Pollution — AP Environmental Science APES Study Guide

For: AP Environmental Science candidates sitting AP Environmental Science.

Covers: Primary and secondary air pollutants, point vs non-point water pollution sources, solid waste management and recycling, and pollution reduction and remediation strategies as required by the APES CED.

You should already know: Algebra 1, basic biology and chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Environmental Science style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Pollution?

Pollution is the introduction of harmful, toxic, or excess substances into the natural environment (air, water, land) that cause adverse change to ecosystem function, human health, or the quality of natural resources. It is the core focus of Unit 7 in the AP Environmental Science CED, accounting for 7-10% of your total exam score, and appears frequently in both multiple-choice questions and free-response questions (FRQs), often paired with units on human health, sustainability, or climate change. Common synonymous terms you may see on exams include environmental contamination and toxicant release.

2. Air pollution — primary and secondary pollutants

Air pollution refers to the presence of harmful substances in the troposphere (the lowest layer of Earth’s atmosphere) at concentrations high enough to harm organisms, damage human-made structures, or alter ecosystem function. Pollutants are categorized into two core groups:

Primary pollutants

These are substances emitted directly from an identifiable source, before any chemical reactions occur in the atmosphere. Common examples include:

• Carbon monoxide (CO) from incomplete combustion of fossil fuels in vehicles and power plants
• Sulfur dioxide ($S{O}_2$) from coal combustion in power plants and industrial smelting
• Nitrogen oxides ($N{O}_x$, a mix of $NO$ and $N{O}_2$) from vehicle exhaust and high-temperature combustion
• Particulate matter ($P{M}_{2.5}$, $P{M}_{10}$) from wildfires, construction, and fossil fuel combustion
• Volatile organic compounds (VOCs) from gasoline evaporation, paint, and industrial solvent use

Secondary pollutants

These form in the atmosphere when primary pollutants react with each other, or with natural atmospheric components like water vapor, oxygen, or sunlight. Common examples include:

• Tropospheric ozone ($O_3$), formed when $N{O}_x$ and VOCs react in warm, sunny conditions
• Smog, a mixture of ozone, particulate matter, and other secondary pollutants
• Acid rain components (sulfuric acid $H_{2}S{O}_4$, nitric acid $HN{O}_3$) formed when $S{O}_2$ and $N{O}_x$ react with atmospheric water vapor

Worked Example: A city records peak summer tropospheric ozone concentrations of 0.12 ppm, well above the EPA’s 8-hour standard of 0.070 ppm. Ozone formation is directly proportional to $N{O}_x$ concentrations under the city’s high-VOC, high-sunlight conditions. If a new vehicle emission policy reduces $N{O}_x$ emissions by 45%, what will the new peak ozone concentration be?

Solution: First, calculate the percentage reduction in ozone: $0.12\text{ ppm}\times 0.45=0.054\text{ ppm}$ reduction. New peak ozone = $0.12-0.054=0.066\text{ ppm}$, which meets the EPA standard.

Examiner note: A very common test question asks you to classify pollutants as primary or secondary. Remember the mnemonic "good up high, bad nearby" to avoid confusing harmful tropospheric ozone (secondary) with beneficial stratospheric ozone that blocks UV radiation.

3. Water pollution — point vs non-point sources

Water pollution is the degradation of surface water (lakes, rivers, oceans) or groundwater quality by contaminants, reducing its suitability for human use (drinking, recreation, agriculture) or aquatic ecosystem health. Sources are split into two regulatory categories:

Point sources

These are discrete, identifiable discharge points where pollution is released directly into water. Examples include:

• Sewage treatment plant outflow pipes
• Industrial factory discharge pipes
• Oil tanker spill sites
• Abandoned mine drainage outlets Point sources are easy to monitor, regulate, and hold polluters accountable for, under the U.S. Clean Water Act’s National Pollutant Discharge Elimination System (NPDES) permit program.

Non-point sources

These are diffuse, widespread sources of pollution that do not come from a single discrete outlet. Examples include:

• Agricultural runoff of fertilizers, pesticides, and animal waste from farm fields
• Urban stormwater runoff carrying oil, sediment, and trash from roads and parking lots
• Atmospheric deposition of acid rain and air pollutants into water bodies Non-point sources are far harder to regulate, as pollution comes from large, distributed areas with multiple actors, making liability difficult to assign. They are addressed primarily through voluntary best management practices (BMPs) rather than mandatory permits.

Worked Example: A rural lake has experienced repeated algal blooms leading to hypoxic (low-oxygen) fish kills. Tests show elevated nitrogen and phosphorus levels. The lake’s watershed is 72% corn and soybean farms, 18% suburban housing, 8% forest, and 2% small businesses with septic systems. What is the most likely source of the nutrient pollution?

Solution: The largest share of the watershed is agricultural land, so non-point source runoff of synthetic fertilizers from farm fields is the most likely cause. Point sources like septic systems contribute less than 5% of total nutrient loads in most U.S. rural watersheds.

