Which radioactive noble gas naturally seeping from rocks is an important source of natural groundwater and indoor air pollution?

Introduction / Context:
Groundwater and indoor air can be contaminated by naturally occurring substances as well as human made pollutants. Some gases originate from radioactive decay of elements in rocks and can seep into soil, water and buildings. This question asks you to identify which noble gas is a recognised source of natural groundwater and indoor air pollution. Understanding such pollutants is important for environmental chemistry and public health.

Given Data / Assumptions:

• The gases listed are noble gases: helium, krypton, radon, argon and neon.
• We are concerned with natural groundwater pollution and indoor air quality.
• One of these gases is radioactive and produced from decay of uranium and thorium in rocks.
• Basic environmental science knowledge is assumed.

Concept / Approach:
Radon is a radioactive noble gas that arises from decay of radium, itself a decay product of uranium and thorium in the Earth crust. Radon can diffuse through soil and enter groundwater or accumulate in basements and enclosed spaces. Because it is radioactive, long term exposure to elevated radon levels increases the risk of lung cancer. Other noble gases like helium, neon, argon and krypton are largely inert and non radioactive in their common isotopes and do not significantly pollute groundwater in a harmful way at normal concentrations. The approach is to recall that radon is the problematic radioactive gas among the noble gases.

Step-by-Step Solution:
Step 1: List the candidate noble gases from the options: helium, krypton, radon, argon and neon. Step 2: Recall that radon is a radioactive noble gas produced by natural radioactive decay chains in rocks and soil. Step 3: Understand that radon gas can dissolve in groundwater and can also seep into houses, especially in areas with uranium rich rocks. Step 4: Recognise that helium, neon, argon and krypton are not significant radioactive pollutants in normal environmental conditions. Step 5: Conclude that the gas responsible for natural groundwater pollution among the given options is radon.

Verification / Alternative check:
Environmental agencies and health organisations identify radon as a major naturally occurring indoor air pollutant. Maps of radon risk often show regions where geological formations lead to higher radon emission. Studies of groundwater quality in certain areas also report elevated radon levels, especially in wells drilled into granite or other uranium containing rocks. No similar warnings are routinely issued for helium or other noble gases in groundwater, supporting the conclusion that radon is the relevant pollutant here.

Why Other Options Are Wrong:
Option A, helium, is inert and non radioactive in its common isotopes and is not considered a serious groundwater pollutant. Option B, krypton, also has stable isotopes and is not a major natural groundwater contaminant. Option D, argon, is present in the atmosphere and can dissolve slightly in water, but it is not harmful at normal concentrations and is not a radioactive pollutant. Option E, neon, is similarly inert and not associated with groundwater pollution. Only option C, radon, matches the description of a radioactive gas that pollutes groundwater and indoor air.

Common Pitfalls:
Students may be tempted to choose argon or helium because they are more familiar names or more abundant in the atmosphere. Another pitfall is to forget that the main reason radon is dangerous is its radioactivity, not its chemical reactivity. Remember that radon is unique among the noble gases in being significantly radioactive in the environment and is specifically monitored in homes and water supplies in some regions.

Final Answer:
The correct gas is Radon, a radioactive noble gas that can pollute groundwater and indoor air when it seeps from rocks and soil.