Why did pollution become a major problem in the Great Lakes during the 20th century?

During the 20th century, pollution in the Great Lakes became a critical issue primarily due to the rapid buildup of human populations and industrialization around the lake basins. This environmental degradation was driven by several key factors:12

• Urbanization and Sewage: The growth of cities led to the discharge of domestic sewage into the lakes. This introduced pathogens and putrescible organic matter, which depleted dissolved oxygen levels necessary for aquatic life.31
• Nutrient Enrichment: Increased concentrations of nitrogen and phosphorus—stemming from sewage and agricultural runoff—accelerated eutrophication. This process promotes excessive algal growth, which, upon decomposition, further depletes oxygen and can lead to the death of aquatic ecosystems.43
• Industrial Contamination: Industrial activities, including mining and pulp and paper production, contributed to the accumulation of toxic substances and chemicals such as chloride, sodium, and sulfate. These pollutants, along with the buildup of sewage sludge on lake bottoms, rendered some fish species unfit for consumption and harmed wildlife.421

These impacts were most acute in the lower Great Lakes, leading to widespread beach closures and necessitating significant international pollution control efforts starting in the 1970s.12