Sewage Pollution and Dissolved Oxygen Depletion

Steps:

• Untreated sewage introduces organic matter to the river.
• Bacteria decompose the organics via aerobic respiration, consuming dissolved oxygen (DO).
• This creates a high biochemical oxygen demand (BOD), lowering DO levels downstream from the spill.
• DO eventually recovers as organic load diminishes through dilution and bacterial die-off.

Why B is correct:

• B depicts a sharp DO drop post-spill followed by gradual recovery, matching the BOD curve where oxygen demand peaks then declines.

Why the others are wrong:

• A shows steady DO levels, ignoring pollution's oxygen-consuming effect.
• C indicates rising DO, which contradicts bacterial oxygen use.
• D shows continuous DO decline without recovery, overstating persistent pollution impact.

Final answer: B