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Introduction

Stormwater is rainfall or snowmelt runoff, which can occur as sheet flow or flow in a conveyance system or downstream waterway. The Clean Water Act, which was developed “to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters” (33 U.S.C. § 1251) regulates stormwater in municipal, construction, and industrial settings under the National Pollutant Discharge Elimination System (NPDES) (40 CFR § 122.3) permit program. Industrial stormwater is derived from precipitation and/or runoff that comes in contact with industrial manufacturing, processing, storage, or material overburden and then runs off site and enters drainage systems or streams. Industrial stormwater does not include direct discharges of wastewater or process water from facilities or stormwater associated with activities exempted from the NPDES program, such as certain agricultural activities.

The NPDES was created to provide a regulatory framework for the control and elimination of the discharge of pollutants to surface waters to restore and maintain the integrity of the nation’s waters. This program was initially focused on reducing point-source discharges of pollutants from industrial process wastewater and municipal sewage into receiving waters, which are more easily regulated because they emanate from identifiable locations on a relatively consistent basis. The added regulation of stormwater in the NPDES program has been challenging. Stormwater is produced throughout a developed landscape, and its production and delivery are episodic. In 2009, the National Research Council (NRC) released a comprehensive report on the Environmental Protection Agency’s (EPA’s) Stormwater Program that covered all sectors of the program, including municipal, industrial, and construction. This study builds on that report, with a focus on industrial stormwater monitoring and management.

THE CLEAN WATER ACT AND INDUSTRIAL STORMWATER MANAGEMENT

The Clean Water Act requires that effluent limits be established to meet state-determined water quality standards. State water quality standards include designated uses, which identify the uses or goals of each water body or segment (such as aquatic life, water supply, and recreation), and numeric or narrative criteria that will protect or restore the designated use. Effluent limits must consider both the technological capability to control or treat the pollutants (technology-based effluent limits or TBELs) and limits necessary to protect the designated uses of the receiving water (water quality-based effluent limits or WQBELs).

TBELs are applied through nationally developed effluent limitation guidelines (ELGs). National ELGs are developed by EPA through a rigorous process to determine the effluent limits that are achievable using the best available technology within the economic means of the industry. The development process includes studies of pollutant levels, industry surveys, and a detailed analysis of technological controls, plus economic considerations. ELGs are then applied nationally so that there is no economic advantage to operating and discharging pollutants in one state over

another. ELGs may be specific to process wastewater discharge or to stormwater discharge, or may be applied to both. Where national ELGs have not been established, a permit writer may develop numeric effluent limits for categories of industries based on best professional judgment. However, these limits must withstand intense industry and public scrutiny as well as be technically defensible in a court of law and, therefore, are more likely at individual sites with extensive data rather than in national or statewide general permits.

WQBELs are established to meet the designated use objectives of individual receiving waters, which are identified, for the most part, by states. Water quality criteria form the basis for WQBELs. A number of complexities refine the designated use criteria, such as specific types of fish and macroinvertebrate populations expected in the water body, the level of exposure to pollutants in drinking water over a lifetime and acceptable cancer risk, and the type and frequency of human immersion expected in a recreational water body. The amount of pollution that a water body can assimilate and still support beneficial use goals is defined through adoption of water quality criteria. Most often, the criteria are pollutant specific and numeric and are designed around a low-flow dry weather condition, with the idea that this condition represents the highest pollutant concentration in a water body. However, stormwater flows will occur during quite different flow and loading conditions than those for which the criteria are typically established. Questions have been raised about the applicability and relevance of these criteria to wet weather conditions, but separate criteria for wet weather allowances have not been developed and implemented for industrial stormwater discharges. WQBELs are established when analyses determine that a discharge causes or has the reasonable potential to cause or measurably contribute to an instream excursion above water quality criteria. For discharges of process wastewater from traditional sources, these WQBELs are typically numeric, and monitoring data are routinely used to inform the analysis for compliance.

