Urban Stormwater Control Measures for Nutrients Management

Identifying Effective Stormwater Control Measures for Improved Water Quality

According to the United States Environmental Protection Agency, 58% of the nation’s rivers and streams and 45%  of our lakes have excess levels of phosphorus, while approximately 60% of our coastal areas and more than 30%  of our estuaries are impaired by nutrients.¹

Nutrients such as total phosphorus, orthophosphate, dissolved phosphorus, total nitrogen, total Kjeldahl nitrogen nitrate, nitrate+nitrite, and ammonia as N are commonly transported in stormwater runoff and are direct contributors to water quality impairments. Excess nutrients from stormwater discharges directly increase aquatic plant growth, harmful algal blooms and anoxia in receiving waterbodies, which adversely impacts fish and aquatic life.²

This article describes the current state of stormwater-sector knowledge about nutrients as a pollutant of concern for urban stormwater runoff and the sewer systems that convey stormwater discharges to our receiving waterbodies. Regulatory and permitting approaches are also described including regional approaches for assessing water quality in the San Diego area and identifying effective stormwater control measures based on these assessments to meet numeric water quality criteria.

While the City of San Diego is currently addressing various pollutants in addition to nutrients, the assessment approach, as described in this article, provides specific steps. These steps can be valuable for other communities as more Municipal Separate Storm Sewer permits either include Total Maximum Daily Loads specific to nutrients or necessitate regional stormwater management approaches for nutrients. The challenges and benefits of these approaches are summarized as well to further assist other communities with preparation for evolving stormwater management requirements.

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Nutrient Pollution in Urban Stormwater

Human activities are a common cause of excessive nutrient loading with specific activities, commonly tied to land use, contributing to high concentrations of nutrients in stormwater runoff. Activities such as urban or residential fertilization, septic systems, yard waste, organic debris, pet waste, trash and food waste, automobile emissions and atmospheric deposition are sources of nutrients discharging to our waterbodies via separate storm sewer outfalls or direct drainage via overland flow.

In the 1990s, early federal guidance for nutrient control (i.e., 1992 TMDL Guidance and 1998 National Nutrient Strategy) focused on wastewater plants, combined sewer overflows, septic tanks, and fertilizers and detergents.

In subsequent decades, the numeric water quality criteria and TMDLs for nutrients began to appear in MS4 permits in Minnesota and Florida and then Virginia and Maryland. Most recently, EPA established new nutrient wasteload allocations in its Massachusetts MS4 permit, and, like the above predecessor states, the cost estimates for communities to comply are significant.

MS4 permits can vary in their approach to expressing water quality-based requirements. While most permits require controls to reduce the discharge of pollutants to the “maximum extent practicable,” many MS4 permits focus on effluent or receiving water quality or loading requirements. For these permits, required watershed or stormwater management plans must demonstrate the implementation of appropriate management actions over time to meet TMDL wasteload allocations or Water Quality-Based Effluent Limits. According to the 2022 National MS4 System Needs Assessment Survey Results, 55% of respondents (643 total) reported that their MS4 permits include TMDL requirements.³

Stormwater Control Measures for Nutrient Management

A stormwater control measure, also known as an SCM, is an action, either structural or non-structural, implemented to manage stormwater by regulating its flow rate, quantity and/or pollution levels. Studies over the past several decades have shown that, after source controls through actions like fertilizer bans and street sweeping, structural SCMs or best management practices are effective at removing nutrients in runoff. Filtration practices such as media filters and high-rate bioretention are most effective for reducing total phosphorus and total nitrogen in stormwater runoff. Detention basins, retention ponds, wetland basins and wetland channels are most efficient at removing dissolved forms of nutrients in stormwater runoff.⁴ Selecting SCMs for nutrient removal can be complicated given the number of different parameters contributing to nutrient pollution and that specific SCMs are known to export nutrients. Bioretention, grass swales or grass strips, for example, may consist of phosphorus-rich soils or media, or cause ammonification or nitrification of organic nitrogen.

Combined with multiple pollutants of concern for our waterbodies (i.e., pathogens also impacting urban receiving bodies for both combined sewer overflow and MS4 communities), it is important to know which nutrients are impacting a receiving waterbody and review data from the International Best Management Practices Database to select SCMs that will address nutrient loads as well as other loadings. This database provides stormwater managers a centralized, online repository for the effectiveness of BMPs, as well as data on urban and agricultural runoff quality. This resource is particularly valuable for selecting SCMs to implement in watersheds that need to address multiple pollutants where competing costs or siting challenges are expected. These challenges can arise as utilities and public works departments try to fit SCMs into tight or constrained urban spaces.

A Multifaceted Approach

A multifaceted approach is of utmost importance in addressing nutrient pollution in stormwater runoff due to the complexity and scale of the issue. Nutrient pollution, primarily caused by excess nitrogen and phosphorus, poses significant ecological and public health risks. Adopting a multifaceted approach recognizes that nutrient pollution stems from diverse sources such as urban and agricultural runoff, sewage systems and industrial discharges. By considering the wide range of sources, this approach ensures that strategies and interventions are tailored to address each specific contributor, maximizing the effectiveness of pollution reduction efforts.

