Lesson 25: Water Pollution & Eutrophication

Warm-Up

A pipe pouring waste into a river is easy to spot and regulate. But most water pollution today doesn't come from a pipe — it drips off millions of lawns, farms, and streets as runoff you can't pin on anyone. That distinction, point vs. nonpoint source, shapes how we fight water pollution. And the most widespread form is almost too ordinary to fear: nutrients. Fertilizer and sewage feed explosive algae growth that, when the algae die and decompose, suffocates the water and creates dead zones the size of small countries — like the one in the Gulf of Mexico. This lesson follows pollution into the water and traces the oxygen crash at the heart of eutrophication.

Core Concept

Point vs. nonpoint sources

• Point source — pollution from a single, identifiable location (a discharge pipe, a factory outfall, a sewage-treatment plant, an oil spill from one tanker). Easier to monitor and regulate.
• Nonpoint source — pollution from diffuse, spread-out sources carried by runoff (fertilizer/pesticides from many farms, oil and salt from roads, lawn chemicals, animal waste). The largest source of water pollution and the hardest to control because it has no single origin.

[DIAGRAM: A river receiving pollution. Point source = a labeled pipe discharging from a factory. Nonpoint source = runoff arrows from farm fields, suburban lawns, and a parking lot, all draining diffusely into the river.]

Nutrient pollution and cultural eutrophication

Water bodies naturally accumulate nutrients slowly over time (eutrophication). Cultural (anthropogenic) eutrophication is the human-accelerated version, driven by nitrogen and phosphorus from fertilizer runoff, sewage, and detergents (recall: phosphorus usually limits freshwater, nitrogen limits marine systems — Lesson 2).

The eutrophication sequence: 1. Excess N and P enter the water (runoff/sewage). 2. Algae and aquatic plants bloom explosively (an algal bloom), often forming a surface mat that blocks light. 3. Algae die; decomposer bacteria multiply and consume oxygen breaking them down. 4. Dissolved oxygen (DO) plummets (hypoxia), creating a dead zone where fish and other aerobic organisms suffocate or flee.

[DIAGRAM: Eutrophication sequence — (1) nutrient runoff enters lake → (2) algal bloom covers surface → (3) algae die, bacteria decompose them → (4) dissolved oxygen drops (hypoxia) → fish die (dead zone).]

Dead zones appear where nutrient-rich rivers meet the sea — the Gulf of Mexico dead zone (fed by Mississippi River agricultural runoff) and the Chesapeake Bay are classic examples.

Dissolved oxygen and the oxygen sag curve

Dissolved oxygen (DO) is essential for aquatic life. It decreases when: temperatures rise (warm water holds less O₂ — thermal pollution), organic waste is decomposed (high biochemical oxygen demand, BOD), or eutrophication occurs.

Below a sewage/organic discharge point, DO drops sharply then gradually recovers downstream as the water re-aerates — the oxygen sag curve. Species present indicate water quality: clean water supports sensitive species (trout, mayfly larvae); polluted, low-DO water favors tolerant species.

Other water pollutants

• Thermal pollution — heated water discharged from power plants lowers DO and stresses aquatic life.
• Oil pollution — spills (Deepwater Horizon, Exxon Valdez) coat organisms, smother habitats, and are toxic; nonpoint oil runoff from roads is also large.
• Sediment — from erosion; clouds water, smothers habitats, blocks light.
• Pathogens — from sewage/animal waste; cause disease (cholera, dysentery); measured by fecal coliform counts.
• Toxic chemicals and heavy metals — mercury, lead, industrial chemicals (bioaccumulate — Lesson 27).

Water-quality indicators and the Clean Water Act

Indicators include dissolved oxygen, BOD, pH, turbidity, nutrient levels, temperature, and indicator species (bioindicators). The US Clean Water Act (CWA) regulates pollutant discharges into US waters, requires permits for point sources, and sets water-quality standards; the Safe Drinking Water Act sets standards for drinking water. (Cite purpose accurately; don't invent limits.)

Why this matters

Point vs. nonpoint classification, the eutrophication → dead-zone sequence, dissolved-oxygen reasoning, and the Clean Water Act are among the most tested Unit 8 items — and eutrophication FRQs appear repeatedly (it also ties back to the nutrient cycles of Lesson 2).

Worked Examples

Example 1 (easy): Classify the source

Classify: (i) a factory discharge pipe, (ii) fertilizer runoff from many farms, (iii) an oil tanker spill.

Solution: (i) Point source (single pipe); (ii) nonpoint source (diffuse runoff); (iii) point source (single identifiable spill).

Interpretation: One identifiable origin = point; diffuse/spread-out = nonpoint.

Example 2 (medium): Eutrophication sequence

Order the steps of cultural eutrophication leading to a fish kill.

Solution: (1) N and P runoff enters the water → (2) algal bloom → (3) algae die and decomposer bacteria consume oxygen → (4) dissolved oxygen drops (hypoxia) → fish die (dead zone).

Interpretation: Nutrients → bloom → death → decomposition → oxygen crash → fish kill.

Example 3 (AP-style): Dissolved oxygen reasoning

A power plant discharges warm water into a river, and a sewage outfall lies just upstream. Explain two reasons dissolved oxygen is low here.

Solution: (1) Thermal pollution — warm water holds less dissolved oxygen, so heating lowers DO. (2) High BOD from sewage — decomposer bacteria consume oxygen breaking down the organic waste, further depleting DO. Both stress or kill aquatic organisms.

Interpretation: Warm water + organic waste both drive DO down (temperature and BOD).

Example 4 (AP-style): Gulf dead zone

Explain how farming in the Mississippi River basin creates a dead zone in the Gulf of Mexico.

Solution: Fertilizer (N and P) applied across the vast basin washes as nonpoint runoff into the Mississippi, which carries it to the Gulf. There, nutrients trigger algal blooms; when the algae die, decomposition consumes oxygen, producing a large hypoxic dead zone where marine life cannot survive.

Interpretation: Distant farm runoff → river transport → coastal blooms → hypoxia → dead zone.

Common Mistakes

• Confusing point and nonpoint sources. Point = single identifiable outlet; nonpoint = diffuse runoff (the larger, harder-to-regulate share).
• Saying eutrophication "adds oxygen." The algal bloom's decomposition removes oxygen — that's the whole problem (hypoxia).
• Forgetting temperature's effect on DO. Warm water holds less dissolved oxygen — the basis of thermal pollution harm.
• Mixing up limiting nutrients. Phosphorus usually limits freshwater; nitrogen usually limits marine systems.
• Inventing Clean Water Act specifics. Cite it as regulating point-source discharges and setting water-quality standards; don't fabricate numeric limits.

Practice Problems

11. (FRQ-style) Describe the full sequence of cultural eutrophication from fertilizer application to a fish kill, and propose two practices to reduce nutrient input.

12. (Data) Downstream of a sewage outfall, DO falls sharply then rises again miles later. (a) Name this pattern. (b) Explain the dip and the recovery.

FRQ Practice — Analyze an Environmental Problem and Propose a Solution (10 pts)

A bay receives runoff from surrounding farms and suburbs and discharge from a wastewater plant. It suffers algal blooms, summer dead zones, and declining fisheries.

(a) Identify the point and nonpoint sources contributing to the problem. (2 pts) (b) Explain the sequence by which nutrients cause the dead zone. (3 pts) (c) Explain why the dead zone is worst in summer. (2 pts) (d) Propose two solutions (at least one targeting nonpoint sources) and justify each. (3 pts)