Water Pollution

💧 Water Pollution

Part 1 of 7 — Sources and Measures of Water Quality

🔑 Key Concept: The point/nonpoint split and the list of major pollutants are covered in Water Resources, Part 1. This topic goes one level deeper. Part 1 asks two practical questions: who can be held responsible for each kind of source (CED 8.1), and how scientists actually measure whether a stream is healthy, using chemical tests and living indicator species (CED 8.2).

📖 Why the Source Type Decides the Remedy

A point source discharges from one identifiable place, so it can be given a permit, sampled at the pipe and fined. A nonpoint source is spread across a landscape, so there is no single pipe to sample and no single owner to fine.

The gray zone

Some sources do not fit neatly, and the exam likes them:

🔑 AP Connection: When an FRQ asks you to classify a source, use the test "can you point to one discharge location?" Then add the consequence: a point source can be permitted and treated at the pipe, while a nonpoint source needs land-management practices across the watershed.

Concept Check 🎯

Reading a Water-Quality Report

Field kits and probes measure a handful of chemical and physical indicators. Each one points toward a likely cause. (Ranges are typical guides; exact standards vary by state and by type of water body.)

Two habits make these numbers useful:

1. Compare upstream with downstream. A single reading means little; a drop in DO or a jump in E. coli between two sites brackets the source.
2. Look for the matching pair. High BOD with low DO points to organic waste. High nitrate and phosphate with very high daytime DO points to an algal bloom. High turbidity after a storm points to erosion.

Worked Example: Percent Oxygen Saturation

How much oxygen water can hold depends on temperature (the solubility table is in Water Resources, Part 3). Comparing what is measured with what the water could hold gives percent saturation:

$\text{percent saturation} = \frac{\text{measured DO}}{\text{DO at saturation}} \times 100$

Site A (illustrative data): water at 20 °C, where saturation is 9.1 mg/L. Measured DO is 6.4 mg/L. $\frac{6.4}{9.1} \times 100 \approx 70\%$ Decomposers are using oxygen faster than it diffuses in from the air.

Site B (illustrative data): a lake at 10 °C, where saturation is 11.3 mg/L. At 3 p.m. the measured DO is 13.0 mg/L. $\frac{13.0}{11.3} \times 100 \approx 115\%$ Water above 100% is supersaturated. That happens when a dense algal bloom photosynthesizes in sunlight. The same lake can fall far below 100% before dawn, when the algae only respire. A big day-night swing is itself a sign of eutrophication (Part 2).

Calculate It 🧮

Enter numbers only, without units. Saturation values: 0 °C = 14.6 mg/L, 15 °C = 10.1 mg/L, 30 °C = 7.6 mg/L.

1) A pond at 30 °C has a measured DO of 5.7 mg/L. What is its percent saturation?

2) An ice-covered lake at 0 °C has a measured DO of 7.3 mg/L. What is its percent saturation?

3) A trout stream at 15 °C reads 101% saturation. What is its measured DO, in mg/L, to the nearest tenth?

Living Indicators: Macroinvertebrates

Every species has a range of tolerance for oxygen, temperature and pollutants (the tolerance curve is covered in Water Resources, Part 4). Species with a narrow range for oxygen make good indicator species: if they are present, the water has been clean for a long time.

Stream macroinvertebrates (insect larvae, crustaceans, snails and worms large enough to see) are the standard choice because they live in one stretch of stream for months to years and cannot swim away from a pollution pulse.

Why use animals at all? A chemical test is a snapshot of one moment, and a short pollution pulse at night can be missed. The animal community integrates conditions over months, so a missing group of sensitive insects records past events the probe never saw.

Worked Example: A Biotic Index Score

One common volunteer method gives each type of organism found (not each individual) a weight: Group 1 = 3 points, Group 2 = 2 points, Group 3 = 1 point. On that scale, a total above 22 is excellent, 17–22 good, 11–16 fair and below 11 poor.

Kick-net sample from a creek (illustrative data):

Notice that the creek still had sensitive types, but no stoneflies, which need the most oxygen, and three tolerant types. A sample with 400 midges and nothing else would score just 1, even though it contains many animals: diversity of sensitive types matters, not abundance.

🔑 AP Connection: For an FRQ that gives a macroinvertebrate table, state the trend and the mechanism in one sentence: "Below the outfall, sensitive stoneflies and mayflies disappear and tolerant worms and midges increase, because organic waste lowers dissolved oxygen."

AP-Style Application 🎯

Part 1 Summary

Next: Part 2 follows the most common cause of low oxygen, excess nutrients, from a farm field to a toxic algal bloom and a coastal dead zone.

💧 Water Pollution

Part 2 of 7 — Eutrophication and Hypoxia in Depth

🔑 Key Concept: The basic cascade (nutrients → bloom → die-off → decomposition → low oxygen) and the oligotrophic/eutrophic contrast are covered in Water Resources, Part 3. This part explains why the cascade happens where it does: which nutrient controls growth, why oxygen loss is concentrated in bottom water, and how two of the best-studied cases, Lake Erie and the Gulf of Mexico, were measured and managed (CED 8.5).

📖 Which Nutrient Limits Growth?

Algae need many elements, but their growth is capped by the one in shortest supply relative to need: the limiting nutrient. Adding more of any other nutrient changes little; adding the limiting one triggers a bloom.

The classic evidence. In the 1970s, researchers at Canada's Experimental Lakes Area split one small lake in two with a plastic curtain. Both halves received carbon and nitrogen; only one half also received phosphorus. By late summer that half had turned bright green with algae, and the other half stayed clear. Results like this supported limits on phosphate in laundry detergents in Canada and many U.S. states.

🔑 AP Connection: If a question asks which nutrient to reduce to stop blooms in a lake, the answer is phosphorus; for a coastal dead zone, it is nitrogen. In practice managers cut both, because water flows from rivers (P-limited) to the coast (N-limited).

Why the Bottom Runs Out of Oxygen

In summer, sunlight warms the surface of a deep lake. Warm water is less dense, so the lake stratifies into layers that do not mix:

Dead algae sink into the hypolimnion, and decomposers there use up its fixed supply of oxygen. Bottom oxygen falls all summer until fall turnover, when the surface cools, the layers reach the same density, and wind mixes oxygen back down.

Worked Example: Reading a DO Depth Profile (illustrative data)

A eutrophic lake in August:

1. Find the thermocline: temperature drops 8 °C between 4 and 6 m, far more than elsewhere, so the thermocline is at about 4–6 m.
2. Find the hypoxic layer: DO is below 2 mg/L from about 8 m down. Fish are squeezed into the warm surface layer, which is a problem for cold-water species such as trout.
3. Percent change from surface to bottom: $\frac{0.2 - 9.8}{9.8} \times 100 \approx -98\%$.

A feedback loop. When bottom water loses its oxygen, phosphorus that was locked in the sediments is released back into the water (internal loading). At turnover that phosphorus mixes upward and feeds the next bloom, which is why lakes can stay eutrophic for years after outside inputs are cut.

Concept Check 🎯

Harmful Algal Blooms: Lake Erie

Some blooms are dominated by cyanobacteria such as Microcystis, which make microcystin, a toxin that damages the liver. These harmful algal blooms (HABs) threaten drinking water and pets as well as fish.

The lesson: point-source controls fixed the first crisis, but the second came from nonpoint runoff, which is far harder to control (Part 1). Wet springs, which wash more fertilizer into the river, produce the largest summer blooms.

The Gulf of Mexico Hypoxic Zone

The Mississippi–Atchafalaya River basin drains about 40% of the lower 48 states, including the Corn Belt. Its nitrogen (mostly nitrate from fertilizer and manure) reaches the Gulf each spring.

• Fresh river water floats on top of denser salt water, and summer warming strengthens the layering, so the bottom water is cut off from the air, just like a lake's hypolimnion.
• The zone is mapped each summer and defined as bottom water with DO below 2 mg/L.
• Its size swings with spring river flow. The largest measured zone was about 22,700 km² (2017), roughly the size of New Jersey. The federal–state task force goal is a five-year average of about 5,000 km² by 2035.
• Fish and shrimp swim away; slow bottom animals die. Shrimp boats must travel farther, raising costs.

Hypoxic zone area in late July (illustrative data, within the measured range):

Reading it: the two largest zones follow the two high-flow springs, because more runoff carries more nitrate. The five-year mean is $\frac{8 + 20 + 15 + 22 + 10}{5} = 15$ thousand km², three times the 5,000 km² goal.

