Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 374

composition on the retention of lead (Pb) and cadmium (Cd). In the particular case shown
in Figure 10.10, humic matter in the form of fulvic acid, a sulide mineral called macki-
nawite (Fe,Ni) 9 S 8 and kaolinite were used as control soil material.
One speaks of the natural attenuation of contaminants as being a set of positive pro-
cesses that mitigates the impact of contaminants in the ground through a reduction of
their intensity—as measured in terms of concentration and toxicity of the contaminants. In
the past, the use of natural attenuation processes had been considered almost exclusively
in connection with remediation of contaminated sites—and more speciically with sites
contaminated with organic chemical contaminants, as shown, for example, in the deini-
tion provided by the USEPA. Little distinction was made between intrinsic remediation
and intrinsic bioremediation. More recently, with a better appreciation of the assimilative
capacity of soils and especially in view of a growing body of research information on
contaminant-soil interactions, more attention is being paid to the use of natural attenuation
as a tool for mitigating and managing the transport and fate of contaminants in the ground.
A contributing factor has also been the accelerating costs for application of aggressive
remediation techniques to treat contaminated sites. The reader is reminded that contami-
nants include all the polluting and health-threatening elements entering into the ground,
such as contaminants, toxicants, leachates, liquid wastes, hazardous substances, etc.
10.6.1 Natural Attenuation by Dilution and Retention
We have, up to now, considered natural attenuation of contaminants as being due to the
processes associated with physical, chemical, and biological properties of soil. We have
considered that these properties contribute directly to the partitioning of contaminants,
i.e., the transfer of contaminants in the porewater to the surfaces of the soil solids. Strictly
speaking, there is another set of processes that arguably can be considered as part of the
natural attenuation capacity of soils—except that we would now have to refer to this as the
natural attenuation capacity of soil-water systems. Although the groundwater and pore-
water aspects of the soil-water system have heretofore been considered only in respect
to their physical and chemical interactions with the soil solids and contaminants, they
attenuate contaminants through processes of dilution. Thus, in addition to the processes
previously described, reduction in concentrations of contaminants can be accomplished by
dilution through mixing of the contaminants with uncontaminated or less contaminated
groundwater. In total, natural attenuation of contaminants in soils includes (a) dilution,
(b) interactions and reactions between contaminants and soil solids resulting in partition-
ing of the contaminants between the soil solids and porewater, and (c) transformations that
reduce the toxicity threat posed by the original polluting contaminants (contaminants).
The likelihood of only one mechanism being solely responsible for attenuation of contami-
nants in transport in the soil is very remote. In all probability, all the various processes or
mechanisms will participate to varying degrees in the attenuation of contaminants—with
perhaps partitioning being by far the most signiicant factor in attenuation of contaminants.
10.6.1.1 Dilution and Retention
In the context of contaminant transport in soils, dilution refers to the reduction in concen-
tration of contaminants in a unit volume as a result of a reduction of the ratio of num-
ber of contaminants n c to the volume V of the host luid. An example of this would be
when the original contaminant load is given as 100 ppm, and dilution with groundwater
reduces this to 50 ppm, the singular process responsible for the decrease or reduction in