Which gas is primarily found in groundwater, is heavier than air, and has a strong odor?

The gas that is primarily found in groundwater, is heavier than air, and has a strong odor is hydrogen sulfide. This gas is often associated with the decay of organic matter and can be produced by anaerobic bacteria in groundwater environments. Its distinctive rotten egg smell makes it easily recognizable.

Hydrogen sulfide is more dense than air, which means it can accumulate in low-lying areas and pose risks to human health. Additionally, its presence in water can indicate underlying issues, such as corrosion of plumbing or the presence of certain bacteria, making it crucial for drinking water treatment operators to monitor and mitigate its effects.

While methane is also a gas that can be found in groundwater, it is typically odorless unless it is mixed with other compounds. Ammonia has a distinctive smell, but it is lighter than air and forms under different conditions, primarily in areas with decaying organic substances. Carbon dioxide, while found in groundwater, is also heavier than air but is not known for a strong odor. Thus, hydrogen sulfide is the most accurate choice given its characteristics.