Furthermore, a multifaceted approach acknowledges the need for a watershed-scale perspective. Stormwater runoff flows through interconnected watersheds, ultimately impacting downstream water bodies. By implementing strategies at various points along the stormwater pathway, such as source reduction measures, green infrastructure, treatment facilities and non-structural strategies, a multifaceted approach can effectively intercept and treat nutrients before they reach sensitive aquatic ecosystems. This comprehensive perspective recognizes the interconnectedness of the stormwater system and ensures a holistic approach to tackling nutrient pollution, ultimately leading to improved water quality and healthier environments for both humans and aquatic life.

While this article focuses on practices based on current permitting goals, partnerships for large-scale or watershed-wide wasteload allocations and TMDL implementation plans are critical for siting SCMs and project delivery as EPA seeks to address the diversity of sources of nutrients to our waterbodies (EPA, 2022).

Mark Doneux, chair of the WEF Stormwater Community and administrator of the Capitol Region Watershed District in Minnesota, is encouraging communities to consider watershed districts to pool resources, pay for SCMs, and take advantage of watershed-based or regional approaches for solving our stormwater challenges.

“In Minnesota, we have employed watershed districts since 1955. Watershed districts have the unique ability to address water resource issues of common concern based on watershed boundaries and not political ones. However, the greatest value of watershed districts comes with sharing and leveraging technologies for stormwater management that are most cost-effective and partnering locally and regionally on projects with multiple benefits for the District and our partners.”

Mark Doneux, WEF Stormwater 
Community chair and Minnesota Capitol Region Watershed District administrator

Understanding and Meeting Water Quality Requirements Beyond Maximum Extent Practicable

The California Regional Water Quality Control Board, San Diego Region, is responsible for implementing and enforcing the MS4 NPDES (National Pollutant Discharge Elimination System) permit in the region. This permit aims to regulate stormwater runoff and protect water quality in diverse landscapes, urban areas and coastal regions of San Diego. The RWQCB collaborates with municipalities and other entities operating MS4s to ensure compliance with the permit's requirements.

The Regional MS4 permit serves as a comprehensive framework for managing stormwater runoff and mitigating its impact on water quality in the region.

Analytical methods application including modeling and monitoring is crucial for evaluating and establishing the necessary connections between stormwater management practices and desired water quality goals. The following steps have been established to guide co-permittees in achieving compliance with the water quality numeric goals of the Regional MS4 Permit:

1. Determine stormwater improvement goals for water quality compliance, using the MS4 NPDES Permit. These goals serve as clear and measurable targets and can be established in collaboration with regulatory bodies or through stakeholder consultation.
2. Characterize existing conditions and estimate existing nutrient loads. This requires utilizing available data and employing modeling approaches to accurately assess the nutrient levels. In addition, it is important to consider all stormwater management practices and system assets that are currently in place or implemented at a specified point in time. This comprehensive characterization may require additional resources, such as historical information, environmental data and improved modeling approaches, to ensure accurate estimations.
3. Calculate required nutrient load reductions. These calculations are essential for developing targeted strategies and measures to achieve the established goals. Based on the existing conditions characterized above, in combination with water quality targets set by TMDLs or other assessments, the MS4 permit, and/or the watershed or stormwater management plan, specific goals can be determined to address impairments caused by stormwater. These goals typically involve specified reductions in stormwater pollutant loads or concentrations, volumes, or peak flows, and serve as benchmarks for effective stormwater management.
4. Identify and implement the most effective strategies and management actions for load reduction. This involves evaluating the effectiveness of various management practices, conducting economic assessments and adopting cost-effective approaches. Through the watershed or stormwater management planning process, opportunities for management actions are identified, including programmatic activities, low-impact development practices and municipal capital improvement projects. These strategies ensure that the selected management approach will lead to the attainment of water quality improvement goals.
5. Monitor the implemented management approach. This includes conducting ambient monitoring of the water body, tracking nutrient loadings and assessing the effectiveness of specific projects. Monitoring progress is crucial for providing feedback and assessing the effectiveness of the implemented measures.
6. Track and assess progress toward goals. This assessment can be conducted through various methods, such as citizen observations, monitoring trend analysis and the development of quantifiable measures. It is important to report the results to stakeholders and the public. Developing quantifiable methods for evaluating progress is an area that requires further resources and research. If progress falls short of expectations, it may be necessary to revisit earlier steps, reassess relationships and strategies, and make necessary adjustments. This assessment process serves multiple purposes, including providing reasonable assurance to stakeholders and regulators, informing future stormwater program enhancements and capital improvement planning, and highlighting quantitative results that support adaptive management, implementation tracking, and progress toward meeting stormwater improvement goals and requirements.

