4

Consideration of Retention Standards in the Multi-Sector General Permit

The majority of this report has focused on improving the monitoring process as a way of confirming appropriate stormwater management and ensuring compliance with the objectives of the Multi-Sector General Permit (MSGP). This chapter focuses on a different approach to ensuring that industrial stormwater is appropriately managed—retention standards. On-site stormwater retention and infiltration are already included within the MSGP as possible stormwater control measures (SCMs). Nevertheless, this committee was asked to evaluate the feasibility of retention standards as both technology-based and water quality-based numeric effluent limitations to establish objective and transparent effluent limitations (see Statement of Task in Chapter 1). The committee was also tasked to discuss whether the appropriate data and statistical methods are available for establishing such standards and the merits and faults of retention versus discharge standards. Retention standards are not assumed to replace monitoring, but would provide another structural approach to control pollutant discharges.

STORMWATER RETENTION

The process of stormwater retention, as envisioned in the committee’s task, involves storing the stormwater on site, with the goal that at least a large portion of it will not be discharged to surface waters but will go elsewhere. Possible fate pathways for stormwater after retention include infiltration (see Figure 4-1), some type of beneficial use, and evapotranspiration. This definition of stormwater retention, which is the focus of the committee’s analysis, differs from other forms of stormwater retention commonly used in stormwater management that aim to hold water on site for later, more gradual release, possibly after treatment.

Storage for stormwater retention can be provided with a pond or engineered facility such as underground tankage or an underground infiltration facility. The latter example is important when land is either expensive or unavailable. Stormwater retention systems are generally designed based on the volumetric capture of a storm event of a specific size. The targeted event would be large enough so that exceedance of this event size would be relatively rare, because exceedance will result in the discharge of untreated or minimally treated stormwater. Retention designs must also consider the time over which the captured stormwater would be removed, typically through infiltration or beneficial use, so that storage is again available to handle the next storm.

Infiltration is an attractive management option for stored stormwater; it has been used widely and successfully in municipal stormwater applications to reduce stormwater impacts on local water bodies and to recharge groundwater. In addition to simple storage with infiltration, novel SCMs that are infiltration based, known collectively as low-impact development or stormwater green infrastructure, including bioretention basins, permeable pavements, and vegetated filter strips and swales, may be employed for stormwater retention and infiltration (Caltrans, 2010). Infiltration of stormwater requires soil and geologic conditions conducive to the infiltration process, including rela-

tively high-permeability soils. However, for industrial stormwater, concerns due to the likely presence of toxic pollutants that could migrate through the soil to groundwater systems require very careful considerations, especially in terms of pretreatment requirements, before infiltrating. These issues are discussed in more detail later in this chapter.

The two other storage recovery pathways are typically minor for industrial stormwater. Evapotranspiration will require vegetation and a large amount of land area, both of which are not common on most industrial sites. Beneficial use of stormwater may be feasible in areas with extreme water shortages, but its applicability will be highly site specific. It is not expected that on-site stormwater harvesting and use (e.g., firefighting, dust control, washing, toilet flushing) will be practiced at many industrial sites due to the water quality treatment requirements and/or likely small or inconsistent water demand from these applications (NASEM, 2016). To be part of a reliable retention system, the demand for reclaimed water would need to be sufficiently consistent to ensure that storage is made available for the next precipitation event in a reasonable period of time.

RETENTION STANDARDS

Stormwater retention standards are commonly used in municipal stormwater applications to reduce overall stormwater volumes and the associated pollutant mass discharge. States and localities routinely select retention standards as the basis of municipal stormwater management requirements for new construction or redevelopment (see Box 4-1). The retention standards listed in Box 4-1 are specifically based on stormwater volume reduction in accordance with the maximum extent practicable standard for municipal separate storm sewer system (MS4) permits rather than water quality-based effluent limits (WQBELs). Water quality benefits will result due to the corresponding reduction in pollutant mass load. This approach specifically aims to reduce discharge loads, with less emphasis on specific pollutant concentrations. Other considerations also drive these standards, such as groundwater recharge. Possible application of retention standards in the regulatory context of the MSGP is discussed later in the chapter.

