Why is runoff in rivers and the ocean such a serious problem?

The main danger resulting from ocean contamination from runoff is the pervasive impact upon the food chain ... Rivers are ecosystems that many communities rely upon for food and drinking water. Contamination makes it dangerous to eat fish that contain PCBs, heavy metals, mercury or other chemicals.

The main danger resulting from ocean contamination from runoff is the pervasive impact upon the food chain. Chemical contaminants do not just poison or contaminate the flesh of fish and large sea creatures: but also smaller animals eaten by fish, and the plankton eaten by the smallest sea creatures.