Aquatic and Terrestrial Pollution
Unit 8 of AP Environmental Science, worth 7–10% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Sources of pollution, human impacts on ecosystems, endocrine disruptors, thermal pollution, solid waste, sewage treatment, LD50 and dose-response, pathogens.
How this unit is tested
This unit blends vocabulary (point vs. nonpoint source, pathogen, endocrine disruptor) with quantitative reasoning (LD50, dose-response curves, BOD). Start by sorting every pollution source you encounter into point source (single identifiable outlet) or nonpoint source (diffuse, widespread), since the AP exam frequently tests this distinction with real-world scenarios like farms, factories, and city streets.
For the toxicology material, get comfortable reading dose-response graphs: know that the x-axis is dose (usually log scale) and the y-axis is percent response (often mortality), and practice pulling the LD50 directly off a curve or a data table. Remember the counterintuitive rule that a LOWER LD50 means a substance is MORE toxic, because less of it is needed to kill half the test population.
For sewage treatment, walk through the sequence physically: primary treatment removes solids by settling and screening, secondary treatment uses microorganisms to digest dissolved organic matter (lowering BOD), and tertiary treatment chemically or biologically strips out remaining nutrients like nitrogen and phosphorus before disinfection and discharge. Link this back to eutrophication — treatment failures at any stage can send excess nutrients or organic matter into waterways.
Finally, connect the human-impact topics (thermal pollution, solid waste, pathogens, endocrine disruptors) back to ecosystem-level consequences: lower dissolved oxygen, food-web disruption via bioaccumulation/biomagnification, and disease transmission. AP free-response questions often ask you to propose a specific, feasible solution — practice naming an actual technology or policy (e.g., cooling towers, tertiary treatment, integrated pest management) rather than a vague fix.
What you have to know
LD50
The dose of a substance, usually expressed in mg of substance per kg of body weight, required to kill 50% of a test population. A lower LD50 indicates a more toxic substance.
Dose-Response Relationship
As the dose of a substance increases, the biological response of an organism generally increases. The threshold dose is the lowest dose that produces a measurable effect; below it, no observable response occurs.
Biochemical Oxygen Demand (BOD)
The amount of dissolved oxygen consumed by microorganisms as they decompose organic matter in water. High BOD indicates heavy organic pollution and predicts lower oxygen available for other aquatic life.
Bioaccumulation vs. Biomagnification
Bioaccumulation is the buildup of a substance within a single organism over its lifetime from its diet and environment. Biomagnification is the increase in concentration of that substance in tissues as it moves up successive trophic levels in a food chain.
Point Source vs. Nonpoint Source Pollution
Point source pollution enters the environment from a single, identifiable location such as a discharge pipe or smokestack. Nonpoint source pollution comes from diffuse, widespread sources such as agricultural or urban runoff and is much harder to monitor and regulate.
14 practice questions
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Which of the following is the best example of nonpoint source water pollution?
- A discharge pipe from a factory
- Agricultural runoff carrying fertilizer from multiple fields into a stream
- Effluent from a single wastewater treatment plant outfall
- An oil spill from one tanker
Show the answer
Answer. Agricultural runoff carrying fertilizer from multiple fields into a stream
Nonpoint source pollution comes from diffuse, widespread inputs rather than a single identifiable outlet. A factory pipe or a single oil spill has one clear source, but fertilizer washing off many fields during a rainstorm has no single entry point, making it nonpoint and much harder to regulate. -
Why is nonpoint source pollution generally harder to regulate than point source pollution?
Show the answer
Answer. Because it comes from many diffuse, widespread locations rather than a single identifiable outlet, making it difficult to monitor, trace to a responsible party, and control with a single permit or filter.
Regulations like discharge permits work well when pollution exits from one pipe or stack that can be measured and controlled. Nonpoint pollution enters through countless small paths (driveways, fields, lawns), so agencies must rely on broader land-use policies and voluntary best practices instead of a single point of enforcement. -
A power plant discharges heated cooling water into a river. What is the primary ecological mechanism by which this thermal pollution harms aquatic organisms?
