Aquatic & Terrestrial Pollution
Complete review of all 12 topics — pollution sources, endocrine disruptors, eutrophication, bioaccumulation, waste management, sewage treatment, and toxicology.
Sources of Pollution
Pollution sources are classified as point source or nonpoint source. Point sources are identifiable, discrete locations (a factory pipe, a sewage outfall). Nonpoint sources are diffuse and harder to regulate (agricultural runoff, urban stormwater, atmospheric deposition).
| Feature | Point Source | Nonpoint Source |
|---|---|---|
| Origin | Single, identifiable location | Diffuse, widespread area |
| Examples | Factory discharge pipe, wastewater treatment plant, oil spill | Agricultural runoff, urban stormwater, road salt, atmospheric deposition |
| Regulation | Easier — monitored via permits (NPDES in the US) | Harder — requires land use management, BMPs |
| Responsibility | Clear — specific polluter identified | Shared — many contributors |
Nonpoint source pollution is now the #1 cause of water quality problems in the US, because point sources have been well-regulated since the Clean Water Act (1972). Agricultural runoff (fertilizers, pesticides, sediment) is the largest nonpoint source contributor.
❌ Point vs nonpoint mix-up: An oil spill from a tanker IS a point source (identifiable origin), even though the oil spreads widely. Nonpoint source means the pollution comes from many diffuse locations (e.g., fertilizer runoff from thousands of farms across a watershed).
❌ Thinking point sources are the bigger problem today. Since the Clean Water Act (1972), point sources are well-regulated. Nonpoint source pollution is now the #1 water quality problem in the US.
Which of the following is the best example of nonpoint source pollution?
- (A) A factory discharging chemicals into a river
- (B) Fertilizer runoff from farms across a watershed
- (C) An oil tanker spilling crude oil into the ocean
- (D) A sewage treatment plant releasing effluent
Human Impacts on Ecosystems
Human activities alter ecosystems through habitat destruction, pollution, resource extraction, and introduction of invasive species. These impacts reduce biodiversity, disrupt ecosystem services, and can trigger cascading effects through food webs.
Clearing forests for agriculture, logging, and development. Releases stored carbon, destroys habitat, increases erosion and flooding. Tropical deforestation accounts for ~10% of global CO₂ emissions.
Impervious surfaces (roads, buildings) increase stormwater runoff and reduce groundwater recharge. Urban heat islands raise temperatures by 1-3°C. Habitat fragmentation isolates wildlife populations.
Monocultures reduce biodiversity. Pesticides kill non-target species. Tillage causes soil erosion. Irrigation can cause salinization and deplete aquifers (Ogallala, Aral Sea).
Strip mining destroys surface habitat. Subsurface mining causes land subsidence. Acid mine drainage (AMD) leaches heavy metals and sulfuric acid into waterways from exposed sulfide minerals.
When sulfide minerals (like pyrite, FeS₂) in mine tailings are exposed to air and water, they oxidize to form sulfuric acid and dissolve heavy metals (iron, copper, arsenic). AMD can sterilize streams for miles downstream and persist for decades after mine closure.
Urbanization most directly affects the water cycle by
- (A) increasing surface runoff and decreasing groundwater recharge
- (B) increasing evaporation rates from reservoirs
- (C) decreasing precipitation in urban areas
- (D) increasing infiltration through permeable surfaces
Endocrine Disruptors
Endocrine disruptors are chemicals that interfere with the hormonal (endocrine) system of organisms. They can mimic, block, or alter hormone signaling, causing reproductive abnormalities, developmental defects, and cancer — often at very low concentrations.
| Chemical | Source | Known Effects |
|---|---|---|
| BPA (bisphenol A) | Plastics, can linings, receipts | Mimics estrogen; linked to reproductive issues, obesity, heart disease |
| Atrazine | Herbicide (corn farming) | Feminization of male frogs at parts-per-billion levels; water contamination |
| Phthalates | Plasticizers in PVC, cosmetics, toys | Anti-androgen effects; reduced sperm count, developmental issues |
| DDT / DDE | Pesticide (banned in US 1972, still used in some countries) | Eggshell thinning in birds (bald eagle near-extinction); bioaccumulates |
| PCBs | Electrical equipment, coolants (banned 1979) | Cancer, immune suppression, neurological damage; persist in environment |
AP questions on endocrine disruptors often focus on DDT and eggshell thinning (Rachel Carson's Silent Spring) or atrazine feminizing frogs. Key point: endocrine disruptors act at extremely low concentrations (parts per billion), which challenges traditional dose-response assumptions.
