Nonpoint Source Pollution: Water Contamination Explained

Nonpoint source pollution (NPS) is a type of pollution that affects water bodies and air quality. Unlike pollution from industrial and sewage treatment plants, NPS pollution comes from many diffuse sources, making it difficult to regulate. NPS pollution is caused by rainfall or snowmelt moving over and through the ground, picking up and carrying natural and human-made pollutants, which are then deposited into lakes, rivers, wetlands, coastal waters, and groundwater. This type of pollution can severely impact communities, particularly the commercial fishing industry, and can also have negative economic effects.

Urban runoff

Urbanization increases the variety and amount of pollutants carried into bodies of water. In urban and suburban areas, the land surface is often covered by buildings, pavement, and compacted landscapes. These impervious surfaces prevent rain and snowmelt from soaking into the ground, leading to increased stormwater runoff. The high volume and velocity of this runoff can cause habitat destruction.

Contaminants from urban runoff include sediment, oil, grease, and toxic chemicals from motor vehicles, as well as pesticides and nutrients from lawns and gardens. Heavy metals from roof shingles, motor vehicles, and other sources also contribute to this type of pollution. Additionally, road salts, bacteria, and nutrients from pet waste and failing septic systems can pollute water bodies.

Thermal pollution is another consequence of urban runoff. Impervious surfaces such as streets and rooftops absorb heat, and when rainwater or snowmelt flows over them, it carries that heat into nearby water bodies, causing thermal pollution. This type of pollution can harm fish and wildlife populations, kill native vegetation, foul drinking water sources, and make recreational areas unsafe and unpleasant.

The impact of urban runoff on water quality is significant. Pollutants from urban areas are carried by stormwater into nearby lakes, rivers, wetlands, and coastal waters. This pollution cannot be attributed to a single activity or group of activities, making it challenging to regulate and address. However, it is crucial to recognize that urban runoff is a significant contributor to NPS pollution and that improving the management of urban and suburban areas is essential to mitigating its effects.

Agricultural practices

Nonpoint source pollution is the leading cause of water pollution in the United States, and agricultural practices are a significant contributor to this issue. This type of pollution is challenging to control because it arises from various everyday activities, and there are no specific solutions or changes to address it effectively.

Agricultural operations account for a large percentage of nonpoint source pollution in the United States. The vast expanse of agricultural land, approximately 1.2 billion acres, means that farming and ranching activities can have a considerable impact on water quality, both locally and across multi-state watersheds.

Farming practices that expose and loosen the soil, such as ploughing, increase the vulnerability of the soil to erosion during rainstorms. This erosion results in the runoff of sediment (loose soil) into nearby bodies of water, causing turbidity (cloudiness) and impairing aquatic life and plant growth.

The use of pesticides, fertilizers, and animal manure in agriculture is another major contributor to nonpoint source pollution. These chemicals can enter groundwater, depending on local land use and geologic conditions, and have been detected even in remote mountain ponds and streams. Increased levels of nitrogen and phosphorus from fertilizers and manure can stimulate algal blooms, leading to hypoxic (low oxygen) conditions that are harmful to aquatic ecosystems and recreational uses of water bodies.

Additionally, bacteria and nutrients from livestock and poultry manure can contaminate drinking water supplies and cause beach and shellfish bed closures. The management of large livestock operations, such as factory farms, is also a concern, as they produce significant amounts of animal waste, which can be a source of nutrient pollution.

To address these issues, organizations like the USDA NRCS have launched initiatives such as the National Water Quality Initiative (NWQI) in 2012. The NWQI aims to reduce the runoff of agriculture-related nutrients, sediment, and pathogens in small high-priority watersheds across the country. Farmers are also encouraged to adopt soil and water conservation practices and work with local conservation partners to minimize the impact of their operations on water quality.

Precipitation

During rainfall or snowmelt events, water flows across various surfaces, including parking lots, roads, highways, and agricultural fields. As it moves, it picks up and assimilates a range of contaminants. For example, water flowing across a parking lot can collect oil and other oil-based chemicals from vehicles, resulting in a rainbow-colored sheen on the water. This contaminated runoff then empties into nearby streams, which carry it downstream to larger water bodies, such as lakes, rivers, or oceans.

The impact of precipitation on NPS pollution is particularly evident in urban and suburban areas. In these settings, impervious surfaces like streets, driveways, and sidewalks prevent stormwater from infiltrating the ground. Instead, the water, along with any pollutants it has picked up, runs directly into storm sewers and waterways. This phenomenon is known as "flashiness" and leads to rapid changes in water levels in urban streams.