Stormwater retention systems are typically sized according to the retention standard and site-specific information such as the drainage area, the runoff coefficient (land use), and infiltration rate. The retention standard can be based on a specific design storm (see Box 4-2), under which the SCM is expected to operate at full efficiency. Retention systems would capture all of the small and mid-sized storms up to a specific design storm and a portion (usually the initial fraction) of the largest storms, resulting in capture of a large fraction of the overall runoff volume and corresponding contaminant load. Events larger than the targeted storm event and some smaller events that enter the storage when it is not completely empty (such as back-to-back rains) will result in overflow of the storage system and discharge of stormwater and industrial pollutants.

Depending on the retention system design, pollutant concentrations in the bypass flow may be less than those that occur early in the storm because the bypass will occur only after substantial prior rainfall, when runoff concentrations are typically lower.

Retention standards may also include a requirement of how long the captured stormwater may be stored prior to infiltration or beneficial use, which affects the volume available to capture any subsequent storms. The 2018 amendment to the 2014 California Industrial General Permit requires that discharge reduction SCMs be sized for an 85th percentile, 24-hour storm as a daily volume for on-site retention and infiltration or beneficial use, meaning that the captured stormwater would need to infiltrate or be used on site fully within 24 hours. Sites with insufficient infiltration rates to meet this requirement can increase the size of their retention storage (California Water Boards, 2018). The recommended maximum storage time for stormwater may be influenced by local rainfall conditions (e.g., frequent back-to-back storms) or concerns over vector (primarily mosquito) control.

MERITS OF AND CONCERNS ABOUT RETENTION FOR INDUSTRIAL STORMWATER

Many states and local governments have developed regulations requiring retention in all new development or significant redevelopment (EPA, 2016c; see Box 4-1). These states, such as Minnesota, New Jersey, and Washington, promote retention by including descriptions of proper infiltration methods in their stormwater manuals. Some states, such as California and Oregon, have developed specific requirements for industrial stormwater retention if it is used as part of stormwater management (OR DEQ, 2017; California Water Boards, 2018). Widespread interest in stormwater retention has mostly focused on common municipal stormwater source areas, such as

roofs, parking lots, and roads. Stormwater runoff from industrial facilities, in contrast, differs in its greater potential number of contaminants that pose a risk to groundwater and their higher concentrations. This section examines the merits and concerns when using retention standards for industrial stormwater.

Merits of Retention

Retention of stormwater is a proven way of reducing the impacts of urbanization on the natural hydrological cycle of a watershed, with benefits for surface water quality and flows. If contaminants in stormwater are treated prior to infiltration or adsorbed or filtered out by the soil matrix upon infiltration, stormwater retention reduces the mass of contaminants discharged to surface water (see Box 4-3).

Some bypass will occur in the late stages of retention of extreme storms, and the amount of bypass would be largely determined by the design storm used to size the retention system (see Box 4-2). The amount of bypass is also affected by the infiltration rate, which can change over time. The retention system can be designed to retain the first flush and, if its capacity is exceeded, bypass runoff will occur later in the storm. If a first flush of pollutants typically occurs at the site, capturing the first part of the runoff with an empty retention system provides a higher proportional mass removal compared to an equivalent volume captured later in the storm event (Han et al., 2006b).

Stormwater retention, by preventing discharge, also reduces the peak rates of runoff. High flow rates and magnitudes of stormwater are common in developed areas that have a significant percentage of impervious surfaces. These high stormwater flows can modify or destroy natural habitats due to erosion, known as hydromodification, and cause flood damage. It is generally desirable to reduce the maximum rate of runoff, and, in some cases, regulatory agencies have required controls that reduce peak flow rates. Stormwater retention with infiltration also increases groundwater recharge, which increases base flows in streams, providing ecological benefits, reduces saltwater intrusion in coastal areas, and potentially benefits water availability for water supply. Given that many industrial sites are highly impervious and may be quite large, if retention is feasible after consideration of the appropriate restrictions, retention could reduce the runoff from a substantial number of acres in a watershed.

