Notes
Water supply
- About 97% of Earth's water is salty; most freshwater is frozen or underground. The water cycle is driven by the sun.
- An aquifer stores groundwater. Overpumping lowers the water table, causes subsidence, and allows saltwater intrusion. A watershed is the land that drains to one body of water.
- Agriculture uses about 70% of freshwater withdrawals. Efficiency measures: drip irrigation, leak repair, reuse, and desalination.
Pollution
- Point sources come from one place; nonpoint sources are diffuse (runoff) and harder to regulate.
- Pollutants: pathogens, nutrients, sediment, toxins, heavy metals, oil, and heat. Biomagnification concentrates toxins in top predators.
- Cultural eutrophication: extra nitrogen and phosphorus, algal bloom, death and decomposition, low dissolved oxygen (hypoxia), fish kills, dead zones.
Treatment and law
- Wastewater: primary (solids), secondary (microbes), tertiary (nutrients and disinfection). Drinking water: coagulation, sedimentation, filtration, disinfection.
- Clean Water Act (1972) and Safe Drinking Water Act (1974) protect US water.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Diffuse runoff is a nonpoint source; an outfall pipe is a point source.
Decomposers consume oxygen as they break down dead algae.
Frozen water is the largest freshwater reservoir.
Drip irrigation is the most efficient common method.
Toxins that are not excreted build up in each trophic level.
Tertiary treatment is the final polishing step.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A small lake surrounded by farms and lawns turns green in summer. In late summer, many dead fish are found along the shore. Tests show high nitrogen and phosphorus levels and very low dissolved oxygen near the bottom.
(a) Name the process that has occurred and identify two likely sources of the nutrients.
Show the answer and scoring guide
Cultural eutrophication. Sources include fertilizer runoff from farms and lawns, animal waste, and leaking septic systems or sewage.
Check off the points you earned:
(b) Explain why oxygen levels dropped, leading to the fish deaths.
Show the answer and scoring guide
Extra nutrients caused an algal bloom. When the algae died, decomposers used up dissolved oxygen as they broke down the dead material. With too little oxygen, the fish could not survive.
Check off the points you earned:
(c) Classify the nutrient runoff as point or nonpoint pollution and propose one solution.
Show the answer and scoring guide
It is nonpoint source pollution because it comes from many scattered places. A solution is planting vegetated buffer strips, reducing fertilizer use, or restoring wetlands to filter runoff.
Check off the points you earned: