In environmental science, the pollution level of a river is commonly assessed by examining which of the following parameters related to the water?

Introduction / Context:
Water pollution affects aquatic life, human health, and the usefulness of river water for drinking, irrigation, and recreation. Scientists and environmental agencies use certain indicators to measure how polluted a river is. One important indicator is related to how much oxygen is dissolved in the water, because aquatic organisms depend on this oxygen for survival. This question asks which parameter is commonly used to assess water quality in a river.

Given Data / Assumptions:

• The question is about measuring or assessing water pollution in a river.
• Options include amounts of dissolved chlorine, ozone, nitrogen, oxygen, and suspended soil particles.
• We assume a basic understanding of dissolved oxygen and pollution effects on aquatic life.
• We focus on a parameter directly influenced by organic pollution and biological activity.

Concept / Approach:
Dissolved oxygen is a key indicator of water quality. Clean, unpolluted water normally has a relatively high level of dissolved oxygen, which supports fish and other aquatic organisms. When organic waste or pollutants enter a river, bacteria use up oxygen while decomposing this material. As a result, the dissolved oxygen level drops. Very low dissolved oxygen can lead to fish kills and indicates severe pollution. While biochemical oxygen demand (BOD) is another important indicator, among the options given, the amount of oxygen dissolved in water directly reflects pollution levels in a simple and widely used way.

Step-by-Step Solution:
Step 1: Recognise that river water should contain enough dissolved oxygen to support aquatic life. Step 2: Understand that when pollutants, especially organic waste, enter the water, microorganisms consume oxygen to break it down. Step 3: As microbial activity increases, dissolved oxygen decreases; lower oxygen means poorer water quality. Step 4: Chlorine and ozone may be used for disinfection but are not primary indicators of organic pollution levels in rivers. Step 5: Nitrogen compounds can contribute to eutrophication, but the question specifically emphasises pollution measurement using a simple dissolved parameter. Step 6: Suspended soil particles cause turbidity, which affects clarity but is not a direct indicator of organic or chemical pollution. Step 7: Therefore, the amount of oxygen dissolved in water is a key measure for assessing river pollution.

Verification / Alternative check:
Environmental science references frequently list dissolved oxygen (DO) as a primary water quality parameter. They explain that healthy rivers have adequate DO levels, whereas polluted rivers with heavy organic loads show depleted DO. While more advanced assessments use indicators such as BOD and chemical oxygen demand (COD), DO remains a straightforward measure used in many monitoring programmes. This supports the choice of dissolved oxygen as the correct answer.

Why Other Options Are Wrong:
Amount of chlorine dissolved in water: Chlorine is mainly used in treated water supplies as a disinfectant and is not a standard indicator for natural river pollution levels. Amount of ozone dissolved in water: Ozone is also a disinfectant and is not commonly measured in natural river settings. Amount of nitrogen dissolved in water: High nitrogen can contribute to algal blooms, but alone it is not the standard simplest indicator of overall pollution in this type of question. Amount of suspended soil particles in water: This affects turbidity rather than directly indicating organic or chemical pollution.

Common Pitfalls:
Students may confuse dissolved oxygen with chlorine or nitrogen because all involve chemicals in water. Others may focus on visible turbidity and think soil particles are the main pollutant. To avoid these mistakes, remember that aquatic organisms need oxygen, and dissolved oxygen is a direct, sensitive indicator of river health and organic pollution levels.

Final Answer:
The correct answer is Amount of oxygen dissolved in water.