Concerns Associated with Retention

When evaluating the potential for stormwater retention at an industrial facility, extreme caution should be used to ensure that infiltration does not result in groundwater contamination or mobilization of existing soil or groundwater contamination. Many common pollutants found in stormwater, such as heavy metals and toxic organics, have some mobility in the soil column (Armstrong and Llena, 1992; Clark et al., 2010; Treese et al., 2012). Without appropriate treatment, as well as spill prevention and containment, industrial stormwater retention can lead to groundwater contamination well beyond the site boundary that is difficult and costly to remediate.

A large percentage of the U.S. population depends on groundwater for water supply, and groundwater contamination of aquifers used as water supplies can cause major health risks. Groundwater contamination from stormwater infiltration has been documented in various locations around the country. For example, groundwater was contaminated with organic chemicals from stormwater from two industrial sites in Florida (Pitt, 1996) and from drywell infiltration of stormwater (Edwards et al., 2016).

Even when retention systems are designed to protect groundwater quality, systems can fail if not designed or maintained appropriately. Failure can occur because of inadequate information on soil infiltration rates, improper retention basin sizing for the design storm, or insufficient treatment and/or pretreatment. Neglecting the appropriate maintenance protocols that enable the infiltration system to function as designed can also lead to failure.

Concerns over potential groundwater contamination have led some states, such as Minnesota and Wisconsin, and some authors (e.g., Pitt, 2011) to suggest limiting the use of retention for industrial stormwater or simply prohibiting the infiltration of industrial stormwater in most cases. Wisconsin prohibits the infiltration of industrial stormwater, with the exception of rooftops, no-exposure facilities, and parking areas of Tier 2 (light) industries (Wis. Admin. Code NR [Natural Resources] § 212.21). Minnesota’s stormwater manual prohibits stormwater infiltration at “potential stormwater hotspots” that might have the potential to produce relatively high levels of pollutants in the case of spills, leaks, or illicit discharges, including storage areas, refueling areas, vehicle storage, and material transfer areas.¹ California allows infiltration of industrial stormwater in its general permit and includes state groundwater protection requirements for on-site compliance (California Water Boards, 2018).

To protect groundwater and surface water, states will need the regulatory authority to address failures in maintenance or performance of industrial stormwater retention facilities. However, not all states have the authority to manage groundwater quality and may lack enforcement capacity if contamination occurs. Because of the potential risks to groundwater, industrial stormwater infiltration is not recommended in these states.

Another disadvantage of retention and infiltration basins is the large amount of land required. Existing industrial facilities may not have available and suitable land in which to construct an infiltration basin, and major retrofits are costly. Retention and infiltration are more likely to be useful for new facilities, where construction would be less expensive.

Infiltrated stormwater also has the potential to mobilize existing contaminants in the subsurface. Extensive infiltration can cause existing groundwater contamination plumes to migrate, thereby shifting or spreading their adverse impacts. This is a particular concern in highly industrial areas, which are likely to have existing contaminant plumes in the subsurface. Infiltration can also cause local or regional groundwater mounding that could saturate contaminated soils currently above the saturated zone or under a protective cap, resulting in a release of stored pollution.

A final challenge is that the regulatory framework under the MSGP requires that discharge from these retention facilities, which is expected to occur only under the heaviest storms, comply with the benchmarks. This is a deterrent to use of retention systems for industrial stormwater, because bypass that exceeds benchmark thresholds under high flow conditions may result even after substantial investments to construct such systems that reduce overall pollutant loads. This issue is discussed in more depth at the end of the chapter.

CONSIDERATIONS FOR RETENTION AT INDUSTRIAL SITES

Successful use of retention/infiltration at an industrial facility for treatment of industrial stormwater depends on a full understanding of the characteristics of the potential stormwater pollutants, selection and thorough evaluation of the infiltration site, and appropriate use of treatment technologies as needed. Certain pollutant or site characteristics will make retention and infiltration inappropriate or cost prohibitive.

