AP Environmental Science Aquatic and Terrestrial Pollution — Worked Answer Explanations

Unit 8 · 12 questions explained

Below is a complete answer key for our AP Environmental Science Aquatic and Terrestrial Pollution practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Aquatic and Terrestrial Pollution practice test and come back here to review, or head back to the Aquatic and Terrestrial Pollution unit overview.

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Question 1 · Easy
Superfund (CERCLA) sites are designated locations where:
- A
  Renewable energy facilities are built on abandoned land
  Why not A: Brownfield redevelopment for solar/wind is separate from the Superfund program.
- B
  Hazardous substances have been released and require federally mandated remediationCorrect
- C
  Nuclear power plants are sited to ensure proximity to cooling water sources
  Why not C: Nuclear plant siting is regulated by the NRC, not Superfund.
- D
  Municipal solid waste landfills operate under strict modern containment standards
  Why not D: Modern Subtitle D landfills are regulated under RCRA; Superfund addresses pre-existing contaminated sites, not operating landfills.
Explanation
The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, 1980), commonly called Superfund, authorizes EPA to identify and clean up sites contaminated with hazardous substances — including abandoned industrial sites, old landfills, and chemical spills. The polluter-pays principle makes responsible parties liable for cleanup costs; where parties cannot be found, the Superfund (now financed by general tax revenue) pays. The Love Canal neighborhood (NY) was a key catalyst for CERCLA.

Key takeaway
Superfund (CERCLA): federal authority to remediate hazardous waste sites; 'polluter pays' principle. Love Canal was the catalyst.
Question 2 · Easy
A river downstream of a cattle feedlot shows high concentrations of nitrates, fecal coliform bacteria, and ammonia. Which source category best describes this pollution?
- A
  Point source pollution
  Why not A: Point sources discharge from a single, identifiable location (pipe, outfall). Feedlot runoff typically disperses across a landscape before entering a waterway — making it nonpoint.
- B
  Nonpoint source pollutionCorrect
- C
  Thermal pollution from feedlot operations
  Why not C: Thermal pollution is warm water discharged into a water body (typically from power plants); feedlot runoff causes chemical, not thermal pollution.
- D
  Natural background pollution from wildlife fecal matter
  Why not D: The concentrations described are far above natural background levels and are attributable to a specific human land use (cattle feedlot).
Explanation
Nonpoint source (NPS) pollution comes from diffuse areas — runoff from agricultural fields, urban streets, construction sites — and cannot be traced to a single discharge pipe. Feedlot runoff carrying manure, nitrates, and pathogens enters waterways across a landscape. The Clean Water Act regulates point sources (via NPDES permits) but NPS pollution is harder to regulate and is now the leading cause of water quality impairment in the U.S.

Key takeaway
Point source: single pipe/outfall. Nonpoint source: diffuse runoff (agriculture, urban runoff) — now the leading U.S. water quality problem.
Question 3 · Medium
Methylmercury in aquatic systems is most hazardous to organisms at the top of the food chain because of:
- A
  Bioaccumulation: mercury accumulates in individual organisms that cannot excrete it
  Why not A: Bioaccumulation describes accumulation in a single organism's lifetime; the APEX predator problem also involves transfer and multiplication across trophic levels (biomagnification).
- B
  Biomagnification: mercury concentration multiplies at each trophic level, reaching dangerous levels in top predatorsCorrect
- C
  Bioavailability: mercury dissolves better in saltwater near the ocean surface, concentrating in large marine predators
  Why not C: Bioavailability affects exposure but doesn't explain the systematic increase in concentration with trophic level.
- D
  Biodegradation: top predators cannot break down mercury, while lower organisms can metabolize it safely
  Why not D: Lower organisms cannot degrade methylmercury either; biomagnification occurs because it accumulates in lipid tissue at every level.
Explanation
Methylmercury (organic mercury produced by anaerobic bacteria from inorganic mercury) is fat-soluble and not excreted. It accumulates in organisms' lipid tissue (bioaccumulation). When zooplankton eat phytoplankton, they concentrate mercury 10-100×. Each trophic level further concentrates it — biomagnification. Tuna, swordfish, and polar bears (top predators) carry mercury concentrations millions of times higher than surrounding water. Minamata disease (Japan, 1950s) illustrated human health consequences.

