With the approach of the federal requirements of the National Pollutant Discharge Elimination System (NPDES) Phase II stormwater rule, many municipalities, consultants, and water resource planners have begun the process of determining what mix of best management practices (BMPs) will be needed to comply with Phase II’s six minimum measures and to obtain permit coverage. This article briefly highlights the various types of BMPs available to consider including in a stormwater “toolbox” for a successful stormwater management plan.
Although particular designs for each type of BMP system are beyond the scope of this article, readers are encouraged to examine a wide range of resources available for detailed information about design considerations for specific applications.
Because Phase II is a narrative rule generally requiring the implementation of BMPs for obtaining permit coverage, selecting the right mix of BMPs is of critical importance. (See “Storm Warning” in the November/December 2000 issue of Stormwater for details of the rule.) EPA’s interim permitting policy for stormwater permits, adopted on August 1, 1996, states that using BMPs, rather than conforming to numeric discharge limits, is generally the most appropriate control unless adequate information exists to establish more-specific requirements. Understanding what types of BMPs are available and how they interact with each other will help provide guidance in selecting the right mix.
According to EPA’s Preliminary Data Summary of Urban Storm Water Best Management Practices, an urban stormwater BMP is a “technique, measure or structural control that is used for a given set of conditions to manage the quantity and improve the quality of storm water runoff in the most cost-effective manner.” Planners need to consider differences in sites and pollutant-reduction goals. For example, BMPs for retrofits will be far different from those for new development. Pollution-reduction goals will likely be established by permitting authorities, and these goals need to be considered in the BMP selection process. Many circumstances call for the use of several BMPs, or a “treatment train,” to accomplish the objectives for a particular application.
In addition to identifying BMPs for each of the six Phase II minimum measures, municipalities must also describe measurable goals for each BMP. A measurable goal is generally a way for both the municipality and the permitting authority to track the implementation of a BMP. Measurable goals can be as simple as when or how often a BMP will be used or can be more complex and performance-based. As planners develop Phase II programs and identify which BMPs to use, they should keep in mind that a measurable goal will need to be established for each BMP. EPA is scheduled to issue guidance on developing measurable goals by November 2001.
BMPs can generally be divided into structural and nonstructural categories. Although this article generally discusses BMPs by type, many BMPs actually fit within more than one category. For example, infiltration can result in the filtration of some pollutants. Another example is the reduction of impervious surfaces. As the amount of impervious surface increases, so does surface flow, which can transport a wide range of pollutants. Encouraging open-space development with reduced imperviousness is properly characterized as a nonstructural BMP. The actual design of a particular site with reduced impervious surfaces, vegetative buffers, and bioretention is a structural BMP.
Many people only have a vague understanding of the range of BMPs available, and with ongoing research, new BMPs are constantly emerging. In fact, the term “best management practices” would be more accurately phrased as “better management practices” because what is “best” varies with each situation. Further clouding the issue of BMP selection is that each locality has its own set of variables and conditions that influence the decision of which BMP or BMP treatment train is the best to use. This broad overview of various BMP types is intended to guide stormwater planners through available options to consider for meeting the Phase II six minimum measures.
|The Tollgate Drainage District stormwater treatment facility in Lansing, MI, is an example of a constructed wetland BMP.|
Structural BMPs are techniques that can be used to address flow control and pollution removal in stormwater runoff. These BMPs include site-specific engineered and constructed systems as well as proprietary systems. Structural BMPs can generally be grouped into several subcategories that include
- infiltration systems,
- detention systems,
- retention systems,
- constructed wetlands and other vegetated systems,
- filtration systems,
- proprietary treatment systems.
Infiltration Systems. Infiltration systems are designed primarily to reduce the quantity of stormwater runoff from a particular site, although as discussed above, some pollutants may also be filtered when this type of BMP is used. Increasing urbanization and use of impervious surface has resulted in substantial increases of surface runoff, causing serious degradation of urban streams and the corresponding negative impacts on aquatic health. The use of infiltration techniques can reduce the amount of surface flow and direct the water back into the ground. Advantages of infiltration techniques include the recharging of groundwater supplies and the removal of certain pollutants such as sediments. Care must be exercised, however, in determining whether infiltration is best for a specific application, especially when groundwater is the source of drinking water in the area. Infiltration can result in groundwater contamination, because soils that allow good infiltration also allow rapid migration of certain pollutants. In these situations, infiltration should not be used without effective pretreatment. Conversely, poorly permeable soils can prevent an infiltration system from functioning. Examples of infiltration techniques include infiltration basins, porous pavement, infiltration trenches, and dry wells.
