Question

Which graph shows the effect of pollution by sewage on the amount of oxygen dissolved in the river?
A.

B.

C.

D.

Answer 1

Sewage is being discharged in rivers and other water bodies without being treated and this causes a risk to the aquatic life and also the life of animals or living organisms who consume this water.

Complete answer:
Dissolved oxygen is the amount of gaseous oxygen that gets dissolved with the water by absorption from the surface. To maintain good quality of water dissolved oxygen levels should be above 7-8 mg/L. This dissolved oxygen is used by the aquatic animals for their respiration. If the levels of dissolved oxygen fall down (below 5 mg/L) which happens due to the increased pollution then water is unhealthy for any kind of use. Decreased in the dissolved oxygen level because aquatic life in a stress like situation where they do not have enough oxygen to breathe and fishes die in masses due to this. This happens because of the different kinds of organic wastes that are dumped into the rivers like untreated sewage, Industrial waste, and agricultural waste which use the dissolved oxygen of the river for their own degradation and slowly finish the dissolved oxygen present.

In most of the underdeveloped or undeveloped nation untreated sewage containing human faeces and waste water from bathing, washing clothes and other human activities which are not industrial is being dumped into the nearby water bodies. This untreated sewage carries waterborne pathogens like cholera, typhoid which can cause serious illness. When sewage enters a water body it uses all the dissolved oxygen and the anaerobic bacteria present in it eat all the organic material and produce hydrogen sulphide which causes a foul smell and leads to the growth of water hyacinth and other weeds which cover the surface of the water body leading to the complete destruction of the aquatic life.

Hence, the correct answer is option (A), where the dissolved oxygen levels are decreasing rapidly and after sometime they become stable when all the dissolved oxygen is consumed.

Additional information:
• Excess levels of dissolved oxygen are also not good as they lead to excessive plant growth, algal blooms which are again not good for aquatic life.
• Temperature and pH also affect the levels of water pollution. Increase in temperature and decrease in pH both cause the decrease in dissolved oxygen levels.
• Water which is continuously moving has good levels of dissolved oxygen as compared to stagnant water bodies like ponds.

Note: When the water is warm the bacteria and aquatic organisms accumulate and use all the dissolved oxygen due to which its levels fall down. If bacteria get organic matter in the water then they rapidly degrade it and multiply themselves using huge amounts of dissolved oxygen.