The herbicide atrazine is classified as an endocrine disruptor because it
- (A) directly kills amphibians through acute toxicity
- (B) interferes with hormonal function, causing feminization in male frogs
- (C) depletes dissolved oxygen in waterways
- (D) increases nitrogen levels through agricultural runoff
Human Impacts on Wetlands and Mangroves
Wetlands (marshes, swamps, bogs) and mangroves (coastal salt-tolerant forests) are among the most productive and ecologically valuable ecosystems on Earth, yet over 50% of the world's wetlands have been lost since 1900, primarily to agriculture and development.
Flood control (absorb excess water), water filtration (remove pollutants/sediment), carbon sequestration (peatlands store 2x more C than all forests), nursery habitat for fish/shellfish, groundwater recharge.
Coastal storm protection (reduce wave energy by up to 66%), prevent erosion, nursery for 75% of commercial fish species, sequester carbon 3-5x faster than terrestrial forests (blue carbon).
Draining for agriculture, filling for development, aquaculture (shrimp farming destroys mangroves), pollution, upstream dams altering water flow, rising sea levels.
Wetlands are sometimes called "nature's kidneys" because they filter pollutants and excess nutrients from water. They also serve as "nature's sponges" for flood control. Destroying wetlands increases flood damage, worsens water quality, and releases stored carbon.
The primary reason that wetland destruction leads to increased flooding downstream is that wetlands
- (A) release stored water during dry periods
- (B) absorb and slowly release excess water during storm events
- (C) increase the rate of evaporation in the watershed
- (D) prevent precipitation from reaching the ground
Eutrophication
Eutrophication is the over-enrichment of a water body with nutrients (primarily nitrogen and phosphorus), leading to excessive algal growth, oxygen depletion, and ecological collapse. It is the most widespread water quality problem globally.
The sequence: Excess nutrients → Algal bloom → Algae die → Decomposition by bacteria → Oxygen depletion (hypoxia) → Fish kills → Dead zone. The Gulf of Mexico dead zone covers ~6,000-7,000 sq miles each summer, fed by Mississippi River agricultural runoff.
Students often say algal blooms kill fish by "poisoning" them. While some algal blooms are toxic (harmful algal blooms / HABs), the primary mechanism in eutrophication is oxygen depletion: bacteria decomposing dead algae consume dissolved oxygen, suffocating aquatic life. This creates hypoxic (low O₂) or anoxic (no O₂) conditions.
In the process of eutrophication, fish kills result primarily from
- (A) toxic chemicals released by fertilizers
- (B) increased water temperature from algal blooms
- (C) depletion of dissolved oxygen by decomposing bacteria
- (D) direct consumption of fish by excessive algal growth
A coastal bay surrounded by farmland experiences annual summer dead zones where fish and shellfish populations collapse.
(a) Describe the complete sequence of events that leads from agricultural activity to the formation of a dead zone.
(b) Explain the role of BOD (biological oxygen demand) in the dead zone formation process.
(c) Propose TWO specific strategies that could reduce the size of the dead zone. For each, explain the mechanism by which it would help.
(b) BOD measures the amount of dissolved oxygen consumed by bacteria during decomposition of organic matter. When the algal bloom dies, the large mass of dead organic material causes BOD to spike dramatically. High BOD means bacteria are consuming oxygen faster than it can be replenished from the atmosphere or photosynthesis, directly driving the oxygen depletion that creates hypoxic/anoxic conditions and kills aquatic life.
(c) Strategy 1: Riparian buffer zones — planting strips of native vegetation along waterways between farmland and the bay. Plant roots absorb excess nitrogen and phosphorus from runoff before it reaches the water, and vegetation slows water flow, allowing sediment and nutrients to settle out. Strategy 2: Precision agriculture / reduced fertilizer application — using soil testing and GPS-guided application to apply only the nutrients crops actually need, reducing the excess that runs off. This directly reduces the nutrient input to the watershed at the source.
Thermal Pollution
Thermal pollution is the discharge of heated water into natural water bodies, primarily from power plant cooling systems (both fossil fuel and nuclear). Water is used as a coolant and returned to rivers/lakes at temperatures 10-15°C above ambient.
Warmer water holds less dissolved oxygen (gas solubility decreases as temperature increases), stresses cold-water species (trout, salmon), increases metabolic rates (organisms need more O₂ but less is available), promotes algal growth, and can disrupt reproduction cycles. Mitigation: cooling towers dissipate heat to the atmosphere before discharge; cooling ponds allow natural cooling.
