Dissolved Oxygen & Biochemical Oxygen Demand WHY WE NEED TO KNOW

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Dissolved Oxygen and Biochemical Oxygen Demand
@What is dissolved oxygen and why is
it important?
The stream system both produces and
consumes oxygen. It gains oxygen from the atmosphere and from plants as a
result of photosynthesis. Running water, because of its churning, dissolves
more oxygen than still water, such as that in a reservoir behind a dam.
Respiration by aquatic animals, decomposition, and various chemical reactions
consume oxygen.





Wastewater from sewage treatment
plants often contains organic materials that are decomposed by microorganisms,
which use oxygen in the process. (The amount of oxygen consumed by these
organisms in breaking down the waste is known as the biochemical oxygen demand
or BOD. A discussion of BOD and how to monitor it is included at the end of
this section.) Other sources of oxygen-consuming waste include stormwater
runoff from farmland or urban streets, feedlots, and failing septic systems.
Oxygen is measured in its dissolved
form as dissolved oxygen (DO). If more oxygen is consumed than is produced,
dissolved oxygen levels decline and some sensitive animals may move away,
weaken, or die.





DO levels fluctuate seasonally and
over a 24-hour period. They vary with water temperature and altitude. Cold
water holds more oxygen than warm water (Table 5.3) and water holds less oxygen
at higher altitudes. Thermal discharges, such as water used to cool machinery
in a manufacturing plant or a power plant, raise the temperature of water and
lower its oxygen content. Aquatic animals are most vulnerable to lowered DO
levels in the early morning on hot summer days when stream flows are low, water
temperatures are high, and aquatic plants have not been producing oxygen since
sunset. Temp


45





5.95







Sampling
and Equipment Considerations





In contrast to lakes, where DO
levels are most likely to vary vertically in the water column, the DO in rivers
and streams changes more horizontally along the course of the waterway. This is
especially true in smaller, shallower streams. In larger, deeper rivers, some
vertical stratification of dissolved oxygen might occur. The DO levelalls, or dam ays are typically higher than
those in pools and-moving stretches. If you wanted to measure the effect
of a dam, it would be important to sample
below tllway, a