Key takeaway
Biomagnification: fat-soluble pollutants (Hg, DDT, PCBs) multiply in concentration at each trophic level → highest in apex predators.
Question 4 · Medium
Which of the following best describes the biological oxygen demand (BOD) of a water sample?
- A
  The total concentration of dissolved oxygen present in the water
  Why not A: Dissolved oxygen (DO) is how much O₂ is in the water; BOD measures how much O₂ will be consumed by decomposition.
- B
  The amount of oxygen required by aerobic microorganisms to decompose organic matter in the waterCorrect
- C
  The concentration of nitrogen and phosphorus needed for aquatic plant growth
  Why not C: Nitrogen and phosphorus are nutrient concentrations, not the BOD, which specifically refers to oxygen demand from decomposition.
- D
  The rate at which oxygen is produced by algae and aquatic plants through photosynthesis
  Why not D: Photosynthesis produces O₂; BOD measures O₂ consumption by decomposition — these are opposite processes.
Explanation
BOD (Biological Oxygen Demand) is a measure of water quality — it quantifies the oxygen that aerobic bacteria will consume as they break down organic matter in a water sample (typically over 5 days at 20°C). High BOD = high organic pollution = low dissolved oxygen remaining = poor water quality. Sewage, agricultural runoff, and paper mill effluent have high BOD.

Key takeaway
High BOD = high organic pollution = bacteria consume O₂ rapidly = low DO = poor water quality.
Question 5 · Medium
Microplastics in marine environments pose ecological threats because they:
- A
  Dissolve in seawater and release chlorine, which acidifies the ocean
  Why not A: Plastics don't dissolve in seawater and don't release chlorine; ocean acidification is driven by CO₂ absorption.
- B
  Are ingested by marine organisms, causing physical harm and acting as vectors for toxic chemicalsCorrect
- C
  Block sunlight from reaching phytoplankton, reducing ocean primary productivity globally
  Why not C: While surface plastic patches exist, the density is not sufficient to significantly reduce global photosynthesis.
- D
  React with saltwater to form phthalate acids that kill coral reefs
  Why not D: Phthalates are plasticizers that can leach from plastics, but they don't react with saltwater to form acids; and coral bleaching is primarily driven by warming, not phthalates.
Explanation
Microplastics (<5 mm) are ingested by filter feeders, fish, seabirds, and marine mammals — either mistaken for food (pellets resemble fish eggs) or consumed incidentally. They physically block digestive tracts, cause false satiation, and transfer toxic chemicals (PCBs, DDT) that adsorb to their surfaces. They also enable invasive species transport and are now found in every marine environment from deep trenches to Arctic ice.

Key takeaway
Microplastics: ingested by marine organisms → physical blockage, false satiation, toxic chemical transfer (bioconcentration on plastic surfaces).
Question 6 · Medium
A city's sewage treatment plant discharges treated effluent into a river. Primary treatment removes ___; secondary treatment removes ___.
- A
  Dissolved nutrients (N, P); large solids and debris
  Why not A: Primary treatment removes physical solids first (not dissolved nutrients); secondary treatment uses biology to remove organic/dissolved matter.
- B
  Large solids and settleable materials; dissolved organic matter via biological treatmentCorrect
- C
  Disease-causing pathogens; heavy metals from industrial wastewater
  Why not C: Pathogen disinfection (chlorination, UV) is tertiary treatment; heavy metals require specialized industrial treatment.
- D
  Heavy metals; nitrogen and phosphorus nutrients
  Why not D: Heavy metal removal and nutrient removal both require advanced (tertiary) treatment beyond standard secondary.
Explanation
Wastewater treatment stages: Primary (physical): screens, grit chambers, settling tanks remove large solids (rags, grit) and ~60% of suspended solids. Secondary (biological): microorganisms in aeration tanks/trickling filters consume dissolved organic matter (reduces BOD ~90%). Tertiary (chemical/physical): removes nutrients (N, P), pathogens (disinfection), and trace contaminants. Most plants do primary + secondary; tertiary is less common and more expensive.

