Effect of groundwater temperature increase on dissolved oxygen availability

[bappa.tester@gmail.com]

I have measured the physico-chemical parameters of a groundwater sample at temperature 10°C. I am trying to see the effect of temperature changes on the availability of dissolved oxygen (DO).

I have added the available parameters along with solution temperature = 10°C. Added REACTION_TEMPERATURE to 11, 12, 13, ......, 99, 100°C.

Run the simulation. Checked the molarity value printed in the output section against each temperature. It shows that amount of DO dissolved oxygen increases with temperature increase. I am expecting an inverse relationship as demonstrated here

https://www.phionics.com/2021/01/07/factors-affecting-dissolved-oxygen/#:~:text=Correction%20Factor%20Table-,How%20Temperature%20Affects%20Dissolved%20Oxygen,water%20molecules%20to%20gain%20energy.

I have considered molarity value of O2.

[dlparkhurst]

I'm not sure how you did the calculation. Here is a script that considers a 1 mmol/kgw solution of NaCl in equilibrium with the atmosphere at temperatures between 0 and 100 C. Results are plotted as mg O2(aq) per kgw.

Code: [Select]

SOLUTION 1
Na 1
Cl 1
END
USE solution 1
REACTION_TEMPERATURE
0 100 in 101
EQUILIBRIUM_PHASES
O2(g) -0.7
USER_GRAPH 1
    -headings               tc O2(aq)
    -axis_titles            "Celsius" "O2(aq), mg/kgw" ""
    -chart_title            "1 mmol NaCl in equilibrium with atmospheric P(O2)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TC
20 GRAPH_Y MOL("O2")*GFW("O2")*1000
  -end
    -active                 true
END


[bappa.tester@gmail.com]

Thank you @dlparkhurst. I have used PHREEQCI with the below inputs. It does not give a proper trend either -

Code: [Select]

USER_GRAPH 1
    -headings               Availability of DO with temperature increase
    -axis_titles            "Celsius" "O2(aq), mg/kgw" ""
    -chart_title            "tc O2(aq)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TC
20 GRAPH_Y MOL("O2")*GFW("O2")*1000
  -end
    -active                 true
EQUILIBRIUM_PHASES 1
    O2(g)     0 -0.7
REACTION_TEMPERATURE 1
    20 100 in 81 steps
SOLUTION 1
    temp      19
    pH        8.2
    pe        4
    redox     pe
    units     mg/l
    density   1
    C(4)      823
    Ca        38
    Cl        1025
    K         23
    Mg        30
    Na        1205
    S(6)      18
    -water    1 # kg
TITLE 771057

END



[dlparkhurst]

This definition is incorrect.

Code: [Select]

EQUILIBRIUM_PHASES 1
    O2(g)     0 -0.7

Should probably be

Code: [Select]

EQUILIBRIUM_PHASES 1
    O2(g)     -0.7 10


[bappa.tester@gmail.com]

Thank you @dlparkhurst.

Could you please let me know the reason of considering the equilibrium phases of O2 here. The sample is a groundwater sample collected from 250 m (below ground level) - not sure whether it is in equilibrium with O2 or not. I have measured the physico-chemical parameters of the water samples (pH, EC, Eh, Temp., alkalinity, Ca, Na, Ca, Mg, Fe, Mn, HCO3, Cl ......) and trying to understand the effect of temperature change (by 1°C from its initial temperature 10°C till 60°C) on dissolved oxygen and other minerals and gases in groundwater.

Could you please let me from where I can get the values of equilibrium phases for different minerals (such as calcite, dolomite, Fe(OH)3 etc.), gases (CO2, O2 etc.).

I appreciate your help on this.

[dlparkhurst]

Sorry, I misunderstood. You seemed to be expecting O2 solubility to decrease with temperature, which I interpreted as expecting the normal response of water in equilibrium with the atmosphere.

If you are working with a closed system at depth, the molality of O2 will probably not change very much with temperature. The saturation indices will change, both because the aqueous speciation will change and because of the temperature dependence of the log Ks for the minerals.

The saturation indices are in the output section Saturation Indices, or they can be printed with SELECTED_OUTOUT or USER_PUNCH with the Basic function SI.

[bappa.tester@gmail.com]

Hello @dlparkhurst,

Thank you for replying.

As I am increasing groundwater temperature from 10°C to 60°C gradually (by 1°C), I am expecting a change in molality of dissolved oxygen. The basic understanding is the amount of dissolved oxygen would be decreased with increased temperature. I am trying to derived the same using PHREEQC modeling with the physico-chemical data of the water sample.

[dlparkhurst]

In a closed system, that is without O2(g) in EQUILIBRIUM_PHASES, the concentration of O2 will not change unless some redox reactions occur. You don't expect the concentration of Cl- to change, it is the same with O2. If O2 is stable in your initial solution, then it will probably stay that way. As I said some redox reactions are possible, but not with the dissolved constituents you present in your SOLUTION.

Where is it that you think the DO will go?

[bappa.tester@gmail.com]

Hello @dlparkhurst,

Concentration of Cl- in groundwater also may get changed with temperature increase due to water-rock interactions. I could see ferric and ferrous ions, Mn, NO2 and NO3, Sulfate ions in my groundwater samples along with microbial contaminants. So radox reactions are also possible.

Can I identify the possible radox reactions in the groundwater system using PHREEQC?

[dlparkhurst]

Now you are asking a different question. Dissolved oxygen is absent in most groundwater systems because it reacts with organic matter or reduced minerals like pyrite or Fe(2)-bearing aluminosilicates. There are a few oxic groundwater systems that lack any reducing compounds, like the Central Oklahoma Aquifer that I worked on many years ago.

To determine the reactions in an aquifer, sometimes you can determine reactions from the changes in water composition as you move downgradient in an aquifer. Mineralogy is also important information; you can develop hypotheses from the minerals and mineral textures in the aquifer.

You can use PHREEQC to calculate saturation indices, and develop hypotheses based on whether minerals should dissolve, precipitate, or are reacting fast enough to maintain near-zero saturation indices.