Summary

Industrial stormwater is derived from precipitation and/or runoff that comes in contact with industrial manufacturing, processing, storage, or material overburden and then runs offsite and enters drainage systems or receiving waters. In 1987, Congress significantly expanded the National Pollutant Discharge Elimination System (NPDES) program through amendments to the Clean Water Act to include industrial stormwater runoff conveyed through outfalls directly to receiving waters or indirectly through municipal separate storm sewer systems. This led to a huge increase in the number of industrial facilities that needed to be permitted as point-source discharges. The Environmental Protection Agency (EPA) developed the Multi-Sector General Permit (MSGP) in 1995 to provide permit coverage for the full range of industrial stormwater facilities, grouped by industrial activity. The 2015 MSGP sets the requirements for industrial stormwater management and monitoring in areas where EPA is the permitting authority, including most of Indian country and some federally operated facilities, all U.S. territories, the District of Columbia, and four states (Idaho, Massachusetts, New Hampshire, and New Mexico). The MSGP also serves as a model for states with delegated permitting authority as they develop their own industrial stormwater general permits.

The various industrial stormwater permitting requirements have come under scrutiny since the program’s inception. The 2009 National Research Council report Urban Stormwater Management in the United States stated that the industrial stormwater program has suffered from poor accountability and uncertain effectiveness at improving the quality of the nation’s waters. That report recommended updates to outdated benchmark monitoring requirements and recommended the use of more sophisticated sampling protocols. These issues resurfaced in a recent settlement agreement made between EPA, industries, and environmental groups regarding revisions to the nationwide MSGP for industrial stormwater. As a result of this settlement agreement, EPA asked the National Academies of Sciences, Engineering, and Medicine to convene a committee to study certain aspects of the industrial stormwater program, with an emphasis on monitoring requirements and retention standards (see Box S-1). EPA will use the results of this study to inform its proposed revisions to the 2015 MSGP, which are anticipated in 2020. The committee was not asked to analyze the financial costs of its recommendations; instead, EPA will assess the costs of possible changes in its proposed revision of the MSGP.

Although the 1995 MSGP was based on sound scientific and public policy principles, the committee found that many of the program elements have been hampered by shortfalls in generating, considering, and acting on new information. This has resulted in missed opportunities for refining the MSGP monitoring requirements in support of improved stormwater management. In this report, the committee recommends updating MSGP benchmark monitoring requirements and thresholds using a periodic review process to incorporate the latest science and monitoring information into each permit revision. Additionally, the committee recommends allowing more sophisticated monitor-

ing methods, training, and support for enhanced data analysis tools within the MSGP. The committee recommends risk-based tiered monitoring requirements to improve the quality of data from the largest, high-risk facilities, while moderating the burden on the lowest-risk facilities. The major conclusions and recommendations are summarized below.

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS

The primary purpose of the MSGP monitoring program is to ensure that industries are complying with the terms of the permit and appropriately managing stormwater on site to minimize discharges of harmful stormwater pollutants to the local environment. Under the MSGP, all industrial facilities are required to conduct quarterly site inspections performed by the permittee, and approximately 55 percent of permittees are required to conduct chemical-specific benchmark monitoring through quarterly grab samples. If the average of the four quarterly samples exceeds the EPA-established benchmark threshold, monitoring must be continued for another year. Sampling continues until the facility’s data show 1 year in which the average of the four quarterly samples meets the benchmark. A benchmark exceedance (based on an average of four

samples) is not a permit violation, unless no corrective action is undertaken and exceedances persist. Chapter 2 includes recommendations to improve the benchmark monitoring requirements and thresholds to improve industrial stormwater management.

EPA should require industry-wide monitoring under the MSGP for pH, total suspended solids (TSS), and chemical oxygen demand (COD) as basic indicators of the effectiveness of stormwater control measures (SCMs) employed on site. These parameters can serve as broad indicators of poor site management, insufficient SCMs, or SCM failure, which can lead to high concentrations of these and other pollutants. Industry-wide monitoring of pH, TSS, and COD would also provide a baseline understanding of industrial stormwater management across all sectors. All permitted facilities are currently required to conduct visual monitoring of quarterly stormwater samples, and these additional analyses are relatively inexpensive, minimizing the additional monitoring cost burden. Replacement of COD with total organic carbon (TOC) should be considered once EPA has adequate data to develop a benchmark threshold level.

