Explanation:

BOD (Biochemical Oxygen Demand):
Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by aerobic biological organisms in water to break down organic material present in a given water sample at a certain temperature over a specific time period. It is a widely used parameter to assess the organic pollution level of water bodies.

High BOD and Contamination with Sewage:
When a water body has a high BOD, it indicates that the water is being contaminated with sewage. Sewage is the wastewater generated from domestic, industrial, and commercial activities. It contains organic matter such as human and animal waste, food scraps, and other organic substances. When sewage enters water bodies, it introduces a large amount of organic pollutants.

Organic Matter and BOD:
Organic matter present in sewage serves as a food source for microorganisms. These microorganisms consume organic matter through the process of aerobic respiration, which requires oxygen. As a result, the dissolved oxygen level in the water decreases. The decrease in dissolved oxygen is directly proportional to the amount of organic matter present. Therefore, high levels of organic matter lead to high BOD values.

Impact of High BOD:
High BOD levels in water bodies have several negative impacts:
1. Decreased Dissolved Oxygen: As microorganisms consume organic matter, the dissolved oxygen level decreases, leading to oxygen depletion in the water. This decrease in dissolved oxygen can harm aquatic organisms, especially those that require oxygen to survive.
2. Eutrophication: High BOD levels contribute to eutrophication, which is the excessive growth of algae and other aquatic plants. The rapid growth of algae reduces light penetration, depletes oxygen during decomposition, and disrupts the natural balance of the ecosystem.
3. Odor and Visual Pollution: Decomposition of organic matter produces foul odors and can cause the water to appear murky and discolored.
4. Harm to Aquatic Life: High BOD levels can result in the death of fish and other aquatic organisms due to oxygen depletion.

Therefore, when a water body has high BOD, it is an indicator that the water is contaminated with sewage, and appropriate measures should be taken to treat and manage the pollution to protect the ecosystem and ensure the water quality.