Remember the key principle: warm water holds less dissolved oxygen. This is why thermal pollution and climate-driven ocean warming both reduce oxygen availability for aquatic organisms. It also explains why cold mountain streams support trout but warm lowland rivers do not.
Thermal pollution from power plant cooling water most directly harms aquatic ecosystems by
- (A) adding toxic chemicals to the water
- (B) decreasing the dissolved oxygen concentration
- (C) increasing the pH to harmful levels
- (D) introducing invasive species through intake pipes
Persistent Organic Pollutants (POPs)
Persistent organic pollutants (POPs) are carbon-based chemicals that resist environmental degradation, bioaccumulate in fatty tissues, travel long distances via the grasshopper effect (repeated evaporation and condensation carrying them toward the poles), and are toxic to humans and wildlife.
| POP | Original Use | Environmental Concern |
|---|---|---|
| DDT | Insecticide (malaria control) | Eggshell thinning in raptors (bald eagle, peregrine falcon); banned in US 1972; still used in some developing countries for malaria |
| PCBs | Electrical insulators, coolants | Cancer, immune suppression, neurological damage; banned 1979; persist in river sediments (Hudson River cleanup: $1.8B) |
| Dioxins | Byproduct of combustion, pesticide manufacturing | Most toxic human-made chemical; carcinogenic at extremely low doses; Agent Orange contained dioxin |
| PFAS ("forever chemicals") | Nonstick coatings, firefighting foam, waterproof fabrics | Virtually indestructible; found in blood of 98% of Americans; linked to cancer, thyroid disease |
POPs evaporate in warm regions, travel through the atmosphere, and condense in cooler regions — repeating this "hop" poleward. This is why Arctic indigenous peoples have high body burdens of POPs despite living far from industrial sources. The Stockholm Convention (2001) targets elimination of the 12 worst POPs ("dirty dozen").
Persistent organic pollutants (POPs) are found in high concentrations in Arctic ecosystems primarily because of
- (A) heavy industrial activity near the Arctic
- (B) intentional disposal of chemicals in polar regions
- (C) atmospheric transport through repeated evaporation and condensation cycles
- (D) ocean currents carrying warm water to polar seas
Bioaccumulation and Biomagnification
Bioaccumulation is the buildup of a substance in an individual organism over its lifetime (intake exceeds excretion). Biomagnification is the increasing concentration of a substance at higher trophic levels in a food chain. Both processes make top predators especially vulnerable to fat-soluble, persistent toxins.
One organism absorbs a toxin faster than it can metabolize or excrete it. Example: a fish living in mercury-contaminated water accumulates mercury in its tissues over years.
Concentration increases at each trophic level. A classic example: DDT at 0.04 ppm in water → 0.5 ppm in zooplankton → 2 ppm in small fish → 25 ppm in large fish → 500 ppm in fish-eating birds (bald eagle).
Mercury (methylmercury from coal plants → fish → humans), DDT (eggshell thinning in raptors), PCBs (orca pods contaminated from fish). All are fat-soluble and persistent.
The AP exam often provides a food chain diagram with concentration data and asks you to identify biomagnification. Remember: top predators (eagles, orcas, tuna, humans) are most at risk. The chemicals must be persistent (don't break down) and fat-soluble (stored in lipid tissues, not excreted in urine).
❌ Bioaccumulation ≠ biomagnification: Bioaccumulation occurs in a single organism over its lifetime (intake > excretion). Biomagnification is the increase in concentration at higher trophic levels through the food chain. Both involve persistent toxins, but they describe different processes.
❌ Not all pollutants biomagnify. Only substances that are fat-soluble AND persistent (not broken down or excreted) undergo biomagnification. Water-soluble pollutants are excreted in urine and do not concentrate up the food chain.
In a food chain contaminated with mercury, which organism would be expected to have the highest tissue concentration?
