Geoscience Reference
In-Depth Information
Fig. 8.16 Kinetics of Ca
dissolution from a gypsum
sample in various salt
solutions (Gobran and
Miyamoto
1985
)
where human actions enhance subsurface alkalinity, as a result of irrigation with
alkali water or poor management of irrigation and drainage, the distribution of
gypsum with depth may be changed by gypsum dissolution. The effective solu-
bility of gypsum depends only on the subsurface water chemistry and the
exchange-phase composition of the solid matrix. The dissolution exchange reac-
tion is
2NaX
þ
CaSO
4
CaX
2
þ
2Na
þ
þ
SO
2
ð
8
:
5
Þ
4
where X is one equivalent of exchanger and indicates that cation exchange removes
Ca
2+
from subsurface water and allows additional gypsum dissolution. Disposal of
alkali water or alkali effluents on land brings an increase in the exchangeable Na
+
in
the solid phase. Under such conditions, the dissolution of CaSO
4
in the subsurface
increases to compensate for the Na
+
that exchanges with Ca
2+
.
Effective gypsum solubility is enhanced through ion-pair formation and elec-
trolyte effects. The amount of water required for dissolution of gypsum in an
alkaline subsurface is likely to be much less than commonly inferred for gypsum
solubility in water. When the alkalinity is associated with an increase in the total
salt content, gypsum dissolution is affected by the presence of electrolytes. An
example is given in Fig.
8.16
, which shows dissolution kinetics of gypsum sam-
ples from Egypt in NaCl and MgCl
2
solutions (Gobran and Miyamoto
1985
). It
appears that the dissolved amounts at equilibrium are related to the ionic com-
position of the aqueous solution. We see that the dissolution in the subsurface
water apparently increases with increasing salt content, as expected from ionic
strength considerations.
8.3.2 Redox Processes
Redox processes affect contaminant solubility and may result from fluctuating
saturation and drying processes in the subsurface due to natural or anthropogenic
