How do fertilizers, pesticides, industrial waste, and sewage affect freshwater quality?

In the context of hydrologic sciences, human activities significantly degrade freshwater quality through the introduction of various pollutants.1

• Fertilizers: Agricultural runoff containing nitrates and phosphates promotes cultural eutrophication. These nutrients trigger rapid algal blooms, which eventually die and decompose. This process consumes dissolved oxygen, leading to hypoxic conditions that can kill fish and other aquatic life.2345
• Pesticides: These toxic chemicals enter water bodies via runoff, particularly when soil erosion is high. They can contaminate slow-flowing rivers, harm non-target organisms, and cause fish kills. Some persistent chemicals, such as certain hydrocarbons, accumulate in the food chain and pose long-term health risks to humans.65
• Industrial Waste: Untreated chemical discharges can alter biological activity and increase water acidity, potentially becoming corrosive to living organisms. Many industrial compounds are difficult to treat and may persist in the environment for decades, accumulating in aquatic tissues.5
• Sewage: Domestic sewage introduces pathogens and putrescible organic matter. Decomposition of this organic waste depletes oxygen levels, while the discharge of untreated sewage further accelerates eutrophication and introduces pharmaceutical residues that harm aquatic ecosystems.25