- (A) Phytoplankton
- (B) Zooplankton
- (C) Small fish
- (D) Fish-eating bird
Solid Waste Disposal
The US generates ~292 million tons of municipal solid waste (MSW) per year (~4.9 lbs/person/day). Disposal methods include landfills, incineration, recycling, and composting. Each has trade-offs regarding cost, environmental impact, and resource recovery.
| Method | Advantages | Disadvantages |
|---|---|---|
| Sanitary Landfill | Cheap, handles all waste types, can capture methane for energy | Leachate can contaminate groundwater; methane emissions (GHG); land use; NIMBY; doesn't recover resources |
| Incineration | Reduces volume by ~90%; generates electricity (waste-to-energy); kills pathogens | Air pollution (dioxins, mercury, PM); toxic ash requires special landfill; expensive; public opposition |
| Recycling | Conserves resources, reduces energy use, reduces landfill volume | Contamination reduces value; not all materials recyclable; market fluctuations; collection costs |
| Composting | Diverts organic waste (~30% of MSW); creates useful soil amendment | Only works for organic waste; odor issues; requires space and management |
Rainwater percolating through landfill waste dissolves toxic substances, creating leachate — a hazardous liquid containing heavy metals, organic compounds, and pathogens. Modern sanitary landfills use clay + plastic liners and leachate collection systems to prevent groundwater contamination, but older dumps lack these protections.
The most significant environmental concern associated with older, unlined landfills is
- (A) air pollution from decomposing waste
- (B) groundwater contamination from leachate
- (C) increased surface temperatures from methane combustion
- (D) soil erosion on the landfill surface
A growing city must choose between expanding its landfill or building a waste-to-energy incineration facility.
(a) Describe ONE environmental advantage and ONE environmental disadvantage of each option (landfill and incineration).
(b) Explain how leachate forms in a landfill and describe the design features of a modern sanitary landfill that prevent groundwater contamination.
(c) According to the waste management hierarchy, identify a strategy that is preferred over BOTH landfilling and incineration, and explain why it is more effective.
(b) Leachate forms when rainwater percolates through decomposing waste, dissolving toxic substances including heavy metals, organic compounds, and pathogens into a hazardous liquid. Modern sanitary landfills prevent groundwater contamination using: (1) clay and synthetic plastic liners (typically HDPE) at the bottom and sides to create an impermeable barrier, (2) leachate collection pipes above the liner that drain leachate to treatment facilities, and (3) a daily soil cover layer to reduce rainwater infiltration and odor.
(c) Source reduction (reduce) is preferred over both options. It sits at the top of the waste management hierarchy because preventing waste from being created in the first place avoids ALL downstream environmental impacts — no need for collection, transportation, processing, air emissions, leachate generation, or land use for disposal. Examples include minimal packaging, durable product design, and digital alternatives to paper.
Waste Reduction Methods
The waste management hierarchy prioritizes strategies from most to least preferred: Reduce → Reuse → Recycle → Recovery (energy) → Disposal. Source reduction (preventing waste creation) is the most effective approach because it avoids environmental impacts entirely.
Most effective strategy. Examples: minimal packaging, digital documents instead of paper, buying in bulk, designing products for longer life. Prevents waste before it's created.
Using items multiple times: refillable water bottles, cloth bags, repurposed containers, second-hand markets. Extends product life without reprocessing energy.
Processing materials into new products: paper, glass, aluminum, some plastics. Aluminum recycling saves 95% of the energy needed for virgin production. US recycling rate: ~32%.
Electronic waste is the fastest-growing waste stream. Contains valuable metals (gold, copper) but also toxic substances (lead, mercury, cadmium). Much is exported to developing countries for informal, hazardous recycling.
For the AP exam, remember that "reduce" is always the best answer when asked about the most effective waste management strategy. Also know that recycling aluminum saves the most energy compared to virgin production (95%), making it the most energy-efficient material to recycle.
According to the waste management hierarchy, which strategy is most effective at reducing environmental impact?
- (A) Recycling materials into new products
- (B) Incinerating waste to generate electricity
- (C) Reducing consumption to prevent waste generation
- (D) Composting organic waste for soil amendment
Sewage Treatment
Sewage (wastewater) treatment progressively removes contaminants through physical, biological, and chemical processes. Untreated sewage contains pathogens, nutrients, organic matter, and chemicals that cause disease and eutrophication.
| Stage | Process | What It Removes |
|---|---|---|
| Primary Treatment | Physical: screening, settling tanks (sedimentation) | Large solids, grit, ~60% of suspended solids; sludge settles out |
| Secondary Treatment | Biological: aerobic bacteria decompose organic matter (activated sludge, trickling filters) | ~90% of organic matter (reduces BOD); most pathogens |
| Tertiary Treatment | Chemical/advanced: filtration, UV disinfection, nutrient removal (N, P) | Remaining nutrients (prevents eutrophication), pharmaceuticals, pathogens; produces near-drinking-quality water |
BOD (Biological Oxygen Demand) measures the amount of dissolved oxygen consumed by bacteria decomposing organic matter. High BOD = heavily polluted water. Secondary treatment dramatically reduces BOD. Treated water with low BOD won't deplete oxygen in receiving waters.