EPA should implement a process to periodically review and update sector-specific benchmark monitoring requirements that incorporates new scientific information. This process should consider updated industry fact sheets, published literature and industry data, advances in monitoring technology, and other available information, so that the monitoring programs adequately address the classes of pollutants used on site and their potential for environmental contamination. The committee reviewed several sectors where data suggest that stormwater pollutants are common, but little or no benchmark monitoring is required. In some cases, this situation resulted from limitations in the original process where industries self-determined what pollutants to monitor in their group applications, and those results were then analyzed to develop benchmark monitoring requirements. Additional information and data gathering for polycyclic aromatic hydrocarbons (PAHs) could help EPA determine if benchmark monitoring is needed for sectors that have the potential to release PAHs. Periodic monitoring reviews would allow EPA to assess changing industry practices that could affect monitoring needs, new analytical technology for pollutant quantification, as well as current toxicological information. Where data gaps remain, additional sector-specific data-gathering efforts should be initiated.

EPA should update the MSGP industrial-sector classifications so that requirements for monitoring extend to nonindustrial facilities with activities similar to those currently covered under the MSGP. Many facilities and activities generating pollutants of concern in stormwater discharges are not included within the MSGP because the facilities themselves are not considered to be industrial, even though the on-site activities (and associated risks) are similar `to those of regulated facilities. These include school bus transportation facilities and fuel storage and fueling facilities. Some states have included these activities in their existing industrial general permits. EPA should examine other facilities with activities similar to regulated facilities and add them to the MSGP so that pollutant risks from these facilities can be appropriately reduced.

Benchmarks should be based on the latest toxicity criteria designed to protect aquatic ecosystems from adverse impacts from short-term or intermittent exposures, which to date have generally been acute criteria. Aquatic life criteria are designed for protection against both short-term (acute) and long-term (chronic) effects on both freshwater and saltwater species. Studies that form the basis of criteria development typically measure acute end points following exposure of aquatic life to consistent pollutant levels for short periods of time, and measure chronic end points following exposure of aquatic life to consistent pollutant levels for longer periods of time. Given the episodic nature of stormwater flow and the likelihood of instream dilution and attenuation, aquatic life criteria based on short-term (acute) or intermittent exposures are typically more appropriate for stormwater benchmark threshold levels than criteria based on long-term (chronic) exposures. Where EPA identifies substantial chronic risks to aquatic ecosystems from intermittent exposures during criteria development, such as for contaminants that bioaccumulate, an equation should be provided to translate chronic criteria for intermittent exposures. In this context, EPA should

• Develop acute aquatic life criteria for benchmarks where they do not currently exist, or where substantial chronic risks to aquatic ecosystems exist from repeated stormwater exposures, develop equations

• to translate chronic criteria based on intermittent exposures.
• Revisit the application of three benchmarks (iron, arsenic, and selenium) that are currently based on chronic and, in some cases, outdated aquatic life criteria.
• Allow permittees with repeated benchmark exceedances to use the latest aquatic life criteria for selenium and copper to evaluate water quality risk on a site-specific basis and discontinue comparisons to national benchmarks, as appropriate. The latest criteria for selenium and copper include equations for calculating toxicity criteria based on short-term exposure, using additional water chemistry and/or flow data.
• Based on little evidence of adverse effects to aquatic organisms at common levels, suspend or remove the benchmarks for magnesium and iron; benchmarks for these metals can be reinstated if/when acute aquatic life criteria are established or benchmarks are developed based on chronic effects from intermittent exposure.
• Express all benchmarks in the units from which they are derived, to improve communication and reduce reporting errors and provide guidance on the expected level of precision in reported results.

