Environmental Engineering Reference
In-Depth Information
suffer if DO concentrations fall below a concentration of 3 to 4 mg/L. Larvae and juvenile fish are more
sensitive and require even higher concentrations of DO (USEPA, 1997). Many fish and other aquatic
organisms can recover from short periods of low DO in the water. However, prolonged episodes of
depressed DO concentrations of 2 mg/L or less can result in “dead” water bodies.
Water absorbs oxygen directly from the atmosphere, and from plants as a result of photosynthesis. The
ability of water to hold oxygen is influenced by temperature and salinity. Water loses oxygen primarily
by respiration of aquatic plants, animals, and microorganisms. Due to their shallow depth, large surface
exposure to air, and constant motion, undisturbed streams generally contain an abundant DO supply.
However, external loads of oxygen-demanding wastes or excessive plant growth induced by nutrient
loading followed by death and decomposition of vegetative material can deplete oxygen.
Oxygen may be produced in stream by aquatic plants. Through photosynthesis, plants capture energy
from the sun to fix carbon dioxide into reduced organic matter as follows:
6CO 2 + 6H 2 O +36ATP = C 6 H 12 O 6 + 6O 2
(1.24)
where 36 ATP reflects the light energy necessary for photosynthesis.
Note that photosynthesis produces oxygen. Plants utilize their simple photosynthetic sugars and other
nutrients (notably nitrogen [N], phosphorus [P], and sulfur [S] with smaller amounts of several common
and trace elements) to operate their metabolism and to build their structures. Most animal lifes depend on
the release of energy stored by plants in the photosynthetic process. This process is known as respiration and
consumes oxygen. The actual process of respiration involves a series of energy converting oxidation-reduction
reactions. This defines the biological oxygen demand.
Sediment oxygen demand (SOD) represents the oxygen demand of respiration of organisms in the
sediment and the benthic decomposition of organic material. The demand for oxygen by sediment and
benthic organisms can, in some instances, be a significant fraction of the total oxygen demand in a stream.
This is particularly true in small streams. The effects may be particularly acute during low-flow and
high-temperature conditions, as microbial activity tends to increase with increased temperature.
Nutrients —In addition to carbon dioxide and water, aquatic plants (both algae and higher plants) require
a variety of other elements to support their bodily structures and metabolism. Just as with terrestrial
plants, the most important of these elements are nitrogen and phosphorus. Additional nutrients, such as
potassium, iron, selenium, and silica, are needed in smaller amounts and generally are not limiting factors to
plant growth. When these chemicals are limited, plant growth may be limited. However, excessive growth
of algae and other aquatic plants instream can result in nuisance conditions and the depletion of DO
during nonphotosynthetic periods by the respiration of plants and decay of dead plant material can create
conditions unfavorable to aquatic life.
Phosphorus in freshwater systems exists in either a particulate phase or a dissolved phase. Both phases
include organic and inorganic fractions. The organic particulate phase includes living and dead particulate
matter, such as plankton and detritus. Inorganic particulate phosphorus includes phosphorus precipitates
and phosphorus adsorbed to particulates. In the aquatic environment, nitrogen can exist in several
forms—dissolved nitrogen gas (N 2 ), ammonia and ammonium ion (NH 3 and NH 4 + ), nitrite (NO 2 - ), nitrate
(NO 3 - ), and organic nitrogen as proteinaceous matter or in dissolved or particulate phases. The most important
forms of nitrogen in terms of their immediate impacts on water quality are the readily available ammonia
ions, nitrites, and nitrates. Only a few life-forms (for example, certain bacteria and bluegreen algae) have
the ability to fix nitrogen gas from the atmosphere. Most plants can use nitrogen only if it is available as
ammonia (NH 3 , commonly present in water as the ionic form ammonium, NH 4 + ) or as nitrate (NO 3 - ).
Toxic chemicals —Toxic organic chemicals (TOC) are synthetic compounds that contain carbon, such
as polychlorinated biphenyls (PCBs) and most pesticides and herbicides. Many of these synthesized
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