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In an engineered wetland, there is groundwater flow from an aquifer with a chemical composition that is rich in magnesium, calcium and sulfate. We are finding that the aquifer is consistently saturated with respect to dolomite and calcite. Similarily, the wetland is also saturated and slightly oversaturated with respect to those minerals, however concentrations of both mg and ca are higher in the wetland than the aquifer. We were expecting concentrations to be similar given saturation with respect to the carbonate minerals. So we are hypothesizing the following:

Alkalinity is much higher in the wetland than in the aquifer, probably due to more extensive CO2 production. Given there is more CO2 in this region, this would enhance the dissolution of dolomite and calcite and therefore cause the increase in both magneaium and calcium. However, we are not sure if those minerals can form in a short period? The system was constructed about one year ago. If the minerals are unlikely to be present and it is by chance that groundwater is oversaturated, then what can be causing the increase in both magnesium and calcium? I know cation exchange should remove those ions from solution and not the other way around.

Any help is greatly appreciated.

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