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  • Writer's pictureKimberley Megis

Agricultural Runoff and water pollution

Agriculture, which accounts for 70 percent of water use worldwide, plays a major role in water pollution. Farms discharge large quantities of agrochemicals, organic matter, drug residues, sediments and saline drainage into water bodies. The resultant water pollution poses demonstrated risks to aquatic ecosystems, human health and productive activities. [1]

Point source pollution such as industrial and sewage wastewaters has been associated with the degradation of lentic ecosystems for decades and is easily identifiable and relatively easily treatable. Non-point source (NPS) pollution associated with storm water runoff from agricultural land uses on the other hand, is more difficult to manage and treat as it is more diffuse.[2]

Point source pollution such as industrial and sewage wastewaters has been associated with the degradation of lentic ecosystems for decades and is easily identifiable and relatively easily treatable.

Today, agricultural runoff is the leading cause of water pollution in the United States. About half of the lakes in the country that have been studied are considered “impaired”, which means they are too polluted to meet standards for swimming and recreation, aquatic life, fish consumption, or more importantly, as drinking water sources. [3]

Chemicals may be transported in runoff from farmlands to aquatic habitats in the form of plant debris, eroded soil particles, true solution, undissolved particles or dissolved in humate material. [4]

Agricultural runoff also contains sediment and heavy metals, as well as animal waste. Runoffs are the result of improper management of animal feeding operations, plowing excessively or the improper, excessive, or poorly timed application of pesticides, irrigation water, and fertilizer.

Consequences for lentic ecosystems

The most prevalent source of agricultural pollution is the soil that is washed off fields. Soil particles and sediment can cloud the water, reducing the amount of sunlight that reaches aquatic plants, or even clog the gills of fish or smother fish larvae. [5]

These particles often carry other pollutants such as fertilizers that carry potassium, phosphorus and nitrogen, pesticides, and heavy metals. Water exposed to agricultural runoff contaminated by fertilizers is subject to algal blooms, which is deadly to most aquatic life. Further, high levels of nitrogen in drinking water have been implicated as causal agents of the potentially fatal condition methemoglobinemia in infants. [6] Insecticides, herbicides, and fungicides do not only kill agricultural pests but also poison fish and wildlife and destroy natural habitats. Finally, runoff from poorly managed animal facilities can carry bacteria, viruses, nutrients, and oxygen-demanding organics and solids that contaminate shellfishing areas and cause water quality problems.

Lake Erie

Lake Erie (Ohio) is notorious for its vibrant green colour, visible from space. Unfortunately, the lake’s colour is the result of the yearly explosion of microscopic life and massive algae blooms. In 2014, the lake left 400,000 people without drinking water after the toxins infiltrated Toledo’s water system. Lake Erie blooms consist of cyanobacteria, known as blue-green algae, that produce the liver toxin microcystin that poses a risk to human and wildlife health. [7]

The nitrogen contaminated lake saw its largest blooms in 2011 and 2015, with severity indexes of 10 and 10.5 respectively, whereas an index above 5 indicates blooms having greater impacts. [8]

The state of Ohio has concluded that 23 percent of the lake’s open waters (662 square miles) are impaired for swimming or any other water-contact recreation. What is even more alarming however, is that one hundred percent of Lake Erie’s surface waters in Ohio are impaired for fish consumption and although many cities in Ohio draw their drinking water from Lake Erie, 99.6 percent of the lake that has been assessed (3,555 out of 3,568 square miles) has been designated by the state as impaired for drinking water.

This not only forces cities to take costly measures in order to treat the water, but also represents a great risk for the inhabitants.

The blooms in Lake Erie are caused by runoff pollution from fertilizers and manure spreading from farm fields into streams that flow into the lake. Since the late 1990s, farms have been poisoning the lake by overusing and misusing nitrogen on land.

Lake Erie is not the only victim of agricultural runoff in the region. Algae blooms occur annually in shallow, sheltered waters like Saginaw Bay in Lake Huron, Green Bay in Lake Michigan and even in the normally pristine waters of Lake Superior near the Apostle Islands.

A European Space Agency (ESA) Envisat satellite.image taken on Oct. 8, 2011, using its MERIS sensor, showing harmful algal bloom in the western basin of Lake Erie. (ESA)

What can we do?

Besides pushing for proper methods of application of fertilizers that would certainly be an important step towards the reduction of agricultural runoff, other programs and initiatives have been slowly put into place. From constructed wetlands designed to mitigate agricultural non-point runoff to bioreactors that receive tile-drainage water and use the biologically mediated process of denitrification to remove nitrate, engineers have been hard at work. Unfortunately, wetlands cannot be built everywhere, and require a significant surface, and bioreactors are not sufficient.

Methods of regenerative agriculture can be used to reduce agricultural runoff. For example, cover crops can reduce soil erosion and hold nutrients in place and no-till practices also reduce soil erosion and improve field-level water infiltration capacity. Both those initiatives have had a positive impact on the management of NPS pollution for farmers who have implemented them. Conservation and regeneration practices could be the key to harmful agricultural runoff.

About the Author

Kimberley has a M.Sc in International Studies (Cooperation, Development, Economics) from the University of Montreal. She is passionate about biodiversity and Indigenous peoples' land rights. Kimberley has been working with NGOs for the past couple of years as a writer, translator and researcher. During her free time, Kimberley loves to venture on long distance hikes in the wilderness.

Other Nomomente articles written by Kimberly Megis:

[1] UNEP. 2016. A snapshot of the world’s water quality: towards a global assessment. Nairobi, United Nations Environment Programme (UNEP)

[2] EPA. “Protecting Water Quality from agricultural runoff” U.S. Environmental Protection Agency.

[3] Jim Barnes, John Graham, and David Konisky, Fifty Years at the USEPA: Progress, Retrenchment, and Opportunities, published by Rowman and Littlefield in 2021. Chapter on the federal Clean Water Act by Jonathan Z. Cannon, former General Counsel for EPA and law professor at the University of Virginia

[4] Willis, G.H. and McDowell, L.L. (1982), Pesticides in agricultural runoff and their effects on downstream water quality. Environmental Toxicology and Chemistry, 1: 267-279.

[5] EPA. “Protecting Water Quality from agricultural runoff” U.S. Environmental Protection Agency.

[6] EPA 2005 Protecting Water Quality from Agricultural Runoff. Agricultural runoff fact sheet 2, US Environmental Protection Agency, EPA 841-F-05-001.


[8] Ibid at 7


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