PFAS Linear Sorption has no effect on PFAS transport

[jebenhack]

I am simulating PFAS transport through the unsaturated zone using HYDRUS-1D and want to simulate linear sorption to the air water interface (Nu_AWI = 0; Beta_AWI = 1; Scal_AWI = 1). I have been running some tests and it seems that the model is not accounting for this mechanism at all. I have ran through a number of Kh_AWI values (1e-5 to 0.1 by an order of magnitude change each time) and there is absolutely no change in the transport of mass through the column. As Kh_AWI gets bigger the concentration coming out of the bottom of the column should likely flatten some and the peak should shift forward in time I am guessing but I see absolutely no change (see screen shots below, only one line shows up because they all plot on top of one another). Changing Kd does affect the transport, however. Does anyone know what could be causing this?

[jebenhack]

*AN UPDATE*

I also ran the program with the PFAS module turned off and get the exact same results which would indicate to me that for some reason the PFAS module is not working at all.

[Jirka]

It would be useful to know, which version of HYDRUS you use. There was one version of 5.02, in which we already implemented into the computational module the two-site kinetic sorption to AWI (the standard option in version 5,.03), while GUI did not reflect it yet. However, we replaced this version immediately upon discovering the problem a few weeks ago.

The Reaction parameters should be as follows:

Untitled.png (10.72 KiB) Viewed 24777 times

where FracA is a fraction of instantaneous sites at AWI (and should be zero if you do not consider kinetic sorption), and GammaAWI is the kinetic sorption rate to kinetic sorption sites at AWI.

J.

[ktrobi]

Is version 5.03 necessary for AWI sorption parameters (such as Kh_AWI and Nu_AWI) to work properly? I'm using 5.02 and essentially having the same problem with my 2D PFAS transport model in the unsaturated zone. There appears to be no changes occurring in my transport model when I run through a wide range of Nu_AWI values in particular.

[Jirka]

I am not aware of any other problems (other than the one discuss above) with version 5.02 with respect to PFAS. You should draw the isotherm to see how impactful nu_AWI is. J.