Calcite aggressive

Considering calcite equilibrium a pHc of 7.076 results, that is 0.376 pH units above the measured pH value of 6.7. The permitted deviation of 0.2 is exceeded. Since pH-pHc is negative, the water is calcite aggressive, i.e., it can still dissolve calcite and present a danger for pipe corrosion. Undersaturation can also be determined without calculation of the pHc, because within initial solution calculations in the PHREEQC output, calcite already shows a saturation index of -0.63 (= 23% saturation). 

In the abandoned well S042" with 260 mg/L and N03" with 70 mg/L exceed the drinking water standards of 240 mg/L, and 50 mg/ L respectively. In the well currently used there are problems concerning the calcite aggressiveness, as modeled in chapter 3.1.5.2. Therefore these parameters must be taken into account while modeling the mixing of the two waters. The following values were obtained (Table 50). 

If a proportion of the water from the old well is too high, the mixed water shows sulfate and nitrate concentrations exceeding drinking water standards (bold), if it is too low, the mixture is calcite aggressive (bold), which may lead to pipe corrosion. The optimum ratio is between 40 60 to 60 40, where the water can be discharged to the drinking water distribution net without any further treatment. 

In this connection equilibrium and aggressive carbon dioxide are distinguished. Equilibrium carbon dioxide is its concentration, at which water is saturated with calcite and the dissolution process is absent. At degassing, the solution loses CO and becomes oversaturated with calcite. Then part of calcite precipitates. Contrary to that, excess of CO facilitates the dissolution of additional portions of calcite, and that is why it is called aggressive carbon dioxide. 

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