Understanding km for scaling the AWI in the PFAS module

[OHBratton]

Hello!

I am currently working with the PFAS module in HYDRUS 2D, and I am having some difficulty understanding how km (also called scal_AWI in the reaction parameters) works for scaling the air-water interface. Section 3.1.4. Sorption to the Air-Water Interface in the technical manual states that km can be used to linearly scale Aaw in equation 3.36 of the manual, but it is not obvious to me what this means.

My initial intuition was that this meant equation 3.36 calculates the value for Aaw, and then inputting km simply scales that value so that HYDRUS instead used km*Aaw as the air water interface instead of just Aaw? However, when I put in a base value of km = 1 HYDRUS gives unreasonable results with negative PFAS concentrations. Instead, I seem to get more reasonable PFAS transport retardation when I put in much lower km values such as km = 0.01.

This led me to think that, instead of my above hypothesis, inputting km might be a way of manually inputting a value for dAaw(theta)/dtheta, such that (dAaw(theta)/dtheta) = km. Is this the case?

Does anyone have any clarifications on how this input variable works?

[Jirka]

HYDRUS calculates the air-water interfacial area Aaw from the pressure head-saturation relationship using an equation according to Bradford et al. [2015] (also Bradford and Leij [1997]), as given by eq. 3.36. Since there exist other models of calculating Aaw (see, e.g., Silva et al., JCH, 2022), I wanted to add some flexibility and added this scaling factor km. km simply multiplies Aaw. Note that version 5.02 prints the final Aaw at the output. J.

[OHBratton]

Thank you Jirka! I believe I understand this parameter much better now