Additional monitoring data collection on the capacity of SCMs to reduce industrial stormwater pollutants is recommended to inform periodic reviews of the benchmark thresholds and identify sectors for which new national effluent limits could help address treatment attainability. Publicly available stormwater data from industrial sites are currently insufficient to determine if there are specific conditions under which industries cannot meet the benchmarks using conventional stormwater treatment systems (e.g., sedimentation, filtration) or if other nontreatment SCMs could reduce concentrations on these sites. Based on limited available SCM performance data, it appears that most standard treatment SCMs can meet the benchmark in at least 50 percent of storm events for TSS and for many pollutants at lower inflow concentrations associated with municipal stormwater. Considering that benchmark exceedance is judged by the average of four sample events, these results suggest that technical achievability is not a major issue for TSS. Limited data suggest that benchmark compliance is more difficult at industrial sites for iron, aluminum, copper, and soft-water conditions for lead and zinc; inadequate data are available for other pollutants. To improve understanding of industrial SCM performance and technical achievability:

• Industries and industry groups should collect scientifically rigorous performance data for common SCMs under typical stormwater conditions to expand the knowledge base and inform future decision making. An appropriate number of storms should be monitored by employing proper quality assurance and quality control to ensure data reliability, and design and maintenance information for the SCMs should be provided.
• EPA should encourage industries to collect these data and make them publicly available, such as uploading to the International Stormwater Best Management Practices database.
• EPA should support maintenance of these data for industrial stormwater, just as they are currently supporting the Industrial Wastewater Treatment Technology national database.

For benchmarks based on aquatic life criteria, the additional high-quality data collected can be used to assess the feasibility of achieving the benchmarks with current technology and practices. For technology-based benchmarks, additional data could inform future benchmark revisions to reflect the state of practice, reducing total loads to the extent practicable.

Because of the paucity of rigorous industrial SCM performance data, the development of new numeric effluent limitations (NELs) is not recommended for any specific sector based on existing data, data gaps, and the likelihood of filling them. Any new NEL that is developed would require extensive new data collection. Several sectors can be identified in recent MSGP data with recurrent high-concentration discharges. However, the decision to develop new numeric effluent limitations would need to be informed by thorough SCM performance data that clearly document attainability issues by sector and include a large number of permittees that cannot achieve the benchmarks under the increased oversight of the additional implementation measure (AIM) process, which is currently in planning.

STORMWATER SAMPLING AND DATA COLLECTION

The current MSGP benchmark monitoring requirement focuses on low-cost, coarse indicators of site problems, and the usefulness of the data can frequently be hampered by its variability. Stormwater monitoring data display variability that originates from many different sources, including the variability of precipitation within and among storms and changes in operations over the course of time. In Chapter 3, the committee discusses and recommends improvements in sampling design and procedures, laboratory analysis protocols, and data management to reduce error and improve the reliability of monitoring results to support improved stormwater management.

EPA should update and strengthen industrial stormwater monitoring, sampling, and analysis protocols and training to improve the quality of monitoring data. Specifically, EPA should

• Consider a training or certificate program in stormwater collection and monitoring to ensure that required sampling and data collection are representative of stormwater leaving the site to the greatest extent possible.
• Stay abreast of advancements in monitoring, sampling, and analysis technology that can provide more or better-quality information for similar or reduced costs and consider these in future revisions of the MSGP.

EPA should allow and promote the use of composite sampling for benchmark monitoring for all pollutants except those affected by storage time. EPA’s disallowance of composite sampling and reliance on grab sampling in the interest of discrete characterization of the highest pollutant concentration is not warranted based on the methods used to derive benchmark thresholds. Multiple composite sampling techniques are available that provide more consistent and reliable quantification of stormwater pollutant discharges compared to a single grab sample. Composite samplers have become common in stormwater monitoring as experience with this approach has increased and costs have declined, and the event mean concentrations that result from composite sampling may reduce the likelihood of exceeding the benchmark compared to first-flush grab sampling. Composite sampling is not appropriate for pollutants for which the results may vary over time with storage, such as those that transform or degrade rapidly or interact with the atmosphere (e.g., pH).

