Geochemical Evolution Along A Flow Path

As long as calcite is present, the pH will be maintained at about 6.5 while calcite in the aquifer matrix is being consumed. This reaction is evident from the higher Ca2+ and HCO concentrations compared with those in background water as represented by MW-36. 

The dissolution of calcite also results in higher calcium concentration and leads to gypsum precipitation  

As a result, sulfate concentrations are reduced, compared with those in upgradient water. 

Groundwater at MW-86 is at the second geochemical phase when calcite in the aquifer is depleted. The fluid pH of the groundwater is reduced to about 4.5 and is buffered by amorphous aluminum hydroxide or kaolinite. 

Precipitation reduces the aluminum and iron concentrations in the fluid to a few milligrams per liter. The fact that there are few observed groundwater pH values between 6.5 and 4.5 supports the hypothesis that solid phases buffer fluid pH at the site. 

---