Solute transport as function of soil water content

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andrew_m
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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
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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
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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 70 times

andrew_m
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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 69 times

Jirka
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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.

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