Models which calculate natural attenuation.
Both your RISC model and the Mackay's models at Trent will model natural attenuation. In one sense, that what the fate and transport models do. The conceptual problem comes not in risk assessment, but in risk management. The "do nothing" option is reworded to "natural attenuation." This is often quite appropriate. However the law or public opinion might not allow doing nothing, hence the problem.
See Level I, II, or III models. They are all equilibrium models and don't handle the unsteady state - variations with time. If you choose a large control volume, though, you can approximate natural attenuation over time. The reaction rate, called "degradation rate" in the RISC model also can be used to model natural attenuation.
Which reminds me, we did not do enough with Mackay's models, but let me refer you to them for your future use. Also, using the method of fugacity is very useful and his book has lots of mathematical and chemistry to enable you to use it.