Environmental Engineering Reference
In-Depth Information
contribute to natural attenuation capability and (b) incorporate NA as part of a scheme to
mitigate and manage the geoenvironmental impacts from discharge and/or containment
of waste products and contaminants.
10.7.1 Enhancement of Natural Attenuation Capability
Successful application of waste discharge and ground contamination impact mitigation
procedures reduces and/or minimizes the damage done to the land environment and its
inhabitants. The main objective of such procedures is to achieve reduction or elimination
of health threats to the biotic receptors through elimination or minimization of ground
contamination, and prevention of contamination of receiving waters and groundwater.
The objectives or desired end points sought in contaminant impact mitigation it very
well with the capabilities of the processes in soils that contribute to natural attenuation.
Accordingly, natural attenuation can be used as tool to provide impact mitigation and
management since the basic processes involved in natural attenuation result in reduction
of concentration of contaminants and toxicity. To increase the capability of natural attenua-
tion as a process tool—i.e., the process tool NA—one can consider enhancement of the natu-
ral assimilative capacity of the soil-water system. These can take the form of geochemical
and biogeochemical aids, bioaugmentation, and biostimulation. Successful enhancement
of NA as a process tool will produce a subsoil with properties that can be considered as
enhanced NA capability.
10.7.1.1 Soil Buffering Capacity Manipulation
The capability of a soil to accept and retain inorganic and some organic contaminants can,
in some instances, be assessed by determining its chemical buffering potential—particu-
larly if the reactions in the soil-water system result in changes in the pH of the system. The
chemical buffering system contributes signiicantly to the carrying capability of the soil,
i.e., the capability of the soil barrier or subsoil to accept and retain contaminants. The main
issue is the ability of the soil-water system to maintain a natural pH level (within accept-
able limits) despite the input of acidic or alkaline contaminant leachates. In situ soil pH
manipulation for the purpose of contaminant impact mitigation requires introduction of
buffering agents generally through leaching methods or via injection. In contaminant-soil
interaction, the chemical buffering system describes the capability of the system to act as
chemical barrier against the transport of contaminants.
The buffering capacity of a soil determines the potential of a soil for effective interaction
with leachate contaminants and is more appropriate for inorganic soils and inorganic con-
taminant leachates. The principal features that establish the usefulness of buffering capac-
ity assessment center around the “acidity” or “alkalinity” of the initial soil-water system
and the solutes in the leachate. Soil conditioning in respect to changes in the natural soil
buffering capacity is usually considered in terms of addition of buffering agents, much
in the same manner as solution chemistry. In the in situ soil conditioning case, however,
addition of buffering agents needs to be effectuated through injection wells or through
leaching, for example, by adding lime to the surface as the leach source to increase the
pH of the soil. In a site contaminated with heavy metals, raising the pH of the soil-water
system would precipitate the heavy metal contaminants and thus make them less envi-
ronmentally mobile and less bioavailable. However, we must recognize that this is not
a permanent solution because if the pH of the system is subsequently reduced by envi-
ronmental forces or external events, the same heavy metals will become mobile again. To
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