Which statement best describes overflow events that occur during heavy rainfall?

• A. Water treatment plant overflow
• B. Sanitary sewer overflows (SSOs) and combined sewer overflows (CSOs)
• C. Overflow from storm drains
• D. Groundwater contamination

Answer: (B)

Overflow during heavy rainfall typically involves sanitary sewer overflows and combined sewer overflows. When rain is intense, water infiltrates and inflows into the sewer system, pushing the flow beyond what the pipes and treatment facilities can handle. In sanitary sewer systems, this can force untreated wastewater to escape through outfalls or manholes to nearby water bodies. In older cities with a combined sewer system, stormwater and wastewater share the same pipes, so heavy rain can cause a surge that leads to discharges of a mixture of stormwater and sewage into rivers or streams. This is why those overflow events are the most relevant during heavy rainfall and have direct implications for water quality and public health. Overflow from storm drains involves only stormwater, not sewage, and groundwater contamination is not an overflow mechanism tied to sewer system capacity in the same way.