Quarterly stormwater event samples collected over 1 year are inadequate to characterize industrial stormwater discharge or describe industrial SCM performance over the permit term. Under the MSGP, if a permittee’s average of four consecutive quarterly samples meets the benchmark, a waiver is granted for the remainder of the permit term. For permittees with average results that meet the benchmark, the MSGP should require a minimum of continued annual sampling, to ensure appropriate stormwater management throughout the remainder of the permit term. Extended sampling over the course of the permit would provide greater assurance of continued effective stormwater management and help identify adverse effects from modifications in facility operation and personnel over time. Given the natural variability and the limitations of grab samples, substantial uncertainty is associated with using the average of only four stormwater samples. EPA should analyze industrial stormwater data and sector-specific coefficients of variation to recommend additional increases in sampling frequency, consistent with EPA’s determination of an acceptable level of error for this indicator of SCM performance. Additional continued monitoring at a lower intensity throughout the permit would also increase the overall sample size and thereby reduce the uncertainty in the monitoring results.

State adoption of national laboratory accreditation programs for the Clean Water Act with a focus on the stormwater matrix and interlaboratory calibration efforts would improve data quality and reduce error. NPDES laboratory accreditation programs and stormwater interlaboratory calibration efforts would improve the comparability and reliability of monitoring data. To support these efforts, EPA should publish guidance and case studies on interlaboratory calibration specifically focused on the stormwater matrix, including the establishment of performance quantification levels for stormwater samples. These efforts would promote similar procedures at a national level to ensure the comparability and reliability of test results reported to permitting authorities.

To improve stormwater data quality while balancing the burden of monitoring, EPA should expand its tiered approach to monitoring within the MSGP, based on facility risk, complexity, and past performance. The committee proposes four categories:

1. Inspection only. Low-risk facilities could opt for permit-term inspection by a certified inspector or the permitting authority in lieu of monitoring. Facilities could be classified as low risk based on facility size (e.g., less than 0.5 or 1 acre of industrial activity), recognizing that size may not fully represent the risk profile, or more accurately based on a detailed assessment of the type and intensity of industrial activities conducted on site, or a hybrid approach.
2. Industry-wide monitoring only. All facilities in sectors that do not merit additional pollutant monitoring would conduct industry-wide monitoring for pH, TSS, and COD. These data would provide broad, low-cost indicators of the effectiveness of stormwater control measures on site.
3. Benchmark monitoring. Sectors that merit additional pollutant monitoring, based on the most recent data and industry literature review, would conduct sector-specific benchmark monitoring in addition to pH, TSS, and COD which would be collected by all facilities with chemical monitoring.
4. Enhanced monitoring. Facilities with repeated benchmark exceedances or those characterized by the permitting authority as large complex sites with high pollutant discharge potential would conduct more rigorous monitoring, in consultation with the permitting authority. These facilities could collect volume-weighted composite samples at multiple outfalls if appropriate. Additional tools and monitoring strategies could be used to assess the water quality impact to receiving waters from stormwater discharge, including wet weather mixing zones, dissolved metal sampling, and site-specific interpretation of water quality criteria, with additional guidance from EPA. EPA should develop “nonrepresentative storm” criteria to exclude monitoring for events that would not be representative of facility stormwater discharge.

This tiered system would improve the overall quality of monitoring data to inform future iterations of the MSGP while balancing the overall burden to industry and permitting agencies.

To improve the ability to analyze data nationally and the efficiency and capability of oversight by permitting agencies, EPA should enhance electronic data reporting and develop data management and visualization tools. Electronic reporting has only been required of permittees since 2016, and the data management capabilities are still developing to make the most use of this information at the national and state levels. Automated compliance reminders, improved checks on missing or unusual data, and data analysis and visualization capabilities would improve the effectiveness of staff oversight and provide new opportunities to analyze trends. EPA should develop national visualization tools that can be used to easily examine data for patterns, trends, and correlations.

CONSIDERATION OF RETENTION STANDARDS IN THE MGSP

Stormwater retention for infiltration or beneficial use minimizes pollutant loads to receiving waters and reduces damaging peak flows while potentially increasing water availability. Yet, infiltration of industrial stormwater, which can contain hazardous pollutants in toxic amounts, can pose serious risks to groundwater; these risks must be managed to prevent groundwater contamination. Chapter 4 discusses scientific and regulatory factors affecting the applicability of stormwater retention standards for industrial stormwater. Based on the potential environmental benefits, particularly in areas of water scarcity, the committee encourages the use of industrial stormwater retention with infiltration or beneficial use under conditions where groundwater is protected.

