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Sources of Pollution & Water Quality

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Point vs. nonpoint sources

Pollution is classified by where it enters the environment. A point source is a single, identifiable discharge point — a factory outfall pipe, a sewage treatment plant, a leaking underground tank. Because you can locate and measure it, a point source is relatively easy to regulate. A nonpoint source is diffuse, entering from a broad area with no single point of origin — agricultural runoff carrying fertilizer and pesticides, urban street runoff, or eroding sediment. Nonpoint pollution is the leading cause of water-quality problems and the hardest to control precisely because it has no single source to plug.

Categories of water pollutants

Water pollutants fall into several types. Pathogens (bacteria, viruses) from sewage and animal waste cause diseases like cholera. Oxygen-demanding wastes (organic matter) fuel decomposers that deplete dissolved oxygen. Plant nutrients (nitrogen, phosphorus) drive eutrophication. Toxic chemicals (heavy metals, pesticides, industrial compounds) poison organisms. Sediment clouds water and smothers habitat. Thermal pollution — warm water discharged from power plants — lowers oxygen and stresses aquatic life. Oil spills coat and poison marine organisms.

Treating wastewater

Sewage treatment removes pollutants in stages. Primary treatment is physical: screening and settling remove solids. Secondary treatment is biological: bacteria break down the remaining organic (oxygen-demanding) waste. Tertiary treatment is chemical/advanced: it removes nutrients like nitrogen and phosphorus and disinfects the water. The Clean Water Act (1972) made it illegal to discharge pollutants from a point source into U.S. waters without a permit and set standards that dramatically improved water quality — though nonpoint pollution remains largely outside its reach.

Pollutant concentration
concentration = mass of pollutant / volume of water
Keep units consistent. 1 g/L = 1,000 mg/L; concentrations in water are often reported in mg/L, which for dilute solutions is approximately equal to ppm.
Worked example

A factory outfall pipe discharges 100 grams of a dissolved pollutant into 10,000 liters of river water each minute. What is the pollutant’s concentration, in milligrams per liter?

  1. 1.Write concentration = mass ÷ volume = 100 g ÷ 10,000 L = 0.01 g/L.
  2. 2.Convert grams to milligrams: 0.01 g/L × 1,000 mg/g = 10 mg/L.
  3. 3.For a dilute water solution, 1 mg/L ≈ 1 ppm, so this is about 10 ppm.
  4. 4.Because the pipe is a single identifiable outfall, this is a point source and its concentration is straightforward to measure and regulate.
Answer: 10 mg/L (about 10 ppm). Being a point source, this discharge can be permitted, monitored, and limited under the Clean Water Act.
Checkpoint

Fertilizer and pesticides washing off farm fields across an entire region into a river are an example of which kind of pollution, and why is it hard to regulate?

Tip

The quickest test for point vs. nonpoint: could you walk up and point at exactly where the pollution enters the water? A pipe or smokestack — yes, point source. Runoff from farms, lawns, and streets across a whole watershed — no, nonpoint source. Nonpoint pollution is the bigger, harder problem.

Checkpoint

During which stage of sewage treatment do bacteria biologically break down the dissolved organic, oxygen-demanding wastes?

On the exam

Two reliable exam facts: nonpoint-source (especially agricultural runoff) is the leading cause of water pollution and the hardest to regulate, and sewage treatment runs primary (physical) → secondary (biological) → tertiary (chemical/nutrient removal). The Clean Water Act targets point sources.

Answer the 2 checkpoints as you read.

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