Drinking Water Regulations - The Drinking Water - page 40

system is used for chemicals that are potential carcinogens. If toxicologi-
cal evidence leads to the classification of the contaminant as a human or
probable human carcinogen, the MCLG is set at zero (Boyce, 1997).
National Primary Drinking Water Regulations
Categories of primary contaminants include organic chemicals , inorganic chem-
icals , microorganisms , turbidity , and radionuclides . Except for some microorgan-
isms and nitrate, water that exceeds the listed MCLs will pose no immediate
threat to public health; however, all of these substances must be controlled,
because drinking water that exceeds the standards over long periods of time
may be harmful.
Note: As we learn more from research about the health effects of various
contaminants, the number of regulated organics is likely to grow. Public
drinking water supplies must be sampled and analyzed for organic
chemicals at least every 3 years.
Organic Chemicals
Organic contaminants for which MCLs are being promulgated are classified
into the following three groupings: synthetic organic chemicals (SOCs), volatile
organic chemicals (VOCs), and trihalomethanes (THMs). Table 3.1 provides a par-
tial list of maximum allowable levels for several selected organic contami-
nants. Synthetic organic chemicals are manmade and are often toxic to living
organisms. These compounds are used in the manufacture of a wide variety
of agricultural and industrial products. This group includes primarily PCBs,
carbon tetrachloride, pesticides and herbicides such as 2.4-D, aldicarb, chlor-
dane, dioxin, xylene, phenols, and thousands of other synthetic chemicals.
Note: A 1995 study of 29 Midwestern cities and towns by the Washington,
DC-based nonprofit Environmental Working Group found pesticide resi-
dues in the drinking water in nearly all of them. In Danville, Illinois, the
level of cyanazine, a weed killer manufactured by DuPont, was 34 times
the federal standard. In Fort Wayne, Indiana, one glass of tap water con-
tained nine kinds of pesticides. The fact is, each year, approximately 2.6
billion pounds of pesticides are used in the United States (Lewis, 1996).
These pesticides find their way into water supplies and thus present
increased risk to public health.
Volatile organic chemicals are synthetic chemicals that readily vaporize
at room temperature. Chemicals used in degreasing agents, paint thinners,
glues, dyes, and some pesticides fall into this category. VOCs include ben-
zene, carbon tetrachloride, 1,1,1-trichloroethane (TCA), trichloroethylene
(TCE), and vinyl chloride.