List any four factors which determine amount of dissolved oxygen in water. Explain in brief harmful effects of nitrate, fluoride and arsenic salts in ground water on humans.

$\text{Solution: }$ 

$\text{Factors determining amount of Dissolved Oxygen in water include the following: }$

$\text{Water temperature:}$

$?$The cold water has the possibility to hold more dissolved oxygen as compared to the warm water. 

$\text{Presence of organic water:}$

$?$Biochemical Oxygen Demand (BOD) is the amount of oxygen required by the microorganisms in milligrams in 5 days to completely decompose the organic wastes present in one litre of polluted water at 20°C. 

$?$BOD shows an inverse relationship with the dissolved oxygen (DO). 

$?$Higher is the amount of oxygen required, the greater is the degree of organic pollutants. 

 $\text{ Salinity:}$

$?$Salty water holds less oxygen than fresh water. 

$\text{Â Vegetation:}$

$?$The presence of vegetation in the water body can elevates the water oxygen content while performing the process of photosynthesis. 

$?$Vegetation shading a stream may decrease water temperatures which indirectly increases the amount of dissolved oxygen. 

$?$Harmful effects of nitrate, fluoride and arsenic salts in ground water on humans:

$?$Ill effects of nitrates: 

$?$Cyanosis (Blue baby syndrome) is the disease associated with the accumulation of nitrates in the human body. 

$?$The contamination of the nitrates by the water supplied to the child's body converts into nitrites which bind to the hemoglobin in the body, forming methemoglobin. 

$?$The compound methemoglobin is unable to carry oxygen.

$?$Ill effects of fluorides:

$?$Excess of fluorides in drinking water can result in fluorosis and can affect the bones and teeth of the human body.  

$?$Ill effects of Arsenic:

$?$Blackfoot disease is a  vascular disease which is caused due to the excess amount of exposure to arsenic. 

$?$Arsenic is circulated in the region through drinking water. 

$?$This damages the vascular system of the human body affecting its lower limbs and severely damaging it.