Dear all,

I am working on a heavy metal migration problem that considers reaction processes. I am working on a heavy metal migration problem that considers reaction processes. I used the inverse solution module of the HYDRUS base module before adding a specific chemical reaction equation using the HP1 add-on module. And the results show that the fitting results of the nonequilibrium transport model (two-site model) is the best. Now I need to substitute the well-fitted parameters of the two-site model into the HP1 module for the next step.

I notice that only the equilibrium adsorption model can be used in module HP1, and the interface for Reaction Parameter input will not be displayed. I tried to modify the model parameters directly in the selector.in file by changing iNonEqul from 0 to 2 and then modifying the parameters Frac, Kd, Beta. It turns out that when I changed the Frac parameter, the program stopped running. When I modify only Kd and Beta parameters, the program runs. It turns out that when I changed the Frac parameter, the program stopped running. When I modify only Kd and Beta parameters, the program runs.

I have referred to the instructions and examples of HP1 provided by you, but I have not found a way to solve this problem, that is, how to use nonequilibrium transport model (two-site model) in HP1. I have seen an example of using the nonequilibrium transport model in HP1 in an article. I'm very much looking forward to getting a solution to this problem from here. Here is my HP1 file.
Best regards,

Bowen

## The question about how to use nonequilibrium transport model (two-site model) in HP1

### Re: The question about how to use nonequilibrium transport model (two-site model) in HP1

In HP1, the standard HYDRUS solver handles only physical processes, i.e., water flow and solute transport. Reactions (including instantaneous and kinetic sorption, which are involved in the two-site sorption model) are handled by PHREEQC, and thus have to be defined in its inputs (and not using the HYDRUS reaction parameters). J.