Which gas, heavier than air and with a strong odor, is often found in groundwater?

Hydrogen sulfide is a colorless gas that is indeed heavier than air and has a distinct, strong odor often described as smelling like rotten eggs. It is commonly found in groundwater, particularly in areas where there is an abundance of organic matter, such as wetlands or near industrial waste sites. The presence of hydrogen sulfide in groundwater can originate from the decomposition of organic materials or from geological sources, such as certain types of rock formations.

The significance of hydrogen sulfide in water quality management lies in its potential health impacts and the challenges it poses for water treatment. At low concentrations, it can be a nuisance due to its odor, but at higher levels, it can be toxic to humans and aquatic life. Identifying and managing its presence in drinking water sources is crucial for ensuring safety and compliance with water quality standards.

In contrast, other gases listed have different attributes: carbon dioxide is commonly dissolved in water but does not have a strong odor; radon, while potentially found in groundwater, is odorless and associated with radioactive decay; and methane, while it can be found in groundwater, is not typically characterized by a strong odor and is lighter than air.