Water pollution in the United States is a complex challenge affecting thousands of water bodies. While many people picture a single factory pipe discharging waste, the reality is far more diffuse. The most significant source of water quality impairment is not a single, easily identifiable outflow. Instead, the leading contributor is a widespread problem caused by rain and snowmelt washing pollutants across the landscape and into waterways.
Point Source Versus Nonpoint Source
Water pollution is categorized into two main types based on its origin: point source and nonpoint source. Point source pollution is discharged from a single, specific location, such as a pipe, ditch, or channel. These sources are often associated with industrial facilities or municipal wastewater treatment plants. The federal Clean Water Act (CWA) heavily regulates these discharges, requiring facilities to obtain permits through the National Pollutant Discharge Elimination System (NPDES).
Nonpoint Source (NPS) pollution, by contrast, comes from multiple, diffuse sources across a broad area. This contamination occurs when rainfall or snowmelt moves over and through the ground, picking up pollutants as it goes. Because NPS pollution cannot be traced back to a single pipe or location, it is significantly more challenging to monitor and regulate under traditional permitting systems.
Identifying the Leading Contributor
Nonpoint source pollution is the leading cause of water quality problems in the US, accounting for almost half of all water pollution. This widespread contamination impairs both surface waters and groundwater, affecting the quality of lakes, rivers, wetlands, and coastal waters. State environmental agencies report that NPS pollution is the primary reason many water bodies fail to meet water quality standards necessary for fishing, swimming, and drinking.
The diffuse nature of NPS pollution makes it a persistent threat to water health. The pollutants carried by runoff are diverse, including sediment, pathogens, and excess nutrients. Nutrients like nitrogen and phosphorus, primarily from fertilizers and animal waste, are widespread stressors, leading to excessive algal growth and oxygen depletion (hypoxia). Sediment acts as a pollutant by clouding the water, smothering aquatic habitats, and carrying other contaminants like phosphorus and pesticides.
Specific Activities Generating Runoff
Nonpoint source pollution originates from various human activities across different land uses. Agricultural activities are the single largest contributor to NPS pollution nationally. Runoff from crop fields and livestock operations carries substantial amounts of sediment, fertilizers, and pesticides into nearby streams and rivers. Poorly managed manure and the overuse of synthetic fertilizers introduce massive nutrient loads that travel across large multi-state watersheds, such as the Mississippi River Basin.
Urban and suburban environments also contribute significantly through stormwater runoff from impervious surfaces like roads and parking lots. Rainwater washing over these areas picks up motor oil, grease, heavy metals, and chemicals from vehicles and asphalt. Lawn chemicals, pet waste, and failing septic systems further add nutrients and bacteria to the runoff that flows directly into storm drains and waterways. Other sources of nonpoint pollution include sediment from construction sites and active mining operations. Atmospheric deposition, where airborne pollutants like mercury settle into water bodies, also contributes to the overall load.
Strategies for Controlling Nonpoint Pollution
Because nonpoint pollution is not managed through the traditional permit system of the CWA, control strategies rely heavily on the voluntary adoption of protective measures. The primary approach involves implementing Best Management Practices (BMPs) designed to prevent runoff and filter pollutants at the source. In agricultural settings, this includes practices like conservation tillage, which leaves crop residue on the surface to reduce soil erosion, and planting vegetated buffer strips along waterways. These riparian buffers absorb fertilizers and pesticides before they can enter the water.
For urban areas, control focuses on green infrastructure solutions that mimic natural processes to manage stormwater. Examples include installing rain gardens, which capture and filter runoff, and using permeable pavement that allows water to soak into the ground. Federal programs, such as the US Department of Agriculture’s Environmental Quality Incentives Program (EQIP), offer financial incentives and technical assistance to landowners for adopting these conservation practices. These coordinated efforts, often involving local watershed organizations, are essential for reducing contamination originating from diffuse sources.