Solute transport as function of soil water content

A discussion forum for Hydrus-1D users.
Post Reply
andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Solute transport as function of soil water content

Post by andrew_m » Thu Jun 13, 2019 11:01 pm

I'm attempting to model the transport of a surfactant solute in a soil during wetting. I believe that as the soil dries out, the solute retreats into pores and then when the soil becomes wet again, the solute starts to diffuse back out of the pores in a nonequilibrium process. However, I am unable to recreate this scenario in Hydrus 1D because I am unable to control the solute behavior as a function of soil water content. Is there an approach in Hydrus 1D that would make this possible? Thanks!

Jirka
Posts: 4726
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Solute transport as function of soil water content

Post by Jirka » Thu Jun 13, 2019 11:24 pm

I guess you will have to use some of the nonequilibrium models implemented into HYDRUS, such as the MIM dual-porosity models. J.

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Fri Jun 14, 2019 12:04 am

Thanks so much for replying so quickly!!

Thanks also for the advice. I have tried the MIM approach previously, but the solute seems to transfer from the immobile to the mobile phase at the same rate (Alpha) regardless of Theta. I've attached images that show an example where a very dry soil is wetted from the bottom and the solute, which is initially entirely in the immobile phase, transfers to the mobile phase at a constant rate regardless of Theta. In this example ThImob is 0.02 and the initial Theta is 0.025. Do you think there is a different approach that would be useful?
Attachments
Water Content.JPG
Water Content.JPG (49.33 KiB) Viewed 159 times

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Fri Jun 14, 2019 12:05 am

Oops, here is the immobile phase concentration profile for the example
Immobile phase concentration.JPG
Immobile phase concentration.JPG (47.92 KiB) Viewed 158 times

Jirka
Posts: 4726
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Solute transport as function of soil water content

Post by Jirka » Fri Jun 14, 2019 12:50 am

You should study the mathematical description of the model. The mass transfer is driven by a concentration gradient, i.e., is equal to Gamma_s=alpha*(c_m-c_im), see eq. 3.32. While the water content of neither domain is directly involved in the mass transfer of solute between the two domains (which is driven by a concentration gradient), the water content of both domains should clearly affect the overall process, since the increase or decrease of concentration in either phase will depend on water contents. If you remove Gamma_s from a small domain, the concentration will change much faster than if you remove Gamma_s from a large domain, and vice versa for addition. J.

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Wed Aug 14, 2019 2:45 am

Thank you again for all the help. It has been fascinating to learn more about Hydrus.

However, I'm still struggling to model my experiment. I packed a very dry soil, with a previously sorbed chemical, into a vertically-oriented column and pumped water up through the column. I would like to model solute desorption as a nonequilibrium process (ideally chemical nonequilibrium), where the solute desorbs only after the wetting front moves past. Below is an image of the experimental progression.

I believe what I'm asking is: is it possible to have no desorption until theta changes from the initial value? Also, are there any examples of similar systems being modeled?

Thanks

Slide1.jpeg
Slide1.jpeg (57.18 KiB) Viewed 34 times

Jirka
Posts: 4726
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Solute transport as function of soil water content

Post by Jirka » Wed Aug 14, 2019 3:39 pm

Note that for non-equilibrium solute transport model, you need to specify the initial condition for the liquid phase concentration and for the kinetically sorbed concentration (concentration on Type-2 sites). While the former one is an equilibrium concentration and thus the equilibrium sorbed concentration will be assigned automatically instantaneously as well, the latter one is kinetic and thus the release of solute from these sites will depend on the mass transfer coefficient, and indirectly on water content. J.

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Wed Aug 14, 2019 4:44 pm

Thanks again.

So, since the water content is always nonzero, the mass transfer coefficient is also always nonzero? Is it possible to change that? Can I set a minimum water content for mass transfer to occur?

I understand that sounds odd and unrealistic, but since my soil has had a long time to dry and equilibrate, I don't believe significant mass transfer is occurring prior to the arrival of the wetting front.

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Wed Aug 14, 2019 5:06 pm

Sorry, the second sentence should read "So, since the water content is always nonzero, mass transfer is also always nonzero?"

Jirka
Posts: 4726
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Solute transport as function of soil water content

Post by Jirka » Wed Aug 14, 2019 6:17 pm

No. The mass transfer is nonzero only when there is non-equilibrium, i.e., when the liquid phase is not in equilibrium with the solid phase (see eq. 3.9). When the soil is dry, there is only very little solute mass in the liquid phase (at equilibrium) and the mass transfer can start only once water content increases and present solute dilutes, introducing nonequilibrium.

You can make any changes you want in the code, which is posted on the HYDRUS website.

J.

andrew_m
Posts: 7
Joined: Fri Jun 07, 2019 3:28 pm
Location: USA

Re: Solute transport as function of soil water content

Post by andrew_m » Wed Aug 14, 2019 8:47 pm

Thanks, that makes sense. At lower water contents, as the solute desorbs the liquid phase solute concentration quickly rises, causing the system to approach equilibrium and the mass transfer rate decreases.

That said, I'm still incapable of reproducing my experimental system. In my system, at t=0, solute-free water enters a dry soil from the bottom and a wetting front propagates upwards. I would like the solute to start desorbing from Type 2 sites only after the wetting front passes by. Is that possible in HYDRUS?

Jirka
Posts: 4726
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Solute transport as function of soil water content

Post by Jirka » Wed Aug 14, 2019 10:56 pm

That's exactly what happens. There is no mass transfer if you have initially an equilibrium. Once water content increases, it dilutes concentration and mass transfer starts. J.

Post Reply