- It lowers dissolved oxygen solubility while simultaneously raising organisms' metabolic rate and oxygen demand
- It raises dissolved oxygen levels, benefiting fish growth
- It decreases the river's biochemical oxygen demand
- It primarily raises the river's pH to dangerous levels
Show the answer
Answer. It lowers dissolved oxygen solubility while simultaneously raising organisms' metabolic rate and oxygen demand
Warm water holds less dissolved gas, so oxygen availability drops. At the same time, higher temperature speeds up the metabolism of ectothermic aquatic organisms, increasing their oxygen requirement. The combination of falling supply and rising demand causes stress or fish kills. -
Define an endocrine disruptor and give one real-world source of exposure.
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Answer. A chemical that interferes with the body's hormone system by mimicking, blocking, or altering natural hormones; common sources include BPA in plastics and certain pesticides like DDT.
Endocrine disruptors bind to or interfere with hormone receptors, which can alter reproduction, development, or growth even at very low concentrations. This is why they are often more concerning than their raw toxicity (LD50) alone would suggest. -
On a dose-response curve, what does a very steep slope near the LD50 indicate about the test population compared to a shallow slope?
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Answer. A steep slope means individuals in the population have similar sensitivity, so a small increase in dose causes a large jump in mortality; a shallow slope means sensitivity varies widely across the population.
The slope of a dose-response curve reflects variability in individual tolerance. If nearly everyone dies within a narrow dose range, the curve rises sharply. If deaths are spread across a wide dose range, some individuals are much more resistant than others, producing a gradual slope. -
In an LD50 bioassay on mice, a dose of 8 mg/kg body weight kills exactly 50 of 100 test mice. What is the LD50 of this substance?
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Answer. 8 mg/kg body weight
By definition, LD50 is the dose at which exactly 50% of the test population dies. Since 50 of 100 mice died at 8 mg/kg, this dose is the LD50, and it would be reported with that unit and the test species/duration. -
Chemical A has an LD50 of 5 mg/kg and Chemical B has an LD50 of 500 mg/kg in the same test species. Which chemical is more toxic?
- Chemical A, because a much smaller dose is needed to kill 50% of the population
- Chemical B, because it requires a larger dose to be lethal
- They are equally toxic since both cause 50% mortality
- Toxicity cannot be compared using LD50 values
Show the answer
Answer. Chemical A, because a much smaller dose is needed to kill 50% of the population
Toxicity and LD50 are inversely related: the lower the LD50, the less substance is needed to cause lethal effects, meaning it is more potent per unit dose. Chemical A requires 100 times less material to achieve the same lethal effect as Chemical B. -
Describe the difference between primary and secondary sewage treatment.
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Answer. Primary treatment physically removes large solids and suspended particles by screening and settling; secondary treatment uses microorganisms to biologically break down dissolved organic matter, reducing BOD.
Primary treatment is mechanical: bar screens catch debris and sedimentation tanks let heavier solids settle out as sludge. Secondary treatment adds a biological step, typically using bacteria in an aeration tank, to digest remaining organic material before the water moves to further treatment or discharge. -
Which stage of wastewater treatment specifically targets removal of dissolved nutrients such as nitrogen and phosphorus before water is discharged?
- Primary treatment
- Secondary treatment
- Tertiary treatment
- Sludge digestion
Show the answer
Answer. Tertiary treatment
Primary treatment removes solids and secondary treatment reduces organic matter and BOD using microbes, but neither reliably strips out dissolved nutrients. Tertiary treatment uses additional chemical or biological processes specifically to remove nitrogen and phosphorus, preventing downstream eutrophication. -
What is biochemical oxygen demand (BOD), and why is a high BOD reading considered a sign of water pollution rather than water quality?
