Issue with Evaporation

[evansmanu]

Dear all,

I am in fix as to how possible I can evaporate my water at a certain percentage say 84%. The problem here is that My rainwater is affected by evaporation and would like to know the effect of evaporation on my mineral phases when performing forward modelling.

My study area has an annual rainfall of 1600 mm and a recharge of about 8% of the average annual rainfall. The rest of the rainfall goes back into the atmosphere and is also washed down into the streams as runoff.

I understand that if you want to include the evaporation phase in a model u can precipitate (H2O(g)) as shown below

EQUILIBRIUM PHASES
H2O(g) 0 0

How should the equilibrium script be if I want to evaporate my water to about 92%

Any comment will be welcomed

[dlparkhurst]

There are several ways that you can produce the effects of evaporation. I am using a 1 mmol solution of NaCl, which, when removing 16% of the water, produces a concentration of 1.19 mmol/kgw.

Given that 1 kg water is 55.5 moles, the first simulation simply removes 16% of the moles of water.

Another alternative is to use MIX with a negative mixing fraction for pure water (SOLUTION 0). This calculation produces a solution with 0.84 kg water.

It may be convenient to end up with approximately 1 L of water, so the next simulation starts with a larger mass of water (~ equal to volume) of 1/0.84 and removes water so that 1 kg water remains, but concentration is again 1.19 mmol/kgw.

The last version is a bit trickier. It sets the activity of water, but in general you will not know the activity of water that corresponds to the evaporation that you want. Here, the activity of water can be calculated in one of the evaporation simulations and used in the final simulation. Otherwise, you would need to iterate on the activity of water to find the evaporation factor that you need. The activity of water approach can be useful if you have very high evaporation factors where the mass of water remaining is difficult to assess because of hydration reactions.

Code: [Select]

SOLUTION 0 Pure water
END
SOLUTION 1
    Na 1
    Cl 1
END
# Remove 16% of mass of water
USE solution 1
REACTION 1
    H2O        1
    -8.88 moles
END
# Negative mix with pure water
MIX 1
    0    -0.16
    1    1
END
# Negative mix and to produce 1 kg water
MIX 1
    0    -0.19047619
    1    1.19047619
USER_PRINT
-start
10 PRINT "Log activity of water: ", LA("H2O")
-end
END
PHASES
H2O_aq
    H2O = H2O
    log_k     0
USE solution 1
EQUILIBRIUM_PHASES 1
    H2O_aq    -1.758e-05 H2O       10
END


[evansmanu]

Thank you very much, Parkhurst. I implemented the first one and it worked for me.
My other concern is that in your code you decided to negate the amount that you want to evaporate. I also do it this way,
REACTION 1
    H2O        -1
    8.88 moles
This is also the case in Example 4--Evaporation and Homogeneous Redox Reactions under the help menu of the PHREEQC interactive program. I hope I am not wrong.

I would also want to know how possible I can plot the sample points on a plotted activity diagram using the USER_GRAPH

Any help from the community will be gladly appreciated

Best
Evans

[dlparkhurst]

The activity of water changes very little in your sample calculation. Here is a plot showing concentration increase in Na and activity change as water is incrementally removed from the solution. I am not sure what plot you have in mind, but perhaps you can modify this example.

Code: [Select]

SOLUTION 0 Pure water
END
SOLUTION 1
    Na 1
    Cl 1
# Remove 16% of mass of water
REACTION 1
    H2O        1
    -8.88 moles in 10 steps
USER_GRAPH 1
    -headings               ah2o Na a(H2O)
    -axis_titles            "H2O removed, moles" "Molality" "Activity of water"
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X -RXN
20 GRAPH_Y TOT("Na")
30 GRAPH_SY ACT("H2O")
  -end
    -active                 true
END


[evansmanu]

Thanks for your prompt reply. However, I still want to know if this reaction script is right.
REACTION 1
    H2O        -1
    8.88 moles
In your case, you negated the amount (-8.88 moles) while in my case I negated the (-1)

[dlparkhurst]

The number of moles added is the product of the stoichiometry of the formula, the coefficient following the formula, and the moles added. Hydrogen in your example would be 2 * -1 * 8.88. The result is the same as 2 * 1 * -8.88.

[evansmanu]

Dear PhreeqC users,

I am trying to simulate the development of the groundwater with rainwater as the initial solution. I also try to determine the development along a demarcated groundwater flow path. I understand that for inverse modeling one needs to know the mineralogical composition of the underlying aquifer materials. Here in my studies, the XRD measurements performed have identified the following mineral phases as primary minerals, including chlorite, biotite, muscovite and albite. There was also calcite and pyrite found in the background.

