Question
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The concentration of fluoride, lead, nitrate, and iron in a water sample from an underground lake was found to be 1000ppb, 40ppb, 100ppm, and 0.2ppm, respectively. This water is unsuitable for drinking due to the concentration of:
(A) Lead
(B) Nitrate
(C) Iron
(D) Fluoride

Answer Verified Verified
Hint: Drinking water is one of the basic needs of life and essential for survival. Water contamination due to pathogenic agents, chemicals, heavy metals, pesticide water disinfectants, and thereby product as a consequence of industrial and agricultural activities become a hazard to human health. According to the World Health Organisation (WHO), Indian Standard Institution(ISI), Central Pollution Control Board (CPCB), and Indian Council Of Medical Research (ICMR) gave some permissible limits in water suitable for drinking purposes.

Complete answer:
The permissible limit of fluoride in drinking water =1-1.5 mg/L
Given,
the concentration of fluoride in underground lake water = 1000ppb (parts per billion)= 1ppm
(since the number of part per million is equal to parts per billion divided by 1000)
The permissible limit of lead in drinking water=0.01mg/L
the concentration of lead in underground lake water(given) = 40ppb=$\dfrac{40}{1000}ppm=0.04ppm$
The permissible limit of nitrate in drinking water=45mg/L
the concentration of nitrate in underground lake water(given)=100ppm
The permissible limit of iron in drinking water=0.3mg/L
the concentration of iron in underground lake water(given)=0.2ppm
from the above observations, the concentrations of given parameters as follows,
nitrate>fluoride>iron>lead , i.e 100ppm > 1ppm > 0.2ppm > 0.04ppm
Hence, This water is unsuitable for drinking due to the concentration of nitrate which concentration is 100ppm.
So, the correct answer is “Option B”.

Note: If water is unsuitable for drinking purposes by the concentration of parameters, then there are methods to estimate or measure the concentrations by experimental methods. For example, to determine dissolved oxygen by “Modern Winkler’s method”, for chloride estimation by argentometric method, for nitrates determination by “Brucine method”, etc. After that various methods used to remove completely or up to permissible limits in drinking water.