WATER QUALITY - Water Quality Engineering in Natural Systems - page 17

Phosphorus in freshwater and marine systems exist
in either an organic or inorganic form. Organic phos-
phorus may be in either particulate or nonparticulate
form. Particulate organic phosphorus includes living
and dead particulate matter, such as plankton and detri-
tus, and nonparticulate organic phosphorus includes
dissolved organic phosphorus excreted by organisms
and colloidal phosphorus compounds. Inorganic phos-
phorus may also be in either particulate or nonparticu-
late form. Particulate inorganic phosphorus includes
phosphorus precipitates, phosphorus adsorbed to
particulate matter, and amorphous phosphorus. Non-
particulate (soluble) inorganic phosphorus includes
H PO
2

2.3.5 Metals

Because of the significant (negative) effects that certain
toxic metals can have on human health, metal pollution
is potentially one of the most serious forms of aquatic
pollution. Metals are introduced into aquatic systems by
many processes, including the weathering of soils and
rocks, atmospheric deposition, volcanic eruptions, and a
variety of human activities, involving mining, industrial
use, and exhaust and tire deposition from automobiles.
urban runoff is a major source of zinc (originating from
tire wear) in many water bodies, and metals tend to
accumulate in bottom sediments. Toxic metals of concern
in water bodies typically include arsenic (Ar), cadmium
(Cd), copper (Cu), chromium (Cr), lead (Pb), mercury
(Hg), nickel (Ni), and zinc (Zn). These metals are some-
times categorized as heavy metals , which is a term that
is not precisely defined, but is commonly taken as metals
with atomic numbers in the range of 21-84. Heavy
metals are sometimes defined as metals with specific
gravities greater than 4-5. Dissolved metals are gener-
ally responsible for toxicity, where dissolved metals are
contained in the fraction of water passing a 0.45- µ m
filter. Environmental conditions, such as pH, tempera-
ture, and salinity can significantly affect metal solubility,
where metal solubilities are lower at near-neutral pH
than in acidic or highly alkaline waters. At toxic levels,
most metals adversely affect the internal organs of the
human body. Specific concerns with several metals are
described below.

− , HPO 2− , PO 3− , which are called orthophosphates ,
are also classified as soluble reactive phosphorus (SrP).
Orthophosphates are salts of phosphoric acid (H 3 PO 4 )
and are readily available to plants and algae. For water
in contact with the atmosphere, the most fully oxidized
state of phosphorus is +5, and phosphorus in the form
of phosphate ( H PO
2

4

− ) from fertilizer, detergents, and
organic wastes becomes adsorbed to sediment, which is
carried to streams during the erosion-sedimentation
process.
Phosphorus undergoes continuous transformations
in the freshwater environment. Some phosphorus will
sorb to sediments in the water column or substrate and
be removed from circulation. Phytoplankton, periphy-
ton, and bacteria assimilate the SrP and change it into
organic phosphorus. These organisms might then be
ingested by detritivores or grazers, which in turn excrete
some of the organic phosphorus as SrP. Continuing the
cycle, the SrP is rapidly assimilated by plants and
microbes.
The measurement of all phosphorus forms in water,
including all inorganic and organic particulate and
soluble forms mentioned above, is known as total phos-
phorus (TP). TP does not distinguish between phospho-
rus currently unavailable to plants (organic and
particulate) and that which is available (SrP). However,
organic and particulate forms are transformed to more
bioavailable forms at various rates dependent on micro-
bial action or environmental conditions. In streams with
relatively short residence times, it is less likely that the
transformation from unavailable to available forms
will have time to occur, and SrP is the most accurate
estimate of biologically available nutrients. In lakes,
however, where residence times are longer, TP generally
is considered an adequate estimation of bioavailable
phosphorus. Phosphorous is usually the limiting nutri-
ent for the growth of algae in streams and lakes. For
lakes in the northern united States to be free of algal
nuisances, the generally accepted upper TP concentra-
tion limit is 10 µ g/l.

4

Arsenic (Ar) is a naturally occurring element in the
environment, and its occurrence in natural waters
(particularly groundwater) is largely the result of
minerals dissolving from weathered rocks and
soils.
Cadmium (Cd) is widely used in metal plating and is
an active ingredient of rechargeable batteries.
Cadmium causes high blood pressure and kidney
damage and is a probable human carcinogen.
Chromium (Cr) is a trace constituent in ordinary
soils, a natural impurity in coal, and is widely used
in the manufacture of stainless steel. Chromium
exists in two oxidation states in the environment,
+3 and +4. Cr +3 is an essential trace element
in human diets, whereas Cr +4 causes a variety of
adverse health effects, including liver and kidney
damage, internal hemorrhage, respiratory disor-
ders, and cancer.
Lead (Pb) was used extensively in several commercial
products before its adverse health effects became
well known. It was incorporated in pigments used
in house paint and in glazes applied to dishware.