The XRD measurements did not reveal the presence of gypsum and halite, which control SO4 and Cl in most inverse models. The concentrations of SO4 in groundwater are quite low and are considered to be from the atmosphere, as is Cl. In formulating the inverse model and reading other solutions prescribed on the platform, I used the following phase boundary conditions by letting primary minerals such as chlorite, albite, muscovite, biotite dissolve. However, I don't have a source and sink for SO4 and Cl and that makes my model problematic.

I've still followed the suggestions from here on the forum to no avail. I've attached the script for any help. In particular, how to include the atmospheric contribution for Cl and SO4 without including gypsum and halite as phases.

Thanks for your support

[dlparkhurst]

I always say that inverse modeling is more difficult than it appears. You are trying to match two conservative species with one evaporation factor, which usually does not work; the conservative species seldom demonstrate exactly the same proportional trends.

I don't think I would adjust Cl for electrical balance if you are relying on evaporation to account for the Cl concentration. You may be attributing analytical error of other analytes to Cl. The inverse modeling algorithm considers the full range of potential analytical errors as specified by the uncertainties.

It is good to have a number of analyses that demonstrate consistent trends in concentrations. With just 4 analyses, it is difficult to establish the trends. I don't think SOLUTION 3 evolves from SOLUTION 2; too many concentrations decrease going from 2 to 3 (ionic strength, pH, Ca, Mg, S, C). Sometimes it is easier to start with the same water (rainwater) for each model, 1-2, 1-3, and 1-4. I think you will still have trouble because the sulfate of solution 3 is considerably less than your rainwater. Assuming you do not have reducing conditions that could precipitate sulfide, you don't really have any way to account for a decrease in sulfate.

Assuming you can determine a sequence of analyses that demonstrate the chemical evolution of water in your system (not easily done), and that sulfate is still problematic, I would just add gypsum and attribute the result to analytical and spatial uncertainty.

I think you will need to add some phases for the alumino-silicate part of your system. You will need secondary phases like silica, gibbsite, kaolinite, or clays to be able to conserve Al and Si predominantly in solids. (One of your Al concentrations looks unrealistically large.)

[MichaelZ20]

The issue may be also high variability in rain composition. I saw variation coefficients of tens percent, sometimes even reaching 100%. Probably, based on the analysis of your data it is worth increasing Cl and SO4 uncertainties and even general uncertainty in rain composition.

[evansmanu]

Dear David and Michael,

Thanks very much for your comments. I have considered using the same initial solution (rainwater) and matching it with the three solutions as inverse model 1 (1 - 2) model 2 (1 - 3), model 3 (1 - 4). I also did not include the halide phase but rather precipitated the rainwater. This gave me a lot of problems until I increased the uncertainty in the Cl and SO4 to 0.6 and allowed the rest with an 0.005 uncertainty.
Still, I had a problem with SO4 (ERROR: inequality not satisfied for S(6) eps+, 1.516046e-04).

Is it correct to add a mineral phase that does not exist in the area of study all the name of achieving a solution?
is it also worth considering the Pure water solution for the entire inverse model?

I have attached the current modified input file for your consideration

[dlparkhurst]

If Cl and SO4 do not change proportionally, you will have trouble trying to model both with just evaporation. Increasing the uncertainties allows each to vary more independently.

I look at inverse modeling as an attempt to determine a consistent trend in geochemical evolution. If you think Cl and SO4 are essentially conservative in your system, then I would probably attribute the variations as different starting conditions. Hopefully, Cl and SO4 are not the key reactants in your system. Garrels and Christ, one of the original inverse models (see example in manual), added NaCl and Gypsum to account for Cl and SO4, but their emphasis was on the other mineral reactions and the difference between a shorter and a longer flow path.

[evansmanu]

Dear David,

In your previous post, you directed me to review the work of Garrels and Christ in the manual I reviewed and I did not find these authors. Instead, I found an example in the Phreeqc manual (Example 16 - Inverse Modeling of Sierra Spring Waters) by Garrels and Mackenzie (1967). I would be grateful if you could point to the inverse modeling of Garrels and Christ in the Phreeqc manual.

Best, Evans

[dlparkhurst]

My bad. I meant Garrels and Mackenzie (Garrels and Christ is a text).

[evansmanu]

Thanks very much for the clarification