A Best Management Practice (BMP) is a method or measure implemented to manage and control water runoff. These practices are designed to reduce the volume and velocity of stormwater while also improving its overall quality before it reaches local waterways. The concept of a BMP represents a foundational element of modern land development and environmental protection planning. These strategies are used across various settings, from large municipal systems to small-scale residential properties, to mitigate the negative effects of urbanization on water systems.
Defining Best Management Practices
Stormwater BMPs are structured responses to the problem of non-point source pollution. This pollution comes from diffuse sources, picking up contaminants as runoff flows across the landscape during and after rain or snowmelt events. The runoff carries pollutants, including fine sediment, excess nutrients like nitrogen and phosphorus, chemicals such as oils and heavy metals, and various forms of debris, all of which harm aquatic ecosystems.
The necessity of these practices stems largely from federal and state environmental regulations. The Clean Water Act mandates the control of pollutant discharges into U.S. waters, enforced through the National Pollutant Discharge Elimination System (NPDES) permit program. Facilities and municipalities are required to develop and implement BMPs as a compliance tool to prevent water quality degradation. BMPs function as a required measure to reduce the release of contaminants and manage runoff volume, ensuring water quality standards are met.
Classifying BMPs: Structural and Non-Structural Approaches
Best Management Practices are broadly categorized into two types based on their nature and application: structural and non-structural. Structural BMPs involve physical engineering solutions, while non-structural BMPs focus on management and behavioral changes. Both approaches are necessary for a comprehensive stormwater management plan, addressing the problem from different angles.
Structural BMPs
Structural BMPs are permanent, engineered installations built into the landscape to manage runoff physically. These systems are designed to intercept, hold, and treat stormwater once it has become runoff. Examples include bioretention cells (shallow depressions filled with specialized soil and vegetation), retention ponds, permeable pavement, and constructed wetlands.
Non-Structural BMPs
Non-structural BMPs are operational and behavioral measures designed to prevent pollution at its source, often without new construction. These measures focus on good housekeeping practices, public education, and planning to minimize the generation of polluted runoff. Examples include regular street sweeping, proper management of hazardous materials, and ordinances regulating fertilizer and pesticide application. These preventative measures are often more cost-effective and can significantly reduce the need for larger structural controls.
How BMPs Remove Pollutants
BMPs employ specific physical, chemical, and biological processes to clean the water. These treatment mechanisms work by manipulating water movement and leveraging natural purification methods. A combination of BMPs is often used for optimal pollutant removal.
Infiltration
Infiltration allows stormwater to soak into the ground, recharging groundwater supplies. Systems like rain gardens and infiltration trenches maximize this process, allowing the soil profile to act as a natural filter. As water passes through the soil, pollutants are filtered out and naturally broken down by soil microbes.
Filtration and Adsorption
Filtration and adsorption rely on media to strain and bind contaminants. In bioretention cells and sand filters, specialized soil mixtures physically trap fine particles. Chemical adsorption causes pollutants like heavy metals to cling to the filter material, and vegetation takes up excess nutrients such as nitrogen and phosphorus.
Settling (Sedimentation)
Settling, or sedimentation, is a process where the velocity of the stormwater is significantly slowed down. In detention and retention ponds, this reduced speed allows heavier suspended solids and attached contaminants to drop out of the water column due to gravity. The water is held for a period of time to ensure adequate settling before the treated water is slowly released.
Long-Term Maintenance and Regulatory Compliance
The effectiveness of any stormwater BMP depends on consistent long-term operation and maintenance. These systems require regular attention to ensure they continue to function as intended. Failure to maintain a BMP results in decreased pollutant removal efficiency and may lead to system failure or water quality violations.
Maintenance activities focus on preserving the system’s capacity and treatment mechanisms. For example, retention ponds require periodic dredging to remove accumulated sediment. Permeable pavements need routine vacuum sweeping to prevent clogging, and bioretention areas need weeding and replanting to ensure healthy vegetation.
From a regulatory standpoint, property owners and municipalities, often through Municipal Separate Storm Sewer System (MS4) permits, are required to establish a formal operation and maintenance plan. This plan includes a schedule for routine inspections, documentation of maintenance activities, and a process for corrective action. Non-compliance due to inadequate maintenance can result in regulatory penalties, underscoring the necessity of treating BMP upkeep as a continuous, legally binding responsibility.