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fluid mixing, pickup command

Tom Meuzelaar

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[admin notice: the below is from the former GWB users group email distribution list. This message was originally posted 6/6/2003]


Posted by: Jean Morrison


Hi all,

I have been using React to model an acid mine drainage mixing zone. I used

input files that titrated a neutral stream into an AMD impacted stream. I

then wanted to model the outgassing of CO2 from the water into the

atmosphere by sliding the fugacity of CO2. I wanted to see how the system

reacted when sliding the fugacity both WHILE mixing and AFTER mixing was



I noticed something strange when I attempted to pickup the system and then

slide log f CO2 to simulate CO2 equilibrating with the atmosphere

downstream from the mixing zone. When I pickup the system some elemental

compositions and aqueous species concentrations at the start of the run

(step 0) are different from endpoint of the previous run (last step of the

output). Specifically, the concentrations of As(OH)4- and Cu+ when the

system are picked up is greater than the initial concentrations in the

system prior to mixing. The pH is increasing and sorption is occurring

during mixing so one would expect the concentrations to decrease. It is

especially surprising that the concentration of these two are greater than

the total amount started with. I am wondering if this is a rounding error

because the concentrations are quite small or is there something else I am

not seeing.


I have attached the initial input file used for mixing (initial.dat).

I then simply picked up the system and slid log f CO2 to ?2.6 (a value I

calculated from data collected in the real system).


For comparison, I then made a new input file manually entering the system

as it was appeared in the last step of the output file from the initial run



I've also attached:

The input file for mixing and sliding simultaneously

Graphs (data exported from GTplot) in an Excel file that compare the

differences between these model runs.


(See attached file: Initial.txt)(See attached file: manual.txt)(See

attached file: mix and slide.txt)(See attached file: CuAs_comparisons.xls)


I would appreciate any insight in explaining why there is a discrepancy in

the As(OH)4- and Cu+ concentrations when the pickup system command is used.






Posted by: Craig Bethke

Hi Jean,


We looked into this and the problem is that after picking up

the results of the mixing path, two basis entries (Cu+ and

As(OH)4-, cuprous copper and arsenite) are present in essentially

zero concentration. The reason for this is that the solution

is oxidizing and the copper is present almost exclusively as

cupric species, and the arsenic as arsenate.


To get around this problem, I recommend that you decouple Cu++

and AsO4--- when you set up your run.


Thanks for the good test input; we will use it to try to make

the pickup facility a little more robust.


Hope this helps,


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