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¹ See https://stormwater.pca.state.mn.us/index.php?title=Potential_stormwater_hotspots.

Pollutant Characteristics

A key factor that distinguishes industrial stormwater management from typical urban stormwater management is the range of potential pollutants and the likelihood of elevated concentrations. The occurrence and concentrations of stormwater pollutants can vary widely by industrial sector or even individual facilities, based on the materials and chemicals used on site. Therefore, before stormwater retention and infiltration are considered, expected stormwater pollutants at a site should be carefully assessed.

Several pollutant characteristics affect the contamination risks associated with infiltration:

• Abundance (high concentrations and high detection frequencies) in stormwater,
• Toxicity,
• Mobility in subsurface soils where infiltration will occur, and
• Persistence.

Contaminant abundance in site runoff and the toxicity of those contaminants are initial considerations in an evaluation of the suitability of retention and infiltration. If the aquifer is (or could be) used as a water supply or is hydrologically connected with such an aquifer, groundwater pollution is a particular concern because of the human health risks. Contaminated groundwater is notoriously difficult to treat due to the inaccessibility and corresponding lack of knowledge about the pollutant sources, aquifer travel pathways, and possible pollutant treatment mechanisms.

Special care must be taken when considering retention and infiltration of highly soluble pollutants with low adsorption to geomedia or vadose zone soils because these pollutants can be highly mobile. Low-molecular-weight polar compounds tend to be highly soluble and move rapidly in the soil column. Soluble pollutants will not be strongly affiliated with particulate matter and will not be significantly removed via sedimentation, filtration, or other particulate matter removal processes. Instead, chemically reactive filter media may be required to adsorb the pollutant. Water chemistry parameters such as pH, salinity, and hardness may also impact pollutant mobility (FAO, 2000). Examples of known groundwater contaminants from stormwater infiltration include nutrients, metals, organics, total dissolved solids/salts, and bacteria (Pitt, 1996; Datry et al., 2004; Boving et al., 2008; Weiss et al., 2008).

Persistence is an additional consideration. Chemicals likely to biodegrade in the subsurface to harmless byproducts would pose a much lower risk than chemicals that do not biodegrade readily and are likely to persist in groundwater for years. For example, simple hydrocarbons can readily biodegrade in aerobic environments, although maintaining aerobic environments in infiltration zones with extended times of inundation can be problematic. Highly chlorinated organic compounds, however, are resistant to aerobic degradations or may degrade to a product that is highly persistent (e.g., trichloroethylene to vinyl chloride). The biodegradation of some stormwater pollutants has been documented in SCMs, such as hydrocarbons in bioretention media (LeFevre et al., 2012a,b). However, reliance on biodegradation would need to be clearly demonstrated and routinely monitored if part of an industrial stormwater management strategy.

Pitt et al. (1994) identified municipal stormwater pollutants with the greatest potential adverse impacts on groundwater assuming sandy soils with high infiltration rates, low soil organic content, and low adsorption potential (considered a worst-case scenario for contaminant mobility). The listing is based on the pollutant information in municipal rather than industrial stormwater and, therefore, does not include all contaminants likely to be found in industrial stormwater. However, it serves as a general guideline for the types of contaminants that pose concerns for retention and infiltration. The contaminants of moderate risk to groundwater from surface infiltration of municipal stormwater included

• Organic compounds, such as low-molecular-weight polycyclic aromatic hydrocarbons (e.g., pyrene and fluoranthene);
• Nutrients, such as nitrate; and
• Chloride from road salt.

These chemicals all have moderate to high mobility in soils. In municipal environments, heavy metals, such as lead, zinc, and copper, tend to occur in low concentrations and sorb to surface soils and sediments and

therefore would be less likely to reach groundwater through infiltration (Dechesne et al., 2004), unless the pH is very low. There are concerns, however, when the groundwater table or a perched lens is near the surface (Squillace et al., 1996; Datry et al., 2004) or when the background soil has a measurable metals content. For example, Ku and Simmons (1986) noted measurable concentrations of chromium in groundwater below stormwater infiltration facilities. Also, deicing salts have the potential to enhance the transport of metals in the subsurface (Kakuturu and Clark, 2015). Extrapolating risk from data from municipal stormwater to industrial settings must be done carefully, considering the many differences in abundance, contaminant occurrence, and site operation.