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Answer. BOD measures the dissolved oxygen consumed by microorganisms decomposing organic matter in water; a high BOD means heavy organic pollution is present and is depleting oxygen available to fish and other aquatic life.
Rather than measuring pollutants directly, BOD measures the oxygen cost of decomposing organic material already in the water. A high value indicates lots of organic waste (sewage, agricultural runoff) is present, which can lead to hypoxic or anoxic conditions harmful to aquatic organisms. -
Explain the difference between bioaccumulation and biomagnification using a specific pollutant example.
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Answer. Bioaccumulation is a toxin building up inside one organism over its life (e.g., DDT accumulating in a fish's fat tissue); biomagnification is that toxin's concentration increasing at each higher trophic level (e.g., DDT reaching its highest concentration in birds of prey at the top of the food chain).
Bioaccumulation happens within a single individual as it absorbs a persistent chemical faster than it can excrete it. Biomagnification is the food-chain-wide consequence: predators eat many contaminated prey, so the toxin concentrates more at each trophic step, famously causing eggshell thinning in raptors exposed to DDT. -
Which of the following is commonly used as an indicator species to detect the likely presence of pathogens in a water supply, rather than testing for every possible disease-causing organism directly?
- Fecal coliform bacteria (e.g., E. coli)
- Daphnia
- Algae
- Zebra mussels
Show the answer
Answer. Fecal coliform bacteria (e.g., E. coli)
Testing directly for every possible pathogen is expensive and slow, so agencies test for fecal coliform bacteria, which live in the intestines of warm-blooded animals. Their presence indicates fecal contamination and therefore a strong likelihood that other pathogens from sewage are also present. -
Describe two negative environmental consequences of an improperly managed solid waste landfill.
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Answer. Leachate can contaminate groundwater with dissolved contaminants, and decomposing waste produces methane gas that contributes to the greenhouse effect and can pose an explosion risk if not captured.
Without an impermeable liner and leachate collection system, rainwater percolating through waste dissolves toxic compounds and carries them into soil and aquifers. Meanwhile, anaerobic decomposition of organic waste generates methane, a potent greenhouse gas that must be vented or captured to prevent buildup and atmospheric release. -
A city wants to reduce the amount of solid waste sent to its landfill. Rank reduce, reuse, and recycle in order of environmental preference and explain the reasoning.
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Answer. Reduce is most preferred, followed by reuse, then recycle, because reducing prevents waste from being created in the first place, reusing extends a product's life without extra processing, and recycling requires energy and resources to reprocess materials into new products.
Each step down the hierarchy requires more energy and resource input: reducing consumption avoids resource use entirely, reusing an item (like a glass jar) needs little processing, while recycling (like melting aluminum cans) still consumes significant energy even though it's better than sending materials to a landfill.
What people get wrong
- Assuming a lower LD50 means a substance is safer. It is the opposite — a lower LD50 means a smaller dose is lethal, so the substance is MORE toxic. Always state the direction of the LD50-toxicity relationship in your answer.
- Misclassifying diffuse pollution as point source because it originates within one city or region. Check whether there is a single identifiable outlet (point source) or many scattered inputs like runoff from streets and lawns (nonpoint source), regardless of how localized the overall area is.
- Treating a high BOD reading as a sign of clean water. High BOD means more organic matter is being decomposed, which consumes more dissolved oxygen — this signals pollution and risk of hypoxia, not water quality.
- Describing thermal pollution as only 'warmer water is bad for fish' without the mechanism. Explain that warmer water holds less dissolved oxygen AND raises organisms' metabolic rate, so oxygen demand rises just as oxygen supply falls — that combination causes fish kills.
- Confusing which treatment stage removes nutrients. Secondary treatment uses microbes to reduce BOD and organic solids; it does NOT reliably remove dissolved nitrogen and phosphorus. Only tertiary treatment specifically targets those nutrients before discharge.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.