How can untreated wastewater and agricultural runoff pollute freshwater sources?

Untreated wastewater and agricultural runoff pollute freshwater sources through several mechanisms that degrade water quality and threaten aquatic ecosystems:

• Pathogens and Organic Matter: Untreated domestic sewage introduces disease-causing microorganisms and putrescible organic substances. As bacteria decompose these organics, they deplete dissolved oxygen, which is essential for the survival of fish and other aquatic life.1
• Cultural Eutrophication: Both sewage and agricultural runoff are rich in nitrates and phosphates. These nutrients trigger rapid algal blooms. When this algae dies, its decomposition further consumes oxygen, leading to eutrophication—a process that can cause the premature death of a water body.1
• Chemical Contamination: Agricultural runoff often carries insecticides, herbicides, and other chemicals that can poison aquatic life and contaminate groundwater. Additionally, irrigation practices can increase the salinity of water supplies, further impairing quality.234
• Toxicity: Nitrates and nitrites in groundwater, originating from agricultural and industrial operations, pose direct poisoning risks to humans and livestock.3