THE GEOCHEMISTRY OF NATURAL WATERS

1
THE GEOCHEMISTRY OF NATURAL WATERS

• THE CARBONATE SYSTEM
• CHAPTER 3 - Kehew (2001)
• The common-ion effect and incongruent dissolution

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LEARNING OBJECTIVES

• Understand sources of CO2 in natural waters.
• Define and understand alkalinity.
• Learn to calculate the solubility of carbonate
  minerals such as calcite.
• Understand the common-ion effect.
• Become familiar with the concept of incongruent
  dissolution.
• Apply these concepts to some case studies.

3
THE COMMON-ION EFFECT - I

• Calcite solubility is governed by the reaction
• CaCO3(s) ? Ca2 CO32- (1)
• Suppose we added a second compound containing
  carbonate, and this compound is more soluble than
  calcite, e.g., Na2CO3. This compound will
  dissolve according to
• Na2CO3(s) ? 2Na CO32- (2)
• To the extent that reaction (2) proceeds to the
  right, by Le Chatliers principle, this will
  force reaction (1) to the left, precipitating
  calcite.

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THE COMMON-ION EFFECT - II

• The effect of adding sodium carbonate to the
  solution can be demonstrated by adjusting the
  charge-balance expression to be
• By repeating the derivation of the equations on a
  previous slide using this charge-balance
  expression we obtain
• Increasing Na concentration leads to decreased
  Ca2 concentration.

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Figure 3-14 from Kehew (2001). Curves showing Ca
concentration in equilibrium with calcite as
increasing amounts of NaHCO3 are added to
solution. Addition of the common ion (HCO3-) in
the form of sodium bicarbonate causes
precipitation of calcite and a consequent
decrease in the concentration of dissolved Ca.
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ANOTHER EXAMPLE OF THE COMMON-ION EFFECT

• Consider a groundwater just saturated with
  respect to calcite. This water encounters a rock
  formation containing gypsum.
• Gypsum is more soluble than calcite it dissolves
  according to
• CaSO42H2O ? Ca2 SO42- 2H2O(l)
• To the extent that this reaction goes to the
  right, it pushes the following reaction to the
  left
• CaCO3(s) ? Ca2 CO32-
• causing calcite to precipitate.

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INCONGRUENT DISSOLUTION OF CALCITE AND DOLOMITE -
I

• Incongruent dissolution - when one mineral
  dissolves simultaneously with the precipitation
  of another.
• Example when calcite and dolomite are both
  encountered along a ground water flow path.
• How do we determine what will happen when both
  dolomite and calcite are present?
• Start by rearranging the KSP for dolomite

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INCONGRUENT DISSOLUTION OF CALCITE AND DOLOMITE -
II

• If a solution were in equilibrium with dolomite
  alone, then the activities of Ca2 and Mg2 would
  be equal so that
• At 10C we have Kdol½ 10-8.355, which is
  exactly equal to Kcal 10-8.355 for this
  temperature. If dolomite had first reached
  equilibrium, then calcite would not be able to
  dissolve because IAP Kcal!

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INCONGRUENT DISSOLUTION OF CALCITE AND DOLOMITE -
III

• However, at other temperatures, in general IAP
  would not be equal to Kcal.
• For example, at 30C we have Kdol½ 10-8.950,
  and Kcal 10-8.510.
• In this case calcite would dissolve, because the
  ion activity product would be less than the
  solubility product for calcite.
• Dissolution of calcite would then cause dolomite
  to precipitate via the common-ion effect.

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INCONGRUENT DISSOLUTION OF CALCITE AND DOLOMITE -
IV

• The latter process would be termed incongruent
  dissolution of calcite.
• At 0C we have Kdol½ 10-8.28, Kcal 10-8.34.
• In this case, calcite would precipitate and
  dolomite would dissolve incongruently.
• We might also get incongruent dissolution because
  calcite dissolves more rapidly than dolomite. In
  this case, Ca2 and CO32- concentrations increase
  more rapidly than Mg2, so calcite may reach
  supersaturation while dolomite is still
  undersaturated.

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SOLUBILITY PRODUCTS FOR CALCITE AND DOLOMITE IN
PURE WATER AT 1 BAR
Source Freeze and Cherry (1979)