Nonpoint source pollution
Nonpoint source pollution refers to the discharges of pollutants that do not originate from a single, identifiable source, unlike point source pollution that comes from discrete conveyances like pipes. This type of pollution is commonly generated through stormwater runoff, which occurs when rainwater flows over land, picking up various pollutants along the way. Common sources of nonpoint pollution include urban areas, where activities such as lawn care, construction, and vehicle maintenance contribute pollutants like nitrogen, phosphorus, and toxic metals. In agricultural regions, practices like fertilizer application, pesticide use, and soil erosion also lead to significant nonpoint source pollutants.
The impacts of nonpoint source pollution can be severe, affecting aquatic ecosystems and drinking water quality. For instance, excess nutrients can result in eutrophication, leading to oxygen depletion and harming fish populations. Controlling nonpoint source pollution is challenging due to its diverse origins, often requiring voluntary best management practices (BMPs) that promote pollution reduction. These BMPs can be structural, like sediment basins, or nonstructural, such as educational outreach. Various countries are adopting watershed management plans and regulatory measures to tackle this issue, emphasizing the importance of community involvement and sustainable practices in reducing pollution and protecting water quality.
Nonpoint source pollution
Summary: Nonpoint source pollution is pollution that enters surface water bodies—such as rivers, streams, lakes, coastal areas, oceans, and groundwater—through diffuse pathways, primarily as a result of precipitation or atmospheric deposition.
A nonpoint source of pollution is generally defined as a source that does not meet the definition of point source. A point source is a discrete conveyance, such as a pipe, from which pollutants are discharged. Examples include discharges from industrial facilities and municipal sewage systems. Nonpoint source pollution is typically generated over land areas as precipitation produces stormwater runoff. As runoff flows from the land, pollutants are picked up, transported with the runoff, and discharged to surface water or into groundwater.
Sources and Pollutants
In the developed world, nonpoint sources can deliver a variety of pollutants. Urban stormwater runoff can contribute a number of pollutants, including nitrogen, phosphorus, sediment, toxic metals, bacteria, pesticides, and oil and grease. These pollutants are produced from the activities and actions in developed areas, including lawn care, improper disposal of pet waste, land development and construction, soil erosion, failing septic tanks, and car maintenance. In agricultural areas, sediment, nitrogen, phosphorus, bacteria, and pesticides are common nonpoint source pollutants. They are derived from agricultural practices such as crop fertilization, pesticide application, and livestock operations, as well as from soil erosion from cropland and pastures. In agricultural areas, growing pressure to increase crop and livestock yields often leads to excessive application of fertilizers and more intensive animal management. Excess nutrients from fertilizers and animal wastes increase the pollutant loads to surface and groundwater. Atmospheric deposition can also be a source of nutrients, metals, and organic contaminants.
The developing world faces exacerbated nonpoint pollution problems due to a lack of infrastructure that is present in the developed world. Often urban areas lack adequate sewer systems, and in some cases sewage and rainwater are transported, untreated, through the same systems and discharged directly to surface waters. Like developed nations, developing countries face increasing pressure to industrialize agricultural processes, and excess nutrients contaminate runoff from agricultural areas and discharge to water bodies. Deforestation can cause soil erosion and represents an important category of nonpoint source pollution in the developing world. Additionally, because of the lack of adequate emissions controls, air pollution can serve as a significant source of nonpoint source pollution, including toxics, to local streams.
Impacts From Nonpoint Source Pollution
Aquatic life can be affected by a number of pollutants. Excess nutrients, such as nitrogen and phosphorus, in surface waters can cause eutrophic and hypoxic conditions. Eutrophication is the process of excess nutrient enrichment of a water body, leading to increased growth of algae and subsequent hypoxic conditions that occur when the excess algal growth consumes oxygen through respiration and decomposition of plants after they die off.
