The gas causing the distinct "rotten egg" odor in many water sources is

The gas responsible for the distinct "rotten egg" odor in many water sources is hydrogen sulfide. This compound is a colorless gas that is primarily produced by the breakdown of organic matter in the absence of oxygen, often occurring in stagnant water, groundwater, and certain industrial processes. When present in water, even at low concentrations, hydrogen sulfide can be detected by its characteristic smell, which is reminiscent of rotten eggs.

In addition to its unpleasant odor, hydrogen sulfide can also have implications for environmental health, specifically concerning water quality and safety. It can be harmful in higher concentrations and may also indicate the presence of other contaminants in the water source.

The other gases listed do not produce a "rotten egg" odor. Carbon dioxide is odorless and is produced through respiration and various chemical processes. Chlorine gas has a sharp, pungent odor often associated with swimming pools and disinfection processes, while hydrogen carbonate does not typically present any distinct smell. Thus, hydrogen sulfide is clearly identified with the "rotten egg" scent, linking it directly to the question's prompt.