A "mixrun" problem

[Hara_Kyp]

Dear everyone,

I am facing a problem regarding the reactive transport modeling. I want to model a RATES reaction for 12 cement minerals over a time period of 1000 years. The model responds quickly when using only KINETICS. When I am trying to add also TRANSPORT keyword things become complicated. I get more than 10000 "mixruns" and the model runs for over 3 hours. I have tried to use less cells and smaller time steps, but the problem persists. Do anyone know what the "mixrun" keyword means and how I can fix this problem?
I present the TRANSPORT code I use. 

TRANSPORT
    -cells                 10
    -shifts                10
    -time_step             31536000 # seconds
    -flow_direction        diffusion_only
    -boundary_conditions   constant flux
    -lengths               10*0.00015
    -diffusion_coefficient 3e-09
    -thermal_diffusion     2   3e-09
    -print_cells           1 10
    -punch_cells           1 10
    -multi_d               false

Thanks in advance

[dlparkhurst]

I think that is simply a result of the explicit numerical method used in PHREEQC. The number of mixruns depends on the diffusion coefficient (~1e-9) times the time interval (1000 yr * 3.15e7 s/yr) divided by the square of the cell length. Doubling the cell length will reduce the execution time by a factor of 4.

[Hara_Kyp]

Thanks a lot for your advice. I'll try it to see how it goes.

[dlparkhurst]

If the calculations are too long, another option is to use the code CrunchFlow.