4. Solid waste and recycling

Solid waste refers to any discarded non-liquid, non-gaseous material, including municipal solid waste (MSW: household and commercial trash), industrial waste, hazardous waste, and electronic waste (e-waste). Waste management follows a standardized hierarchy, ordered from most to least environmentally preferable:

1. Source reduction: Reducing waste generation before it is created, e.g., designing products with minimal packaging, phasing out single-use plastics
2. Reuse: Using a product multiple times for its original purpose, e.g., reusable water bottles, glass jar food storage
3. Recycling: Processing discarded materials into new products, e.g., melting aluminum cans to make new cans, pulping paper to make new paper products
4. Energy recovery: Incinerating non-recyclable waste to generate electricity, reducing landfill volume by 90% but requiring air pollution controls to avoid toxic emissions
5. Disposal: Sending waste to lined landfills, the least preferable option, as landfills emit methane (a greenhouse gas 28x more potent than $C{O}_2$ over 100 years) and can leach toxic chemicals into groundwater if not properly maintained.

In the U.S., the overall MSW recycling rate is ~32%, with aluminum having the highest recycling rate (~68%) because recycling aluminum uses 95% less energy than producing new aluminum from bauxite ore. E-waste, which contains toxic heavy metals like lead, mercury, and cadmium, must be disposed of as hazardous waste, not regular MSW.

Worked Example: A city generates 220,000 tons of MSW per year, with 55% sent to landfills, 30% recycled, 10% incinerated for energy, and 5% composted. If a new curbside composting program diverts 80% of the 30% of MSW that is organic food/yard waste from landfills, what is the new percentage of waste sent to landfills?

Solution: First, calculate total organic waste: $220,000\times 0.3=66,000$ tons. 80% diverted = $66,000\times 0.8=52,800$ tons. Original landfill waste: $220,000\times 0.55=121,000$ tons. New landfill waste: $121,000-52,800=68,200$ tons. New landfill percentage: $\frac{68,200}{220,000}\times 100=31\%$.

5. Pollution reduction and remediation strategies

Pollution reduction refers to policies and practices that prevent pollution from being released in the first place, while remediation refers to the process of removing or neutralizing existing pollution from an affected environment to reduce its harm.

Core reduction strategies

• Air pollution: Catalytic converters on vehicles reduce $N{O}_x$, CO, and VOC emissions; scrubbers on coal power plants remove $S{O}_2$ and particulate matter; baghouse filters capture industrial particulate matter. Reducing $N{O}_x$ and VOC emissions also reduces secondary pollutants like tropospheric ozone and acid rain.
• Water pollution: NPDES permits for point sources; agricultural BMPs like cover crops, riparian buffer strips, and no-till farming to reduce runoff; urban BMPs like green roofs, permeable pavement, and rain gardens to capture stormwater.
• Solid waste: Extended Producer Responsibility (EPR) laws that require manufacturers to pay for end-of-life disposal/recycling of their products; curbside recycling and composting programs; single-use plastic bans.

Core remediation strategies

• Bioremediation: Using living organisms to break down or absorb pollutants, e.g., oil-eating bacteria for marine oil spills, sunflowers to absorb lead from contaminated soil. It is low-cost and low-disruption but slower than other methods.
• Physical remediation: Dredging contaminated sediment from waterways; capping landfills with impermeable material to prevent leachate and methane release; pumping contaminated groundwater to the surface for treatment.
• Chemical remediation: Injecting chemicals to neutralize toxic pollutants, e.g., adding lime to acidic mine drainage to raise pH and precipitate heavy metals.

Worked Example: A former industrial site has soil contaminated with 1400 ppm of lead, well above the EPA safe limit of 400 ppm. Phytoremediation using sunflowers can remove 50 ppm of lead per growing season per acre. How many growing seasons will it take to reduce lead levels to the safe limit?

Solution: Total lead to remove = $1400-400=1000$ ppm. Number of seasons = $\frac{1000}{50}=20$ growing seasons.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Classifying tropospheric ozone as a primary pollutant, or confusing it with beneficial stratospheric ozone. Why students do it: Both are called ozone, so students mix up their sources and effects. Correct move: Use the mnemonic "good up high, bad nearby" and always confirm the atmospheric layer when answering ozone questions; tropospheric ozone is secondary, stratospheric ozone is a natural beneficial substance.
• Wrong move: Classifying agricultural runoff as a point source of water pollution. Why students do it: Students assume farms are a single identifiable source, but runoff comes from the entire field, not a discrete outlet. Correct move: If pollution comes from a pipe, tanker, or single fixed outlet, it is point source; all diffuse, area-wide pollution is non-point.
• Wrong move: Listing recycling as the top priority waste management strategy. Why students do it: Recycling is widely promoted, so students forget the waste hierarchy prioritizes preventing waste entirely first. Correct move: Always cite source reduction > reuse > recycling when answering waste management questions, as source reduction avoids the energy and resource cost of processing waste entirely.
• Wrong move: Miscalculating waste diversion rates by counting diverted waste as part of landfill totals. Why students do it: Students forget to subtract recycled, composted, and incinerated waste from total waste before calculating landfill percentages. Correct move: Use the formula $\text{Landfill percentage}=\frac{\text{Total waste}-\text{Diverted waste}}{\text{Total waste}}\times 100\%$ to avoid errors.
• Wrong move: Classifying $S{O}_2$ and $N{O}_x$ as secondary pollutants because they cause acid rain. Why students do it: Students link the pollutants to their secondary byproducts. Correct move: Primary pollutants are emitted directly; secondary pollutants form in the atmosphere. Acid rain is secondary, but its precursors $S{O}_2$ and $N{O}_x$ are primary.