Worked Example: Nutrient Load

Managers track load, the mass of nutrient a river delivers per unit time, not just its concentration. A dilute river with a huge flow can deliver more than a concentrated creek.

$\text{load} = \text{concentration} \times \text{flow}$

A shortcut: 1 mg/L is the same as 1 g/m³, because 1 m³ holds 1,000 L.

A river carries total phosphorus at 0.4 mg/L with a flow of 100 m³/s (illustrative data).

1. Concentration in g/m³: $0.4\ \text{mg/L} = 0.4\ \text{g/m}^3$
2. Load per second: $0.4\ \text{g/m}^3 \times 100\ \text{m}^3\text{/s} = 40\ \text{g/s}$
3. Load per day: $40\ \text{g/s} \times 86{,}400\ \text{s/day} = 3{,}456{,}000\ \text{g} \approx 3{,}500\ \text{kg/day}$

A 40% reduction target means cutting this to $3{,}456 \times 0.6 \approx 2{,}070$ kg/day.

Calculate It 🧮

Enter numbers only, without units or commas.

1) A creek carries phosphorus at 0.5 mg/L with a flow of 20 m³/s. What is its load in grams per second?

2) Using your answer to 1), what is the load in kilograms per day? (There are 86,400 seconds in a day.)

3) A hypoxic zone shrinks from 20,000 km² to 5,000 km². By what percent did it shrink?

Cutting Nutrients at the Source

🔑 AP Connection: A strong FRQ solution names one practice, its mechanism and a drawback: "Planting cover crops after harvest takes up leftover nitrate before spring rains wash it away, but it adds seed and labor costs for the farmer."

AP-Style Application 🎯

Part 2 Summary

Next: Part 3 turns from chemical overload to physical pollutants: oil, heat and sediment.

💧 Water Pollution

Part 3 of 7 — Oil, Heat and Sediment: Physical Pollutants

🔑 Key Concept: How oil, heat and sediment harm organisms (oiled feathers, lower dissolved oxygen, blocked light) is covered in Water Resources, Part 3. This part works through real cases and engineering choices: what made two famous oil spills different, why every cleanup method has a cost, how power plants choose a cooling system, and how sediment is measured (CED 8.2, 8.4, 8.6).

📖 Two Spills Compared

Units. Oil is measured in barrels, and 1 barrel = 42 gallons. Converting the Deepwater Horizon release: $3.2 \times 10^{6}\ \text{barrels} \times 42\ \frac{\text{gal}}{\text{barrel}} \approx 1.34 \times 10^{8}\ \text{gal}$ That is about $134 \div 11 \approx 12$ times the Exxon Valdez spill. Yet the Exxon Valdez is remembered as severe because cold water slows evaporation and microbial breakdown, and oil lingered under the beach gravel for decades.

Cleanup Methods and Their Trade-offs

Worked Example: Where did the oil go? (federal estimate, August 2010)

Oil physically removed by people: $17 + 5 + 3 = 25\%$. Three quarters of the oil was dispersed, evaporated, dissolved or left in the environment. Later studies revised some of these shares, but the conclusion holds: prevention beats cleanup.

Concept Check 🎯

Oil in Marshes, Mangroves and Reefs

Coastal wetlands and reefs are where oil does its longest-lasting damage (CED 8.2, 8.4).

Cleanup in these habitats is especially hard: trampling and digging can do more damage than the oil, so responders often protect them with booms and then let natural breakdown work.

Cooling Systems and Thermal Shock

Steam power plants (coal, gas and nuclear) must condense their steam with cooling water. There are two basic designs:

Thermal shock is harm from a sudden temperature change. Fish that gather in a warm plume in winter can die of cold shock if the plant shuts down. In Florida, manatees cluster at power-plant outflows in winter, so utilities plan for their warm water when plants close.

Worked Example: Mixing a Warm Discharge (illustrative data)

A river flowing at 50 m³/s and 18 °C receives a discharge of 10 m³/s at 30 °C. Once fully mixed, the temperature is a flow-weighted average:

$T_{\text{mix}} = \frac{(50 \times 18) + (10 \times 30)}{50 + 10} = \frac{900 + 300}{60} = 20\ \text{°C}$

A 2 °C rise sounds small, but near the outfall, before mixing, fish meet water close to 30 °C, where saturated water holds only about 7.6 mg/L of oxygen.

