Water Pollution Sources

Why This Matters

Water pollution isn't just about dirty water—it's about understanding how human activities disrupt aquatic ecosystems and threaten public health. On the AP Environmental Science exam, you're being tested on your ability to trace pollutants from their sources through environmental systems, predict their ecological impacts, and evaluate solutions. The concepts here connect directly to biogeochemical cycles, ecosystem disruption, eutrophication, bioaccumulation, and environmental policy.

Every pollution source in this guide illustrates a broader principle: point vs. nonpoint sources, nutrient loading, toxicity and persistence, or thermal dynamics. Don't just memorize what each pollutant is—know why it causes harm, how it moves through systems, and what distinguishes it from similar sources. That's what earns you points on FRQs.

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Point Sources: Traceable and Regulated

Point source pollution comes from a single, identifiable location—a pipe, outfall, or discharge point. Because these sources are traceable, they're easier to monitor and regulate under laws like the Clean Water Act.

Industrial Effluents

• Heavy metals and synthetic chemicals—factory discharge often contains mercury, lead, cadmium, and organic solvents that persist in aquatic systems
• Bioaccumulation risk means these toxins concentrate as they move up food chains, magnifying harm to top predators and humans
• Clean Water Act permits regulate discharge, but enforcement gaps and aging infrastructure remain significant challenges

Sewage and Wastewater

• Pathogens and nutrients enter water when sewage is untreated or inadequately processed, spreading waterborne diseases like cholera and giardia
• Biochemical oxygen demand (BOD) increases as bacteria decompose organic matter, depleting dissolved oxygen and suffocating aquatic life
• Eutrophication driver—excess nitrogen and phosphorus from sewage accelerate algal growth, creating hypoxic conditions

Thermal Pollution

• Heated discharge from power plants and industrial cooling raises water temperature, reducing dissolved oxygen capacity
• Species composition shifts occur as cold-water species like trout decline while warm-tolerant species dominate
• Synergistic effects combine with other stressors—warmer water holds less oxygen precisely when decomposition rates increase

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Nonpoint Sources: Diffuse and Difficult to Control

Nonpoint source pollution comes from many scattered locations across the landscape. Because it's diffuse and often weather-dependent, it's much harder to regulate than point sources.

Agricultural Runoff

• Nutrient pollution from nitrogen and phosphorus fertilizers is the leading cause of eutrophication in freshwater and coastal systems
• Pesticides and herbicides can be toxic to non-target organisms, disrupting aquatic food webs and contaminating drinking water
• Dead zones like the Gulf of Mexico hypoxic zone form when excess nutrients trigger algal blooms that decompose and deplete oxygen

Urban Stormwater Runoff

• Impervious surfaces like roads and parking lots prevent infiltration, increasing runoff volume and pollutant transport
• Mixed contaminants—oil, heavy metals, bacteria, and debris wash directly into waterways, often without any treatment
• Green infrastructure solutions including permeable pavement, rain gardens, and bioswales slow runoff and filter pollutants naturally

Atmospheric Deposition

• Mercury and sulfur compounds released from coal combustion settle into water bodies through rain and dry deposition
• Biomagnification of mercury in fish tissues creates human health risks, particularly for pregnant women and children
• Long-range transport means pollution sources hundreds of miles away can contaminate remote lakes and streams

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Persistent and Accumulating Pollutants

Some pollutants don't break down easily—they persist in the environment and accumulate in organisms over time. Understanding persistence and bioaccumulation is essential for predicting long-term ecosystem impacts.

Plastic Pollution

• Microplastics (particles < 5mm) form as larger plastics fragment, entering food webs when ingested by filter feeders and fish
• Persistence is extreme—plastics can take hundreds of years to degrade, accumulating in gyres and sediments
• Adsorption of toxins means microplastics can concentrate hydrophobic pollutants like PCBs, delivering chemical doses to organisms

Mining Activities

• Acid mine drainage (AMD) occurs when exposed sulfide minerals react with water and oxygen, producing sulfuric acid that leaches heavy metals
• Sediment loading smothers stream substrates, destroying habitat for benthic invertebrates and fish spawning
• Legacy contamination from abandoned mines continues polluting watersheds decades after operations cease

Landfill Leachate

• Complex chemical mixture—leachate contains heavy metals, organic compounds, ammonia, and pathogens from decomposing waste
• Groundwater contamination risk is high if liner systems fail, threatening drinking water aquifers
• Modern landfill design requires composite liners and leachate collection systems, but older unlined sites remain pollution sources

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Acute Events: Sudden and Catastrophic

Some pollution events are sudden rather than chronic, causing immediate and severe ecosystem damage. These events test your understanding of ecological resilience and recovery.

Oil Spills

• Immediate toxicity kills organisms through smothering, hypothermia (when oil destroys insulation), and poisoning from ingestion
• Habitat destruction affects marshes, mangroves, and coral reefs that may take decades to recover
• Dispersants trade-off—chemical dispersants break up surface slicks but may increase toxicity to subsurface organisms

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Quick Reference Table

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Self-Check Questions

1. Which two pollution sources are most directly responsible for eutrophication and dead zone formation, and what specific nutrients do they contribute?

2. Compare point source and nonpoint source pollution: Why is nonpoint source pollution generally harder to regulate, and which examples from this guide illustrate that challenge?

3. A lake downstream from both agricultural land and an old mining site shows fish die-offs. What different pollutants might each source contribute, and how would their mechanisms of harm differ?

4. Thermal pollution and eutrophication both reduce dissolved oxygen in water bodies. Explain the different mechanisms by which each causes oxygen depletion.

5. An FRQ asks you to propose solutions for reducing water pollution in a coastal watershed with farms, cities, and industrial facilities. Which pollution sources would require regulatory approaches vs. land management changes, and why?