Also available are proprietary infiltration controls that can be placed under parking lots where permeable soil conditions exist. As with designed structures, pretreatment might be needed for various contaminants and sediments, depending on conditions.
Detention Systems. These BMPs are designed to temporarily hold stormwater runoff for gradual release into receiving waters. Detention systems are used primarily to reduce peak discharges to prevent flooding, streambank erosion, and channel alterations. They are generally not very effective for removing pollutants unless combined with other BMPs. Although some particulate settling can occur within certain detention designs, most of these sediments resuspend with subsequent wet-weather events and thus end up in receiving waters. Many detention systems incorporate characteristics normally utilized with retention ponds, such as permanent pools, to prevent subsequent scouring. Examples of detention systems include detention basins, underground tanks, and oversized pipes.
Retention Ponds. Retention ponds, or wet ponds as they are commonly called, are intended to capture and hold runoff from entering receiving waters. Because retention systems are designed for permanent containment of stormwater, they can be an excellent BMP for removing sediment and other pollutants, thus providing both water-quantity and water-quality control. Most of these systems are in the form of ponds or basins, and when certain types of aquatic vegetation or aerators are added, the systems can actually provide further water treatment. As with all BMPs, regular maintenance is essential to maintain a healthy retention pond. Clay siltation can result in a substantial loss of infiltration, resulting in a sharp increase in overflow from the basin during wet-weather events. Without maintenance, retention ponds will eventually fill in and become ineffective. In addition, certain pollutants can become concentrated in the area, potentially requiring remediation.
Constructed Wetland Systems. Constructed wetlands are a very effective BMP for both pollutant removal and runoff storage. When properly designed, they incorporate the processes of sediment removal, microbial decomposition, and aquatic plant uptake. Sites for constructed wetlands must be carefully selected to ensure that sufficient waters are available in dry weather to sustain the wetlands. Areas with shallow groundwater levels are ideal. Heavy sediment loads can quickly degrade a constructed wetland. Pretreatment of sediment flows must be considered if this is the case. Generally, natural wetlands should be preserved and not used as a BMP because changing hydrology can significantly degrade a natural wetland.
Other wetland BMPs include wetland basins and channels. These BMPs do not necessarily have to have open waters and can instead be in the form of wetland meadows that have surface water only for a short period of time after a rain event.
Filtration Systems. Filtration systems are BMPs that use media to remove particulates from runoff. They are typically used when circumstances limit the use of other types of BMPs, such as where space is limited–particularly in a highly urbanized setting–or when it is necessary to capture particular industrial or commercial pollutants (e.g., hydrocarbons). In these circumstances, other BMPs might be cost-prohibitive or not as effective. Filtration devices can also work well as pretreatment systems for other types of BMPs. For example, infiltration systems that move water directly to ground aquifers might require pretreatment for certain contaminants to maintain effective wellhead protection of drinking-water supplies.
Filtration systems can be either designed into a site plan, such as sand filter systems, or proprietary devices, such as catch-basin inserts or in-pipe systems. Many different filtration media are available, such as sand, peat, absorbents, and activated carbon. The choice depends on the particular application.
When considering filtration systems, planners need to consider flow rates. As a result of the volume of water being moved in a wet-weather flow, filters generally focus on treating the first 0.5—1 in. of runoff and allow bypass for high-flow events. Filters should incorporate presettling sediment chambers to remove sediments that can clog the filters and reduce flow rates and effectiveness. An effective filtration system should be able to demonstrate removal efficacy for specific contaminants. Again, as with all BMPs, regular maintenance is essential.
Bioretention and Vegetated Systems. Bioretention and vegetated systems, such as buffers and swales, are variations of infiltration and filtration systems. The media in these systems are actually natural vegetation and soil beds that allow ponding and gradual infiltration. The vegetation and underlying soils can filter a variety of pollutants from runoff. In addition, these systems can be used to reduce the quantity of flow. This category of BMP includes large bioretention systems, swales, rain gardens, grass filter strips, and even green roofs. The use of these “natural” systems in site development can significantly cut down on surface runoff and reduce the need for other more costly structural BMPs.
Proprietary Systems. Proprietary systems can be divided into several general categories. Separator systems remove sediment, debris, and surface oils and grease through various hydrodynamic designs. Effective separator systems trap and separate pollutants to prevent them from being reintroduced into runoff, which can result from “scouring” or other actions prompted by the powerful energies created from heavy volume of stormwater runoff.
