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As Cr 2 O 7 2 oxidizes substances not oxidized by O 2 , the COD is usually
greater than the BOD and to some extent overestimates the threat posed
to oxygen content.
3.3.5.2 Synthetic Organic Chemicals. A large number of organic
compounds are synthesized for agricultural use, mainly as pesticides,
and for industrial use as solvents, cleaners, degreasers, petroleum
products, plastics manufacture, etc. 157 Many organic micropollutants
percolate into the soil and accumulate in aquifers or surface waters.
They can contaminate drinking water sources via agricultural runoff to
surface waters or percolation into groundwaters, industrial spillages to
surface waters and groundwaters, runoff from roads and paved areas,
industrial waste water euents leaching from chemically treated sur-
faces, domestic sewage euents, atmospheric fallout, and as leachate
from industrial and domestic landfill sites. 157 In the last few years,
emerging chemical contaminants such as pharmaceuticals and personal
care products have also been detected in freshwaters. 158 In a widely
publicized study, Kolpin and co-workers 159 investigated the occurrence
of pharmaceuticals as well as other organic wastewater contaminants in
a national reconnaissance study of 95 contaminants in 139 streams in 30
states of the USA in 1999-2000. As many as 38 of the 95 targeted
compounds were found in a single water sample, although the average
number of compounds in a given sample was seven and the concentra-
tions of individual compounds were typically much less than 1 mgL 1 .
Three classes of compounds (detergent metabolites, plasticizers, ster-
oids) had the highest concentrations and the most frequently detected
compounds were coprostanol (faecal steroid), cholesterol (plant and
animal steroid), N,N-diethyltoluamide (insect repellent), tri(2-chlor-
oethyl)phosphate (fire retardant) and 4-nonylphenol (non-ionic deter-
gent metabolite) (Figure 8). 160 Although measured concentrations were
generally low and rarely exceeded drinking water guidelines or aquatic-
life criteria, little is known about the potential interactive effects that
may occur from complex mixtures of organic wastewater contaminants
in the environment.
3.3.5.2.1 Pesticides. Many pesticides in aquifers have resisted degra-
dation and are more likely to persist there because of reduced microbial
activity, absence of light, and lower temperatures. 161 Numerous aqui-
fers, for example in eastern England, have been found to exceed the
Maximum Admissible Concentrations (MAC) guidelines for total pes-
ticides in drinking water (0.5 mgL 1 ), due primarily to the presence of
herbicides of the carboxy acid and basic triazine groups. Gray 157 has
listed the 12 pesticides most often found in UK drinking waters in two
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