zenolevy

Hi Brian, Thank you- this has been hugely helpful. I will look into how the rate laws I used were derived. I have one final problem that is holding me up on this project. I wanted to incorporate DO and have Fe(OH)3 precipitate instead of Fe(OH)2. I added DO and Fe+++ (I made Fe+++ starting concentrations negligible) to the basis, and suppressed minerals I thought to be unlikely to form under these conditions. However, my model fails to converge when DO is included. When I run the model Fe(OH)3 (ppd) is supersaturated and is swapped for Fe(OH)4-, and the model fails to converge soon after. This should (hopefully) be the last query on this thread, and I thank you for your help thus far. Zeno siderite_firstorder_customratelaws_DO.x1t

Hi Brian, Thank you, this was very helpful. I want to confirm that the custom rate law equation in GWB calculates a dissolution rate in the units of mol/sec, is that correct? I think the problem with my siderite rate law was that I was using the "mass" helper function, instead of the "moles" function. I rewrote my rate law in this form: (9.77e-12) * (moles("Siderite") * mw("Siderite")) * (1 - QoverK) and it seems to be working... However, my calcite rate law does not seem to be working (6.75e-10) * (QoverK - 1) I am still getting a negative and extremely small precipitation rate. When I check "System Parameters" in xtplot I get low decimals fractions for Q/K, Calcite; but when I check "mineral saturation" for calcite I have Q/K's in the 1000's, which makes more sense since I am running very high Ca and alkalinity waters through the model domain, and siderite is dissolving. Any idea why this might be? I am attaching my script. Thanks, Zeno siderite_firstorder_customratelaws.x1t

Hello, I am trying to simulate high-pH leachate (~12) flowing through a reactive porous medium composed of siderite (starting out with 20% by volume). I have rate laws that are in the form of mol/sec: siderite dissolution: (rate_con + 5.55e-22 * activity("H+")) * (mass("siderite")) * 115.86) * (1 - Q/K) with a rate_con of 9.77e-12 mol/g/sec (note - the equation I have specifies the middle term as mass of siderite in grams - so I manually altered it to multiply the mass("siderite") term by the gram formula weight since this command returns mass in terms of moles) and calcite dissolution: rate_con * (Q/K - 1) with a rate_con of 6.75e-10 mol/sec I am trying to send the high Ca and pH leachate through a 30 m permeable reactive barrier and achieve pH control by dissolution of siderite, precip of calcite, and precip of iron hydroxides, which I will assume as equilibrium controlled because this is a very fast reaction. My main problems are my rate constants are not in the form specified by GWB (mol/cm2 sec), and I don't have surface areas in my equations. However, GWB requires inputs for both these variables. I am getting a dissolution rate of 0 for siderite and precipitation rate of -6e-11 for calcite when I run the model, which is not surprising because the form I am using does not appear to be compatible with GWB. I am attaching my x1t script (GWB v. 9). Heeeellllpppp!!!!! Zeno siderite_firstorder.x1t