sulfate reduction rate calculation from given pore water sulfate profile

[Gst932364]

Hi, I started studying Phreeqc with the aim of getting the most out of my pore water datasets.

Es. I have a SO4 concentration profile for a sediment core and I want to calculate the sulfate reduction rate. Given sedimentation rate of 0.01 cm/yr

How would you do that on Phreeqc?
Example:

Depth (cm)   SO4 (ppm)
10   2800
20   2600
30   2400
40   2200
50   1900
60   1400
70   900
80   400
90   0

thank you for your help

[dlparkhurst]

The sedimentation rate is pretty slow relative to diffusion and reaction, so I think you can ignore sedimentation. You may need to consider the change in porosity over a meter, but I'm going to ignore it.

A simple model is diffusion with first-order reaction of sulfate, as given below. Eventually, it reaches steady state. However, your sulfate profile is more linear than exponential, so you will have to consider why. Does steady state apply? Is the reactive matter evenly distributed or are there layers of reactive matter?

SOLUTION 0-20  SEAWATER
        units   ppm
        pH      8.22
        pe      8.451
        temp    25.0
        Ca              412.3
        Mg              1291.8
        Na              10768.0
        K               399.1
        Cl              19353.0
        Alkalinity      141.682 as HCO3
        S(6)            2712.0
        O(0)            1.0     O2(g) -0.7
END
KINETICS 1-20
CH2O
    -formula  CH2O  2
    -m0       1
    -tol      1e-008
-steps       3.15e7
END
RATES
CH2O
10 m_so4 = TOT("S(6)")
20 k = .3/(3.17e7) # per year
30 rate = k*m_so4
40 moles = rate * TIME
50 SAVE moles
END
TRANSPORT
    -cells                 20
    -shifts                4
    -time_step             25 y 1 # seconds
    -flow_direction        diffusion_only
    -boundary_conditions   constant closed
    -lengths               0.1
    -punch_frequency       1
USER_GRAPH 1
    -axis_titles            "Depth, m" "Sulfate, mg/kgw" ""
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 GRAPH_X dist
20 GRAPH_Y TOT("S(6)")*GFW("SO4")*1000
  -end

[Gst932364]

Thank you so much for your reply dlparkhurst, I will run some tests and adjust the script to my case studies.