does not have contact with the kinds of minerals that can neutralize the acid and keep the
pH from falling low enough to damage aquatic life. Lakes in some parts of Canada and
in New England in the U.S. are especially susceptible to this kind of damage.
There are not many direct pollutant sources of excessively high alkalinity, which is
due to salts such as sodium carbonate, Na 2 CO 3 , that tend to raise the pH to levels high
enough to be harmful to aquatic life. Some soils and rocks associated with mining have
high alkalinity, and human activities can cause this alkalinity to be leached into water.
Water salinity is due to dissolved salts, such as sodium chloride and calcium chloride.
Each pass of water through a municipal water system adds salinity, especially from
NaCl flushed into water from recharging water softeners. Irrigation also adds salinity
and is responsible for the high levels of salt in California’s Salton Sea, an artificial body
of water with no outlet to the ocean created artificially by runoff from irrigated lands.
Fertilizers are salts, and they get into runoff water during irrigation.
Water Pollutants That Are Just Too Nutritious
Some inorganic species are pollutants, not because they are toxic, but because they
are very nutritious for algae in water. Algae and other plants require a number of different
inorganic nutrients. Those required in the greatest quantity are inorganic phosphorus
(H 2 PO 4 - , HPO 42 - ), nitrogen (NH 4 +, NO 3 - ), and potassium (K+).
The plant fertilizers described above can get into water from a number of sources,
including fertilizers put on soil to enhance crop growth, from some industrial pollutants,
and — especially in the cases of phosphates and nitrates — from the degradation of
sewage in wastewater. So what is wrong with having nutrient-rich water? If the levels of
nutrients are too high, algae grow too well and generate too much biomass. This material
eventually dies and decays, which uses up all the oxygen in the water and clogs a body
of water with dead plant matter. This unhealthy condition of excessive plant growth is
called eutrophication, derived from the Greek word meaning “well-nourished.”
Eutrophication is usually curtailed by limiting phosphate input into bodies of water
and streams. The reason for doing this is that phosphorus is usually the limiting nutrient,
much like the limiting reactant in a chemical reaction (see Section 4.10). So if phosphate
levels are cut down, algal growth and resulting eutrophication are curtailed. Around 1970
the most common source of pollutant phosphorus was from phosphates added as builders
to household detergents and discharged with sewage. Treated wastewater discharged to
streams and other natural waters added phosphate that caused eutrophication. By the
application of green chemistry (though not known as such then) other chemicals were
found that could substitute for phosphate in detergents without causing eutrophication.
7.13. Organic Water Pollutants
A whole host of water pollutants are organic compounds, which include virtually
all carbon-containing compounds. These may be nontoxic, highly biodegradable
materials, such as waste food in sewage, that are nevertheless bad for water because of
the dissolved oxygen consumed when they degrade (see below). At the other extreme
Chap. 7. Water, The Ultimate Green Solvent 177