Key takeaway
Primary = physical solids removal. Secondary = biological BOD reduction. Tertiary = nutrients, pathogens, advanced contaminants.
Question 7 · Medium
Solid waste in landfills undergoes anaerobic decomposition that produces:
- A
  Primarily CO₂ and water through aerobic respiration
  Why not A: Aerobic respiration produces CO₂ + H₂O, but landfills are anaerobic (oxygen-limited); the product is methane, not CO₂.
- B
  Landfill gas containing ~50% methane (CH₄) and ~50% CO₂Correct
- C
  Hydrogen sulfide (H₂S) gas as the primary emission
  Why not C: H₂S is produced in some landfills (from sulfate-reducing bacteria) and causes odors, but methane and CO₂ dominate landfill gas composition.
- D
  Nitrous oxide (N₂O) as the dominant greenhouse gas from decomposition
  Why not D: N₂O is produced during nitrification/denitrification in soils and wastewater treatment; methane is the dominant GHG from landfill anaerobic decomposition.
Explanation
Anaerobic bacteria in oxygen-depleted landfill interiors ferment organic waste, producing landfill gas (LFG) — approximately 50% methane (CH₄) and 50% carbon dioxide (CO₂). Landfills are the third-largest source of anthropogenic methane in the U.S. Many modern landfills capture LFG for electricity generation (landfill-to-energy). Uncaptured methane is flared or vented — methane is 80× more potent than CO₂ over 20 years.

Key takeaway
Landfill anaerobic decomposition: ~50% CH₄ + 50% CO₂. Modern landfills capture methane for electricity. Third-largest U.S. methane source.
Question 8 · Medium
Lead contamination in drinking water systems (e.g., Flint, Michigan crisis) most commonly enters drinking water through:
- A
  Natural bedrock dissolution as water flows through lead-bearing rock formations
  Why not A: Natural lead bedrock dissolution is rare and not the mechanism in municipal systems; the primary source is anthropogenic infrastructure.
- B
  Leaching from lead service lines, lead solder in plumbing, and lead paint dust in old buildingsCorrect
- C
  Industrial discharge of lead directly into municipal water treatment plants
  Why not C: Industrial discharge is regulated under the Clean Water Act; it is not the typical pathway for lead in treated tap water.
- D
  Atmospheric deposition of leaded gasoline combustion residues into reservoirs
  Why not D: Leaded gasoline was phased out in the U.S. by 1996; atmospheric deposition is no longer a significant ongoing source.
Explanation
Lead enters drinking water primarily from the infrastructure: lead service lines (pipes connecting the main to homes, common in pre-1986 construction), lead-tin solder joints in copper plumbing (used before 1986), and brass fixtures. Corrosive water (low pH, low alkalinity) dissolves lead from these surfaces. In Flint, a switch to a more corrosive water source without corrosion inhibitors caused massive leaching from lead pipes. Lead causes irreversible neurological damage, especially in children.

Key takeaway
Lead in drinking water: from infrastructure (lead pipes, old solder, brass fixtures), not source water. Corrosive water accelerates leaching.
Question 9 · Medium
DDT was banned in the United States in 1972 primarily due to evidence that it:
- A
  Was acutely toxic to humans at typical agricultural application concentrations
  Why not A: DDT has relatively low acute toxicity to humans at typical concentrations; the primary concern was ecological — wildlife impacts through biomagnification.
- B
  Bioaccumulated and biomagnified through food chains, causing eggshell thinning in raptors and population crashes in birds of preyCorrect
- C
  Directly caused acid rain by reacting with sulfur in the atmosphere
  Why not C: DDT does not react with atmospheric sulfur; acid rain is caused by SO₂ and NOₓ emissions from combustion.
- D
  Destroyed stratospheric ozone by releasing chlorine radicals when photolyzed
  Why not D: CFCs release chlorine radicals that destroy stratospheric ozone; DDT is not a CFC and is not associated with ozone depletion.
Explanation
DDT (dichlorodiphenyltrichloroethane) is lipophilic and resists breakdown — it biomagnifies dramatically through food chains. At the top: bald eagles, peregrine falcons, and ospreys accumulated DDT metabolites (DDE) that interfered with calcium metabolism, causing thin eggshells that cracked under incubating parents. Populations crashed. Rachel Carson's Silent Spring (1962) documented this and catalyzed the environmental movement. The bald eagle recovered after the 1972 ban.

