3
$\begingroup$

I have a problem with my project and I was hoping I would find some clarification.

I did some modeling with phreeqc to calculate saturation indices and found that calcite was saturated in an engineered aquifer. The time of sampling of groundwater was roughly 1 year after construction was completed. Gypsum is present in the aquifer, so there are high calcium concentrations. However, I don't think the mineral calcite is actually forming insitu. I think the concentrations of calcium and carbonate are just at the right combination to be in equilibrium with calcite, so it is not actually contributing to calcium concentrations. I should add that the pH of the aquifer is between 7-8. I just don't think any sufficient quantities of calcite would form in a system with lots of water moving through it.

Is this a correct interpretation?

$\endgroup$
1
$\begingroup$

Why don't you think calcite is forming in-situ? The saturation index does not guarantee that calcite has precipitated (or dissolved) but there are a number of reasons to think it has. First, calcite does not have a large kinetic barrier to precipitation. The kinetics are fairly rapid. Rapid is a relative term but the formation of calcite in caliche and speleothems indicates that it does not take much for it to precipitate. Secondly, are you suggesting that it is merely coincidental that the SI indicates saturation? Do you have a number of water samples from different locations that are all in equilibrium with calcite? How would that occur without reacting to form or dissolve calcite? What else could be buffering the pH to the value at equilibrium between your solution and calcite?

Have you looked at the gypsum SI? Does it remain undersaturated? If so you would expect it to be dissolving which would move calcite towards supersaturation.

One possibility is that you in equilibrium with both gypsum and calcite. In that case, they will fix the ion concentrations in solution and pH so you get no further precipitation (assuming the system is closed to carbon dioxide).

$\endgroup$
  • $\begingroup$ Thanks so much for your reply. The modeling work tells me that calcite is favored to be present, however I am finding that in one region of the aquifer calcium concentrations are consistently higher than other regions. All regions are saturated with respect to calcite. Saturation indicates no net dissolution or precipitation, so where is the additional calcium coming from in the region with higher calcium concentrations? I propose it is ion exchange because there is some clay in the area of the aquifer with higher calcium. Also, bicarbonate is always higher in this region.. $\endgroup$ – user378381 Jan 4 '18 at 1:20
  • $\begingroup$ There is a region with higher Ca and higher bicarbonate but it is still at saturation with calcite. So you need to figure out what else is different so calcite remains saturated at these concentrations. Check that the solution chemistry hasn't changed enough to make a bid difference in the Ca activity coefficient. Look at the carbonate speciation modelled. CO3 activity will have to be lower than other areas to be at calcite saturation. The only way for this to happen is to have a drop in pH/increase in Pco2. Consider if you should be modelling the system as open or closed to carbon dioxide. $\endgroup$ – haresfur Jan 4 '18 at 23:55
  • $\begingroup$ You are right. I did some checking and found that pH is indeed lower in the region with higher Ca, so that is why it is still at saturation. It is 1 pH unit lower compared to the other regions, which are typically around 8. Thanks again $\endgroup$ – user378381 Jan 5 '18 at 0:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.