Rigorous permitting, (pre)treatment, and monitoring requirements are needed along with careful site characterization and designs to ensure groundwater protection in industrial stormwater infiltration systems. In lieu of other information on the attenuation of contaminants in groundwater before they are transported to the site boundary, infiltrated water should be required to meet primary drinking water standards for inorganic chemicals and organic

chemicals, and secondary standards for chloride and total dissolved solids. Water quality should be monitored and evaluated in the infiltration device or at the base of the vadose zone. Many water quality treatment options are available, ranging from natural removal employing in situ soils to standard SCMs to advanced treatment. Industries considering infiltration should evaluate whether potential stormwater contaminants from routinely occurring pollutants as well as accidents and spills are compatible with infiltration and what technologies are required to remove these contaminants prior to infiltration. Chemicals covered by the Safe Drinking Water Act and unregulated chemicals with known human health risks at concentrations of concern should be evaluated. Meeting stringent water quality requirements may make infiltration cost prohibitive at sites with contaminants that pose a high risk of polluting groundwater. Other factors influencing the feasibility of a retention and infiltration system include the land available, soil infiltration rate, soil chemistry, and depth to groundwater.

Site-specific factors and water quality-based effluent limits render national retention standards for industrial stormwater infeasible within the existing regulatory framework of the MSGP. Retention with infiltration or beneficial use is already allowed within the MSGP as one of many possible SCMs. However, the suitability of retention with infiltration or beneficial use is based on site-specific factors that cannot be generalized nationally into retention standards. Issues such as the design storm size, stormwater quality, receiving water quality goals, and site conditions must be known to ensure performance reliability. Additionally, although retention could be designed using site-specific factors as a technology-based effluent limit, industrial stormwater must also comply with water quality-based effluent limits, which are typically concentration based. It is impractical to design stormwater retention to capture all potential rainfall events, and for storm events that exceed the design standard, discharge or bypass will occur that may exceed the benchmarks.

EPA should consider incentives to encourage industrial stormwater infiltration or capture and use where appropriate. The most significant incentive would be assurance that installation of infiltration in accordance with EPA guidance for determining the appropriate design storm provides relief from the corrective action process associated with episodic exceedances of benchmark thresholds during bypass situations. This could be done through a number of regulatory measures, including a mixing-zone allowance, establishment of allowable frequencies of stormwater discharge at levels above benchmark threshold, development of water quality standard exceedance allowances for extreme weather events, or establishment of separate water quality criteria for major wet weather events. Finally, EPA could develop guidance and cases studies for demonstrating that exceeding the benchmark during storms with precipitation amounts greater than the design storm do not result in an exceedance of water quality standards.

EPA should develop guidance for retention and infiltration of industrial stormwater for protection of groundwater. The guidance should include information on applied water quality, treatment offered within the infiltration zone, monitoring requirements, natural attenuation of pollutants, groundwater use designations, and possible impacts of pollutant dilution or mobilization in the subsurface. Because of the potential risks to groundwater, industrial stormwater infiltration is not recommended in states that lack the legal authority to manage and enforce groundwater quality.

OVERARCHING MESSAGE

An overarching theme within the report’s recommendations is that the MSGP should incorporate the best available science in the MSGP process. Science continues to improve our understanding of the environmental and human health impacts of industrial stormwater. Technologies for water quality monitoring, stormwater treatment, and modeling are advancing at rapid rates, and new data can inform understanding of the performance of stormwater control measures. New tools are being developed to improve toxicological assessments and data management and visualization. As electronic reporting of industrial stormwater monitoring data becomes fully implemented and integrated for all states, large amounts of valuable industrial stormwater data will be available for analysis, evaluation, and identifying areas for improvement. In general, EPA has been slow to adopt new knowledge into its MSGP permit revisions, but the MSGP should not be a static enterprise. Both permitted facilities and

the nation’s waters would be best served by a progressive and continuously improving MSGP based on analysis of new data and focused data-gathering efforts, advances in industrial stormwater science and technology, and structured learning to develop and evaluate permit improvements.