Is it possible to correct charge imbalance in acid mine waters?

[Juan Sebastian]

Hello,

I recently read an article indicating that in acid mine/rock drainage (pH = 1-3), it's common to encounter a high percent error in charge balance calculations due to the exclusion of H+ ions or acidity. The authors concluded that charge imbalance in acid waters is model-dependent and applied corrections by incorporating protons estimated through titration and acidity speciation. They used PHREEQC to refine their calculations, but I lack some of their measurements needed to properly estimate the charge imbalance.

Is there a theoretical way in PHREEQC to correct for charge imbalance in very low acid waters without requiring these additional measurements (i.e., acidity titration)?

I have a sample with this issue, yielding a percent error of -66.66% using the WATEQ4F.dat database.



SOLUTION 1
pH 2.000000
pe 10.352000
temp 42.800000
-units mg/L
Na 402.943233
K 1038.472893
Mg 28848.877736
Ca 550.174358
Alkalinity 0.000000
S(6) 246666.67
Cl 236.666667
Al 7648.121721
As 41.176146
B 19.064162
Ba 0.016402
Cd 0.067811
Cu 0.573103
Fe 2011.838793
Li 68.412078
Mn 81.797433
Ni 1.162858
P 32.229078
Pb 1.916603
Rb 0.000000
Si 63.320155
Sr 40.798466
Zn 13.500483
SAVE solution 1
END

Is there a theoretical way to make this correction? It would be very helpful.

The paper is "Nordstrom et al. (2009) Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: IV Acid?sulfate waters. Applied Geochemistry (24), 191-207"

Thanks,

Sebastian

[dlparkhurst]

First, two questions. Is the pH of 2.0 simply an estimate?, and do you trust your SO4-2 analysis? It would be reassuring if you had multiple analyses with similar analytics.

The concentration of SO4-2 is much larger than any other ion, so assuming the cation analyses are reasonably accurate, the only cation to balance the sulfate is H+. You can always charge balance the solution on pH. The result is a pH of about 0.3.

Another issue is the redox state of the Fe. Your current calculation pe generates Fe(2) and Fe(3), but mostly Fe(2). Unless you have measured the individual redox states, I don't know of any way to know the true distribution, but it will make a difference in the saturation indices of Fe(3) minerals. You can investigate the effect by setting the concentration as Fe(3) instead of Fe.

[Juan Sebastian]

Thank you for your response!

Unfortunately, we don?t have replicates of this sample. The pH was measured in the field using a multiparameter instrument, and the SO₄?⁻ concentration might be related to strong evaporation, which of course would also affect the concentrations of the other ions.

On the other hand, I only have total Fe, so I?ll try adjusting the redox state as you mentioned to evaluate discrepancies in the saturation index.

Thank you for your support!

[Leonardo]

Hi Juan,

I remember reading that paper a while back. It talked about an "old way" (or maybe the most used "method) to estimate error, where you'd use a spreadsheet to convert element concentrations to meq/L. The problem is, that calculation usually misses the H⁺ cation. So, below a pH of around 3, you'd expect a negative error because you're leaving out the H⁺. To fix that, you can measure the H⁺ concentration with a titration. Or, you could use PHREEQC, which handles speciation (like Fe?⁺ and Fe?⁺, or H⁺) and that's the better way to go for acidic waters where speciation is more relevant.

But I don't think that's your problem, since you're already using PHREEQC, not doing the error calculation manually. . The issue is that you have an excess of anions relative to cations. Therefore, either the sulfate concentrations are overestimated, or the cation concentrations are underestimated. In the latter case, I doubt any precipitation occurred at a pH of 2 (or lower) in the cation bottle. And if you trust your sulfate and anion concentrations, the most plausible explanation is that the pH measurement is incorrect and as David suggested, back-calculating the pH by charge-balancing the error, would be the way to go.

With that much sulfate and the acidity, you'd expect a lower pH, like David said, based on the charge balance.

I hope it helps everyone reading this post.

Cheers