Water pollution is an effect produced by human activities rather than natural environmental processes and takes the form of either mineral enrichment of static water bodies, or sewage discharge in to freshwater rivers.

I'll deal with the effects produced when sewage is discharged into rivers in this writeup, since there is an excellent writeup by pimephalis on the causes and effects of eutrophication. They key difference is that eutrophication is due to mineral ions already present in the pollutant, whilst in sewage discharge they are produced by microbial action.

What is organic effluent

Since sewage is a rather imprecise term, the name organic effluent is often preferred, and refers to incompletely processed sewage which still retains some organic materials in it. Broadly speaking we can say organic effluent contains

  • Waste water from industry
  • Organic solids such as food or animal slurry
  • Human faeces from domestic settlements
  • Human or animal urine

Pollution as a temporary process

When discharged in to a river, this organic effluent immediately causes both chemical and biological changes. These changes are severe, but temporary - an important distinction when compared to pollution of static water bodies which is permanent. Clearly it is therefore logical to only discharge organic effluent in to moving bodies; usually the environmental protection laws of a nation requires this, but enforcement is notoriously lax.

Measuring pollution

Often an underpaid but essential job, monitoring of pollution of rivers is left to bodies such as the Environmental Protection Agency, who employ trained scientists in the field of ecology to look at rivers exceeding set limits for pollution.

Study of the environment is the field of ecology and a person undertaking it is known as an ecologist. It is important to make a distinction between these people and environmentalists who campaign for change in the law to protect the environment. It is generally considered scientifically amoral to be in the pay of either an environmental group or industrial lobby group whilst practicing ecology, due to the potential for a conflict of interest.

Biotic indices

The first way of determining pollution in a water body is to look at the indicator species present. Since we know what species should be present in a normal river, it's possible to determine much by their absence. There are also a number of species which thrive in polluted water, again determining their numbers gives us an indication of pollution. Upon comparing these population numbers, it is possible to arrive at a biotic index for the river, allowing various water sources to be compared in terms of pollution.

Biochemical Oxygen Demand

Secondarily, ecologists try to measure the Biochemical Oxygen Demand of a water body, which is a measure of how much oxygen is being removed from the water at any given time by the organisms present. Since in a stable ecosystem any products removed should be in balance with products replaced, a high value for BOD suggests pollution is present in the river. Whereas we measure the biotic index through extensive biological testing; the BOD of a water body can be determined quickly through quicker chemical analysis. This makes it a suitable measure for checking water quality, with a followup biotic index being taken if the results indicate the need.

Effects of pollution

Chemical changes

It makes most sense to consider the chemical changes first, as these preclude and cause most of the biological changes taking place downstream. There are six major chemical factors we want to measure to get a good idea of the changes occuring:


So summarily looking at the above it's possible to say the first 250m of river after the discharge of organic effluent has little oxygen and high concentrations of toxic ammonia. It is therefore of little surprise that variations in the biotic index occur, as few species can survive in such conditions. After this distance, conditions begin to improve and plants can now survive. The river has however far from recovered.

Biological Changes

The chemical variations described above cause division of the river in to three main areas:

To survive in the toxic first 250m after the outfall, an organism must be able to deal with low oxygen concentrations and the presence of ammonia. For this stretch of the river we see four peaks in species, each fulfilling a niche environment and thriving due to lack of interspecific competition: Bloom area
Now that ammonia has largely been converted to nitrates, at 250m we see a massive increase in algae populations as they uptake the nitrates and phosphates and use them for growth. This rapid increase in numbers covers the river in an algal bloom. This effect in static water would cause eutrophication, but in moving water it serves to help remove much of the organic products contaminating the water. After a peak, numbers of algae and other protoctists begin to fall to normal expected levels as nutrient levels stabilise.

Recovery area

Finally, as oxygen levels recover and toxins have been removed, freshwater species begin to return. Key indicator species here include mayfly larvae, stonefly larvae, shrimp and in large rivers trout. At this point we are up to a kilometer down the river and both chemical and biological factors are broadly normal.

Conclusions

A little disclaimer

First it is necessary to point out that the distances quoted are averages, at best, and vary widely depending on river conditions. More turbulent rivers will cope better with pollution much better than slow moving mountain streams, the prior being clean after as little as 500m compared to 3-4km for the slowest streams.

Secondly the above species listed are by no means an exhaustive list - they are merely chosen as they are found in the vast majority of rivers. Species may vary widely in the tropics, but the same concepts will predominate.

Finally, I've simplified a lot details concerning Biochemical Oxygen Demand. If you want to know more about this factor, the relevant node would be an excellent place to start, but expect to do extensive research to get the full and complex picture.

So pollution's ok then?

I hope in the course of the writeup I haven't left you with the impression that water pollution is an acceptable outcome since rivers eventually recover. Recovery merely means that normal species are now present. It does not account for the damage caused to land surrounding the river due to the collapse of associated ecosystems depending on river flora and fauna. Many other chemicals present in organic effluent in trace amounts may cause lasting and irreperable damage; a good example would be hormones present from contraceptive pills causing birth defects in fish. Pollution is a very bad outcome, usually the result of very bad planning with regards to sewage treatment. Ideally all sewage should be treated fully so that organic effluent output could be zero. Given the cost to industry this is an unlikely outcome.

For the purposes of impartiality, I have no connections with environmental or industrial lobby groups.

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