Industrial Stormwater Permitting

Although industrial stormwater discharges were included in some individual NPDES permits in the 1970s and 1980s, stormwater permitting was generally limited to relatively large industrial sites with other discharges of process wastewater. At that time, a large number of industrial stormwater discharges had been deemed to be nonpoint sources or sources of diffuse pollution and were unpermitted. In 1987, Congress significantly expanded the 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. Congress provided timelines for expanding industrial stormwater permit coverage and required EPA to report back with information regarding classes of industrial stormwater discharges that were not widely permitted, the nature and extent of pollutants in those discharges, and procedures and methods specific to industrial stormwater discharge control. Congress also clarified that permits authorizing discharges of stormwater associated with industrial activity were required to meet all applicable provisions of the established permitting program, including TBELs and WQBELs.¹

The congressional expansion of industrial stormwater permitting meant a large increase in the number of industrial facilities that needed to be permitted as point-source discharges. In 1990, EPA promulgated these requirements, including details around the use of general permits for administrative efficiency. The general permit approach is an administratively efficient and cost-effective alternative to the individual permit application method. It reduces the administrative burden on the permitting authority and on the permit applicant. Instead of each applicant having to characterize representative samples of stormwater discharge, EPA allowed industry groups to submit a group application and characterize their wet weather discharges based on monitoring data collected from a subset of these facilities.

Multi-Sector General Permit

EPA issued the first Multi-Sector General Permit (MSGP) in 1995 as a 5-year permit. It was subsequently revised in 2000, 2008, and 2015, and the current MSGP extends through 2020. The MSGP provides permit coverage through submittal of a “notice of intent,” self--

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¹ Water Quality Act of 1987, Pub. L. No. 100-4, 101 Stat. 7 (1987).

certified implementation of a stormwater pollution prevention plan (SWPPP), and implementation of stormwater control measures (SCMs) to reduce pollution levels in the discharge (see Box 1-1). The 2015 MSGP provides permit coverage for industrial sectors listed in Box 1-2, grouped by general industrial activity descriptions and standard industrial classification (SIC) codes.² EPA includes a separate group AD for facilities not covered elsewhere, which may be designated by the EPA administrator or a state with delegated permitting authority. Industrial facilities with no industrial activity exposed to rain, snow, snowmelt, and/or runoff can apply to be excluded from the permit coverage.

Under the MSGP, TBELs are provided either through a limited number of ELGs or through a suite of narrative requirements, some of which are specified for particular sectors (discussed further in the next section). WQBELs in the MSGP are narrative and require the discharge “to be controlled as necessary to meet applicable water quality standards.” EPA states that compliance with TBELs and other permit terms and conditions are expected to result in compliance with water quality criteria and standards. The ambiguity of such compliance expectations for industrial stormwater discharges raises questions of enforceability, public involvement, and permittee liability. More specific requirements have been developed locally in situations where industrial stormwater discharges flow to water bodies that do not meet established water quality criteria and standards. These water bodies are considered impaired and the impairment is addressed through development of a total maximum daily load (TMDL). Development of a TMDL is a process that includes identification of the pollutant causing the impairment, the sources of the pollutant, and controls needed to restore the water body to the point where it meets its designated use.

The original strategy for the MSGP envisioned a broad tool for control of industrial stormwater discharges that, over time, would lead to improved control measures, more specific numeric effluent limitations based on monitoring evidence, and reduced pollutant discharges to receiving waters (EPA, 1992b). In 1990, EPA (EPA, 1990, p. 48002) outlined an escalating tiered implementation strategy to reduce the discharge of industrial stormwater pollutants. The strategy included general permits that incorporated basic pollution prevention strategies, site inspections, and reporting (Tier 1); watershed permits (Tier 2); industry-specific permits (Tier 3) for sectors shown to be significant sources of stormwater pollutants; and individual permits (Tier 4) tailored to specific facilities that are significant sources of stormwater pollutants. The original implementation strategy has not been realized. Rather than move coverage to watershed permits, industry-specific permits, and individual permits, EPA has continued to provide coverage under a single permit, the MSGP.

The MSGP sets the requirements for industrial stormwater management in areas where EPA is the permitting authority, including most of Indian country,³ some federally operated facilities, all U.S. territories, the District of Columbia, and four states (Idaho, Massachusetts, New Hampshire, and New Mexico).⁴ As of September 2018, the MSGP covered 2,174 facilities (R. Urban, EPA, personal communication, 2018). In most of the country, the MSGP serves as a model for states with delegated permitting authority to adopt their own industrial stormwater general permits. Although some states do not venture beyond the requirements of the MSGP, others tailor their permit to address unique geographic conditions (see Appendix A). For example, states may alter the stormwater sampling frequency, require monitoring of additional water quality parameters, and/or specify use of certain SCMs.

