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Using XT to model tritium transport

Tom Meuzelaar

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[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 3/26/2004]


Posted by: Craig Cooper


I'm using XT to try and model tritium transport in an unsaturated system (our measured data seem to indicate tritium retention). Rather than programming in tritium as a new basis species and duplicating all reactions that involve H+, I thought I could do this by defining tritium as a stable isotope and then using the isotope fractionation routines in React. However, X1T doesn't seem to have the ability to deal with isotopic fractionation.


Does anybody have any suggestions for how to proceed?





Posted by: Craig Bethke


Hi Craig,


Xt does not currently consider isotope fractionation, but even React's isotope model doesn't account for radioactive decay.


Since 3H is a trace isotope, there's probably an easier way to set up your problem than messing around with the isotope model.

• Set a new basis species to represent tritium, perhaps 3H or 3H1HO.

• Set a new redox species to represent the decayed isotope, 3H(x) or 3H1HO(x).

• Write reactions by which the species reacts or sorbs and include these in your thermo or surface reaction datasets. In this way, you can model not only reaction but fractionation.

• Decouple the tritium from its decay product.

• Set up a redox kinetic rate law for this couple, using one of the custom rate law options. The rate law will actually be the radioactive decay equation. In this way, you model radioactive decay.


Hope this helps,



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