Water Quality Management - River Dynamics and Integrated River Management - page 587

the minimum values required in a lake. On the other hand, slow moving clear-water streams are susceptible
to eutrophication. In rivers, excessive plant growth can create a number of undesirable conditions, such
as thick slime layers on rocks and dense growths of aquatic weeds (Fig. 9.18).
Fig. 9.18 Harvesting aquatic weeds in the Juma River west of Beijing, China
Methemoglobinemia —One of the most common forms of nitrogen in water is nitrate (NO 3 ), which is
itself not particularly dangerous. Since the gastric juices of young infants lack sufficient acidity, nitrate
reducing bacteria can grow in their upper intestinal tracts. When they ingest nitrate, the nitrate can be
reduced to nitrite before the nitrate is completely absorbed in the bloodstream. Nitrites have a greater
affinity for hemoglobin in the bloodstream than does oxygen, and when nitrites replace that needed oxygen,
the nitrites react with the hemoglobin to form methemglobin. This condition, known as methemoglobinemia,
results in ineffective oxygen carrying by the blood. The resulting oxygen starvation causes a bluish
discoloration of the infant; hence, it is commonly referred to as the “blue baby” syndrome. In extreme cases
the victim may die from suffocation. Usually after the age of about 6 months, the digestive system of a
child is sufficiently developed that this syndrome does not occur.
Clark et al. (1977, p. 243) stated the evidence that links nitrates with methemoglobinemia and death in
infants is not strong, but is widely cited in the literature. Despite this uncertainty of the link between
nitrates and methemoglobinemia, most States in the U.S. have decided better safe than sorry and have set
a 10 mg/L standard for nitrates in drinking water. Because of intensive use of fertilizers in agricultural
areas in the Midwestern U.S. many communities, such as Bloomington and Decatur, Ill., have had to
come up with innovative solutions to reduce nitrate concentrations in the drinking water supplies.
Ammonity toxicity —In natural water bodies total ammonia exists in two forms: the ammonium ion,
NH 4 + , and the ammonia molecule, NH 3 , also known as un-ionized ammonia. The un-ionized form of
ammonia has been proven to harm aquatic animals. In the 1970s, concentrations above 0.05 mg/L were
considered dangerous to fish and other aquatic life. Hence, most State standards from the 1970s through
1990s limit un-ionized ammonia concentrations to very low levels, e.g., Colorado = 0.02 mg/L and
Illinois = 0.04 mg/L.
Generally, in natural water bodies, with low concentrations of background total ammonia the pH and
temperatures are such that the ammonium ion is the dominant form. pH increases due to sewage wastes
and eutrophication, and, thus, ammonia toxicity could be important in polluted water bodies. Normally
models compute and monitoring programs measure the total ammonia concentration, whereas the State
ammonia toxicity standards used to be (and in some cases still are) for un-ionized ammonia. Thus, to