INDUSTRIAL STORMWATER MONITORING IN THE MSGP

Three types of monitoring are specified under the MSGP, intended to promote sound stormwater management and provide indicators of compliance and the effectiveness of stormwater controls:

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² The North American Industrial Classification System (NAICS) has since supplanted the SIC codes for commercial activity in North America, and EPA provides a translation from SIC to NAICS on its website.

³ Indian country is defined as “a) all land within the limits of any Indian reservation under the jurisdiction of the United States Government … ; b) all dependent Indian communities within the borders of the United States … ; and c) all Indian allotments” (18 U.S.C. § 1151).

⁴ Idaho recently received NPDES authorization and will begin issuing its own stormwater permits in 2021.

1. Visual monitoring, where samples of runoff are collected and observed visually for certain water quality characteristics (e.g., color, turbidity, and oil sheen);
2. Benchmark monitoring, where stormwater samples are collected and analyzed in a laboratory for specific pollutants and compared to benchmark thresholds identified in the MSGP; and
3. ELG monitoring, where stormwater samples are analyzed for specific pollutants that are compared for compliance with national ELGs (see also Table 1-1).

The monitoring requirements complement quarterly site inspections that must be performed and documented by permittees.

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 harmful discharges of stormwater pollutants to the local environment. Monitoring observations can signal shortfalls in stormwater management, and exceedances can be cause for review and reconsideration of SCM selection and implementation. Monitoring results can also be used to quantify improvement in stormwater quality on site based on implementation of stormwater control measures and to identify pollutants not being successfully controlled.

At a program level, MSGP monitoring data should also provide an indication over time whether the quality of industrial stormwater across the country is improving to meet the objectives of the MSGP (EPA, 2015d). Additionally, MSGP monitoring would, ideally, inform future decision making and updates to future general permits, such as refinements in benchmark thresholds over time based on the capabilities of treatment technology. The various types of MSGP-required monitoring are summarized in Table 1-1 and discussed in more detail below.

Visual Monitoring

The MSGP requires all permittees to conduct quarterly visual assessment of stormwater samples from each outfall. For events that result in a discharge, samples must be taken within the first 30 minutes of discharge (if feasible) resulting from a storm event that

TABLE 1-1
MSGP Monitoring Requirements

occurs at least 72 hours following a previously measurable event. The sample is then inspected visually for color, odor, floating or settled solids, suspended solids, oil, sheen, and other indicators of stormwater pollution. These results are documented by the permittee and summarized in an annual report to EPA. If evidence of stormwater pollution is observed, corrective actions are required (EPA, 2015d).

Benchmark Monitoring

EPA recognized the greater cost burden of analytical monitoring over visual monitoring and required analytical monitoring only of sectors that demonstrated a potential to discharge pollutants at concentrations of concern. For the most part, EPA determined which industry sectors required benchmark monitoring using industry-supplied baseline data during a 1992 group application process. The industry group leaders were given the discretion to identify which facilities to sample and for which pollutant. The sampling data requested included a mandatory list of pollutants (pH, oil and grease, biological oxygen demand, chemical oxygen demand, total suspended solids, total nitrogen, nitrite and nitrate, and total phosphorus), commonly referred to as the baseline sampling (EPA, 1992c). Industry groups were asked to select other pollutants to analyze for based on lists of pollutants that they deemed to be representative of the industry subsector activity (see Appendix B). The data collected were presumed to be representative of discharges without the implementation of SCMs because, at the time, those discharges were unpermitted. EPA compiled and analyzed the data by industry sector, and where industries were found to contain a wide range of industrial activities or potential pollutant sources, the industries were subdivided further and the data compiled on a subsector basis.

Based on the group application data, EPA required benchmark monitoring for industrial sectors where pollutants were identified in stormwater at concentrations of concern to receiving waters that could be reduced through implementation of SCMs. EPA also required benchmark monitoring for a few industries that had a high potential for contamination from stormwater discharge that was not adequately characterized by the data generated through the group application process. The different sectors with specific required benchmark monitoring are listed in Table 1-2. The benchmark monitoring requirements in the 1995 MSGP have for the most part carried over to the current 2015 MSGP.

