it is again.   I have water quality analyses from several monitoring wells at a contaminated site and I want to use that data to do mineral saturation modeling.   I have the total Fe and Fe2+ concentration for three sampling events. The Fe2+ data is suspect because the Fe2+ concentration in groundwater is often higher than the total Fe concentration and that can't be right. It appears there is no option to enter total iron into GSS and REACT. In fact, unless I decouple the Fe2+/Fe3+ redox pair, the only option to input Fe data is for Fe2+.  I decouple the redox pair and now I can enter the concentration for Fe3+ and Fe2+.  Since I don't have the Fe3+ concentration, I want to assume that the total Fe concentration is mostly in the form of Fe3+ because the groundwater environment has an Eh of over 300 mv. I notice the program won't run if I don't enter in something for the Fe2+ concentration so I'm entering a very small concentration (1.0e-5 mg/l). Again, I don't want to use the Fe2+ data from the monitor well samples because I don't think it's valid.  Here are my questions:

1) Why is the Fe2+ concentration the only species available unless I decouple the Fe2+/Fe3+ redox pair? 

2) Do you think my assumption that I can consider most of the Fe to be in the Fe3+ state because the groundwater environment is it's strongly oxidizing? 

 

 

 

[Jia Wang]

Hello Gregg,

React and all other GWB programs assume redox reactions by default are in equilibrium. In doing so, you will only need to supply a total bulk concentration for the total component. Keeping the redox reaction in equilibrium, any concentration you set for the Basis species, which is Fe++ in thermo.tdat, is distributed between Fe++ and Fe+++ species, provided that some constraint on the oxidation is present (e.g. oxygen fugacity, Eh). Also note that when you decouple a redox couple, both species become available to add to the basis but it does not mean both are required, as they are now constrained as separate pools of masses in the calculation.

In some environmental conditions, redox species are often times not at equilibrium with each other. In those cases, then it becomes necessary to decouple the relationship between species of different oxidation. When a redox pair is decoupled, you can then supply the program with the concentration of each group of species, such as Fe+++ and Fe++, separately. This method requires that you supply accurate measurements of the concentration of each oxidation state. Please see section 7.3 Redox disequilibrium to see more details regarding modeling reactions with redox disequilibrium.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Gregg W. Jones]

Hi Jia - that's a big help - thanks!

[Jia Wang]

You're welcome. I am glad that was helpful.

Best regards,
Jia