Urban Stormwater and Controls for Nutrient Challenges

Numeric water quality criteria for nutrients in MS4 permits will always point communities toward a watershed or sub-watershed scale approach for SCM implementation. Implementing watershed-wide structural BMPs comes with its fair share of challenges. Reporting data can be a significant hurdle. Collecting and analyzing data from numerous BMPs across a watershed can be complex and time-consuming. It requires establishing robust monitoring systems and ensuring data consistency and accuracy.

Local pilots, demonstrations and post-construction monitoring or data analysis can help communities determine capital investments. The International BMPs Database 2020 Summary Statistics Report spells out BMP-related data needs to provide pollutant removal effectiveness values for nutrients.⁵

Particularly for nutrients, more data is needed relating to green infrastructure and their impact on potential nutrient export, how enhanced engineered media mixes (i.e., iron or biochar) may have positive effects on both nutrients and other pollutants of emerging concern and if nutrient recovery may be useful where soils are nutrient deficient (i.e., for a circular economy, to support agriculture). 

Another challenge is the timeline for implementation, which is often identified in permits by regulatory agencies. Implementing SCMs at a watershed scale involves numerous projects and stakeholders, each with their own priorities and schedules. Coordinating and aligning these efforts can be time-intensive and may require overcoming logistical hurdles. Additionally, securing funding resources for large-scale implementation can pose challenges, especially when considering the diverse range of sites and their site-specific considerations, such as land availability, soil conditions and local regulations.

Furthermore, evolving regulatory criteria present a challenge. Stormwater regulations and permits often undergo updates and revisions to address emerging concerns and scientific understanding. Keeping up with these changes, including stricter pollutant reduction, monitoring or reporting requirements, and ensuring compliance across a watershed, can be demanding for project planners and implementers. Staying informed and adaptable to evolving regulatory requirements is essential to the successful implementation of watershed-wide stormwater SCMs. 

Potential future regulations in stormwater management may involve the adoption of new technologies and practices to improve stormwater quality. Costs of compliance with stormwater regulations can vary depending on factors such as the size of the watershed or community, the condition of the stormwater infrastructure, the level of pollution reduction required and the implementation of SCMs. Compliance costs may include infrastructure upgrades, maintenance expenses, monitoring and reporting activities, stakeholder outreach and staff training.

Benefits and Opportunities

Despite the challenges, there are opportunities associated with implementing watershed-wide stormwater SCMs for nutrient management. Recent permits written by EPA are purposefully general to enable communities to take advantage of flexible and broad watershed approaches, like credit trading, to meet water quality goals and TMDL requirements. EPA’s 2022 Memorandum, Accelerating Nutrient Pollution Reductions in the Nation’s Waters, provides direction and guidance on these broader approaches for nutrient management including One Water strategies for which stormwater is an important component.⁶ 

One of the primary benefits of implementing SCMs at a watershed scale is improved water quality and ecosystem health. By implementing SCMs across a watershed, the overall nutrient and pollutant loads entering water bodies can be significantly reduced. This, in turn, improves water quality, enhances aquatic habitat, and protects the health of plants, animals and humans dependent on these ecosystems.

Implementing SCMs at a watershed scale often allows for multiple benefit projects. Many SCMs, such as constructed wetlands, permeable pavement and stormwater ponds, provide additional advantages beyond nutrient reduction. They can help manage stormwater volume, reduce erosion, enhance groundwater recharge and create green spaces that improve aesthetics and provide recreational opportunities. By integrating multiple benefits into a watershed-wide approach, communities can achieve more comprehensive and sustainable stormwater management solutions.

Additionally, a potential future challenge that a watershed-wide approach can help address is climate change resilience. As climate patterns shift, communities may face increased stormwater challenges, such as more frequent and intense rainfall events. Implementing SCMs at a watershed scale can help build resilience and adaptability to these changing conditions by effectively managing stormwater runoff and minimizing its impacts on downstream areas.

About the Authors

Contact julie.stein [at] hdrinc.com (Julie Stein) or at +1 (212) 542-6073 and leila.talebi [at] hdrinc.com (Leila Talebi) or at +1 (619) 307-9123 for more information.

References:

1. EPA National Aquatic Resource Surveys; EPA Nutrient Pollution – Where This Occurs: Coasts and Bays.
2. National Research Council. 2009. Urban Stormwater Management in the United States. Washington, DC: The National Academies Press.
3. WEF. June 2023. 2022 National Municipal Separate Storm Sewer System (MS4) Needs Assessment Survey Results.
4. Clearly, et al. International BMP Database 2020 Summary Statistics Report 2020.
5. The Water Research Foundation. 2020. The International BMP Database 2020 Summary Statistics Report 2020 (pdf). International Stormwater BMP Database: 2020 Summary Statistics
6. Fox, Radhika. Technical Memorandum: Accelerating Nutrient Pollution Reductions in the Nation’s Waters (pdf). Published by the United States Environmental Protection Agency, April 5, 2022.