Thermomineral Water and Geothermal Energy - Hydrogeology: Objectives, Methods, Applications - page 82

Table 8 Solubility of Common Gases in Water (from Detay, 1993).

Gas Solubility in mg·L -1

Nitrogen (N 2 ) 23.3

Oxygen (O 2 ) 54.3

Carbon dioxide (CO 2 ) 2 318

Methane (CH 4 ) 32.5

Hydrogen (H 2 ) 1.6

Hydrogen sulfi de (H 2 S) 5 112

One notes, in this table, the high solubility of CO 2 and H 2 S, which
explains the prevalence of sparkling mineral waters and the sulfurous smell
common to hot springs.
Enrichment with gases has effects on mineralization. The chapter on
karstifi cation (A.8) details the importance of CO 2 in the dissolution of
limestone. The CO 2 , which acidifi es water, normally comes from the soil
atmosphere. Karst then evolves from top to bottom and forms epigenetic
cavities. CO 2 can also originate at depth, resulting in the formation of a
hypogenetic karst. The cavities then form from bottom to top. Speleologic
exploration has enabled the study of a number of non-functioning
hydrothermal systems (caves of Rhar Kab Shrir, Algeria; Collignon, 1988),
the high temperature and the presence of gases preventing the study of
active systems.
Upon their exit, thermomineral waters, subject to a decrease in
temperature and pressure, experience a shift in their chemical equilibrium
towards precipitation reactions. Water degasses and becomes sparkling; a
whole series of mineral deposits form near the spring. The accumulations
of white tuff at Pamukkale (Cotton Castle), in Turkey, are one of the most
famous examples of hydrothermal calcite deposits.
A similar mechanism depends on H 2 S, originating at depth, to carve out
the caves in the southwest of the United States (Hill, 1987). By combining
with H 2 S from hydrocarbon deposits, groundwater becomes acidic,
dissolves limestone, and carves out galleries. The acid corrosion creates
gypsum, which is deposited in the passageways and allows the formation
of remarkable concretions (Hill & Forti, 1997).
H 2 S + 2 O 2 → 2H + + SO 4 2-
2H + + SO 4 2- + CaCO 3 + 2 H 2 O → Ca SO 4 , 2H 2 O (gypsum)↓ + CO 2 ↑
The cave systems are created per ascensum , and can have a characteristic
caulifl ower shape, or can form labyrinths. The gypsum created in the
reaction is deposited inside the passageways of numerous cavities in the
Carlsbad region (New Mexico, USA). One example, the Lechuguilla system,
extends over more than 150 km and reaches a depth of 500 m.