of the contamination. Social and economic considerations will also affect reme-
diation options. Cost is always a key issue, and this might need to be balanced
against the chances of successful remediation. It might be important to know how
long the remediation will take or whether the remediation technique is sustain-
able (see below).
Remediation options are broadly physical, chemical or biological in approach.
Physical remediation includes dig-and-dump, incineration or containment of
contaminants on site. Generally, these approaches provide a guaranteed ‘quick-
fix’, but at a cost. Some of these options are expensive (incineration), while others
simply pass the problem on without addressing the root problem of contamina-
tion (dig and dump and containment). Soil washing with, for example, surfactants
(Box 4.15) and/or solvents is a chemical remediation option, as is the addition of
chemically active reagents to promote contaminant degradation and/or immobi-
lization. Overall, physical and chemical remediation options are non-sustainable
because typically they alter a soil’s structure, chemistry or biology.
Biological options are described by the term bioremediation, ‘the elimination,
attenuation or transformation of polluting or contaminating substances by the use
of biological processes, to minimize the risk to human health and the environment’.
In contrast to physical and chemical methods, biological options generally main-
tain soil integrity with respect to its chemical and biological elements. However,
bioremediation often takes months or years to complete, and success cannot be
guaranteed. Although bioremediation is driven by biological vectors, much chem-
istry (biochemistry) is involved. Biodegradation occurs under both aerobic and
anaerobic conditions, although aerobic degradation is generally faster and more
extensive for most contaminants (Section 4.6.5). Thus, aerobic conditions are
generally promoted during bioremediation strategies, for example air-venting.
The same physical and chemical properties that dictate a contaminant’s fate in
soils (Box 4.14) also dictate the amenability of contaminants to bioremediation.
In general, lighter molecules that are non-halogenated (e.g. without Cl) and with
high polarity are more readily biodegraded. This is because lighter and simpler
molecules are inherently more biodegradable and because polar molecules are
more soluble, and hence available for degradation. The bioavailability of a con-
taminant may also be influenced by its length of contact with the soil; longer
contact usually lowering bioavailability. Toxicity is also an issue because some
compounds are highly toxic to microbes (e.g. pentachlorophenol (PCP), the
fungicide used in wood preservers).
Bioremediation is either donein situ, with contamination treated where it
occurs, or ex situ, where contaminated soil is removed by excavation prior to treat-
ment. In some cases treatment can be ex situon-site, i.e. where soil is excavated
but treated on site in heaps. In situand ex situapproaches have their advantages
and disadvantages, as outlined in Table 4.11.
In situ bioremediation by biostimulation—Exxon Valdezoil spillageBiostimulation is the promotion of favourable conditions to facilitate the degra-
dation of contaminants by in situmicroorganisms. Stimulation can be achieved
The Chemistry of Continental Solids 131