A recent national analysis of urban waterways found that almost every single sample of surface water contained at least one pesticide and generally two or more. When people water their gardens or rainstorms occur, runoff from yards carries pesticides into streams where they can harm aquatic life and contaminate drinking water supplies.

Although scientific measurements are used to define water quality, it's not as simple as designating water either good or bad. That determination is typically made depending on the intended use for the water. It could be used for drinking, car washing, irrigation, or other purposes. Runoff is a major source of water pollution. As water runs along a surface, it picks up litter, petroleum, chemicals, fertilizers, and other toxic substances. From California to New Jersey, beaches in the U.S. are regularly closed after heavy rainfall due to runoff that includes sewage and medical waste.

Water pollution affects the entire biosphere of organisms and plants that live in a body of water along with organisms and plants exposed to that water. In almost all cases, the effect is damaging for not only individual species and populations, but also for the natural biological communities that live in them. Poor water quality poses a health risk for humans and can also pose a health risk for entire ecosystems.

There are more than 25,000 pesticides containing more than six hundred active ingredients registered in the United States. More than 16,000 different products are on the market. The chemicals in these products all have characteristics that determine risk. Neonicotinoid insecticides are known to pose significant risks to pollinators. These insecticides are hydrophilic and move readily into both surface and ground water. Most neonicotinoids are persistent and continue to be toxic and likely to contaminate water for weeks to months after application. The runoff from rainstorms carries these residues of garden chemicals into creeks and wetlands where they harm aquatic life.

Years of U.S. Geological Survey research shows that urban and suburban pesticide application is a significant source of water contamination. The highest level of Imidacloprid detected in California is in urban storm water. Runoff occurs when there is more water than land can absorb. Runoff comes from both natural processes and human activity, the most familiar type of natural runoff being snowmelt. Mountains that cannot absorb water from heavy snowfalls produce runoff that becomes streams, rivers, and lakes. Glaciers, snow, and rain all contribute to this natural runoff.

Runoff from human activity comes from two sources: point sources and nonpoint sources. Point source pollution is any source that empties directly into a waterway such as a pipe from a sewage treatment plant, factory, or home. Regulations determine what type of runoff and how much industries are allowed to release. These regulations vary from state to state. A nonpoint source is any source where runoff doesn't go directly into a waterway. Nonpoint sources of runoff can be urban, suburban, or rural. In these areas, rainwater and irrigation wash chemicals into local streams. Runoff from nonpoint sources includes lawn fertilizer, car exhaust, and substances like motor oil and gasoline. Farms are a large nonpoint source of runoff since rainwater and irrigation drains fertilizers and pesticides into bodies of water.

Surfaces that don't absorb water increase runoff. Roads, parking lots, and sidewalks are impervious surfaces. Materials such as car washing products, litter, and gasoline spillage all become runoff. These chemical pollutants can harm an entire ecosystem. Tiny microbes such as algae and plankton absorb pollutants in the runoff. Shellfish or fish eat them or directly absorb the pollutants. Animals such as birds then consume the fish which increases the level of pollutants in their bodies. Biomagnification means organisms higher up the food chain, including people, have a higher concentration of pollutants in their bodies than lower organisms such as seagrass or algae. When people eat foods like oysters, they may be ingesting runoff from farms, sewage treatment plants, and city streets.

(photo of point-of-origin stormwater filtration unit)

Runoff is an economic threat as well as an environmental one. Agribusiness loses millions of dollars to runoff every year. In the process of erosion, runoff washes away fertile topsoil. Farmers rely on topsoil to grow crops and tons of it are lost to runoff every year.

The Pesticide Data Program of the United States Department of Agriculture is the largest tester of pesticide residues on food sold in the United States. Since 1991, it has tested more than 60 different types of food for more than 400 different types of pesticides via samples collected close to the point of consumption.

Takeaway: People can limit runoff pollution in many ways. Farmers and gardeners can reduce the amount of fertilizer and sprays they use. Since soil acts as a natural sponge that filters and absorbs many harmful chemicals, urban areas can reduce the amount of paved surfaces. Communities can plant more native plants and shrubs and, in turn, prevent more runoff from going into sewers and waterways.

(Credits: https://water.usgs.gov/nawqa/pnsp/usage/maps/about.phphttps://water.usgs.gov/nawqa/pnsp/usage/maps/county-level/; https://xerces.org/; https://www.nationalgeographic.org/encyclopedia/runoff/; photos top to bottom: [email protected]; Songwut [email protected]; [email protected]; [email protected];By Ryxhd123 (http://www.transpo.com/envirosafe.htm) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons); [email protected])