automobiIe.exhaust), water and tobacco smoke. Since all of the above sources except
water are, for the most part, unregulated concentrations of lead greater than 0.05 mg/l
in drinking water constitute grounds for rejection of the supply.
(7) Mercury. Mercury is highly toxic to humans. Continued ingestion or
large amounts can cause damage to the brain and central nervous system.
(8) Nitrate. Serious and occasionally fatal poisonings in infants have
occurred following ingestion of well waters shown to contain nitrate (NO3). Nitrate
poisoning appears to be confined to infants during their first few months of Iife. Adults
drinking the same water are not affected, but breast-fed infants of mothers drinking such
water maybe poisoned. Cows drinking water containing nitrate may produce miIk
sufficiently high in nitrate to result in infant poisoning. Waste from chemical fertiIizer
plants and field fertiIization may be sources of pollution.
(9) Selenium. Selenium is known to produce "alkali disease" in cattle. Its
effects, Iike those of arsenic, may be permanent. Recent reports indicate that selenium
may increase the incidence of dental caries. Of greater importance in Iimiting the
concentration of selenium is its potential carcinogenicity (tendency to produce cancer).
(10) Silver. The chief effect of siIver in the body is cosmetic, which consists
of a permanent blue-gray discoloration of the skin, eyes, and mucous membranes.
Evidence also indicates that siIver, once absorbed, is held indefinitely in the tissues,
particularly the skin, without evident loss through normal channels of elimination. It is
also believed that certain vegetables, when cooked in water containing.siIver, combine
chemically with the silver. Thus, a person could possibly ingest the silver from several
liters of water by eating such vegetables.
b. Secondary Standards. The following chemicals are among those in
Table 4-3. These chemicals should not be present in a water supply in excess of the
listed concentrations where other, more suitable supplies are available or can be made
available.
(1) Chloride. sulfate and dissolved solids. The importance of chloride,
sulfate, and dissolved solids as they affect water quality hinges upon their taste, laxative
effect, and hardness.
(a) Taste. There is quite a variation between individuals as far as the
taste threshold (concentration at which the taste can be detected) and the concentration
at which taste becomes objectionable. More than 100 public water supplies in the
United States provide water with more than 2,000 ppm total dissolved solids. Although
newcomers and casual visitors would find these waters almost intolerable, and although
some of the residents use other supplies for drinking, many are able to tolerate, if not
enjoy, these highly mineralized waters. The maximum recommended Iimits in Table 4-3
are just below the median taste threshold obtained from tests.
MD0160
4-5
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