Agricultural areas are also susceptible to NPS pollution from precipitation. Nutrient runoff, for instance, occurs when rainwater washes over agricultural fields or forests, picking up fertilizers, pesticides, and other pollutants. This polluted stormwater then drains into nearby water bodies, contributing to NPS pollution. Additionally, the leaching of nitrogen compounds from fertilized lands is another form of NPS pollution resulting from precipitation.

The effects of NPS pollution on water quality can be detrimental. For instance, an overabundance of nutrients from fertilizers and animal waste can lead to excessive algae growth in lakes and streams. When the algae eventually die off, they are broken down by bacteria, which consume oxygen and create an oxygen-deprived environment, making it challenging for fish and other aquatic life to survive.

Atmospheric deposition

Nonpoint source (NPS) pollution is a type of diffuse contamination that affects water or air and does not originate from a single discrete source. Atmospheric deposition is one of the ways in which NPS pollution reaches bodies of water. Atmospheric deposition refers to the process by which pollutants are deposited from the atmosphere onto the Earth's surface through wet or dry deposition. Here's how it works:

Wet Deposition

Wet deposition occurs when pollutants in the air are dissolved in precipitation, such as rain or snow, and then deposited onto the Earth's surface. This can include acid rain, which is formed when sulphur dioxide and nitrogen oxides emitted from industrial sources and vehicles react with water vapour in the atmosphere. When the polluted rain or snowmelt flows over the ground, it picks up and carries away additional pollutants, which are then deposited into lakes, rivers, and other water bodies. Wet deposition can also occur through fog, cloud water, and dew, which can absorb pollutants from the air and deposit them onto surfaces.

Dry Deposition

Dry deposition, on the other hand, involves the direct settling of pollutants from the air onto the Earth's surface. This can include particulate matter, such as dust, soot, and pollen, as well as gaseous pollutants like nitrogen oxides and volatile organic compounds. These pollutants may be emitted directly from sources such as smokestacks, vehicle emissions, and agricultural activities, or they may be formed through chemical reactions in the atmosphere. Once deposited on the ground, these pollutants can be picked up by wind and blown into water bodies or washed into them during rainfall or snowmelt.

Combined Effects

Control and Mitigation

Controlling and mitigating atmospheric deposition as a source of NPS pollution can be challenging due to the diverse sources and long-range transport of pollutants. However, implementing regulations and technologies to reduce emissions from industrial sources, vehicles, and agricultural practices can help minimize the deposition of pollutants into water bodies. Additionally, using porous paving materials in parking lots and highways can help reduce runoff and allow for the filtration of rainwater before it reaches groundwater.

Drainage

One example of how NPS pollution reaches water bodies through drainage is via urban runoff. In urban areas, rainwater washes off parking lots, roads, and highways, collecting contaminants such as fertilizers, pesticides, metals, solvents, and petroleum products. This contaminated stormwater then flows through storm drain systems and is discharged into nearby lakes, rivers, or streams. The vast network of pipes beneath urban streets allows this polluted runoff to bypass sanitary sewers and treatment facilities, directly entering water bodies and degrading their quality.

Agricultural practices also contribute to NPS pollution through drainage. The application of pesticides and fertilizers to crops can result in excess chemicals reaching adjacent water bodies during rainfall or irrigation events. Additionally, the management of livestock in confined areas can lead to the runoff of animal waste, which contains nutrients that can pollute waterways. These nutrients contribute to the over-fertilization of lakes and streams, leading to excessive algae growth. When the algae eventually die, they are broken down by bacteria, which consume oxygen and create an inhospitable environment for fish and other aquatic life.

To mitigate the impact of NPS pollution through drainage, several strategies can be employed:

• Buffer Strips: Implementing buffer strips, which are areas of grass or vegetation between impervious surfaces (such as parking lots or roads) and water bodies, helps absorb pollutants before they reach aquatic ecosystems.
• Retention Ponds: Constructing retention ponds in drainage areas creates an aquatic buffer, allowing contaminants to settle and become trapped, preventing them from reaching natural water bodies.
• Porous Pavement: Using porous materials for pavement allows rainwater and stormwater to drain into the ground, reducing the volume of runoff that directly enters nearby water bodies.
• Wetland Construction: Restoring or constructing wetlands can help slow down runoff and provide natural filtration to absorb contaminants before they reach water bodies.

By implementing these drainage management strategies, the impact of NPS pollution on water quality can be significantly reduced, protecting aquatic ecosystems and ensuring safer drinking water supplies.

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