Phosphate is a nutrient from fertilizers and sewage that enters water through runoff. It causes algal blooms, reducing water quality and harming aquatic ecosystems.

Phosphate occurs naturally in rocks and organic matter but is also widely used in agriculture and industry. It enters water sources primarily through agricultural runoff and wastewater discharge. While not directly harmful to human health at levels typically found in drinking water, excess phosphate can severely impact aquatic ecosystems. It promotes algal growth, leading to eutrophication—a process that depletes oxygen in water bodies, harming fish and other aquatic life. Water quality regulations often target phosphate to protect environmental health rather than for direct human health concerns.