Secondary sewage treatment primarily reduces water pollution by
- (A) filtering out heavy metals and pharmaceuticals
- (B) using chemicals to remove nitrogen and phosphorus
- (C) using bacteria to decompose organic matter and reduce BOD
- (D) physically screening out large solid debris
Lethal Dose 50% (LD₅₀)
LD₅₀ (Lethal Dose 50%) is the dose of a substance that kills 50% of a test population. It is used to compare the acute toxicity of chemicals. A lower LD₅₀ = more toxic (less substance needed to kill). LD₅₀ is measured in mg/kg of body weight.
| Concept | Description |
|---|---|
| Dose-Response Curve | S-shaped curve showing the relationship between dose and response (% affected). The threshold is the dose below which no effect is observed. |
| ED₅₀ | Effective Dose 50% — the dose that produces a desired effect in 50% of the population |
| Threshold Dose | The minimum dose needed to produce a measurable effect; below this, no response occurs (for most toxins) |
| Acute vs Chronic | Acute toxicity = immediate, short-term, high-dose effects. Chronic toxicity = long-term, low-dose, cumulative effects (harder to study) |
| Synergistic Effects | Combined effect of two chemicals is greater than the sum of individual effects (e.g., asbestos + smoking → lung cancer risk multiplied) |
The AP exam may give you LD₅₀ values for multiple chemicals and ask which is most toxic. Lower LD₅₀ = more toxic. Example: botulinum toxin LD₅₀ ~0.001 mg/kg (extremely toxic) vs. table salt LD₅₀ ~3,000 mg/kg (low toxicity). Also expect questions about interpreting dose-response curves and identifying threshold doses.
❌ Lower LD₅₀ = MORE toxic, not less: Students frequently reverse the relationship. A chemical with LD₅₀ of 5 mg/kg is far more dangerous than one with LD₅₀ of 5,000 mg/kg — it takes much less to kill.
❌ Confusing acute toxicity (LD₅₀, short-term, high dose) with chronic toxicity (long-term, low-dose exposure). LD₅₀ only measures acute lethality. A chemical can have a high LD₅₀ (low acute toxicity) but still cause cancer or organ damage with chronic exposure.
Chemical X has an LD₅₀ of 5 mg/kg and Chemical Y has an LD₅₀ of 500 mg/kg. Which statement is correct?
- (A) Chemical X is more toxic because a smaller dose is lethal
- (B) Chemical Y is more toxic because it has a higher LD₅₀ value
- (C) Both chemicals have equal toxicity
- (D) Toxicity cannot be determined from LD₅₀ values alone
Comprehensive Practice Questions
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
A lake near agricultural land develops an algal bloom, and fish-eating birds in the area are found with high DDT concentrations. Which pair of processes best explains these two observations?
- (A) Thermal pollution and bioaccumulation
- (B) Eutrophication and biomagnification
- (C) Acid deposition and endocrine disruption
- (D) Point source pollution and biodegradation
A community discovers that an old, unlined landfill has been leaching chemicals into the groundwater. Testing reveals Chemical A (LD₅₀ = 10 mg/kg) and Chemical B (LD₅₀ = 2,000 mg/kg) in the water supply. Which statement is correct?
- (A) Chemical B is more acutely toxic and should be prioritized for cleanup
- (B) Both chemicals are equally dangerous because they are in drinking water
- (C) Chemical A is more acutely toxic because a smaller dose is lethal to 50% of a test population
- (D) LD₅₀ values cannot be used to compare toxicity of different chemicals
A river receives discharge from a power plant (heated cooling water) and also drains a watershed with extensive corn farming.
(a) Classify each pollution source as point or nonpoint. Justify your classification.
(b) Explain how the combination of thermal pollution and nutrient runoff could create synergistic harm to aquatic life in the river.
(c) Describe how BOD measurements taken upstream and downstream of the farm runoff entry point would differ, and explain why.
(b) The heated cooling water (thermal pollution) reduces dissolved oxygen in the river because warm water holds less dissolved gas. Simultaneously, nutrient runoff from corn farming causes eutrophication — algal blooms that, when decomposed by bacteria, further consume dissolved oxygen (increased BOD). Together, these two stressors create a synergistic effect: the thermal pollution lowers the baseline oxygen level while the eutrophication-driven decomposition demands even more oxygen, potentially pushing the river into severe hypoxia that neither stressor alone would cause.