The depletion of dissolved oxygen from excessive algal growth can result in oxygen concentrations in the water column that are insufficient to support aquatic life, such as fish and aquatic insects. Negative impacts to aquatic life can, in turn, affect humans through the decline in fisheries’ populations. Examples of large hypoxic zones include the Gulf of Mexico in the United States, the Pearl River Delta in China, and the Black Sea in Eastern Europe.
Aquatic life can also be negatively affected by runoff containing toxic pollutants, oil and grease from transportation activities, and emissions from industrial processes. Toxic pollutants can originate from a number of sources, including abandoned mines and urban runoff.
Nonpoint source pollution can also negatively affect drinking water supplies and recreational waters. Nitrates, which frequently originate from agricultural activities, can contaminate surface and groundwater, making them unsuitable as sources of drinking water. Bacteria from failing septic systems, animal waste, and illicit sewage discharges can contaminate potable water supplies and cause beach closings due to violations of recreational health standards.
Nonpoint Source Pollution Control
Unlike point source pollution, which is often regulated through discharge permits, nonpoint source pollution is challenging to address, given the diverse sources and the lack of regulatory authority for controlling their discharges. Nonpoint source pollution is typically controlled through the voluntary or incentive-based implementation of best management practices (BMPs) by public and private landowners. BMPs are pollution control or treatment practices designed to reduce pollution from runoff. BMPs are generally categorized as structural or nonstructural controls and can be used separately or in combination to prevent nonpoint source pollution.
Structural BMPs are designed to retain or filter runoff in order to decrease the amount of water flow after a storm event and to remove pollutants from the runoff. For example, common structural BMPs used to reduce sediment transport from agricultural land to water bodies include the use of sediment basins that collect and store runoff to trap debris. The use of green infrastructure techniques (also referred to as low impact development) has become increasingly popular in urban areas as a way to engage the natural infiltration capabilities of vegetation to prevent nonpoint source pollution. The principle behind green infrastructure is to decrease impervious areas (including paved areas such as parking lots) and increase pervious areas that naturally filter runoff (permeable pavement, rain gardens, and constructed wetlands, for example).
Nonstructural BMPs are preventive actions that seek to increase the number of people who implement BMP practices, such as education and outreach activities and land-use policies. For instance, education programs can promote the environmental benefits or cost savings of new practices in an effort to increase the number of users.
Countries also address nonpoint source pollution through the development of watershed management plans and the regulation of pollutants. Many countries and transboundary river commissions require local governments to monitor water quality and address impaired water bodies through the development of watershed management plans that include specific management activities to decrease pollution. In the United States, the Clean Water Act requires that a total maximum daily load (TMDL) be developed for a water body that does not meet water quality standards. A TMDL is the maximum amount of pollution tolerated by a water body without causing impairment. The pollution is allocated to all the identified sources, including nonpoint sources. States and local jurisdictions then develop watershed management plans to address the required load reductions from nonpoint sources, often through the implementation of BMPs.
The regulation of pollutants is another method to address those pollutants considered particularly harmful to human health. The Canadian government, for instance, regulates the pesticide industry through the Pest Control Products Act and Pest Management Regulatory System, in order to reduce the incidence of nonpoint pesticide pollution.
Water quality trading is a relatively new approach to address nonpoint source pollution, designed to implement market-based approaches to improve water quality across a region or watershed. Trading schemes address specific water quality parameters (such as phosphorus, nitrates, and salinity) and allow those entities that have higher pollution abatement costs to trade or buy pollution reduction credits from those sources with lower pollution abatement costs. An example of a trading scheme that is focused entirely on nonpoint pollution reduction is the Lake Taupo Nitrogen Trading Program in New Zealand.
Bibliography
American Society of Civil Engineers and U.S. Environmental Protection Agency. “International Stormwater Best Management Practices (BMP) Database.” http://www.bmpdatabase.org/.
Dosi, C., and T. Tomasi, eds. “Nonpoint Source Pollution Regulation: Issues and Analysis.” Economics, Energy and Environment 3 (2010).
U.S. Environmental Protection Agency. “What Is Nonpoint Source Pollution?” http://www.epa.gov/owow‗keep/NPS/index.html.