Simulating EC at Differing water contents

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ansleybrown1337
Posts: 3
Joined: Tue Feb 26, 2019 5:36 pm
Location: USA

Simulating EC at Differing water contents

Post by ansleybrown1337 » Tue Feb 26, 2019 6:32 pm

Hey All,

I'm have ECe data collected from saturated paste extracts (SPE), as well as EC of actual pore water (ECpw) data that was collected simultaneously to observe differences of EC at different moisture contents in gypsiferous soils.

I'm trying to identify inflation of ECe due to excess gypsum dissolution in the preparation the the SPE by using the major ion chemistry model in hydrus. This process occurs in 3 steps:
1. Simulate the EC from the pore water extract at the moisture content of the field at the same time of sample extraction
2. Simulate EC of the SPE at the moisture content measured of the saturated paste in the lab (to check for model accuracy)
3. Simulate the pore water extract at the inflated moisture content of the corresponding SPE sample taken congruently with pore water extraction
(this will be an EC without the effects of excess gypsum dissolution).
4. Compare simulated ECe and simulated ECpw values

However, when I try to simulate a soil with my given ions at saturated paste moisture (e.g 0.7 cm3/cm3) versus a field moisture (e.g. 0.4 cm3/cm3), the EC hardly changes when I run the simulation. I think it has something to do with the way I'm simulating the soil. Here is a basic summary of my simulation, and I've also attached the corresponding scenario files.

Inputs:
Major Ion Chemistry:
Solution Ions: Ca, Mg, Na, K, Total Alkalinity (defined as HCO3 + CO3), SO4, Cl
Adsorption Ions: Ca, Mg, Na, K
Precipitated Ions: CaSO4 (gypsum)
Units: cm and minutes
Total Simulation Time: 10 minutes
Soil Profile Depth: 1 cm
Boundary Conditions: constant flux of 0 cm per hour (i.e. no water passing boundary, but also wet)
Initial condition: Uniform moisture content of SPE or field at time of pore water extraction.

Thank you for any help you might have!
Attachments
Hydrus Gypsum.zip
Contains project folder and .h1d file
(11.43 KiB) Downloaded 22 times

Jirka
Posts: 4712
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Simulating EC at Differing water contents

Post by Jirka » Thu Feb 28, 2019 8:30 pm

I'm not really sure what you are trying to do, but here are few comments anyway.
Concentrations of Solution Ions are given in meq/L.
Concentrations of Adsorption Ions and Precipitated Ions are given in meq/kg.
CEC must be equal to the sum of Concentrations of Adsorption Ions (which it is not in your project).
Solution ions will react with adsorption and precipitation ions based on thermodynamic constants and whatever water contents and bulk densities (to convert all to volume of soil) are specified.
Kinetic precipitation/dissolution is considered only for calcite (not for gypsum) and then you must specify Calcite Surface Area (which is zero in your case).

J.

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