The uncertainty of stormwater product and practice efficacy is well documented and has been a major challenge in the value proposition associated with stormwater infrastructure investments. In short, why would we expect public tax/rate payers to invest in infrastructure that provides treatment that may not be clearly effective
The principles of stormwater runoff treatment are relatively clear and easily understood: pollutants delivered and discharged in association with urban runoff generation and conveyance can be treated through well-known processes, such as sedimentation, filtration, infiltration and at times through chemical additives. However, the chaotic nature of flows and pollutant loading creates challenges in providing certain and consistent treatment capacity compared with the more predictable pollutant loadings associated with municipal wastewater discharges. Consistent treatment performance to address stormwater pollution has been and continues to be difficult even though our understanding of these challenges and technologies to provide treatment have evolved over the last 30 years.
What is the record of performance in the stormwater sector thus far?
The value proposition facing the stormwater sector hinges on the performance of investments made to address stormwater pollution. The data paints at best a mixed picture for the stormwater sector. A 2023 article noted that event mean concentrations of pollutants reported in the National Stormwater Quality Database show a decrease in total suspended solids, total phosphorus, total Kejdahl nitrogen and some heavy metals over the last 40 years but also attributed these reductions not to stormwater treatment controls but to advancements in cleaner manufacturing practices, compliance with Clean Air Act requirements, and other source controls.
To contrast, the American Society of Civil Engineers gave the stormwater sector its second grade of “D” in a row in its 2025 report, noting that the miles of impaired rivers has increased from 424,000 to 703,00 between 2010 and 2022. And in a 2022 Government Accountability Office report focused on gauging progress over the 50 years since the passage of the Clean Water Act, it is noted that “while the sources of pollution that created situations (such as the Cuyahoga River catching on fire) have largely been addressed, threats from more dispersed sources—such as stormwater runoff that carries pollutants into our waters—will require further actions. Emerging threats resulting from a changing climate also need to be addressed.”
The role of the STEPP program in performance
One of the major outcomes of the recent US Supreme Court case of the City/County of San Francisco v. U.S. EPA that resulted in a close 5-4 ruling for the City/County of Francisco is that the responsibility of NPDES permittees is to ensure their own discharges do not contribute to water quality violations rather than meeting “end result” conditions of meeting water quality standards in receiving waters. Permits are expected to become more specific and actionable by providing treatment details – perhaps even effluent limits – so permittees can ensure compliance with less ambiguity. If revised permits indeed go this direction, the performance of stormwater control measures (SCMs) implemented in their programs will likely face increased scrutiny to demonstrate their actual effectiveness.
The Stormwater Testing and Evaluation for Products and Practices (STEPP) program, launched in 2024 by the National Municipal Stormwater Alliance (NMSA), is the right program at the right time as it was established specifically to provide increased confidence in the performance of stormwater infrastructure to bolster the value of stormwater management investments. STEPP is an environmental technology verification program tailored to the stormwater sector that works to establish performance testing standards for common categories of public domain and proprietary stormwater products and practices and provide third-party reviews of tested technologies to ensure that the performance data produced is trusted and consistent with testing standards. In effect, STEPP is the tool we need to show stormwater practices work as advertised.
STEPP is a program that is national in scope, but it is built upon highly successful state-based programs, such as the Washington State Technology Assessment Protocol for Ecology (TAPE) and New Jersey Corporation for Advanced Technology (NJCAT) programs, which are continuing to provide technology verification services as STEPP continues to evolve. The STEPP program provides verification services starting with trash capture devices and hydrodynamic separators tested in the lab environment with other pollutant filter technologies to be added by mid-2026. STEPP is also currently offering “concurrence” approvals, which is a process to approve technologies verified in other verification programs that use the same or equivalents testing standards. Field testing verification is being developed currently through close coordination with Washington TAPE and Department of Ecology staff.
To help improve STEPP, NMSA is currently gathering input from the sector on performance evaluation programs and issues through a survey that can be accessed at: https://www.surveymonkey.com/r/STEPP-MS4s-2025_2026. This survey should take less than ten minutes to complete – NMSA appreciates all input provided. Also, you can learn more about STEPP and see information on first concurrence approval granted by STEPP by going to the program website at https://stormwatertesting.org/.
With the need to verify performance in the stormwater sector becoming a driving issue, the STEPP program is meeting the moment by providing trusted data on the efficacy of stormwater products and practices as well as developing critical information on how this information can be effectively used within stormwater programs.
