Final answer:
During heavy rain, stormwater runoff can overwhelm a combined sewer system and cause untreated sewage to be discharged into surface waters.
Step-by-step explanation:
During heavy rain, stormwater runoff from the surface of the ground can overload a combined sewer system and cause untreated sewage to be discharged into surface waters. Combined sewer systems use a single set of underground pipes to collect both sewage and stormwater runoff from streets for treatment.
This can be a major source of water pollution, as the excess stormwater can overwhelm the capacity of the sewer system and lead to untreated sewage being released into nearby water bodies. This can negatively impact the quality of the water and the health of the ecosystems that depend on it.
To protect water resources, it is important to manage stormwater runoff effectively and maintain the capacity of sewer systems to handle both stormwater and sewage.