Measuring Sediment

Sediment does more than cloud water. Soil particles carry attached phosphorus and pesticides into streams, and dams trap sediment that would otherwise rebuild deltas and beaches downstream.

Calculate It 🧮

Enter numbers only, without units or commas.

1) A spill releases 50,000 barrels of crude oil. How many gallons is that? (1 barrel = 42 gallons)

2) A river flowing at 90 m³/s and 16 °C receives a 10 m³/s discharge at 26 °C. What is the fully mixed temperature, in °C?

3) Using the August 2010 table, what percent of the Deepwater Horizon oil was captured, burned or skimmed?

AP-Style Application 🎯

Part 3 Summary

Next: Part 4 follows chemicals that do not break down, through case studies of mercury, PCBs, DDT and hormone mimics.

💧 Water Pollution

Part 4 of 7 — Persistent Toxins: Case Studies

🔑 Key Concept: The definitions of endocrine disruptors, POPs, bioaccumulation and biomagnification, and the mercury and DDT basics, are covered in Water Resources, Part 4. This part tests those ideas against real cases: what happened, how the cause was traced, what policy followed, and why fish advisories single out large predatory fish (CED 8.3, 8.7, 8.8).

📖 Minamata: Mercury in a Fishing Town

Why the town was hit so hard: methylmercury biomagnified from plankton to fish and shellfish, and fishing families ate seafood at almost every meal. More than 2,000 people were officially recognized as patients, and many more were affected.

Mercury today. Factories no longer dump methylmercury directly. Most human emissions now come from small-scale gold mining (miners use mercury to separate gold) and burning coal. Inorganic mercury falls into lakes and wetlands, where bacteria in low-oxygen sediments convert it to methylmercury, the form that enters food webs.

PCBs in the Great Lakes and the Hudson

PCBs (polychlorinated biphenyls) were used as insulating fluids in electrical transformers and capacitors because they do not burn or break down, the same property that makes them persistent. U.S. production ended in 1979.

• From the late 1940s to the late 1970s, two capacitor plants on the Hudson River in New York released an estimated 1.3 million pounds of PCBs. The river became a Superfund site, and contaminated sediment was dredged out from 2009 to 2015.
• In the Great Lakes, PCBs still sit in bottom sediments. States issue fish consumption advisories that limit meals of large, fatty, long-lived fish such as lake trout and salmon.

Worked Example: A PCB Food Web

PCB concentrations in a Great Lakes food web (rounded from a classic study; treat as illustrative):

Factor from water to gull eggs: $\frac{124}{0.000002} = \frac{1.24 \times 10^{2}}{2 \times 10^{-6}} = 0.62 \times 10^{8} = 6.2 \times 10^{7}$

The eggs carry about 62 million times the concentration in the water. The gulls are at the top of a long aquatic chain, and each step concentrates the fat-soluble PCBs again.

Concept Check 🎯

DDT, Eagles and a Global Treaty

The recovery data. U.S. Fish and Wildlife Service estimates of bald eagle nesting pairs in the lower 48 states:

$\frac{9{,}800}{417} \approx 23$

The population grew about 23-fold. The DDT ban removed the main cause of eggshell thinning, while Endangered Species Act protection of nests and habitat also helped: real recoveries usually have more than one cause.

The global response. The Stockholm Convention on Persistent Organic Pollutants (adopted 2001, in force 2004) began with twelve POPs, including DDT, PCBs, dioxins and several pesticides, and has added more since. It contains a deliberate exception: countries may still use DDT indoors to control malaria-carrying mosquitoes, because for some regions the disease risk outweighs the environmental risk. (The United States signed the treaty but has not ratified it.)

Global Distillation and Hormone Mimics

Global distillation (the "grasshopper effect"). Many POPs evaporate in warm regions, travel on winds and condense in cold regions, hopping poleward over years. Arctic food webs are long and rich in fat (seal blubber), so some Arctic communities that eat traditional foods carry some of the highest PCB levels measured in people, far from any factory or farm that used them.

Endocrine disruptors in the field:

These cases show why endocrine disruptors are hard to regulate: effects can appear at very low doses, show up in the next generation, and come from mixtures rather than one chemical.

Worked Example: Mercury in a Serving of Fish

Fish advisories rest on biomagnification: methylmercury builds up in large, long-lived predators near the top of aquatic food chains. In fish, 1 ppm is the same as 1 µg of mercury per gram of fish, so the mercury in a serving is the serving mass times the concentration.

