Geoscience Reference
In-Depth Information
Examples of NAPL droplet formation in water are described by Yaron-
Marcovich et al. (
2007
). Solutions of benzene, toluene, xylene, trichloroethylene,
and a mixture of them were prepared in excess in freshwater and saltwater, and
solution stability was examined. High organic concentrations were found to remain
stable in both freshwater and saltwater. In saltwater, for example, toluene and
xylene concentrations remained as high as 14 and 26 times their theoretical sol-
ubilities, respectively, over a period of six days, while in freshwater, their con-
centrations remained 8 and 30 times their solubilities over the same period. This
phenomenon is attributed to the presence of stable organic droplets. An image of
organic droplets in saline solution captured by optical microscope is presented in
Fig.
8.25
.
A similar effect was observed in experiments reported by Dror et al. (
2003
)ina
saline solution simulating seawater. They found that the concentration of a mixture
of organic contaminants may be much higher than expected from theoretical
considerations, accounting for the salting-out effect (Table
8.5
). This is because
the total concentration of organic mixture in natural saline (or sea) water is given
not only by the dissolved contaminant and electrolyte concentration but also by the
presence of mechanically dispersed organic droplets formed under various envi-
ronmental conditions. This apparent solubility and concentration is called the
carrying capacity of the aqueous solution (CCAS). It may be observed from
Table
8.5
that, in saline water, the low-density hydrocarbons (benzene and tolu-
ene) have a CCAS value much greater than the CCAS in freshwater.
8.4 Contaminant Retention in the Subsurface
As mentioned previously, the retention of contaminants on geosorbents may occur
by surface adsorption on or into the colloid fraction of the solid phase and by
physical retention as liquid ganglia or as precipitates into the porous media. The
type of retention is defined by the properties of the solid phase and the contami-
nants as well as by the composition of the subsurface water solution and the
ambient temperature.
8.4.1 Contaminant Adsorption
Contaminant adsorption includes retention on the porous medium solid phase, as a
result of cation exchange processes, and surface retention of neutral molecules,
due to van der Waals forces.
Sodic water reaching the land surface through irrigation or disposal of sewage
effluent leads to soil deterioration, caused by the exchange process between Na
+
from the water and clay-saturating cations. The sodium-calcium exchange process
leads to retention of Na
+
in the solid phase, affecting not only the earth material
