Agricultural Runoff

June, 2025

Agricultural runoff is the largest source of groundwater pollutants in the world. It makes rivers and lakes unswimable, destroy many aquatic habitats, dries up water supplies, and poses major health risks to the surrounding population. [2]

What is Agricultural Runoff?

Agricultural Runoff is the water the runs over farms and fields without getting absorbed into the ground. All the water that is drained from a field and ends up in a creak or something of the likes is agricultural runoff. In the US, "half a million tons of pesticides, 12 million tons of nitrogen, and 4 million tons of phosphorus fertilizer" [1] are applied to crops every year. A significant portion of these figures don't stay in the fields. Instead, they are picked up my running water and moved into the nearest bodies of water.

Agricultural runoff is considered a non-point pollution source, meaning it doesn't come from a single point. [4] As such, it's very difficult to prevent agricultural runoff from making its way into our bodies of water, because it comes from everywhere.

What are the main contributors to Runoff?

The three main contributors are fertilizer loss, soil erosion, and animal manure. [3] Soil erosion happens when the soil is tilled to oxidize the dirt. It breaks up the structure of the soil and it makes it very susceptible to being picked up by running water and carried with the flow.

Animal waste runoff is particularly a problem surrounding "concentrated animal feeding operations" (CAFO), this is just the regularity term for factory farming, or intensive farming. These all refer to the practice of farming with a focus on "high product output with low production costs" often to the detriment of the animals. [5] These facilities generate an incredible amount of manure and animal waste which then needs to be disposed of.

To manage waste, CAFOs typically store the manure in treatment lagoons, holding ponds and underground pits. [2] It's also used as a fertilizer which is sprayed onto crops. Unlike human waste which is all treated, there are no regulated treatment facilities for animal waste. So as it is stored, if it rains too heavily, the treatment lagoons and holding ponds can overflow and carry the waste into nearby bodies of water. Additionally, just like regular fertilizer, the waste fertilizer can be picked up by rainwater and carried also.

What are the effects?

In the Gulf of Mexico, there is a dead zone spanning 6,000 square miles where there is so little oxygen, marine life cannot survive. [1] The main reason for this dead zone is agricultural runoff.

Aquatic life can be impacted by the fertilizer and pesticides, which are often toxic to them. The toxicity can be lethal, but can also manifest as sub-lethal effects like reproductive failure and birth abnormalities. These chemicals can also build up in the tissue of the fish, and become dangerous to predators who consume them, like birds and humans.

For inland bodies of water like rivers, agricultural runoff kills them by overfeeding the water with nutrients. The excess of nutrients leads to a process called eutrophication. Eutrophication is when excess nutrients, mainly phosphorus and nitrogen build up in the water and algae begins growing. This rapid growth can lead to algal blooms, the green paste that can be found floating in water. This covers a large portion of the surface area and blocks light from penetrating the water. Due to the lack of sun light, the algae and other aquatic plants die and decompose, removing a lot of oxygen from the water, which leads to dead zones.

The local river near my village has suffered from this effect, and is now not safe to swim or fish in. It's brown and mucky and contact should generally be avoided.

Efforts to Prevent Runoff

Though there are some efforts to reduce agricultural runoff, such as planting shrub barriers around fields to catch more of the fertilizer before it makes its way into the water, the main source of pollution comes from factory farming operations. The animal waste from these operations is unregulated and untreated and this is one of the biggest levers at our disposal to help rid of agricultural runoff.

[1] Agricultural Runoff: Causes, Effects, and Solutions for Cleaner Water. (2024, July 9). Kraken Sense. https://krakensense.com/blog/agricultural-runoff

[2] Agricultural Runoff—An overview | ScienceDirect Topics. (2011). https://www.sciencedirect.com/topics/earth-and-planetary-sciences/agricultural-runoff

[3] US EPA, O. (2015, July 7). Nonpoint Source: Agriculture [Overviews and Factsheets]. https://www.epa.gov/nps/nonpoint-source-agriculture

[4] US EPA, O. (2015, September 15). Basic Information about Nonpoint Source (NPS) Pollution [Overviews and Factsheets]. https://www.epa.gov/nps/basic-information-about-nonpoint-source-nps-pollution

[5] Intensive farming. (n.d.). BC SPCA. Retrieved June 30, 2025, from https://spca.bc.ca/programs-services/farm-animal-programs/farm-animal-production/intensive-farming/