REGIONAL STORMWATER BMPS: A SOLUTION FOR MEETING FUTURE TMDL WASTELOAD ALLOCATIONS IN SOUTHERN CALIFORNIA
Urban runoff is one of the last major waste streams to be regulated, and poses unique challenges with respect to meeting Total Maximum Daily Loads (TMDLs). Recent guidance from the U.S. EPA indicates that future TMDL numerical wasteload allocations (WLAs) will be established for stormwater,
and that those WLAs will be enforced through NPDES permit conditions. In California, many municipal stormwater permits require implementation of numerous smaller, site-specific best management practices (BMPs), the collective effectiveness of which is difficult to determine. Consequently,
municipalities will face an enormous challenge in demonstrating compliance with future numerical WLAs for stormwater. Regional (watershed-based) stormwater BMPs may provide a viable solution for meeting the permit requirements and increased control required for TMDL wasteload allocations.
The Construction Industry Coalition on Water Quality (CICWQ) has been proactive in developing regional BMPs for implementation in Los Angeles County and has developed this study to examine the feasibility of a regional approach.
Regional systems for managing stormwater are currently in
use in various parts of the country and are gaining momentum in southern California. Regional BMP facilities, such as detention and/or infiltration basins or wetlands, can provide higher levels of treatment more reliably. Regional BMP facilities are also more cost-effective to construct
and maintain when compared on a cost per acre basis. These facilities make it possible to manage urban runoff from a larger watershed or sub-watershed area, including all existing land uses as well as new development or redevelopment areas. Regional BMPs can be effective in helping cities
meet TMDL wasteload allocations, for the following reasons. First, regional facilities can be optimally located and sized to reduce pollutant loads from all tributary areas within a sub-watershed, rather than small discrete portions, resulting in greater water quality benefits. Second, regional
facilities can address both wet-weather and dry-weather flows (dry-weather flows are suspected to carry a large portion of urban runoff-related pollutants). Besides improving water quality, some systems could result in the creation of urban habitat, greenspace, and water conservation, such
as through the use of BMP treatment trains that employ biofiltration and infiltration. Especially in economically disadvantaged areas, this can provide significant community benefit in addition to meeting TMDL requirements. Without proper maintenance, stormwater BMPs lose their ability to
remove pollutants and no longer provide benefits for water quality. Poorly maintained facilities can also contribute to vector problems. There is a much higher likelihood that regional stormwater BMPs will be maintained properly over time, preventing future failure. Therefore, fewer regional
facilities operated by a municipal or quasi-municipal agency have a greater assurance of being consistently properly maintained so they operate in perpetuity.
CICWQ proposes to conduct a local model project based on credible engineering and scientific bases to illustrate the applicability
of regional solutions in the Los Angeles area. This model project will provide a real-world example of how a regional, watershed-based solution compares to alternative on-site approaches. For the model project, runoff from a watershed undergoing redevelopment (along with runoff from adjacent
developed areas) will be routed from the City of Los Angeles storm drain system into the regional BMP. The treatment process will most likely be a “treatment train,” or series of treatment systems arranged within the selected location. For example, the runoff could be routed from
the existing storm drain system through a grassed swale into a sand filter, with overflows directed into a depressed area built to maximize infiltration. The remaining treated effluent could then be discharged back into the storm drain.
In summary, a comprehensive regional, or watershed,
approach to stormwater treatment provides the greatest overall benefit and water quality improvement and TMDL compliance.
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