Key takeaway
DDT ban: biomagnification → DDE → eggshell thinning → bird of prey population collapse. Rachel Carson's Silent Spring was the catalyst.
Question 10 · Medium
Which waste management strategy is HIGHEST on the preferred waste management hierarchy (reduces the most environmental impact)?
- A
  Landfilling with methane capture
  Why not A: Landfilling is the lowest preferred option in the waste hierarchy, even with methane capture.
- B
  Recycling
  Why not B: Recycling is preferred over disposal but ranked below source reduction (generating less waste in the first place).
- C
  Source reduction (preventing waste generation)Correct
- D
  Incineration with energy recovery
  Why not D: Waste-to-energy incineration recovers some value but produces air emissions and ash; it ranks above landfilling but well below source reduction.
Explanation
The waste management hierarchy (EPA): 1. Source reduction/prevention (best) → 2. Reuse → 3. Recycling → 4. Composting → 5. Energy recovery (incineration) → 6. Landfilling (worst). Source reduction prevents waste before it's created (e.g., using less packaging, buying durable goods) and avoids all downstream environmental impacts. Each step down the hierarchy adds energy and environmental costs.

Key takeaway
Waste hierarchy: source reduction > reuse > recycling > composting > energy recovery > landfill. Source reduction is always preferred.
Question 11 · Hard
A farmer applies excess nitrogen and phosphorus fertilizer to fields near a lake. After a heavy rain, runoff enters the lake. Over the following weeks, which sequence of events is most likely to occur?
- A
  Algae die → bacteria decompose algae → oxygen levels rise → fish thrive
  Why not A: Bacterial decomposition of algae CONSUMES oxygen, not produces it — this is the core mechanism of eutrophication-driven fish kills.
- B
  Algal bloom → algae die → bacterial decomposition → oxygen depletion (hypoxia) → fish killsCorrect
- C
  Nitrogen directly toxifies fish before algae can grow
  Why not C: Nitrogen at agricultural concentrations is not directly lethal to most fish; eutrophication's oxygen depletion is the primary mechanism.
- D
  Phosphorus precipitates out immediately, preventing algal growth
  Why not D: Phosphorus can bind to sediments under some conditions, but excess dissolved phosphate promotes algal growth before settling.
Explanation
Eutrophication sequence: nutrient runoff (N+P) → algal bloom → algae block sunlight from submerged plants → algae die → aerobic bacteria decompose dead algae → BOD rises → dissolved oxygen (DO) crashes below 2 mg/L (hypoxia) → fish, invertebrates, and other aerobic organisms suffocate. Dead zones in the Gulf of Mexico at the Mississippi River mouth are a classic example.

Key takeaway
Eutrophication: nutrients → algal bloom → algae die → decomposition → oxygen depletion → dead zone/fish kill.
Question 12 · Hard
The Exxon Valdez oil spill (1989) and Deepwater Horizon spill (2010) differed in that Deepwater Horizon released oil:
- A
  From a surface tanker, making containment and skimming easier than a deepwater blowout
  Why not A: Exxon Valdez was a tanker spill; Deepwater Horizon was a subsurface wellhead blowout — the opposite of what this choice states.
- B
  From a subsurface wellhead (~1,500 m below sea surface), making containment far more difficult than a surface spillCorrect
- C
  In an arctic environment where cold temperatures accelerated biodegradation
  Why not C: The Deepwater Horizon spill was in the Gulf of Mexico. Exxon Valdez was in Alaska's Prince William Sound. Cold temperatures slow, not accelerate, biodegradation.
- D
  Only refined petroleum products, which are less toxic to wildlife than crude oil
  Why not D: Both spills involved crude oil; there is no basis for claiming one was refined products or that refined products are less harmful.
Explanation
Deepwater Horizon released crude oil from a blowout at ~1,500 meters depth for 87 days (4.9 million barrels total, the largest marine oil spill in U.S. history). The depth made containment and cleanup far more difficult than surface spills — booms and skimmers couldn't reach the source. Dispersants (Corexit) were injected at depth, dispersing oil into fine droplets throughout the water column, creating novel deepwater contamination and toxicity concerns.

Key takeaway
Deepwater Horizon: subsurface blowout at 1,500 m depth = containment far harder than surface tanker spill. Dispersants mixed oil through water column.