Eutrophication and soil pollution
Explanation
Discharging untreated sewage into water bodies introduces high loads of organic matter, nutrients (specifically nitrates and phosphates), and pathogens. This influx fundamentally alters the chemical and biological characteristics of the aquatic ecosystem.
Analysis of Options:
- Eutrophication and soil pollution is Correct: The primary consequence of sewage discharge is nutrient enrichment, leading to eutrophication. The excess nitrogen and phosphorus trigger rapid algal blooms, which deplete dissolved oxygen upon decomposition. Additionally, when such polluted water is used for irrigation or floods adjacent lands, it leads to soil pollution by depositing heavy metals, salts, and pathogens.
- Increased freshwater availability is Incorrect: Pollution degrades water quality, rendering it unfit for consumption or agricultural use, thereby effectively reducing the availability of usable freshwater.
- Reduction in water-borne pathogens is Incorrect: Untreated sewage is a primary vector for water-borne pathogens (bacteria, viruses, and protozoa), leading to diseases such as cholera, typhoid, and dysentery.
- Stabilization of aquatic ecosystems is Incorrect: The introduction of sewage destabilizes aquatic ecosystems by increasing Biological Oxygen Demand (BOD) and causing hypoxia (dead zones), leading to the death of aquatic organisms.
Key Takeaway: Untreated sewage discharge primarily causes eutrophication due to nutrient loading and increases Biological Oxygen Demand (BOD), leading to oxygen depletion and ecosystem collapse.