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THE ROLE OF STORMWATER BMPS IN MITIGATING THE EFFECTS OF NUTRIENT OVERENRICHMENT IN THE URBAN WATERSHED
Citation:
O'Shea*, M, M Borst*, AND C. Nietch**. THE ROLE OF STORMWATER BMPS IN MITIGATING THE EFFECTS OF NUTRIENT OVERENRICHMENT IN THE URBAN WATERSHED. Presented at Ninth Triennial International Conference on Urban Storm Drainage, Portland, OR, 9/8-13/2002.
Description:
Nutrient overenrichment from agricultural and urban point and nonpoint sources, including urban stormwter, is a leading cause of impairment to our nation's rivers, lakes, and coastal waters. For waters that do not currently meet existing water quality standards, The USEPA's TMDL program tasks authorities with developing and implementing control plans for point and nonpoint pollutant loads influencing receiving waters failing to meet designated use criteria. For diffuse pollutant sources like urban stormwater runoff, these plans will rely heavily on the use of BMPs to reduce the impacts of stormwater-associated stressors on impaired waterbodies. While the implementation of BMPs for the control of stormwater associated pollutants is already an important component of the recently promulgated Phase II stormwater regulations and many surface source water protection plans, TMDL-mandated load allocations may result in additional treatment requirements, e.g., numeric effluent limits for specific pollutants exceeding ambient water quality standards. The TMDL effort, along with the eventual adop-tion of more protective nutrient criteria and eutrophication-associated endpoints, will only increase the importance of obtaining consistent, seasonal, and species-specific BMP effectiveness data to ensure that controls implemented to reduce nutrients have the desired impact of reducing eutrophication in impaired receiving waters. Many structural BMPs (e.g., wet and dry detention ponds), when appropriate designed and constructed, capture and treat urban runoff to remove particulate-associated pollutants, Field monitoring programs show these same structures provide relatively little reduction in the loadings of dissolved constituents. For receiving waters impacted by eutrophication, these controls will provide minimal or even negative removals of the bioavailable nutrient species most responsible for the impairment of aquatic life and recreational usage. In overfertilized receiving waters, solids control in the absence of targeted nutrient reductions may additionally reduce productivity-limiting turbidity, or other limiting factors, potentially resulting in counterintuitive declines in water quality. Infiltration-based controls may satisfactorily capture particle-sorbed nutrients, but, for nonsorbed species, there is the potential for both groundwater contamination and eventual delivery to the receiving water through groundwater migration. Commonly employed vegetated systems combine biological uptake with sedimentation to control both particle-phase and dissolved nutrients. Yet, biologically-induced transformation can lead to negative removls of some species since nutrient uptake rates and ambient cycling vary seasonally. As productivity also varies seasonally, ecosystem response to loadings will depend not only on the magnitude, but also the timing of inputs. At USEPA/NRMRL's UWMB stressor-specific research into the control of pollutants by conventional BMPs is, in the case of nutrients, focused on understanding how physical, biological and chemical processes within stormwater controls can influence the complex nature of nutrient speciation. Ongoing and planned research targets understanding, defining, and modeling the key mechanistic processes governing nutrient removal in stormwater BMPs. Findings will be used to: enhance the removal of biologically important nutrient species, predict BMP effluent based on influent concentrations, improve removals at existing BMPs, and predict efficiencies under extreme episodic events and variable O&M practices.