Water pollution

Eutrophication is the natural ageing of a lake by nutrient enrichment of its water.
Streams draining into a lake bring nutrients such as nitrates and phosphates, which encourage the growth of aquatic organisms and algal blooms.
Organic remains accumulate at the bottom, the lake becomes shallower and warmer, and it is gradually converted into land.
Pollutants from sewage and agricultural and industrial wastes greatly speed up this process (cultural or accelerated eutrophication).
It is a phenomenon of standing fresh water bodies and is not seen on mountains or deserts; the open ocean is too large and well mixed.
Hence eutrophication is often seen in fresh water lakes.

Domestic sewage contains a large amount of biodegradable organic matter.
Micro-organisms in the receiving water body decompose this organic matter, and in doing so they consume a lot of oxygen.
As a result there is a sharp decline in dissolved oxygen downstream from the point of sewage discharge (high BOD), which causes mortality of fish and other aquatic creatures.
Although the impurities are only about 0.1%, they include pathogens and make the water unfit for drinking.
Hence the increased microbial activity uses up dissolved oxygen.