Geoscience Reference
In-Depth Information
However, not all existing data obtained under laboratory conditions can be used in a
subsurface environment, where instead of pure water, an electrolyte solution that
may contain surface active organic compounds exists. As a consequence, the vali-
dation of Henry's law for specific subsurface conditions must be carried out in
specially designed experiments that mimic as closely as possible the natural
environment.
7.2 Volatilization from Subsurface Aqueous Solutions
Water evaporation and contaminant volatilization from subsurface aqueous solu-
tions are two companion processes that affect contaminant partitioning between
the liquid and gaseous phases. Temperature-induced evaporation may affect the
concentration of the natural constituents of the subsurface water and thus affect
contaminant dissolution in this water.
Water evaporation occurs when the vapor pressure of the water at the surface,
which is temperature dependent, is greater than the water pressure in the subsurface,
which is dependent on relative humidity and temperature. The isothermal evapora-
tion process is described by Stumm and Morgan (
1996
) via a ''reaction progress
model,'' in which the effects of the initial reaction path are based on the concept of
partial equilibrium. Stumm and Morgan (
1996
) describe partial equilibrium as a state
in which a system is in equilibrium with respect to one reaction but out of equilibrium
with respect to others. As an example, Stumm and Morgan (
1996
) indicate (Fig.
7.1
)
that water with a ''negative residual alkalinity'' (i.e., 2[Ca
2+
][[HCO
3
-
]) tends to
increase its calcium hardness as a result of water evaporation into the atmosphere,
and consequently its alkalinity and pH decrease. Eventually, after excessive evap-
oration, the water may reach saturation, becoming a Na
+
,SO
4
2-
,orCl
-
brine.
From Fig.
7.1
, it may be observed that residual alkalinity initially increases
with water evaporation, concomitant with a pH increase, and that negative residual
alkalinity and pH decrease. Because evaporation leads to a change in the quality of
water, the water properties as a solvent for organic contaminants are also changed.
This
fact
should
be
considered
when
dealing
with
contaminant
partitioning
between phases.
The molecular weight of water vapor (MW = 18) is less than that of air
(MW & 29). As such, the diffusion of water vapor into the surrounding
atmosphere, which consists of a mixture of water and air, leads to a buoyant force
with upward macroscopic movement. The natural evaporation phenomenon is not
only the effect of heat transfer but also a buoyancy-induced motion. The system is
at steady state when the vapor pressure of the water at the surface is less than that
in the air above, and the resulting condensation is governed by the slow process of
molecular diffusion and laminar flow.
Volatilization of contaminants from subsurface aqueous solutions into the
subsurface gas phase or the (above ground) atmosphere is controlled by the vapor
pressure. Compounds with high vapor pressure tend to accumulate in the gas
