Agricultural Runoff: Leading Nonpoint Source Of Water Pollution

Nonpoint source ?(NPS)? pollution is the leading cause of water quality problems. Unlike pollution from industrial and sewage treatment plants, NPS pollution comes from many diffuse sources and is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves, it picks up and carries away natural and human-made pollutants, depositing them into lakes, rivers, wetlands, coastal waters, and groundwater. NPS pollution can damage aquatic habitats, harm aquatic life, and reduce the capacity of water resources to be used for drinking water and recreation.

Nonpoint Source Pollution Characteristics and Values Table

Urban and suburban areas

In urban and suburban areas, much of the land surface is covered by buildings, pavement, and compacted landscapes. These surfaces are usually impervious, meaning that water runs off them without being absorbed into the soil. This increases the volume and velocity of stormwater runoff, which picks up and carries pollutants such as oil, pet waste, pesticides, herbicides, fertilizers, road salt, bacteria, and sediment.

Other factors contributing to nonpoint source pollution in urban and suburban areas include construction sites, where disturbed soil and discarded materials (plastics, wood, oils, trash) can be carried away by runoff waters. In suburban areas, lawn care chemicals and pet wastes often end up in runoff, further contaminating water sources.

The impact of urbanization on water pollution is significant, with pollutants harming fish and wildlife populations, killing native vegetation, fouling drinking water sources, and making recreational areas unsafe and unpleasant. The term "low-impact development" (LID) refers to systems and practices that aim to protect water quality and aquatic habitats by managing stormwater through natural processes.

The EPA and other organizations provide resources and programs to address nonpoint source pollution in urban areas, including the Stormwater Manager's Resource Center (SMRC), which offers technical assistance on stormwater management issues.

Agricultural operations

Nonpoint source (NPS) pollution is the leading cause of water quality issues. NPS pollution is caused by rainfall or snowmelt moving over and through the ground, picking up and carrying natural and human-made pollutants, and depositing them into water bodies. Agricultural operations are a significant contributor to NPS pollution.

Agricultural practices, including animal feeding operations and the use of fertilizers and pesticides, have a considerable impact on water quality. Intense cultivation and "factory" livestock operations have led to increased nitrogen, phosphorus, and pesticide residues in surface and groundwater. These high levels of "nutrients" threaten the health and biological diversity of waterways, resulting in the loss of aquatic life and their habitats, shellfish contamination, and seasonal dead zones.

Agricultural runoff, a major nonpoint source category of pollution, occurs primarily during storm flow conditions when rainwater washes pollutants like oil, pet waste, pesticides, herbicides, fertilizers, and bacteria into nearby water bodies. This runoff can stimulate algal blooms, leading to hypoxic conditions harmful to aquatic life and impacting recreational activities. Excessive sedimentation from erosion can overwhelm aquatic ecosystems, smother breeding areas, and degrade coastal and marine ecosystems, including coral reefs.

Pollutants from agricultural operations can also infiltrate groundwater, degrading drinking water sources. Nitrogen and phosphorus from fertilizer and manure can enter water bodies, causing algal blooms and hypoxic conditions harmful to aquatic life and recreational activities. Pesticide runoff poses risks to aquatic life, fish-eating wildlife, and drinking water supplies.

To mitigate the impact of agricultural operations on water pollution, nutrient management practices can be implemented. This includes targeted fertilizer and manure application through soil testing and crop-specific calibration to minimize runoff. Drip irrigation improves pesticide and nutrient control in irrigation water, and storing livestock manure in lagoons or protected areas reduces runoff risks. Adopting conservation practices and modifying land use can also help reduce the agricultural impact on water quality.

Atmospheric inputs

Nonpoint source (NPS) pollution is the leading cause of water quality problems and is primarily caused by land runoff, precipitation, atmospheric deposition, drainage, seepage, or hydrologic modification. Atmospheric deposition, a type of NPS pollution, is the process by which air pollution leads to water pollution.

In the atmosphere, water particles mix with carbon dioxide, sulphur dioxide, and nitrogen oxides, forming a weak acid. As air pollution increases, water vapour absorbs more of these gases, becoming even more acidic. This acidic water vapour falls as acid rain, which pollutes marine habitats such as rivers and lakes, harming aquatic life. Acid rain is caused by emissions of SO2 and NOx, which make water vapour in the atmosphere acidic. These emissions can come from power plants, industrial plants, and vehicles, among other sources. The combined emissions from multiple sources can have significant ecological impacts, as seen in the damage caused to Canadian ecosystems by emissions from the eastern United States and to Swedish ecosystems by emissions from Belgium, Germany, and Poland.

