Water Pollution: Nutrients

Rain barrels can help prevent nutrient pollution from entering storm water systems. Leanna Rathkelly/Moment Mobile/Getty

According to the Environmental Protection Agency, over half of the nation’s streams and rivers are polluted, and of those, 19% are impaired by the presence of excess nutrients.

What Is Nutrient Pollution?

The term nutrient refers to sources of nourishment supporting organism growth. In the context of water pollution, nutrients generally consist of phosphorus and nitrogen which algae and aquatic plants use to grow and proliferate.

Nitrogen is present in abundance in the atmosphere, but not in a form that is available to most living things. When nitrogen is in the form of ammonia, nitrite, or nitrate, however, it can be used by many bacteria, algae, and plants (here's a nitrogen cycle refresher). Generally, it is the overabundance of nitrates that causes environmental problems.

What Causes Nutrient Pollution?

  • Some common agricultural practices lead to excess nutrients in water bodies. Phosphorus and nitrates are important components of the fertilizers used in agricultural fields – they are present in both synthetic fertilizers and natural ones like manures. If the crops do not pick up all of the fertilizer applied, or if rain has a chance to wash it away before they are absorbed by plants, the excess fertilizer is flushed into streams. Another major source of nutrients also comes from the way agricultural fields are only used seasonally. Most crops are present in the fields over a relatively short growing season, and the rest of the year the soil is left exposed to the elements. Meanwhile soil bacteria is feasting on decaying roots and plant debris, releasing nitrates. Not only do bare fields cause sediment pollution, but this practice allows the massive release and washing away of nitrates.
  • Sewage can carries nutrients to streams and water. Septic systems, especially if older or improperly maintained, can leak into streams or lakes. Households connected to municipal sewer systems also contribute to nutrient pollution. Wastewater treatment plants sometimes function improperly, and are periodically overwhelmed during heavy rain events and release sewage into rivers.
  • Stormwater. Rain falling in urban or suburban areas picks up nutrients from lawn fertilizer, pet waste, and various detergents (for example, the soap used to wash one’s car in the driveway). The stormwater is then canalized into municipal drainage systems and released into streams and rivers, loaded with phosphorus and nitrogen.
  • Burning fossil fuels release nitrogen oxides and ammonia into the air, and when those are deposited in water, they can contribute significantly to the excess nutrient problem. Most problematic are coal-fired power plants and gas- or diesel-powered vehicles.

What Environmental Effects Do Excess Nutrients Have?

Excess nitrates and phosphorus encourage the growth of aquatic plants and algae. Nutrient-boosted algae growth leads to massive algae blooms, visible as a bright green, foul smelling sheen on the water’s surface. Some of the algae making up the blooms produce toxins that are dangerous to fish, wildlife, and humans. The blooms eventually die off, and their decomposition consumes a lot of dissolved oxygen, leaving waters with low oxygen concentrations. Invertebrates and fish are killed when oxygen levels dip too low. Some areas, called dead zones, are so low in oxygen that they become empty of most life.

A notorious dead zone forms in the Gulf of Mexico every year due to agricultural runoff in the Mississippi River watershed.

Human health can be affected directly, as nitrates in drinking water are toxic, especially to infants. People and pets can also become quite ill from exposure to toxic algae. Water treatment does not necessarily solve the problem, and can in fact create dangerous conditions when chlorine interacts with the algae and produces carcinogenic compounds.  

Some Helpful Practices

  • Cover crops and no-till farming protect agricultural fields and mobilize nutrients. The cover plants die out in winter, and the following growing season they give back those nutrients to the new crop.
  • Maintaining well vegetated buffers around farm fields and next to streams allows plants to filter out nutrients before they enter the water.
  • Keep septic systems in good working order, and conduct regular inspections.
  • Consider your nutrient inputs from soaps and detergents, and reduce their use whenever possible.
  • In your yard, slow the water runoff and allow it to be filtered by plants and soil. To accomplish this, establish rain gardens, keep drainage ditches well vegetated, and use rain barrels to harvest roof runoff.
  • Consider using pervious pavement in your driveway. These surfaces are designed to let water percolate into the soil below, preventing runoff.

For More Information

Environmental Protection Agency. Nutrient Pollution.