diffusion speed and diffusion coefficient

[Mr.white]

I have a question: if I define the length of each grid and the diffusion time in the transport settings, does that determine the diffusion speed of water? Then what is the diffusion coefficient defined in transport? I would like to understand the difference between diffusion speed and diffusion coefficient . Thank you.

[dlparkhurst]

You'll have to look at some texts to get a better idea of the diffusion process. The flux of a constituent through an area depends on the gradient of the concentration and the diffusion coefficient. A larger gradient increases the flux, and a constituent with a larger diffusion coefficient, but the same gradient will have a larger flux.

[Mr.white]

Okay, thank you very much. I just checked some textbooks and realized that I should be using advection to simulate the process of the water sample moving from the wellhead to the well bottom. I set up a 3000m long cell. Does that mean I can control the rate by changing the time?
SOLUTION 0
    temp      25
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 801 as HCO3
    C(4)      639
    Ca        12
    Cl        11177
    K         2407.2
    Mg        7
    Na        5616.8
    S(6)      17
    -water    1 # kg
SOLUTION 1
    temp      70
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Cl        0.0001
    K         0.0001
    Mg        0.0001
    Na        0.0001
    -water    0.1 # kg
TRANSPORT
    -time_step             864000 # seconds = 10 days
    -lengths               3000

[dlparkhurst]

The advective velocity of water in TRANSPORT is the distance between two adjacent cell nodes divided by the time step.