Gaseous pollutants, such as sulfur compounds (e.g., SO2 and sulfur trioxide), carbon monoxide, nitrogen compounds (e.g., nitric oxide, NO2, and ammonia), organic compounds (e.g., hydrocarbons), and odorous substances, contribute to the formation of acid rain and the pollution of water bodies. Volatile organic compounds (VOCs), released from burning fuel, solvents, paints, and other common products, are a particular concern. When emissions of nitrogen oxides and hydrocarbons react with sunlight, they form ground-level ozone, which has negative health effects.

While the Clean Water Act has successfully reduced pollution discharges from industries and municipalities (point sources), NPS pollution has become the largest source of water contamination. NPS pollution comes from various sources, including oil, pet waste, pesticides, herbicides, fertilizers, road salt, bacteria, and sediment. These contaminants are picked up by rainwater and snowmelt, which carry them into water bodies such as lakes, rivers, and streams.

Urban runoff is a significant contributor to NPS pollution, with discarded trash, trash booms, and other debris washing into water bodies and degrading their ecological, cultural, and recreational value. Agricultural practices also play a role in NPS pollution, with fertilizer and pesticide use impacting water quality.

Highway runoff

Nonpoint source (NPS) pollution is the leading cause of water quality problems. NPS pollution is caused by rainfall or snowmelt moving over and through the ground, picking up and carrying away natural and human-made pollutants, and depositing them into lakes, rivers, wetlands, coastal waters, and groundwater. One significant source of NPS pollution is highway runoff.

The Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 includes provisions for mitigating water pollution due to highway runoff. Under ISTEA, states can use federal funding for runoff pollution control devices and best management practices (BMPs) to prevent polluted runoff from reaching lakes, rivers, and bays. The National Estuary Program (NEP), established by the Clean Water Act, is another EPA program that helps control roadway pollution by focusing on point sources and runoff pollution in targeted, high-priority estuaries.

In addition, the Federal Insecticide, Fungicide, and Rodenticide Act's pesticides program regulates pesticides that may threaten ground and surface waters. The Clean Water Act's Section 319 programs also assist states in developing nonpoint source controls. Key management measures for roads, highways, and bridges include protecting areas that provide important water quality benefits or are susceptible to erosion or sediment loss. This involves coordinating erosion and sediment controls with the Federal Highway Administration (FHWA), the American Association of State Transportation Officials (AASHTO), and state guidelines.

Despite these efforts, highway outfalls, which are likely to number in the hundreds of thousands or millions across countries, remain largely unmonitored and uncontrolled. This lack of routine monitoring by relevant authorities means that the full impact of highway runoff on aquatic ecosystems and human health is not well understood.

Forestry and mining operations

Nonpoint source (NPS) pollution is the leading cause of water quality problems today. NPS pollution arises from land runoff, precipitation, drainage, seepage, and hydrologic modification. As rainwater and snowmelt move across the ground, they pick up and carry natural and human-made pollutants, depositing them into water bodies. Common sources of NPS pollution include oil, pet waste, pesticides, herbicides, fertilizer, road salt, bacteria, sediment, and discarded trash.

Mining activities can also have severe impacts on water quality. Mining consumes, diverts, and pollutes water resources. Poorly constructed roads during exploration, sedimentation, and the disturbance of water during mine construction can all contribute to water pollution. Mine waste rock and tailings can contaminate water with toxic chemicals and heavy metals, requiring long-term management even after mine closure.

Acid mine drainage (AMD) is a significant issue in mining operations. Water seeping out of mine openings can become highly acidic and contaminated with zinc, copper, or arsenic. Abandoned subsurface mines can contribute to NPS pollution as the water that leaks out of them often contains high levels of contaminants. AMD has affected several streams and water bodies in the United States, including in Colorado and the mid-Atlantic and Appalachian regions.

The extraction of critical minerals, such as lithium and cobalt, which are essential for renewable energy technologies, can also strain water supplies. At least 16% of the world's critical mineral mines are located in areas facing high water stress, where agriculture, industry, and households already compete for limited water resources. Without proper management, mining activities in these regions can further deplete freshwater supplies and contaminate nearby water sources with waste and residual chemicals.

While mining practices have improved in recent years, significant environmental risks remain. The impacts of mining on water resources depend on factors such as local terrain sensitivity, mineral composition, technology used, and the environmental commitment of the mining company. To mitigate the effects of mining on water quality, comprehensive planning, conscientious companies, new technologies, and improved environmental risk assessments are essential.

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