Palouse agriculture slice

Nonpoint Pollution from Agriculture

Landowner Practices Can Cause Pollution

The early emphasis on point sources of pollution meant that Ecology focused on discharges from pipes.

This photo shows a less obvious source of pollution. Livestock next to a stream can trample and denude vegetation, which causes streambank erosion and polluted runoff.

Historically, Washington State, along with the other states, focused on point sources of pollution to eliminate water pollution. Point sources are those sources that discharge pollutants directly into the state’s waterways through pipes or other conveyance systems.


The Department of Ecology (Ecology) uses the permit program authorized by the federal Clean Water Act to regulate the pollutants discharged from these sources. Point sources include sewage treatment plants, factories, and stormwater from urban areas, construction sites, industrial sites, and roads. Ecology’s work with these dischargers has resulted in a substantial cleanup of the state’s waters.

However, despite the successes, studies still indicate that persistent water quality problems remain.

"Nonpoint" pollution

According to the National Water Quality Inventory, more diffuse “nonpoint sources” of pollution – such as runoff, erosion, and stream modification caused by agricultural practices – are now identified as the leading source of stream pollution in the United States.

Washington State water quality data and studies mirror national reports, and indicate that activities on some agricultural lands are a significant source of pollution.

The federal Clean Water Act requires Ecology to identify diffuse sources of pollution, such as runoff from agricultural lands, and develop the necessary controls to prevent it. State law further requires that landowners implement practices to ensure these kinds of water pollution are prevented.

For example, in Western Washington, commercial shellfish operations in the Samish River watershed have been repeatedly shut down due to bacterial contamination. In Central Washington, nitrate pollution threatens drinking water supplies in the lower Yakima basin.

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How does “nonpoint” pollution happen?

Most land drains to surface waters such as streams, lakes and wetlands. Whether a property is next to a large river, small intermittent stream, drainage ditch, or is just draining to ground water, the water from precipitation will likely find its way from the property to that surface or ground water.

Human activities, such as construction and farming, tend to alter soil structure, natural drainage patterns, and native vegetation. These alterations of the land change the nature of the local water cycle (hydrology) by increasing the amount of precipitation that does not soak into the ground, and instead runs off the land. This surface flow can carry pollutants such as fertilizer, pesticides, animal waste, and excess soil into nearby streams. Also, increasing the runoff and changing soil structures may lead to increased erosion. When excess soil enters streams, it may change the properties of the water, smother prime aquatic habitats, and further increase a stream’s power to erode. This in turn, can cause even more soil or sediment to enter and be transported by the water.

At animal confinement areas, there can be significant erosion and polluted runoff problems. Runoff contains sediment, nutrients, and bacteria.

Livestock degrade riparian areas by trampling and denuding vegetation, which causes streambank erosion and generates polluted runoff. Livestock congregation in the riparian area can also lead to manure accumulations and direct discharges.

In addition to increased runoff and erosion, human activities introduce harmful or even toxic substances into the environment. Often these substances are easily transported to surface waters by the increased runoff and erosion caused by changing the land cover.

When these changes occur on or near streambanks (known as the riparian area), the harm to water quality may be compounded. Loss of stream-side vegetation may increase water temperatures and increase the risks of erosion.

Ultimately these changes to the land, and subsequently the water, impair human, fish, and wildlife uses of the water. Prime habitats can be destroyed, the water may be unfit for key species such as salmon to survive, and pollutants may render our waters unsafe to recreate in and ground waters unhealthy to drink.

Agricultural-related water quality risks can be classified into three broad categories:

  • Stream Modification
  • Stormwater Runoff
  • Groundwater Contamination







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Last updated July 2012