Compare two fish (illustrative mercury levels):

1. One 120 g serving of tuna at 0.35 ppm: $120\ \text{g} \times 0.35\ \frac{\mu\text{g}}{\text{g}} = 42\ \mu\text{g}$
2. One 120 g serving of sardines at 0.02 ppm: $120 \times 0.02 = 2.4\ \mu\text{g}$
3. Comparison: $42 \div 2.4 = 17.5$, so the tuna serving carries about 17.5 times as much mercury.

Interpretation: tuna is a large predator high on the food chain, so methylmercury has biomagnified in its tissue. That is why advisories favor small, short-lived fish low on the food chain over large predators, especially for pregnant women and young children.

Calculate It 🧮

Enter numbers only, without units or commas.

1) Using the PCB food-web table, by what factor does the concentration increase from rainbow smelt to lake trout?

2) King mackerel, a large predator, averages about 0.73 ppm mercury; tilapia, which feeds low on the food chain, about 0.01 ppm. For equal 100 g servings, how many times as much mercury is in the king mackerel?

3) How many micrograms of mercury are in a 150 g serving of swordfish at 0.4 ppm?

AP-Style Application 🎯

Part 4 Summary

Next: Part 5 asks how scientists decide how much of a toxin is too much: dose-response curves, safety margins and risk assessment.

💧 Water Pollution

Part 5 of 7 — Dose, Response and Risk

🔑 Key Concept: The definitions of LD50, ED50 and threshold, and how to read one LD50 from a table, are covered in Water Resources, Part 4. This part goes further: comparing whole dose-response curves, the aquatic LC50 test, acute versus chronic exposure, and why it is hard to link a pollutant to a human health problem (CED 8.12, 8.13, 8.14).

📖 "The Dose Makes the Poison"

Nearly any substance is harmful at a high enough dose, and many are harmless at a low one. Approximate oral LD50 values in rats show the range:

The right-hand column multiplies the LD50 by 70 kg. It is only a rough scale: species differ, which is why lab results carry uncertainty when applied to people.

Comparing Two Dose-Response Curves

The LD50 is one point on a curve. The shape of the whole curve matters too. Percent of test fish showing a harmful effect (illustrative data):

Reading the curves:

1. ED50: P reaches 50% at 20 mg/kg. Q passes 50% between 20 and 40 mg/kg, so its ED50 is about 30–35 mg/kg. At the 50% point, P is the more potent chemical.
2. Threshold: P has none of its effect at 5 mg/kg, so it has a threshold between 5 and 10 mg/kg. Q already affects 15% at 5 mg/kg, so it shows no clear threshold.
3. Slope: P's curve is steep (10% to 90% between 10 and 40 mg/kg), so a small increase in dose sharply raises harm. Q's is shallow.
4. Low doses: at 5 mg/kg, Q harms more fish than P. A chemical with a higher ED50 can still be the bigger concern at the low doses found in the environment.

Regulators often assume that carcinogens have no threshold: any dose carries some risk, scaled to the dose. For many other toxins they assume a threshold and set limits below it.

Concept Check 🎯

LC50: Toxicity Tests for Water

For aquatic organisms the dose is the concentration in the water, not a mass swallowed. The LC50 (lethal concentration 50%) is the concentration that kills half the test organisms over a set time, often 96 hours for fish or 48 hours for water fleas (Daphnia).

96-hour test of an industrial solvent on minnows, 20 fish per tank (illustrative data):

The LC50 lies between 4 mg/L (30%) and 8 mg/L (70%). Since 50% is halfway between 30% and 70%, the LC50 is about $\frac{4 + 8}{2} = 6$ mg/L. The 0 mg/L tank is the control: it shows that handling and tank conditions alone did not kill fish.

Discharge permits often require that a pipe's effluent be diluted well below the LC50 for local species, because an LC50 still kills half the population and sublethal harm (slower growth, fewer eggs) begins at lower concentrations.

Acute and Chronic Exposures

Standard tests such as the LD50 examine one chemical over a short time, but people and fish in real streams face low doses of many chemicals for years. That gap is one reason it is hard to connect a pollutant to a health problem (below).

Classify the Exposure 🔽

Choose the term that fits each situation.

