1. Fertilizers: Fertilizers contain nitrates and phosphates, which are essential nutrients for plant growth. However, when these nutrients enter waterways in excess, they can cause algal blooms. These blooms deplete oxygen in the water, harming fish and other aquatic life.

2. Heavy Metals: Heavy metals, such as mercury, lead, and arsenic, are toxic to humans and wildlife. They can enter water sources through industrial waste, mining activities, or even the breakdown of older paint and pipes. These metals accumulate in the food chain, posing health risks.