TABLE 1-2
Industrial Sectors and Subsectors and Their Benchmark Monitoring Requirements

The benchmarks were established as “the pollutant concentrations above which EPA determined represents a level of concern. The level of concern is a concentration at which a stormwater discharge could potentially impair, or contribute to impairing water quality or affect human health from ingestion of water or fish.” The benchmarks were also viewed by EPA “as a level, that if below, a facility represents little potential water quality concern” (EPA, 1995). For the baseline sampling pollutants, EPA used a mix of approaches to establish technology-based benchmark thresholds (see Table 1-3). For example, for total suspended solids and nitrate plus nitrite, EPA derived benchmarks from the median of the National Urban Runoff Program data. For other pollutants, EPA’s benchmark thresholds are largely based on published national or state water quality criteria, using EPA acute criteria where they exist and chronic criteria if no acute criteria exist. Aquatic life water quality criteria provide the basis for 15 of the 23 parameters in the 2015 MSGP for which benchmarks have been established.

Industries required to perform benchmark monitoring (see Table 1-2) must sample pollutants quarterly in the first year of permit coverage. A benchmark sample is collected as a grab sample within the first 30 minutes of stormwater discharge after a rainfall (if feasible) that results in an actual discharge from the site and with an interceding dry period of at least 72 hours. The reported results typically reflect pollutant concentrations for an individual sample but can reflect the average concentration for an outfall for all sampled

TABLE 1-3
Sources of MSGP Benchmark Values

NOTE: FW = freshwater; LOEL = lowest observed effect level; NTU = nephelometric turbidity unit; SW = saltwater.

^a “New criteria are currently under development, but values are based on existing criteria.”

^b “These pollutants are dependent on water hardness where discharged into freshwaters. The freshwater benchmark value listed is based on a hardness of 100 mg/L. When a facility analyzes receiving water samples for hardness, the permittee must use the hardness ranges provided in Table 1 in Appendix J of the 2015 MSGP and in the appropriate tables in Part 8 of the 2015 MSGP to determine applicable benchmark values for that facility. Benchmark values for discharges of these pollutants into saline waters are not dependent on receiving water hardness and do not need to be adjusted.”
SOURCE: EPA, 2015c.

separate runoff events that occurred during the quarterly monitoring period. Facilities that are required to conduct monitoring but have no stormwater discharge during the reporting period are required to report “no discharge.” If the average of the four quarterly results exceeds any of the benchmarks, the monitoring must be continued for another four quarters until the average does not exceed the benchmark. Sampling results exceeding benchmarks (based on an average of four samples) is not a permit violation, unless no corrective action is undertaken and exceedances persist. Instead, an exceedance necessitates that the facility operator investigate stormwater control measures and make necessary improvements. Any corrective action taken must be documented as a modification to the facility’s SWPPP. If a facility determines that no further pollutant reductions are technologically or economically feasible and benchmark exceedances continue to occur, perhaps due to natural background conditions, run-on from adjacent properties, or other factors, permittees may apply for permission to reduce monitoring frequency or eliminate it (also termed an “off-ramp”).

Effluent Limitation Guidelines

EPA has established numeric ELGs for stormwater for 10 subcategories of industrial facilities (see also Table 1-1 and Appendix C);⁵ these subsectors are required to monitor at least once per year at each outfall containing the discharges subject to the ELG. An exceedance of a numeric ELG in a single sample is deemed a violation of the MSGP and subject to enforcement action. If an exceedance of an ELG is detected, it must be reported to EPA, and corrective actions are required. After an exceedance, additional monitoring is required at least quarterly until the discharge is within compliance. The numeric effluent limitations in ELGs tend to be substantially higher than benchmark thresholds (with the exception of total suspended solids). ELGs are based on extensive data collection on the performance and capacity of treatment technology.

CONTEXT FOR THE STUDY

The various industrial stormwater permitting requirements have come under scrutiny since the program’s inception. It is widely recognized that the monitoring program suffers from a paucity of useful data and from inconsistent sampling techniques. Benchmark monitoring has been variously described as overly burdensome to industries and producing data that go unutilized. Some stakeholders question whether benchmark exceedances serve as useful indicators of the effectiveness of implementation of stormwater control measures or potential water quality problems. If problems are observed, others express concern about a lack of enforcement mechanisms to ensure that the issues are effectively addressed. State and local stormwater programs face a shortage of resources to review monitoring data and conduct routine compliance inspections. For these reasons, the NRC concluded that “the stormwater program has suffered from poor accountability and uncertain effectiveness at improving the quality of the nation’s waters” (NRC, 2009).