Pitt et al. (1994) offered general guidelines for infiltrating industrial stormwater to minimize risk to groundwater, assuming that specific evaluation of contaminant mobility and/or treatment is not provided to remove the pollutants:

• Runoff from industrial areas with substantial outdoor storage or with substantial uncovered outdoor operations with heavy machinery use should not be treated by infiltration. Such sites may be expected to have high concentrations of soluble pollutants and potentially wide varieties of contaminants (especially organic compounds). Although much is now known about organic chemical fate and transport, many emerging compounds still have poorly understood treatment, mobility, and toxicity characteristics.
• Runoff from critical source areas, such as vehicle service facilities and large parking areas, require adequate (pre)treatment to reduce groundwater contamination potential before infiltration.
• Snowmelt should be diverted from infiltration devices because of its potential for having high concentrations of soluble salts that are effectively transported through soils to the groundwater. In soils containing clay or high organic matter content, salt can also reduce the soil permeability and render infiltration devices inoperable.

Industrial stormwater containing pollutants with low toxicity, low concentrations, and limited mobility in the subsurface environment will pose the lowest infiltration contamination risks.

Site Suitability

The suitability of a site for detention/infiltration will depend on the stormwater management and treatment processes envisioned on the site. A site evaluation would include determination of the suitability of the site for infiltration and an assessment of the risk of groundwater contamination.

Much work has gone into defining the appropriate physical characteristics of municipal stormwater and highway runoff infiltration systems (NASEM, 2015; WDNR, 2017).² Determining the suitability of a site will include conducting infiltration rate measurements of the native soils. The average infiltration rate will determine the size of the device relative to the size of the drainage area. Many existing industrial sites may lack the land to site an appropriately sized infiltration basin; some sort of subsurface infiltration gallery with underground storage may instead be employed (see Figure 4-1b).

Information about depth to groundwater or perched lenses, depth to bedrock, soil properties, and existing subsurface infrastructure is also necessary. Many industrial areas are located near waterfronts or in low-lying areas, where infiltration would be inappropriate if the depth to groundwater is shallow. Some states specifically prohibit municipal stormwater infiltration systems where depth from the bottom of the infiltration system to groundwater (i.e., seasonal high water table) or bedrock is low (e.g., 3 feet in Minnesota [MPCA, 2015]; 2 feet in Pennsylvania [PA DEP, 2006], 10 feet in Orange County, California [County of Orange, 2013]). Additionally, if the soil texture will not support sufficient infiltration rates or if the rate is too high, leaving inadequate contact time for treatment, the site is not suitable without soil amendments. Some states also prohibit stormwater infiltration if the groundwater is protected as a drinking water supply.

Soil chemical properties, such as soil organic matter content and soil cation exchange capacity (CEC), will control the attenuation characteristics of stormwater pollutants. Adsorption of pollutants, under equilibrium conditions, can be described by a partitioning coefficient, K_d, which describes the ratio of the concentration of

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² See also https://www.pca.state.mn.us/water/minnesotasstormwater-manual.

pollutant adsorbed to the concentration in solution. The higher the K_d, the greater extent the pollutant will be adsorbed to the soil and the less will remain in solution where it could be transported to groundwater. Values of K_d depend on pollutant characteristics but also on the water chemistry and characteristics of the adsorbent. The sorption of hydrophobic organic compounds will primarily depend on the organic matter content of the soils, with K_d linearly related to the soil fraction organic matter (Schwarzenbach et al., 1993). Specific chemical characteristics of the natural organic matter will play a minor role in the adsorption of pollutants because most natural organic matter has a variety of sorption and ion-exchange sites. Soils with low organic matter content would not be expected to provide significant attenuation and removal of organic pollutants. Values of K_d for various pollutants have been tabulated based on soil properties (e.g., Sauvé et al., 2000). Contact time with the soil is another important parameter that influences removal of pollutants. Although K_d can provide a gross estimate of potential removal given sufficient contact time, the time-based interaction of pollutants with soil will determine the actual fraction of pollutant removed by the soil. Thus, slower infiltration rates will usually result in higher fractions of pollutant removal than faster infiltration rates.

