The graph plotted below is based on the data collected by biology students with respect to the levels of oxygen at the specific points in the river flowing outside their city. Which point in the graph indicates the entry of untreated sewage in the river ?

The correct option is Point (i).

Detailed Explanation:
Let us analyze the graph provided in the image:

• The y-axis represents the Level of the substances in the river water.
• The x-axis represents the Distance along River, indicating the direction of water flow from left to right.
• The solid line represents Dissolved oxygen.
• The dashed line represents Mineral ions.
• Four specific positions along the river are marked as (i), (ii), (iii), and (iv).

When untreated sewage is discharged into a river, it triggers specific biological and chemical changes:

1. High Mineral Concentration: Untreated sewage contains high levels of organic waste and dissolved mineral ions (such as nitrates and phosphates). Consequently, the concentration of mineral ions is at its peak near the point of discharge.
2. Oxygen Depletion: The organic matter in the sewage serves as a food source for aerobic bacteria and other decomposers. These microorganisms multiply rapidly and consume dissolved oxygen from the water during aerobic respiration to break down the organic pollutants. This causes the dissolved oxygen levels to drop to their minimum.

Looking at the graph, at Point (i):

• The concentration of mineral ions (dashed line) is at a maximum level.
• The level of dissolved oxygen (solid line) is at its lowest level.

Immediately downstream of Point (i) (towards Point (ii) and Point (iii)), the dissolved oxygen starts to recover and rise because the organic waste gets decomposed, and the water undergoes self-purification (aeration). At the same time, the mineral ion concentration decreases as these nutrients are absorbed by growing aquatic plants and algae.

Therefore, Point (i) represents the point of entry of untreated sewage into the river.