Best Management Practices (BMPs) and Nonpoint Source (NPS) Pollution

Introduction to NPS Pollution and BMPs

Nonpoint source (NPS) pollution refers to diffuse contamination of water that does not originate from a single discrete source. It is a major cause of water quality impairment in rivers, lakes, and coastal areas. Best Management Practices (BMPs) are strategies and structures designed to reduce or prevent NPS pollution, especially in urban, suburban, and agricultural environments.

• Nonpoint Source (NPS) Pollution: Pollution that comes from many diffuse sources, such as runoff from agricultural fields, urban streets, and suburban lawns.

• Best Management Practices (BMPs): Methods, measures, or practices used to prevent or reduce the amount of pollution generated by nonpoint sources.

• Examples of BMPs: Rain gardens, vegetated filter strips, permeable pavements, infiltration basins, and constructed wetlands.

Learning Objectives

• Identify common BMPs used to reduce NPS pollution in various environments.

• Explain how selected BMPs reduce pollutant loads or improve stormwater infiltration.

• Compare the costs, benefits, and feasibility of different BMPs in various land use contexts.

• Demonstrate how to plan or assess a BMP using a field checklist, photo documentation, or site sketch.

• Recommend appropriate BMPs for a hypothetical site based on land use, soil conditions, and observed pollution risks.

Types of BMPs for Stormwater Management

Overview of BMP Categories

BMPs for controlling urban runoff can be classified into several categories, each targeting different aspects of stormwater management. The main categories include:

• Infiltration practices

• Vegetated open channel practices

• Filtration practices

• Retention ponds or vaults

• Detention ponds

• Wetlands

• Other practices (e.g., water quality inlets)

Infiltration Practices

Infiltration practices are designed to temporarily store runoff and allow it to infiltrate into the soil, reducing surface runoff and promoting groundwater recharge. These practices are effective in reducing pollutant loads by filtering water through soil.

• Infiltration Basins: Large, shallow impoundments that collect and temporarily store runoff, allowing it to infiltrate over several days. Basins are typically flat-bottomed and may be vegetated to enhance infiltration and prevent erosion.

• Infiltration Trenches: Shallow, gravel-filled ditches that collect runoff and allow it to gradually infiltrate into the ground. Suitable for small drainage areas or sites with permeable soils.

Key Considerations: The success of infiltration practices depends on keeping the soil and side slopes of the basin uncompacted and free of sediment. Vegetation helps maintain infiltration rates and prevent erosion.

Vegetated Open Channel Practices

These include grassed swales and vegetated filter strips, which slow down runoff, promote infiltration, and filter out pollutants.

• Grassed Swales: Shallow, vegetated channels that convey stormwater while filtering pollutants and promoting infiltration.

• Vegetated Filter Strips: Strips of dense vegetation (such as grass or shrubs) planted along the edges of impervious surfaces to intercept and filter runoff before it reaches water bodies.

Filtration Practices

Filtration practices use soil, sand, or other media to remove pollutants from stormwater as it passes through.

• Sand Filters: Structures that filter runoff through sand beds to remove suspended solids and pollutants.

• Bioretention Cells (Rain Gardens): Landscaped depressions that collect and filter runoff using soil and plants.

Permeable Pavements

Permeable pavements are designed to allow water to pass through the surface, reducing runoff and promoting infiltration. They are an alternative to traditional impervious surfaces like asphalt and concrete.

• Porous Asphalt and Concrete: Specially designed mixtures with high permeability, allowing water to infiltrate through the pavement into the underlying soil.

• Modular Pavement: Consists of individual blocks made of pervious material (e.g., sand, gravel, or soil) interspersed with strong structural material such as concrete. Used in low-traffic areas like overflow parking lots and access roads.

Example: A parking lot constructed with porous asphalt allows rainwater to infiltrate, reducing surface runoff and the risk of flooding.

Retention and Detention Ponds

These are engineered basins designed to temporarily or permanently store stormwater runoff, allowing sediments and pollutants to settle before water is released or infiltrated.

• Retention Ponds: Hold water permanently and provide habitat for wildlife while treating stormwater.

• Detention Ponds: Temporarily store runoff and release it slowly to reduce peak flow rates and prevent downstream flooding.

Constructed Wetlands

Constructed wetlands are engineered systems that mimic natural wetlands to treat stormwater by filtering pollutants through vegetation and soil.

• Benefits: Provide water quality improvement, wildlife habitat, and aesthetic value.

Comparison of BMPs

The selection of BMPs depends on site-specific factors such as soil type, land use, climate, and pollutant sources. The table below summarizes the main types of BMPs, their functions, and typical applications.

Key Equations and Concepts

• Runoff Volume Calculation: The volume of runoff generated from a site can be estimated using the Rational Method:

• Where = peak runoff rate (cubic feet per second), = runoff coefficient (dimensionless), = rainfall intensity (inches/hour), = drainage area (acres).

• Infiltration Rate: The rate at which water enters the soil, typically measured in inches per hour. Influenced by soil type, compaction, and vegetation cover.

Applications and Site Planning

Planning and Implementing BMPs

Effective BMP implementation requires careful site assessment, design, and maintenance. Key steps include:

• Conducting a site assessment to identify pollution sources and drainage patterns.

• Selecting appropriate BMPs based on site conditions and land use.

• Designing BMPs to maximize pollutant removal and minimize maintenance needs.

• Monitoring BMP performance and maintaining structures to ensure long-term effectiveness.

Example: A rain garden is designed for a residential site to capture and treat roof runoff, reducing the amount of pollutants entering nearby streams.

Summary Table: Resource Guide for BMPs

The following table summarizes key resources for further study on BMPs and NPS pollution control:

Conclusion

Understanding and implementing BMPs is essential for reducing NPS pollution and protecting water quality in both urban and agricultural settings. By selecting appropriate BMPs and maintaining them properly, communities can significantly reduce the impact of stormwater runoff on the environment.