Pollutants, such as heavy metals, metalloids, and phosphorus, will adsorb onto soils via specific bonding mechanisms with chemical sites on the soil matrix. Important factors controlling K_d include the CEC, hydrous oxide content, clay content, and organic matter content. The stormwater pH and other chemical parameters can be controlling factors for attenuation of ionic pollutants (Stumm and Morgan, 1995). Soils with low CEC, hydrous oxide content, clay content, and organic matter content would not be expected to provide significant removal of ionic pollutants. Phosphorus removal will only occur if the background phosphorus level in the soil is low. Many common organic soil amendments have high phosphorus contents, resulting in phosphorus leaching rather than removal. Metals may also sorb to colloidal material or form complexes with organic or inorganic ligands, which can enhance their transport in the subsurface (Fein, 1996; Nowack et al., 1997). These processes and their impact on removal are poorly understood in stormwater.

Unless pretreatment is provided to reduce all pollutants below levels of concern, dry wells or subsurface injection are not appropriate for industrial stormwater infiltration because these systems provide little to no removal of contaminants. Pitt and Talebi (2012) found no statistically significant concentration reductions in stormwater contaminants (nutrients, heavy metals, pesticides, herbicides, bacteria) after infiltrating through at least 4 feet of underlying rock and soil beneath dry wells. Dry wells are only appropriate for disposal of high quantities of water that are of good quality and, as such, are unlikely to be appropriate for industrial runoff.

In addition to site-level analyses, regional analyses of potential effects on stormwater infiltration on existing soil or groundwater contamination may be needed. To reduce the likelihood of mobilizing existing contaminants, known soil contamination sites and groundwater contamination plumes in the region should be inventoried, and the potential impacts of increased groundwater levels should be carefully examined.

On-Site Treatment Options

Removal of particulate matter from runoff is necessary for any infiltration system at an industrial facility. Particulate matter removal protects the system by reducing the risk of the infiltration system media clogging, and it also removes the fraction of influent pollutants that are associated with those particles.

If the infiltrating soil characteristics are insufficient to remove the anticipated stormwater pollutants before they reach groundwater, a wide range of additional treatment options can be employed (see Box 1-1). The treatment performance of conventional treatment SCMs for industrial stormwater is summarized by Clark and Pitt (2012) and discussed in Chapter 2 and Appendix D. Soluble pollutants can be difficult to remove, unless an absorbent highly specific to that chemical is used. Extrapolating performance of SCMs from municipal stormwater to industrial settings where pollutants and concentrations are not comparable will require careful analysis of the unit processes themselves and their treatment efficiencies across a wide range of concentrations and water chemistries.

Any treatment of industrial stormwater will result in accumulation of the removed industrial pollutants in

the SCMs. Less-mobile pollutants, such as lead, copper, zinc, and hydrophobic organic contaminants, will generally accumulate in sediments at the point of retention and infiltration or sorb onto geomedia (DiBlasi et al., 2009; Jones and Davis, 2013). Persistence of pollutants in the shallow soil varies depending on the contaminant and the local conditions. Depending on pollutant toxicity and mobility, these soils/sediments may need to be managed to control risks to human health and the environment.

Models such as the Seasonal Soil (SESOIL) compartment model can be used to simulate the water transport, sediment transport, and the fate of the pollutants in the subsurface beneath infiltration facilities. SESOIL has been used to support performance results from dry pond industrial stormwater infiltration (Eppakayala, 2015) and as a screening tool to evaluate groundwater contamination potential of infiltrating MS4 stormwater (Clark and Pitt, 2007).

