Groundwater in a fractured granite has the composition listed in Table 1. The wa
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Question
Groundwater in a fractured granite has the composition listed in Table 1. The water has acquired this composition as a result of dissolution of the most soluble mineral(s) in the granite. What are the most likely source(s) of the calcium and bicarbonate ions in this groundwater? Explain. A complete explanation will incorporate appropriate geochemical reactions.
Table 1. Water quality data for a fractured granite groundwater.
Parameter concentration (mg/L)
Calcium
10
Potassium
1.5
Magnesium
6.1
Sodium
5.8
Bicarbonate
62
Sulfate
8.3
Chloride
2.1
pH (standard units)
6.8
Calcium
10
Potassium
1.5
Magnesium
6.1
Sodium
5.8
Bicarbonate
62
Sulfate
8.3
Chloride
2.1
pH (standard units)
6.8
Equations and Data The following thermodynamic constants are given for 25C: -6.35 H2CO3 H+ HCO 3 Ki 10 K2 10 0.33 HCO 3 CO 3 Kw-10 4.00 H20 H+ OH- KHF10 .47 H20+ CO2 H2CO3 KI 102. H3PO4 (s) H20 H+ H2PO4 H2PO (aq) H20 H+ HPO K2 10 7.20 HPO42-(aq) H20 H++ PO43- K3-10 2.32 Some mineral solubility products: Formula Mineral name Solubility product, BESO4 Barite 10 BaCO3 Withe rite 10 CaCO3 Calcite 10 CaSO4 Gypsum Hydroxyapatite Ca(H3O)2(UO202(SiO4 2.3H2O Uranophane More chemical reactions: Gibbsite- Al20 3H20 2Si(OH) AJ2Si20s (OH 5H20 kaolinite Na-montmorillonite- 2,33S 2010 (OH)2 AI2Si20s (OH) HNat kaolinite Ca-montmorillonite- Cao, 33A O20(OH) kaolinite illite- Ko.6M 2SA12.30Si AI2Si20s (OH kaolinite Biotite- KMg3AISi3010 (OH kaolinite Albite- NaAISi3O Al2Si20s (OH)4. kaolinite Albite- O10(O 3.67 Na-montmorillonite Microcline KAISi,O 9H20 Al kaolinite Anorthite- CaAl2Si20 2H+ H2O A12Si20s (OH)4 Ca2+ kaolinite Andesine- Nae, s 2.5 kaolinite *Solid phases are underlined. $Si(OH).Explanation / Answer
the main sources of calcium and bicarbonate ions is calcium carbnate ,
CaCO3on reactions with acids give calcium ions and carbon dioxide and water.
CaCO3(s) + 2H+(aq) Ca2+(aq) + CO2 + H2O
carbon dioxide and water react with each other to give carbonic acid which isa great source of bicarbonate ions.
CO2 + H2O H2CO3
H2CO3 HCO3 + H+
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