7. Practice Questions (AP Environmental Science Style)

Question 1

A mid-sized U.S. city reports the following annual air pollutant emissions: 13,000 tons of $N{O}_x$ from vehicles, 7,500 tons of $S{O}_2$ from coal power plants, 16,000 tons of VOCs from industrial solvents and gasoline evaporation, and 8,500 tons of tropospheric ozone formed in the atmosphere. (a) Classify each of the four pollutants as primary or secondary. (b) A new electric vehicle mandate is projected to reduce $N{O}_x$ emissions by 60% over 10 years. Assuming ozone formation is directly proportional to $N{O}_x$ levels, calculate the projected total annual tropospheric ozone emissions after 10 years. (c) Identify one additional air pollutant reduced by the switch to electric vehicles, and describe one associated human health benefit.

Solution

(a) Primary: $N{O}_x$, $S{O}_2$, VOCs; Secondary: tropospheric ozone (b) Ozone reduction = $8500\times 0.6=5100$ tons. New ozone levels = $8500-5100=3400$ tons per year. (c) Additional pollutant: $P{M}_{2.5}$ from fossil fuel combustion in gas vehicles. Health benefit: Reduced rates of childhood asthma, lung cancer, and cardiovascular disease in populations living near high-traffic corridors.

Question 2

A coastal watershed has seen a 45% increase in hypoxic dead zone frequency in the adjacent ocean over 20 years. The watershed is 65% agricultural row crops, 22% suburban development, 10% forest, and 3% industrial facilities with NPDES discharge permits. (a) Identify the most likely non-point source of nutrient pollution causing the dead zones, and describe the process by which this pollution leads to hypoxia. (b) Describe one BMP to reduce agricultural nutrient pollution, and one BMP to reduce urban stormwater pollution. (c) Explain why point source regulations alone cannot solve the dead zone problem in this watershed.

Solution

(a) Non-point source: Agricultural runoff of nitrogen and phosphorus fertilizers. Process: Excess nutrients cause dense algal blooms; when algae die, they are decomposed by aerobic bacteria that consume dissolved oxygen in the water, creating low-oxygen conditions that kill fish and aquatic organisms. (b) Agricultural BMP: Planting riparian buffer strips of native vegetation along streams to absorb nutrients before they reach the ocean. Urban BMP: Installing permeable pavement in parking lots to capture stormwater and allow it to infiltrate into the ground instead of flowing into waterways. (c) Point sources make up only 3% of the watershed, so even 100% elimination of point source nutrient pollution would leave 65% of the watershed as unregulated non-point agricultural sources, which contribute the vast majority of nutrient loads.

Question 3

A town generates 180,000 tons of MSW per year, with 45% sent to landfills, 28% recycled, 17% incinerated for energy, and 10% composted. (a) Calculate the total annual amount of waste diverted from landfills. (b) A new EPR law for packaging reduces total waste generation by 12% and increases the recycling rate to 38%. Assuming incineration and composting rates remain the same percentage of the new total waste, calculate the new annual amount of waste sent to landfills. (c) Describe one environmental advantage and one economic disadvantage of the EPR law.

Solution

(a) Diverted waste percentage = $28+17+10=55\%$. Total diverted waste = $180,000\times 0.55=99,000$ tons per year. (b) New total waste after 12% reduction = $180,000\times 0.88=158,400$ tons. New landfill percentage = $100-(38+17+10)=35\%$. New landfill waste = $158,400\times 0.35=55,440$ tons per year. (c) Environmental advantage: Reduced plastic pollution in oceans and landfills, as manufacturers are incentivized to use reusable or recyclable packaging. Economic disadvantage: Higher prices for packaged goods for consumers, as manufacturers pass EPR compliance costs on to customers.

8. Quick Reference Cheatsheet

9. What's Next

This pollution unit connects directly to several high-weight APES topics you will cover next: you will apply pollution reduction frameworks to the climate change unit (reducing greenhouse gas emissions is a form of global air pollution control), the human health unit (pollutants are the leading cause of environmentally mediated disease), and the sustainability unit (pollution prevention is a core pillar of sustainable development). FRQs frequently combine pollution with these topics, so practice linking the concepts you learned here to real-world scenarios like national climate policy or urban public health interventions to maximize your score.

If you have questions about any of the concepts in this guide, from classifying pollutants to calculating waste diversion rates, you can ask Ollie, our AI tutor, for additional practice problems, step-by-step explanations, or custom study plans tailored to your APES exam timeline. You can also find more study guides for other APES units on the homepage, including full-length practice FRQs and multiple-choice quizzes to test your knowledge before exam day.