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).

Explanation:
When untreated sewage enters a river, it dramatically alters the aquatic environment. The graph illustrates these changes in the levels of two key components—Dissolved oxygen (represented by the solid line) and Mineral ions (represented by the dashed line) — plotted against the Distance along River on the x-axis.

Here is the step-by-step breakdown of why Point (i) indicates the entry of untreated sewage:

1. High Concentration of Mineral Ions: Sewage is highly concentrated with organic waste and dissolved mineral ions (such as nitrates and phosphates). At Point (i), the dashed line representing mineral ions is at a very high level, reflecting the immediate influx of these nutrients from the sewage discharge.

2. Decline and Minimum of Dissolved Oxygen: Untreated sewage contains a large amount of organic matter. Decomposers, such as aerobic bacteria, rapidly multiply to break down this organic waste. During this active decomposition process, these bacteria consume vast amounts of oxygen for respiration. As a result, the dissolved oxygen level (the solid line) plummets to its lowest level around Point (i).

3. Downstream Recovery (Self-Purification): As we move further downstream from Point (i) towards point (ii) and point (iii):
- The mineral ions are absorbed and utilized by aquatic plants and algae for growth, causing the mineral ion level (dashed line) to decrease.
- Photosynthesis by these growing plants and algae, combined with the reduction of organic material, allows the dissolved oxygen level (solid line) to recover and rise back up.

Therefore, the point of entry of untreated sewage is marked by the sharpest drop in dissolved oxygen and a peak in mineral ions, which corresponds to Point (i).