What is a sign of potential sewage contamination in water supplies?

A foul smell in water supplies is a significant indicator of potential sewage contamination. This odor is often associated with the breakdown of organic matter and the presence of bacteria from sewage. When waste enters a water source, it can lead to anaerobic conditions, producing gases like hydrogen sulfide, which is notorious for its rotten egg smell. This foul odor serves as a warning sign that the water may be unsafe for consumption and could harbor pathogens responsible for waterborne diseases.

The other alternatives do not specifically indicate sewage contamination. Clear appearance might suggest clean water but does not guarantee safety from contaminants. The presence of heavy metals can be due to industrial pollutants or mineral deposits and doesn't necessarily link to sewage. High mineral content might indicate certain geological formations but isn't a direct sign of sewage pollution. Therefore, the presence of a foul smell stands out as the primary indicator of potential sewage issues in water supplies.