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[OLD] Leachate through a clay layer

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From: Katie Aguilar

Subject: Leachate through a clay layer

I have written the users group before for this same problem, but have moved on to a new avenue/ barrier to completion. As a refresher: I am modeling leachate through a clay layer. The contaminants of concern are metals, there are no organics in this problem. I am trying to model the natural attenuation in clay as the leachate runs through a set volume of clay with a CEC = 0.6 eq/g. I eventually want to include the ion exhange script and FeOH script to allow for ion exchange and adsorption. However, right now I am reconciling the basic model. I have brought my leachate to equilibrium, and have added the soil minerals. We are not sure what is out there exactly, but are assuming (reasonable, informed assumptions here) that kaolinite, quartz and goethite exist as soil minerals.

Two problems:

1) When I run the system, it indicated a drop in sodium in the leachate, even when I add ion exchange. I know that the level of Na in my leachate should increase as the sodium ions in the soil are replaced with ions in the leachate. Does anyone have experience with this and have a suggestion?

2) I have quartz in my system. When I run the model, it has about 50 moles of quartz precipitating out. I can not suppress the species to prevent this from happening as it is in my basis. I know, though, that my time line does not allow for the generation of quartz in my system. Does anyone have experience at surpressing the precipitation of minerals that are in the basis? Attached is the script. I am using the thermodynamic database thermo_minteq_gwb4.

I am running GWB (React) Release 4.0.2 on Windows 2000, and have attached the script. I am also using the default thermo dataset.

Katie

# React script, saved Fri Jan 16 2004 by kaguilar

data = "C:Program FilesGwbGtdatathermo_minteq_gwb4.dat" verify

temperature = 25

swap PbCrO4 for Pb++

swap Calcite for CO3--

swap Cerargyrite for Ag+

swap Barite for Ba++

swap Kaolinite for Al+++

swap Goethite for Fe+++

swap Quartz for H4SiO4

1.00000004 kg free H2O

total mol AsO4--- = 8.07563367e-6

total mol SeO4-- = 2.52051463e-6

free mol PbCrO4 = 1.93505021e-6

total mol Hg(OH)2 = 2.49264669e-9

total mol H+ = .000811677887

free mol Calcite = 3.76854276e-5

total mol CrO4-- = 2.11436482e-5

total mol Mg++ = .00390753537

free mol Cerargyrite = 3.79105577e-6

free mol Barite = 1.77546743e-6

total mol O2(aq) = .00015625

total mol Cl- = .000814193318

balance on Na+

total mol Na+ = .0223492309

total mol K+ = .00749322285

total mol SO4-- = .0288026584

total mol Ca++ = .0107906579

total mol Cd++ = 1.08541505e-6

free kg Kaolinite = 2.61

free kg Goethite = .125

free kg Quartz = 3.69

react 2.61 kg of Kaolinite

react .125 gram of Goethite

react 3.69 gram of Quartz

itmax0 = 5e4

From: Craig Bethke

Subject: Re: Leachate through a clay layer

Regarding your silica question, you need to determine whether or not your initial system is in equilibrium with quartz. If it is, but the mineral is unlikely to form to any significant extent over the reaction path, then on the Reactants pane set quartz as a kinetic mineral, using a small or zero-valued rate constant. If it is not, set the initial silica concentration by swapping a less stable mineral, such as tridymite, into the basis in place of aqueous silica. Or, constrain the basis using a measurement of silica concentration. Then you can suppress quartz.

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