Match List-I (Factor) with List-II (Effect on self-purification) and select the correct code:

Question

Match List-I (Factor) with List-II (Effect on self-purification) and select the correct code:

List-I

i. High DO

ii. High organic load

iii. Sunlight

iv. Low temperature

List-II

a. Retards

b. Promotes

c. Enhances O2 production

d. Slows reactions

Choose the correct match:

1. A. i-b, ii-a, iii-c, iv-d (Correct answer)
2. B. i-a, ii-b, iii-d, iv-c
3. C. i-b, ii-d, iii-a, iv-c
4. D. i-c, ii-a, iii-b, iv-d

Correct Answer

Option A — i-b, ii-a, iii-c, iv-d

Detailed Solution & Explanation

The correct answer is i-b, ii-a, iii-c, iv-d.

Key Points

• Each factor either helps or hinders a stream's recovery:
  • i. High DO - b. Promotes. Oxygen is what aerobic micro-organisms need to stabilise the waste.
  • ii. High organic load - a. Retards. Demand outstrips supply, oxygen is exhausted, and the reach turns anaerobic.
  • iii. Sunlight - c. Enhances oxygen production. Algal photosynthesis adds dissolved oxygen during daylight, and ultraviolet light also inactivates pathogens.
  • iv. Low temperature - d. Slows reactions. Biochemical breakdown proceeds more slowly in cold water.

Additional Information

• Low temperature is genuinely double-edged and worth understanding rather than memorising: it slows the biological reactions that stabilise waste, but it also raises oxygen saturation, so cold water carries a larger oxygen reserve. The option here isolates the reaction-rate effect.
• Sunlight's contribution is also two-sided over a day. Algae release oxygen while photosynthesising but consume it by respiration at night, which is why algal-rich waters show a pronounced diurnal DO swing and their lowest oxygen just before dawn.
• The factors that promote self-purification are broadly turbulence, high flow velocity, shallow depth, sunlight and dilution — everything that increases reaeration or reduces concentration.
• Those that retard it are high organic load, low DO, stagnant conditions, toxic substances that kill the decomposing organisms, and heavy suspended matter that blocks light.