Output interpretation

[Kellen]

Hello,

I have used phreeqc to calculate saturation indices for calcite and dolomite along a groundwater flowpath in a layered aquifer. I used default phreeqc.dat database for low ionic strength water. All outputs show saturation with respect to calcite and dolomite, which indicates equilibrium with the mineral phase. However, in the chemical analyses there are always considerably higher calcium and magnesium concentrations in one layer than the other. So I am not really sure how to interpret this, because we would expect no net dissolution or precipitation if the groundwater is saturated with respect to the minerals. My thinking is there might be some reverse ion exchange occuring in the layer with higher cation exchange capacity?

Grateful for any help. Thanks.

[dlparkhurst]

SI=0 really means that the minerals are reacting fast enough to maintain equilibrium. The classic example is dedolomitization, where gypsum dissolution drives dissolution of dolomite and precipitation of calcite. Calcite and dolomite remain close to equilibrium, while irreversible dissolution of gypsum drives the reaction. Certainly other reactions can have effects on the dissolution or precipitation of calcite: ion exchange, organic decomposition, gas exchange, and many others.