Geoscience Reference
In-Depth Information
sludge during the effluent treatment process. Their effect on the properties of the soil
and subsurface regime depends on their interaction with the system solid phase and
their persistence and distribution with depth. As a result, physical properties of the
pristine solid phase matrix, such as water retention capacity, bulk density, porosity,
and structure, may be changed irreversibly. Both effluents and sludge are charac-
terized by a high organic matter content which may vary from 1 to 70 %. As a
function of their origin, effluents, treated wastewaters, and the resulting sludge may
contain chemical contaminants that may cause irreversible changes to the soil-
subsurface solid phase (Yaron et al.
1984
; Feigin et al.
1991
;Levyetal.
2010
). Here,
we consider surfactants that originate from effluents to illustrate irreversible changes
in soil-subsurface properties.
Surfactants, or ''surface active agents,'' are organic compounds of natural or
anthropogenic origin that reduce the surface tension of water. Surfactants may
interact with the porous medium solid phase, and as a consequence, they may
affect the soil-subsurface water transmission properties. Surfactants are organic
compounds comprising hydrophobic and hydrophilic components. Surfactants may
be hydrophilic when the surfactant is ionic or highly polar. Surfactants may be
anionic, cationic amphoteric, or nonionic. Due to an amphipathic structure, sur-
factant molecules exhibit a specific orientation at interfaces within multiphase
systems where they concentrate. In these cases, the interfacial tension is reduced.
Surfactants form micelles at critical solution concentrations.
When sewage water contaminated by surfactants reaches the land surface, e.g.,
in an irrigated field or in a disposal site, several physical interactions between the
surfactant and the soil-subsurface solid phase occur (Tumeo
1997
, and references
therein). The surfactant induces changes in hydraulic conductivity, due to the
alteration of the liquid phase properties and to a direct effect on the physical
properties of the solid phase. While changes occurring in the liquid phase are
reversible, surfactant-induced alteration of the solid phase becomes irreversible
under specific conditions; this leads to an irreversible change in the soil-subsur-
face water transmission properties. Here, we present some examples from the
literature that show the irreversibility of changes induced by anthropogenic sur-
factants on soil-subsurface water transmission properties.
The types of surfactant and soil control surfactant-induced effects on soil-
subsurface water transmission properties. Allred and Brown (
1994
) performed
laboratory tests on hydraulic conductivity of sandy and loam soils leached with
nonionic, anionic, cationic, and amphoteric surfactants. It is apparent that the ionic
surfactants caused a greater hydraulic conductivity decrease than the nonionic
surfactants. Maximum hydraulic conductivity decreases were 47 % for the sandy
soil and more than two orders of magnitude for the loamy soil (Fig.
18.32
).
Similarly, Liu and Roy (
1995
) found that maximum adsorption and precipita-
tion of the anionic surfactant sodium dodecylsulfate in a calcium-saturated sub-
surface soil (from Louisiana, USA) occurred when the surfactant concentration
was close to the critical micelle concentration. Cation exchange between soil and
this sodium surfactant led to the release of Ca
2+
, subsequent precipitation of
calcium, and irreversible decrease in hydraulic conductivity.