Filtration devices are catch-basin inserts or in-pipe designs that remove various pollutants. Effective designs should use nonleaching media, incorporate prefiltration sedimentation removal chambers to reduce plugging, and be accessible for regular maintenance. In addition, filtration devices need to be designed with overflow bypass to prevent flooding caused by high flow rates or plugging of the filters. A properly designed filtration system can be a useful device for urban hot-spot applications where a particular pollutant is being targeted. It also can be cost-effective where land use does not allow other economical BMP options. This is particularly true with existing sites in urban settings. Proprietary systems can be effective pretreatment devices for infiltration systems and other BMPs.
|Properly designed BMP systems can help control quantity of flow during storms and reduce flooding.|
Determining which proprietary control will be successful can be a challenge, however, and planners need to closely examine vendor claims to ensure the device will function as advertised. Some vendor-supplied devices are now being evaluated under EPA’s Environmental Technology Verification (ETV) program. This program, which is administered through NSF International, has nearly completed a standardized protocol for testing wet-weather flow technology and in-drain devices for source water protection. When testing is completed, results will be posted by EPA and likely on the American Society of Civil Engineers database as well. In addition, the Civil Engineering Research Foundation is providing verification on certain devices through its Environmental Technology Evaluation Center program. It is hoped that the ETV will provide a national database where planners can go to verify vendor claims regarding proprietary BMP performance.
Examples of separator-type systems include:
- BaySaver Separation System
- CDS Technologies
- Downstream Defender
- Stormceptor System
- StormTreat System
- Vortechs Stormwater Treatment System
Examples of filtration systems include:
- DrainPac Storm Drain Filter
- Hydro-Kleen Filtration System
- Ultra Urban Filters
Examples of infiltration systems include:
- Storm Chamber
These lists are intended as examples and are not endorsements by the author or Stormwater. Many other proprietary technologies are available. Planners should contact individual vendors for details regarding their products.
Using BMPs for sediment control during construction activities is of critical importance in preventing degradation of water quality. A host of soil and erosion controls are currently used in the construction industry to contain sediment on-site until completion of vegetative stabilization and appropriate impervious surfaces. Controlling pollutants from construction activities must be considered in conjunction with site development. Although detailed discussions of soil and erosion techniques are beyond the scope of this article, planners need to address this issue for Phase II permit requirements.
Nonstructural BMPs are practices designed to prevent pollutants from entering stormwater runoff or to reduce the volume of runoff. The most important nonstructural BMP is public education and outreach. Many people are unaware that most stormwater runoff entering storm drains is not treated and flows unchecked into receiving waters such as streams and lakes. In addition, they are unaware of the means by which individuals and businesses can reduce negative human impacts through their own effort. Understanding the sources of pollutants and how they can end up in stormwater runoff is at the root of changing perceptions on how stormwater should be handled. Examples of techniques for pollution prevention at the source include
- proper disposal of hazardous substances;
- storm-drain stenciling;
- minimizing use of fertilizers, pesticides, and herbicides;
- preventing lawn waste, such as clippings and leaves, from entering stormwater runoff;
- reducing exposure of materials and products to wet-weather impacts through proper storage;
- good housekeeping measures by commercial and governmental entities, including training of personnel.
Other examples of nonstructural BMPs include
- street sweeping;
- catch-basin cleaning;
- identification and removal of illicit connections into storm drains from industrial and sanitary wastewater systems;
- identification of leaking septic or other onsite treatment systems;
- reduction in impervious surfaces or low-impact development;
- ordinances and rulemaking for stormwater management, including maintenance requirements.
These are just a few examples of nonstructural BMPs that can be included in a stormwater management plan to help meet the minimum measure requirements of the Phase II rule.
Determining which BMPs will be most effective for a specific locale requires an understanding of the area draining to a proposed BMP as well as the affected watershed. By gathering stakeholders to evaluate and determine designated uses and pollutants of concern, planners can better identify those areas of critical concern. By developing an inclusive stormwater management plan, planners may then select various BMPs for specific applications. Sources for detailed design considerations of BMPs can be obtained through EPA, the Center for Watershed Protection, individual states, or local consultants who are familiar with these issues. By preparing for the Phase II stormwater rule now, stormwater planners will be able to make better use of BMPs to help achieve the desired goal of improved water quality.