Quite a few sewer systems have insufficient capacity for draining off water volumes when it rains. When the water in the pipelines rises above a given level, it floods basements and other low-lying areas. As most people know, this causes damage, fouling and contamination.
Many waste water treatment plants are overloaded as the clearing basins are not big enough, causing excess contaminated water volumes to flow into watercourses and lakes. When it rains, many overflow structures also divert large volumes of diluted waste water into watercourses and lakes.
If an oil separator is overloaded, the contents are whirled round and flow directly into and contaminate watercourses or lakes.