5 2 detection and monitoring of pollution
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Direct measurement is performed by monitoring the level of the pollutant itself, e.g. nitrates in a lake or temperature levels in a lake or stream. An indirect method would monitor the effects of the pollutants on other factors, e.g. dissolved oxygen, B.O.D., presence or absence of indicator species (see 5.2.3)


Some direct measurements might include:

  • measuring temperature in several locations along the length of a river, or at different times during the year with thermometers as an indication of thermal pollution. Take baseline measurements and then monitor in a systematic manner to determine changes.
  • measuring nitrate levels/ammonia levels/chloride levels as an indication of nutrient overload. Use the same process as above
  • measuring TSS (Total Suspended Solids) or TDS (Total Dissolved Solids) as an indication of material entering the lake or stream. (see next slide)
  • BOD is the measure of the amount of dissolved oxygen that is used by aerobic bacteria to break down the organic matter in a specific volume of water.
  • Therefore the greater the amount of organic matter (sewage, agricultural run-off, fertilizer etc.) in the body of water, the higher the BOD will be. The less organic matter, the lower the BOD. It is not uncommon for the BOD of incoming water to a sewage treatment plant to be in the range of 120. After treatment the BOD of the same water is around 5.

The standard technique for measuring the BOD of body of water is the 5-Day BOD test or BOD5. This is done by measuring the DO (dissolved oxygen) on day 1 and and again on day 5 while keeping the samples at 200C in the dark during that time. This ensures that only respiration is going on and not photosynthesis. Refer to Topic 2 under Gross and Net primary productivity


Factors Controlling BOD and DO

  • A major source for DO comes from the atmosphere
  • When the surface of the water is disturbed as in rapids or water falls, then a greater the amount of oxygen will become dissolved in it.   
  • Temperature is a major factor. An increase in temperature  a decrease in DO. i.e the colder the water the greater the concentration of dissolved oxygen in the water.
  • Low DO from man-made source of heating bodies of water (effluent from nuclear power plants used to cool the reactors)
  • BOD is affected by respiration or lack of respiration by microorganisms and by the increase in organic matter (plant and animal) and organic waste and sewage.
  • Organic effluent can be treated by pumping it with large volumes of air.
indirect measurement
Indirect measurement
  • Indirect measurement involves the monitoring and measurement of organisms in the ecosystem and more specifically indicator species or index species. These are species that by virtue of their abundance or absence will indicate the level of pollution in that ecosystem.

Some important indicators are:

  • fish
  • bottom fauna
  • algae
  • bacteria

Comparison of diatoms to blue-green algae provides similar information

  • Measuring the coliform levels can also indicate the presence of sewage dumping in the ecosystem.

Overall the diversity of the whole system is often the best indicator while a general rule to follow is that presence is better evidence than absence.