13.2 Water pollution sources and effects

Types of Water Pollution

Water pollution enters ecosystems through many pathways, and the type of source determines how easy it is to track and regulate. The pollutants themselves range from well-studied nutrients and heavy metals to newer concerns like microplastics. Understanding both the sources and the contaminants is the foundation for tackling water quality problems.

Point Source and Non-Point Source Pollution

The most fundamental distinction in water pollution is where it comes from.

Point source pollution originates from a single, identifiable location. Think of a pipe discharging wastewater from a factory or a sewage treatment plant outfall. Because you can literally point to the source, these are easier to monitor, regulate, and treat. Examples include:

• Sewage treatment plant discharges
• Industrial facility outflows
• Oil spills from tankers or pipelines

Non-point source pollution comes from diffuse sources spread over a large area. There's no single pipe to regulate, which makes it much harder to control. Major non-point sources include:

• Agricultural runoff — Excess fertilizers, pesticides, and animal waste wash off fields during rain events and flow into nearby streams and rivers. This is one of the largest contributors to nutrient loading and eutrophication worldwide.
• Urban runoff — Stormwater picks up oil, heavy metals, litter, and lawn chemicals from roads, parking lots, and residential areas.
• Atmospheric deposition — Pollutants released into the air (like sulfur and nitrogen compounds) settle back onto land and water surfaces.

Industrial effluents are a common point source. These wastewater discharges from manufacturing and processing facilities can contain toxic chemicals, heavy metals, and organic compounds. Strict regulations govern these discharges in many countries, but accidental spills and improper disposal still occur.

Emerging Contaminants

Beyond conventional pollutants, two categories deserve special attention because of their persistence and growing impact.

Heavy metals such as lead, mercury, cadmium, and arsenic enter waterways through industrial discharge, mining operations, and runoff. What makes them especially dangerous is bioaccumulation: organisms absorb these metals faster than they can excrete them, so concentrations increase at each level of the food chain. A top predator like a tuna or an eagle can carry metal concentrations thousands of times higher than the surrounding water. Health effects include neurological damage, reproductive failure, and developmental problems in both wildlife and humans.

Microplastics are plastic particles smaller than 5 mm. They come from two main sources: the physical breakdown of larger plastic debris in the environment, and manufactured microbeads found in some personal care products like exfoliating scrubs. Marine organisms from zooplankton to whales ingest these particles, which can cause physical damage to digestive systems and introduce adsorbed toxic chemicals into tissues. The long-term ecological and human health effects are still an active area of research, but microplastics have now been detected in drinking water, seafood, and even human blood.

Effects on Aquatic Ecosystems

Nutrient Pollution and Algal Blooms

Eutrophication is the process by which excess nutrients, primarily nitrogen and phosphorus, fuel explosive growth of algae and aquatic plants. The sequence works like this:

1. Nutrients enter a water body from agricultural runoff, sewage, or industrial waste.
2. Algae and aquatic plants grow rapidly in response to the nutrient surplus.
3. Dense algal growth blocks sunlight from reaching submerged vegetation, reducing water clarity.
4. When the algae die, bacteria decompose the organic matter and consume large amounts of dissolved oxygen in the process.
5. Oxygen levels drop, creating hypoxic (low-oxygen) or anoxic (no-oxygen) conditions.

These oxygen-depleted areas are often called dead zones because most fish and invertebrates cannot survive in them. The Gulf of Mexico dead zone, fed by nutrient runoff from the Mississippi River watershed, regularly exceeds 15,000 square kilometers in summer.

Some algal blooms also produce toxins. Harmful algal blooms (HABs) include red tides (caused by dinoflagellates) and cyanobacterial blooms in freshwater lakes. These toxins can kill fish, sicken marine mammals, and contaminate drinking water supplies.

Thermal and Acidic Pollution

Thermal pollution occurs when heated water is released into the environment, most commonly from power plant cooling systems and industrial processes. Warmer water holds less dissolved oxygen, which stresses aquatic organisms. It also raises metabolic rates, meaning organisms need more oxygen at the exact time less is available. Temperature-sensitive species like trout and salmon are particularly vulnerable because they require cold, well-oxygenated water.

Acid rain forms when sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) from fossil fuel combustion react with water vapor in the atmosphere, producing sulfuric and nitric acids. This acidic precipitation lowers the pH of lakes and streams, with several cascading effects:

• Acidified water leaches toxic metals like aluminum from surrounding soils, further harming aquatic life.
• Many fish species cannot reproduce below a pH of about 5.0, and eggs and larvae are especially sensitive.
• Species diversity declines sharply in acidified waters, with sensitive organisms like mayflies and freshwater mussels disappearing first.

Boreal lakes and high-altitude streams are particularly vulnerable because their surrounding soils and bedrock often lack the calcium carbonate that would naturally buffer (neutralize) the acidity.