(c) BOD measurements downstream of the farm runoff entry point would be significantly higher than upstream. This is because the agricultural runoff adds large amounts of organic matter (dead plant material, fertilizer-stimulated algal growth) to the river. Downstream bacteria must decompose this additional organic load, consuming more dissolved oxygen per liter of water. Higher BOD indicates more heavily polluted water with more organic matter for bacteria to decompose.
An Arctic indigenous community that relies on marine mammals for food is found to have elevated blood levels of PCBs and DDT, despite living far from industrial activity.
(a) Explain how these persistent organic pollutants reach the Arctic from distant industrial sources.
(b) Distinguish between bioaccumulation and biomagnification, and explain how both processes contribute to the high contaminant levels found in the community's food supply.
(c) Identify ONE specific endocrine-disrupting effect of DDT on wildlife, and name the landmark book that first brought public attention to DDT's environmental impacts.
(b) Bioaccumulation occurs within individual organisms: a single seal absorbs PCBs and DDT from contaminated fish throughout its lifetime, and because these chemicals are fat-soluble and persistent, they accumulate in the seal's blubber faster than they can be excreted. Biomagnification occurs across trophic levels: phytoplankton absorb small amounts, zooplankton accumulate more by eating many phytoplankton, fish accumulate more by eating many zooplankton, and marine mammals at the top of the food chain have the highest concentrations. The indigenous community, eating these top predators, receives the most concentrated doses.
(c) DDT causes eggshell thinning in birds of prey (raptors). DDT metabolite DDE interferes with calcium metabolism, producing thin-shelled eggs that crack during incubation. This nearly drove the bald eagle and peregrine falcon to extinction. Rachel Carson's landmark book Silent Spring (1962) first brought widespread public attention to DDT's environmental impacts, leading to its US ban in 1972.
High-Frequency Common Mistakes — Full Unit 8
- 🐧Bioaccumulation ≠ biomagnificationBioaccumulation = one organism over time. Biomagnification = increasing concentration up the food chain. Both involve persistent, fat-soluble toxins, but they describe different scales.
- 🌊Point source ≠ nonpoint sourcePoint = identifiable pipe or outfall (factory, sewage plant). Nonpoint = diffuse runoff from many sources (farms, roads, cities). Nonpoint is now the #1 US water quality problem.
- 💉Lower LD₅₀ = MORE toxicStudents frequently reverse this. LD₅₀ of 5 mg/kg is far more dangerous than 5,000 mg/kg. Less substance needed to kill = more toxic.
- 🦠Eutrophication kills via oxygen depletion, not poisonThe main kill mechanism is: algae die → bacteria decompose → oxygen consumed → fish suffocate. Not algal toxins (though HABs exist, that's a separate concept).
- 🌡️Warm water holds LESS dissolved oxygenGas solubility decreases as temperature increases. This is why thermal pollution harms aquatic life — organisms need more O₂ (higher metabolism) but less is available.
- 📈BOD: higher = more pollutedHigh biological oxygen demand means lots of organic matter for bacteria to decompose, consuming dissolved oxygen. High BOD = heavily polluted, low DO water.
- ♻️"Reduce" beats "recycle" on the hierarchyThe waste hierarchy is Reduce > Reuse > Recycle > Recovery > Disposal. Source reduction is always the best answer for most effective waste strategy.
- 💣Acute toxicity ≠ chronic toxicityLD₅₀ only measures acute (short-term, high-dose) lethality. A chemical with high LD₅₀ can still cause cancer or organ damage from chronic low-dose exposure.
- 🧬DDT eggshell thinning, not direct poisoningDDT doesn't kill birds directly — its metabolite DDE disrupts calcium metabolism, producing thin eggshells that crack during incubation. This is an endocrine-disrupting effect.
- 🌎Grasshopper effect moves POPs to the ArcticPOPs evaporate in warm regions and condense in cold regions, hopping poleward. This explains why Arctic peoples have high POP body burdens despite no local industry.
- 💧Leachate vs runoff confusionLeachate = toxic liquid formed when rain percolates THROUGH landfill waste. Runoff = water flowing OVER surfaces picking up pollutants. Different pathways, different sources.
Focus on bioaccumulation vs biomagnification (tested almost every year with food chain data), the eutrophication sequence (nutrients → bloom → death → decomposition → hypoxia), and LD₅₀ interpretation (lower = more toxic). Also master point vs nonpoint source classification and the waste management hierarchy. These five concepts cover the highest-frequency exam questions for Unit 8.