Civil Engineering Reference
In-Depth Information
Another group of SOCs of concern are polychlorinated biphenyls (PCBs), most
often sold under the trade name Arochlor. These represent a group of remarkably
refractory compounds that were produced in great volume (prior to their ban in the
1980s) and commonly used as high temperature stabilizers for transformer oils. Re-
sistant to both chemical and biological degradation, these compounds are environmen-
tally persistent and continue to contaminate the sediments of many waterways in
urban-industrial settings. Drinking-water MCLs have been established for a number of
pesticides, herbicides, and PCBs. Table 1-5 presents MCLs, and suspected health
effects, for all currently regulated SOCs.
Much of the concern about pesticides and herbicides relates to the ability of aquatic
plants and animals (including humans) to store and concentrate these substances in
their tissues, particularly fatty tissues. With some pesticides there can be a remarkable
magnification effect, as the concentration of the pesticide increases with trophic level
in the food chain. Ultimately, the levels achieved can be several thousand times greater
than the ambient concentration of the pesticide in the water body. The chlorinated
SOCs have been singled out for special consideration because of their resistance to
biological degradation (environmental persistence). These compounds in particular
have a remarkable longevity that in some cases span decades and make their inappro-
priate usage a true environmental hazard. Research to develop more effective and
environmentally acceptable pesticides has produced many new classes and types of
compounds. Table 3-5 presents a chart of the major classes of insecticides and her-
bicides, their structure and information relative to their toxicity and environmental
persistence.
NATURAL ORGANIC MATTER
The Most Problematic Organics Are Natural
Diverse organic compounds generated by biological processes both in a water body
(autochthonous material) and in the surrounding watershed (allochthonous material)
are found in all surface and many ground waters. Collectively, these compounds, along
with some organic compounds that enter the water as a result of human activities, are
referred to as
natural organic matter
(NOM). The concentrations of NOM and of the
subgroups of molecules that contribute to it are usually quantified in terms of the
amount of carbon in the molecules. Typically, values are reported as the concentration
of dissolved organic carbon (DOC) in a sample. The more inclusive term
total organic
carbon
(TOC) includes particulate and colloidal material.
Natural waters can contain a considerable quantity of organic matter (see Fig.
3-1), a substantial portion of which may be particulate material—largely vegetative
debris. The more important fraction is the dissolved organic carbon, which consti-
tutes the reactive organic content, and is responsible for the majority of reactions of
interest in water treatment processes. Natural waters generally contain between 2 and
10 mg / L of DOC.
Since the first application of gas-liquid chromatographic techniques and their sub-
sequent coupling with mass spectrometry, an ever-growing number of the dissolved
organic compounds have been identified. However, despite the more extensive appli-
cation of these sophisticated analytical techniques, semiquantitative estimations carried