Among dozens of recommendations for improving the stormwater program, the 2009 NRC report recognized that many of the benchmark monitoring requirements and effluent guidelines for certain industrial subsectors were based on incomplete and outdated information (NRC, 2009). The report recommended that “Industry monitor the quality of stormwater discharges from certain critical industrial sectors in a more sophisticated manner, so that permitting authorities can better establish benchmarks and technology-based effluent guidelines.” The report also noted the lack of a nationwide compilation and analysis of industrial benchmark monitoring data, which could be used to better understand typical stormwater concentrations of pollutants from various industries. Additionally, the report recommended a risk-based approach for industrial stormwater monitoring requirements so as to not unduly burden those industrial facilities with limited exposure to runoff, while also not allowing high-risk sites to escape the more intensive monitoring that would be necessary to ensure compliance with effluent limitations.

These issues resurfaced in a recent settlement agreement made between EPA, industries, and environmental groups regarding revisions to the nationwide

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⁵ ELGs have been established for specific constituents in stormwater for cement manufacturing, petroleum refining, steam electric power generation, timber products processing, coal mining, hard rock mining, mineral mining and processing, and airports.

MSGP for industrial stormwater. The agreement requires the parties to suspend all legal actions against EPA regarding the revisions to the MSGP until the National Academies of Sciences, Engineering, and Medicine have conducted a study on certain aspects of the industrial stormwater program. In particular, the agreement asked the National Academies’ committee to:

1. Suggest improvements to the current MSGP benchmarking monitoring requirements. Areas to examine could include
   • Monitoring by additional sectors not currently subject to benchmark monitoring;
   • Monitoring for additional industrial-activity-related pollutants;
   • Adjusting the benchmark threshold levels;
   • Adjusting the frequency of benchmark monitoring;
   • Identifying those parameters that are the most important in indicating whether stormwater control measures are operating at the best-available-technology or best-conventional-technology level of control; and
   • New methodologies or technologies for industrial stormwater monitoring.
2. Evaluate the feasibility of numeric retention standards (such as volumetric control standards for a percent storm size or standards based on percentage of imperviousness).
   • Are data and appropriate statistical methods available for establishing such standards as both technology-based and water quality-based numeric effluent limitations?
   • Could such retention standards provide an effective and scientifically defensible approach for establishing objective and transparent effluent limitations?
   • What are the merits and faults of retention versus discharge standards, including any risks of groundwater or surface-water contamination from retained stormwater?
3. Identify the highest-priority industrial facilities/subsectors for consideration of additional discharge monitoring. By “highest priority” EPA means those facilities/subsectors for which the development of numeric effluent limitations or reasonably standardized stormwater control measures would be most scientifically defensible (based on sampling data quality, data gaps and the likelihood of filling them, and other data quantity/quality issues that may affect the calculation of numeric limitations).

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.

EPA’s proposed revisions of the 2015 MSGP will also address other provisions of the legal settlement that will increase the importance of the benchmark thresholds. The settlement required that EPA develop requirements for “Additional Implementation Measures” (AIMs) “substantially similar” to those detailed in Box 1-3. AIM would set specific actions that must be taken upon different levels of exceedance of the benchmarks or repeated exceedances. The specifics of the AIM tiers and the consequences of exceedances have not been finalized, but repeated exceedances of annual averages or large repeated exceedances could require additional structural stormwater control measures if feasible. If exceedances continue, an individual permit may be required. These requirements would provide stronger consequences to benchmark exceedances, thus increasing the significance of the benchmark thresholds.

The committee’s report and its conclusions and recommendations are based on a review of relevant technical literature, briefings, and discussions at its five in-person meetings and three web conferences, and the experience and knowledge of the committee members in their fields of expertise. The committee received briefings from a range of experts, including federal, state, and local government officials; practitioners; industry representatives; environmental organizations; and academics (see the Acknowledgments).

OUTLINE OF THE REPORT

Following this Introduction, the Statement of Task is addressed in three subsequent chapters of this report. In Chapter 2, the committee discusses benchmark

monitoring requirements and benchmark thresholds. Chapter 3 identifies opportunities for improving industrial stormwater MSGP monitoring, including evaluations of sampling methods, laboratory analysis, and data management. The committee recommends a new tiered approach to monitoring to provide improved stormwater management while reducing burden for small, low-risk facilities. In Chapter 4, the committee evaluates the merits and concerns associated with retention standards for industrial stormwater under the MSGP framework.