Why Cause and Effect Is Hard to Prove

The CED notes that it can be difficult to establish cause and effect between pollutants and human health problems (CED 8.14; the core examples are in Water Resources, Part 5):

• Toxicology doses lab animals in controlled experiments, so it can show cause, but results must be extrapolated to people.
• Epidemiology studies patterns of disease in human populations. It deals with real people but finds correlations, which can be confounded by smoking, diet, income or other exposures.
• Latency: some diseases appear decades after exposure (mesothelioma typically 20 or more years after asbestos), so the cause may be long gone.
• People are exposed to many chemicals at once, and deliberately dosing people with a suspected toxin would be unethical.

Worked Example: Dose from Drinking Water

The daily dose from drinking water, in mg per kg of body mass: $\text{dose} = \frac{\text{concentration (mg/L)} \times \text{water drunk (L/day)}}{\text{body mass (kg)}}$

A well contains a solvent at 0.35 mg/L. An adult of 70 kg drinks 2 L/day (illustrative data).

1. Dose: $\frac{0.35 \times 2}{70} = \frac{0.70}{70} = 0.01$ mg/kg per day
2. Compare the dose with the threshold on the solvent's dose-response curve. If the threshold is 0.005 mg/kg per day, this dose is twice the level at which effects begin, so the town should act: treat the water or switch sources.

🔑 AP Connection: In FRQ calculations, carry the units: (mg/L × L/day) ÷ kg gives mg/kg per day. The units cancel only if every number is in the right form, which is how graders check your setup.

Calculate It 🧮

Enter numbers only, without units or commas.

1) A 20 kg child drinks 1 L/day of water containing 0.1 mg/L of a contaminant. What is the child's daily dose, in mg/kg per day?

2) The contaminant's dose-response curve shows no effect below 0.01 mg/kg per day. How many times smaller than that threshold is the child's dose? (number only)

3) Using the minnow table above, what is the LC50 of the solvent, in mg/L?

AP-Style Application 🎯

Part 5 Summary

Next: Part 6 follows sewage beyond the treatment plant, into septic systems, combined sewers and the laws that protect water.

💧 Water Pollution

Part 6 of 7 — Sewage, Septic Systems and Water Law

🔑 Key Concept: The stages inside a treatment plant (primary, secondary, tertiary, disinfection) and the CED's disease list are covered in Water Resources, Part 5. This part looks at what happens outside the plant: overflowing sewers in storms, the septic systems that serve rural homes, how health officials test for pathogens, and the two federal laws that set the rules (CED 8.11, 8.14, 8.15).

📖 When the Sewer Overflows

Many older cities, mostly in the Northeast and Great Lakes region, built combined sewer systems: one pipe carries both household sewage and storm runoff to the treatment plant. In dry weather this works. In heavy rain, the flow exceeds what the pipes and plant can handle, and the system is designed to spill the excess, untreated sewage mixed with stormwater, straight into the nearest river or harbor. This is a combined sewer overflow (CSO).

What happens to the sludge? The solids removed at the plant are often digested by anaerobic bacteria, which produce methane-rich biogas that can power the plant. Treated sludge (biosolids) may be spread on farmland as fertilizer if it meets limits for pathogens and heavy metals; otherwise it is landfilled or incinerated.

Worked Example: How Much Overflow? (illustrative data)

A storm drops 2 cm of rain on a 5 km² combined-sewer district, and 60% of the rain runs off into the sewers.

1. Convert units: $2\ \text{cm} = 0.02\ \text{m}$ and $5\ \text{km}^2 = 5 \times 10^{6}\ \text{m}^2$
2. Rain volume: $0.02\ \text{m} \times 5 \times 10^{6}\ \text{m}^2 = 1 \times 10^{5}\ \text{m}^3$
3. Runoff: $1 \times 10^{5} \times 0.60 = 6 \times 10^{4}\ \text{m}^3 = 60{,}000\ \text{m}^3$
4. If the plant can accept 20,000 m³ during the storm, the overflow is $60{,}000 - 20{,}000 = 40{,}000\ \text{m}^3$ of untreated, diluted sewage.

Septic Systems: Treatment in the Backyard

About one in five U.S. homes, mostly rural, is not connected to a sewer. They use a septic system:

A septic system fails when the soil cannot finish the treatment: a high water table, saturated or very sandy soil, an overloaded or unpumped tank, or a drain field too close to a well or lake. Then nitrate and pathogens reach groundwater, and nutrients seep into nearby lakes. Dense lakeside cottages on septic systems are a classic nonpoint source of lake phosphorus.

