Halite aquifer and Brine

[DFhidro]

Hi!
Could anyone help me? I have the following brine concentration:

SOLUTION 1
    pH        6.638
    units     mg/l
    density   1.226
    Ca        270.2
    S(6)      26914.08
    Cl        180997.65
    Na        65260

If I calculate the SI of Halite with those concentrations, I get -0.58. If I sampled brine from a halite aquifer, I would expect my SI to be 0 (or near 0). How should I interpret that?

- Is the sampled brine out of equilibrium with the host aquifer, causing the SI to be negative now?
- Is it possible for the brine to be undersaturated with respect to halite if it is in contact with a halite aquifer?


Should I use Equilibrium_Phase to get the "Real" Brine concentration assuming a Halite aquifer? (Halite SI --> 0)

Regards,

[dlparkhurst]

I would like to see more analyses, but my first guess is that your analysis is in error. There is a 33% charge imbalance, which can most likely be assigned only to the major ions Cl and Na, and the calculated density is low compared to the measured density. If you have specific conductance, you can compare it to the calculated specific conductance (pitzer.dat); my guess is that the calculated specific conductance is low relative to measured. If you charge balance with Na, the concentration of Na doubles from 3.2 to 6.4; the resulting solution is in equilibrium with halite; and the calculated density is close to measured.

Code: [Select]

SOLUTION 1
    pH        6.638
    units     mg/l
    density   1.226 calc
    Ca        270.2
    S(6)      26914.08
    Cl        180997.65
    Na        65260 charge

[DFhidro]

I didn't add the rest of the ions in my sample because I want to compare only the behavior of Na and Cl with Halite. Is it necessary to include all the ions? Could you still compare the precipitation/dissolution result of halite if I don't include all the ions I have?

[dlparkhurst]

You should probably include the majors--Ca, Mg, Na, K, Cl, SO4, and Alkalinity, but I doubt the missing ions would compensate for the charge imbalance, and I don't think it will make much difference in your SI.