During a field test, where would hydrogen sulfide be found in a manhole?

• A. Near top
• B. Near bottom
• C. Evenly dispersed
• D. Not at all because manhole lids have pick holes

Answer: (B)

Hydrogen sulfide is heavier than air, so in a confined space like a manhole it tends to settle toward the bottom. This means its concentration is highest near the floor, where gas produced by sewage can accumulate, especially when ventilation is limited. Sampling from the bottom is more likely to detect the presence and level of H2S than sampling near the top, where diluting air reduces concentration. The idea that it would be evenly dispersed isn’t typical in a stagnant manhole, and the notion that lids with holes prevent it isn’t accurate because gas can still collect at the bottom despite openings.