Concept Check 🎯

Waterborne Pathogens and Indicator Bacteria

Two landmark cases

• London, 1854. Physician John Snow mapped cholera deaths and found them clustered around one public water pump on Broad Street. He persuaded local officials to remove the pump handle, and the outbreak, already waning, ended. His map is a founding example of epidemiology, decades before scientists identified the cholera bacterium.
• Milwaukee, 1993. Cryptosporidium passed through one of the city's water treatment plants, and an estimated 400,000 people became ill, the largest documented waterborne outbreak in U.S. history. Utilities nationwide improved filtration and monitoring afterward.

Indicator bacteria. Testing water for every pathogen would be slow and expensive. Instead, labs test for fecal coliform bacteria or E. coli, harmless gut bacteria that are always present in feces. Finding them means sewage or manure is present, so pathogens may be too.

Worked Example: Is the Beach Open?

A technician filters 10 mL of lake water and grows 19 E. coli colonies on the plate. Counts are reported per 100 mL: $19\ \text{colonies} \times \frac{100\ \text{mL}}{10\ \text{mL}} = 190\ \text{per 100 mL}$ That exceeds about 126 per 100 mL, so the beach would get an advisory until follow-up samples come back clean.

Calculate It 🧮

Enter numbers only, without units or commas.

1) A storm drops 3 cm of rain on a 2 km² combined-sewer district, and half of it runs off into the sewers. How many cubic meters of runoff enter the sewers?

2) The treatment plant can accept 12,000 m³ during the storm. How many cubic meters overflow?

3) A 25 mL beach sample grows 40 E. coli colonies. What is the count per 100 mL?

The Clean Water Act and the Safe Drinking Water Act

By the late 1960s, some U.S. rivers were so polluted that the oily Cuyahoga River in Cleveland caught fire (1969, not for the first time). Congress responded with two laws that divide the work:

Case Study: Flint, Michigan

Why it matters: lead is a neurotoxin; health agencies state that no safe blood-lead level in children has been identified. Lead reached the tap not from the river itself but from the pipes, because the water chemistry changed. Flint is a majority-Black city with high poverty, and residents' complaints were dismissed for months, so it is also a central environmental justice case.

🔑 AP Connection: Match the law to the water. Pollution entering a river from a pipe is a Clean Water Act issue; contamination in water from a public tap is a Safe Drinking Water Act issue.

AP-Style Application 🎯

Part 6 Summary

Next: Part 7 puts the whole topic together in AP exam-style practice.

💧 Water Pollution

Part 7 of 7 — AP Exam Practice and Synthesis

🔑 Key Concept: The APES exam has three free-response questions: FRQ 1 designs an investigation, FRQ 2 analyzes data, and FRQ 3 solves an environmental problem with calculations. Water Resources, Part 6 practiced FRQ 1 and FRQ 3 with thermal pollution and irrigation. This part practices FRQ 2 with a stream survey and FRQ 3 with a nutrient-reduction budget, drawing on Parts 1–6 of this topic.

📋 FRQ 2 Practice: A Stream Survey

A county surveyed four sites along a creek on the same August morning. Site B is just below a dairy farm's manure lagoon outlet; Site C is just below a small town's sewage plant, which uses primary and secondary treatment only. (Illustrative data.)

Questions

(a) Identify the site that serves as the reference (control) and explain why.

(b) Calculate the percent decrease in DO from Site A to Site C.

(c) Describe the trend in the biotic index from Site A to Site D, and explain it using the DO data.

(d) Explain why Site B has the highest E. coli count but Site C has the highest nitrate.

(e) Explain why DO recovers by Site D.

(f) Propose one practice at the dairy farm that would reduce its impact, and describe how it works.

Calculate It 🧮

Use the FRQ 2 table. Enter numbers only.

1) (b) Percent decrease in DO from Site A to Site C:

2) By how many points does the biotic index fall from Site A to Site C?

3) How many times higher is the E. coli count at Site B than at Site A?

Model Answers for FRQ 2

(a) Site A, upstream of both discharges. It shows the creek's condition without the farm or the sewage plant, so changes at the other sites can be attributed to those sources.

(b) $\frac{9.0 - 3.6}{9.0} \times 100 = \frac{5.4}{9.0} \times 100 = 60\%$ decrease.