Infiltrating industrial stormwater can carry high risks. Risks to groundwater from infiltration of industrial stormwater can be greatly reduced by requiring that infiltrated water meet stringent water quality requirements, such as those for drinking water, as defined by the Safe Drinking Water Act. This recommendation would put numeric limits on many pollutants of concern, including many heavy metals, a number of synthetic organic compounds, and nitrate. The use of drinking water standards as cleanup goals for contaminated groundwater is well established. The 2018 amendments to the California equivalent of the MSGP allows infiltration of industrial stormwater if the water meets drinking water quality standards by the time it reaches the base of the unsaturated zone (California Water Boards, 2018). California’s amended permit includes all primary maximum contaminant levels (MCLs)³ as well as secondary standards for total dissolved solids, chloride, specific conductance, and sulfate.⁴ However, drinking water standards may not provide a sufficient screening tool because many industrial chemicals that may be highly toxic are not regulated under the Safe Drinking Water Act. EPA’s drinking water Contaminant Candidate List 4 should also be considered when assessing risks of infiltration to groundwater.⁵ If pollutants on this list (but not regulated under the Safe Drinking Water Act) or emerging chemicals of concern to human health are present in stormwater, careful consideration of pollutant removal or treatment options is needed.

In lieu of other information on contaminant attenuation in the groundwater of an industrial site, the committee recommends that industrial stormwater infiltrated to groundwater be treated to meet primary drinking water standards for inorganic chemicals and organic chemicals, and secondary standards for chloride and total dissolved solids. If the aquifer is not suitable for use as a public water supply, this requirement could be relaxed with concurrence of state and local public health agencies. Additionally, other pollutants of concern that may not currently be regulated by the Safe Drinking Water Act should be treated to drinking water risk levels. The industrial facility would need to ensure that this level of quality is met through monitoring, either before the stormwater is applied to the infiltration area or after passing through the infiltration/treatment media at the base of the unsaturated zone.

Some degree of stormwater treatment, possibly advanced treatment, would be required at most industrial sites to meet drinking water quality standards. This may include adsorption of toxic organic compounds via activated carbon or another specialty adsorbent. If the stormwater exceeds drinking water limits for total dissolved solids, chloride, specific conductance, and/or sulfate, costly technologies, such as reverse osmosis or other desalination processes, would be required, likely making infiltration economically unfeasible.

Requiring that infiltrating stormwater meet drinking water standards holds industries to a higher infiltration standard than MS4s. However, such requirements acknowledge the wide range of pollutant types, concentrations, toxicities, and properties expected in industrial stormwater. Stormwater from areas that are not part of the industrial activity would not have to meet the drinking water requirement to be infiltrated; segregation of such stormwater is highly encouraged.

EPA guidelines for infiltrating industrial stormwater would help ensure that industries implement this stormwater management option in a way that is effec-

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³ See https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations.

⁴ See https://www.epa.gov/dwstandardsregulations/secondarydrinking-water-standards-guidance-nuisance-chemicals.

⁵ See https://www.epa.gov/ccl/chemical-contaminants-ccl-4.

tive in reducing surface water pollution while being protective of groundwater. Such guidance would ideally include the necessary treatment options and costs for different pollutant source areas, considering concentrations, toxicity, persistence, and potential for adsorption onto or ion exchange with the geomedia. The potential for dilution or attenuation in the subsurface could also be addressed.

Design Considerations

Readily available stormwater manuals (e.g., CASQA, 2010) provide details on the proper installation of infiltration systems. Additionally, a growing body of practitioners has experience and knowledge to evaluate where industrial stormwater retention and infiltration retention systems are appropriate. To evaluate the feasibility of retention, a number of data sets are required. As discussed previously, knowledge of the stormwater contaminants (types and concentrations) is necessary on both a chronic and an episodic basis. Site conditions, including soil properties, depth to groundwater table, rainfall information, and land availability, are also required. If these properties are favorable, then a preliminary design of a retention system can made. Details must include a design storm and consideration on how that design affects compliance with benchmarks for any bypass.

Many design models, such as WinSLAMM and P8, are available to add confidence to the sizing of an infiltration system. These models can describe both the retention and water quality benefits of an infiltration system. The models can maximize the benefits of a system by accounting for important variables, such as soil type, drainage area size, and rainfall patterns.

