The pollution of streams and rivers is a complex issue driven by the movement of water across the landscape, which carries various contaminants into waterways. While pollution can come from many different sources, the dominant contributor today is known as Nonpoint Source (NPS) Pollution. This diffuse form of contamination, often generated by rainfall or snowmelt runoff, represents the largest remaining threat to water quality in rivers, lakes, and streams across the nation. Understanding the distinction between pollution types and the mechanics of how they enter the environment is the first step toward effective remediation.
Identifying Nonpoint Source Pollution
Nonpoint Source Pollution is defined by its dispersed origin, meaning it cannot be traced back to a single, identifiable discharge point like a pipe, ditch, or smokestack. Instead, NPS pollution occurs when water runs over or through the ground, picking up and carrying away pollutants from a wide area before depositing them into a water body. This transport mechanism, primarily surface runoff, makes this type of pollution difficult to monitor and regulate.
Point Source Pollution (PSP), in contrast, comes from a specific, discrete conveyance, such as a factory’s effluent pipe or a municipal sewage treatment plant. Historic environmental legislation, like the Clean Water Act of 1972, successfully established permitting systems to control and drastically reduce PSP. This regulatory success in managing industrial and municipal discharges elevated the previously unregulated, diffuse NPS pollution to the position of the most widespread water quality problem. Because NPS pollution results from everyday activities across broad landscapes, its control relies more on state-level management and voluntary practices than federal permits.
Major Land Use Sources of Runoff
The primary activities that generate Nonpoint Source Pollution are tied directly to how land is managed and used within a watershed. Agricultural operations and urban/suburban development are consistently identified as the two largest contributors to the problem. Both land uses create conditions that increase runoff volume and load it with contaminants specific to that environment.
Agricultural runoff is a major source, as rain or irrigation water moves across large fields, picking up excess fertilizers, pesticides, and herbicides. Nutrients like nitrogen and phosphorus, applied to boost crop growth, are easily washed away, while manure from livestock operations contributes bacteria and additional nutrients. The tilling of soil also increases erosion, leading to the transport of massive amounts of sediment into streams.
Urban and suburban areas contribute a different but potent mix of pollutants, largely due to the prevalence of impervious surfaces like roads and parking lots. These surfaces prevent rainwater from soaking into the ground, leading to high-volume, rapid-flow stormwater runoff. This urban runoff collects oil, grease, heavy metals like zinc and copper from vehicles, and toxic chemicals, washing them directly into storm drains that often empty untreated into rivers. Residential activities, such as applying lawn chemicals and failing septic systems, also contribute nutrients and pathogens.
The Harmful Components of Runoff
The material components within runoff inflict specific and measurable harm on aquatic ecosystems. Three major categories of pollutants—nutrients, sediment, and pathogens—cause distinct forms of environmental degradation once they enter a stream or river.
Nutrients
Excess nutrients, primarily nitrogen and phosphorus from fertilizers and animal waste, drive a process called eutrophication. This over-enrichment triggers rapid growth of algae, known as blooms, which eventually die and are broken down by bacteria. The decomposition consumes large amounts of dissolved oxygen, leading to hypoxia or “dead zones” where fish and other aquatic life cannot survive.
Sediment
Sediment, consisting of fine soil particles, is the largest volume pollutant by weight and physically alters the river environment. High sediment loads make the water cloudy, reducing the sunlight necessary for submerged aquatic vegetation to grow. This material settles on the streambed, smothering the habitats where aquatic insects live and where fish lay their eggs, effectively destroying spawning grounds.
Pathogens
Pathogens, including bacteria and viruses from sources like pet waste, livestock operations, and failing septic systems, pose a direct health risk to humans. Elevated levels of fecal coliform bacteria can force the closure of recreational areas and contaminate shellfish beds.
Strategies for Controlling Diffuse Pollution
Controlling Nonpoint Source Pollution requires a localized approach focused on managing the land and the water before it runs off. The primary method for achieving this is the implementation of Best Management Practices (BMPs), which are techniques designed to prevent or reduce the generation and transport of pollutants. These practices focus on capturing, filtering, and slowing down runoff across the landscape, rather than treating water at a single point.
In agricultural settings, BMPs include precision agriculture, which uses technology to apply fertilizers only where and when they are needed, reducing nutrient waste. Conservation tillage, which involves leaving crop residue on the soil surface, helps reduce erosion and the resulting sediment and phosphorus runoff. The use of vegetated riparian buffers—strips of trees and grass planted along stream banks—is also effective, as the plants stabilize the soil and filter out pollutants before runoff reaches the water.
In urban environments, solutions focus on green infrastructure to manage stormwater. Techniques include installing permeable pavement that allows water to soak through, and using rain gardens and bioswales. These are landscaped depressions designed to capture and filter runoff from streets and rooftops, reducing the volume and velocity of the water entering storm drains. While the implementation of these solutions is often voluntary or incentivized through state and local programs, their widespread adoption is the most effective way to mitigate the diffuse nature of nonpoint source pollution.