(c) The index falls from 24 (excellent) at A to 9 and 8 (poor) at B and C, then partly recovers to 16 (fair) at D. Organic waste raises BOD, decomposers use up oxygen, and DO falls to 3.6–4.5 mg/L, too low for sensitive insects such as stoneflies and mayflies; only tolerant worms and midges remain.

(d) Manure enters Site B untreated, so it carries large numbers of fecal bacteria. The sewage plant disinfects its effluent, which cuts E. coli, but secondary treatment does not remove most nitrogen, so dissolved nitrogen, much of it as nitrate, passes through. (Tertiary treatment would remove it.)

(e) Over 15 km, decomposers use up most of the organic matter (BOD falls to 3 mg/L), so oxygen demand drops while oxygen keeps dissolving from the air and from photosynthesis. This is the recovery zone of the oxygen sag curve.

(f) Any one, with its mechanism: a vegetated buffer strip between the lagoon and the creek slows runoff so solids and bacteria settle and are filtered; covered manure storage keeps rain from overflowing the lagoon; spreading manure only on dry, unfrozen fields at crop-uptake rates keeps nutrients on the land; fencing cattle out of the creek stops direct manure input.

🔑 AP Connection: FRQ 2 rewards answers that cite the data. Write "DO falls from 9.0 to 3.6 mg/L" instead of "DO goes down".

📋 FRQ 3 Practice: A Nutrient Reduction Plan

A river basin delivers 1,200 metric tons of nitrogen per year to a coastal bay with a summer dead zone. Cropland contributes 70%, sewage plants 20% and other sources 10%. The state's goal is to cut the total load by 30%. (Illustrative data.)

Two practices are available:

• Upgrade sewage plants to tertiary treatment: removes 75% of the nitrogen in their effluent, at a cost of $18 million per year.
• Pay farmers to plant cover crops: each hectare keeps 20 kg of nitrogen out of the river, at a cost of $100 per hectare per year.

Questions

(a) Calculate how many metric tons of nitrogen per year must be removed to meet the goal.

(b) Calculate how many metric tons per year the sewage plant upgrade would remove.

(c) Calculate how many hectares of cover crops are needed to remove the rest of the target.

(d) Calculate the cost per kilogram of nitrogen removed for each practice, and identify which is cheaper.

(e) Explain one reason the cover crop estimate might fall short in a given year.

(f) Describe one benefit of the cover crops other than reducing nitrogen.

Calculate It 🧮

Use the FRQ 3 information. Enter numbers only, without units or commas.

1) (a) Metric tons of nitrogen per year that must be removed:

2) (b) Metric tons per year removed by the sewage plant upgrade:

3) (c) Hectares of cover crops needed for the rest of the target:

Model Answers for FRQ 3

(a) $0.30 \times 1{,}200\ \text{t} = 360\ \text{t}$ per year.

(b) Sewage plants deliver $0.20 \times 1{,}200 = 240$ t; the upgrade removes $0.75 \times 240 = 180$ t per year.

(c) Remaining: $360 - 180 = 180\ \text{t} = 180{,}000\ \text{kg}$. Hectares: $\frac{180{,}000\ \text{kg}}{20\ \text{kg/ha}} = 9{,}000\ \text{ha}$.

(d) Sewage upgrade: \frac{\18{,}000{,}000}{180{,}000\ \text{kg}} = $100$per kg. Cover crops:$\frac{$100\ \text{per ha}}{20\ \text{kg per ha}} = $5$ per kg. Cover crops are 20 times cheaper per kilogram, which is why nonpoint programs are attractive even though they are harder to guarantee.

(e) The program is voluntary, so fewer farmers may enroll; a wet spring can wash out more nitrate than the crops capture; or a late harvest leaves too little time for the cover crop to grow.

(f) Cover crop roots hold soil against erosion, which also keeps sediment and attached phosphorus out of the river; they add organic matter that improves soil fertility and water infiltration.

Misconceptions to Avoid

AP-Style Application 🎯

Match the Evidence to the Problem 🔽

Choose the pollution problem that best explains each set of data.

Part 7 Summary

🔑 AP Connection: Across every water-pollution question, the winning sentence has three links: source → mechanism → effect on organisms. "Manure from the feedlot adds organic matter, decomposers use up dissolved oxygen, and sensitive stoneflies disappear."