REGULATORY CONTEXT FOR RETENTION STANDARDS

Retention with infiltration is an attractive method for stormwater control from industrial facilities when the contaminants do not pose a risk to groundwater and where land is available to install infiltration SCMs. In general, hydrological and statistical methods and data are available (or could be readily obtained) for determining retention requirements to achieve specific objectives for pollutant mass reduction, given site-specific information. Given the site-specific nature of local rainfall patterns and stormwater production and quality, however, it is not possible to recommend a nationwide standard for retention.

As described in Chapter 1, the MSGP must be written to include technology-based effluent limits (TBELs) and WQBELs. Therefore, if numeric retention standards were to be included in the MGSP, it would be within the context of functioning as a TBEL or WQBEL, which is notably distinct from how retention standards have been applied in MS4 permits, which has been in accordance with the maximum extent practicable standard. Given the site-specific nature of the suitability of retention with infiltration at industrial sites, numeric retention standards as a TBEL could not be established in EPA’s MSGP or as best-available technology in an effluent limitation guideline. However, retention with infiltration is already an appropriate allowance within the 2015 MSGP requirement to “select, design, install, and implement control measures (including best management practices) to minimize pollutant discharges” (EPA, 2015d). Because retention with infiltration reduces the overall volume of a discharge, it is an effective means to minimize pollutant discharges through reduction in pollutant mass.

Nonetheless, because of the variable nature of rainfall and stormwater, no retention system can be constructed to contain all stormwater from all events. In some cases bypass discharges that occur in storms beyond the design storm size may be below benchmark thresholds, and in those cases there is a high level of assurance that the discharge that relies on infiltration as a treatment SCM also complies with WQBELs. In other cases, the bypass concentration may exceed the respective benchmark, which will be problematic to industrial facilities desiring to implement retention/infiltration, triggering corrective actions. Some degree of regulatory relief during large-event bypass would need to be implemented to encourage industrial stormwater retention where it is safe and appropriate. The most significant incentive would be assurance that installation of a well-designed retention system provides relief from the corrective action process associated with episodic results above benchmark thresholds associated with bypass.

At least one state has recognized the benefits of industrial stormwater retention in reducing the

pollutant load on water bodies and has adapted its permit to facilitate the practice. In Oregon, permittees can request a mass reduction waiver if they have implemented stormwater retention with infiltration or beneficial use, if these practices can be shown to reduce the mass discharge of pollutants below the equivalent mass discharge of the benchmarks. Permittees are required to provide data and analysis of this mass discharge reduction and to take corrective actions by reviewing their SCMs and whether additional pollution controls are needed (OR DEQ, 2017).

EPA could encourage infiltration by specifically addressing the uncertainty associated with bypass during events that exceed design conditions. If retention systems are relied on to meet a WQBEL, a wet weather accommodation could be included that considers dilution or assimilative capacity during extreme storms (see Chapter 3). Allowable frequencies of stormwater discharge at levels above benchmark thresholds could be derived from allowances of frequencies of exceedance of water quality criteria and the duration of exposure upon which the criteria are based. EPA could also develop a water quality standard exceedance allowance for extreme weather events or, as was contemplated in the past (EPA, 1995), establish separate water quality criteria for wet weather events.

If EPA wants to encourage the use of retention with infiltration as a means to reduce pollutant loads and peak flows, assuming infiltration is suitable based on groundwater considerations, it should develop additional guidance on appropriate design storm standards, perhaps in consideration of regional precipitation patterns. Additionally, EPA should develop guidance and cases studies for demonstrating through the Additional Implementation Measure process that discharges above a benchmark threshold that occur only in storms larger than the design storm do not result in an exceedance of water quality standards.

CONCLUSIONS AND RECOMMENDATIONS

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. 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 and 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, receiv-

ing 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 TBEL, industrial stormwater must also comply with WQBELs, 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 bypass that exceeds benchmark thresholds. 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 thresholds, 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 case 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.

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