types of stormwater hotspots
Figure 1:

Five types of potential stormwater hotspots include commercial, industrial, institutional, municipal and transportation related land use. Source: University of Minnesota Design Center

Potential Stormwater Hotspots (PSHs) are defined as commercial, industrial, institutional, municipal, or transportation-related operations that produce or can produce relatively high levels of stormwater pollutants. PSHs include locations where there is a potential risk for spills, leaks or illicit discharges. It is important to note that designation as a PSH does not imply that a site is a hotspot, but rather that the land use and associated on-site activities have the potential to generate higher pollutant runoff loads compared to other land uses. Designation as a PSH serves as a useful reminder to designers and reviewers that more careful consideration of the site is warranted. Ultimately, a PSH site designation may dictate that certain practices and/or design criteria are promoted or discouraged.

Overview and designation of PSHs

PSHs can be categorized by different land uses. Five land use categories that may be classified as PSHs include commercial, industrial, institutional, municipal, or transportation-related operationser. A description of the major land use category is provided below. Note that some of these land uses fall under the requirements for Phase II NPDES industrial stormwater permits.

Commercial PSHs

Commercial PSHs consist of a small group of businesses associated with a specific activity or operation that generates higher pollutant loads in a subwatershed. Examples include:

  • animal care services
  • building material
  • commercial car washes
  • convenience stores
  • laundries and dry cleaners
  • lawn care companies
  • gas stations
  • nurseries and garden centers
  • petroleum wholesalers
  • fast food restaurants
  • shopping centers
  • vehicle maintenance and repair
  • wholesale food and beverage

Each kind of commercial hotspot generates its own blend of stormwater pollutants, which can include nutrients, hydrocarbons, metals, trash and pesticides. Commercial PSHs typically have a great deal of vehicle traffic, generate waste or wash water, handle fuel or repair vehicles, or store products outside. While commercial PSHs are quite diverse, they are often clustered together. Most commercial PSHs are unregulated, although a few are regulated under the NPDES industrial stormwater permit program by local ordinance or by federal/state law if they handle even small quantities of hazardous material.

Industrial PSHs

Industrial PSHs are a major focus for pollution prevention if they use, generate, handle or store pollutants that can potentially be washed away in stormwater runoff, spilled, or inadvertently discharged to the storm drain system. Examples of industrial land use activities include:

  • auto recyclers
  • boat building and repair facilities
  • recycling centers and scrap yards
  • warehouses

Each type of industrial PSH generates its own blend of stormwater pollutants, but as a group, they generally produce higher levels of metals, hydrocarbons and sediment. Many industrial operations are regulated under the NPDES industrial stormwater permit program, although individual owners or operators may be unaware of their permit status.

Institutional PSHs

Institutional PSHs include larger, privately-owned facilities that have extensive parking, landscaping, or turf cover. In addition, institutions may contain fleet vehicles and large maintenance operations. Examples include:

  • cemeteries
  • churches
  • colleges
  • corporate office parks
  • hospitals
  • private schools
  • private golf courses

By and large, institutional PSHs are not regulated. The most common pollutants generated by institutional PSHs are nutrients and pesticides applied to maintain grounds and landscaping. In addition, large parking lots can produce stormwater runoff and associated pollutants, and are natural targets for stormwater retrofitting. Institutional landowners can be important partners in subwatershed restoration, given the importance of their stewardship practices on the open lands they maintain.

Municipal PSHs

Municipal PSHs include many local government operations that handle solid waste, wastewater, road and vehicle maintenance, bulk storage areas for road salt and sand, and yard waste. Examples include: composting facilities

  • fleet storage and school bus depots
  • landfills/solid waste facilities
  • local streets and storm drains
  • pesticide use in rights-of-way
  • public golf courses
  • public schools
  • public works yards
  • maintenance depots
  • solid waste facilities
  • wastewater treatment plants

Many of these municipal operations are regulated PSHs in MS4 communities. Municipal PSHs must prepare the same pollution prevention plans and implement source control practices as any other regulated PSHs. Municipal PSHs can generate the full range of stormwater pollutants, including nutrients, hydrocarbons, metals, chloride, pesticides, bacteria, and trash. It is common in Minnesota for each municipality and many commercial centers to store a stockpile of road salt. Although these piles are generally not subject to regulation unless they cause a documented water quality problem, MS4 municipal programs should take responsibility for managing these piles in a pollution free manner.

Transport-related PSHs

Examples of transportation-related uses include:

  • airports
  • bus depots
  • rental car lots
  • railroad stations and associated maintenance facilities
  • ports
  • highway maintenance facilities
  • trucking companies and distribution centers

Many, but not all, transportation-related uses are regulated PSHs. They tend to generate higher loads of hydrocarbons, metals, and sediment in stormwater runoff, can be associated with large areas of impervious cover, and have extensive private storm drain systems. Fluid leakage from these sites can be a major source of contamination, as can the addition of sand and salt during the cold weather season. Road surfaces are not automatically considered as PSHs unless they have been shown locally to be such sources.

land use activities for stormwater hotspots
Figure 2:

Examples of potential pollutant generating land uses and land use activities. Note the list is not all-inclusive. Each of these examples is included in the bulleted list within the discussion for the five land use categories (commercial, industrial, institutional, municipal, and transport related.

Pollutant Generating Operations/Activities

common operations to assess pshs
Figure 3:

Six common operations for assessing PSHs include management of turf, waste, loading docks, downspouts, vehicle fueling, and parking lots.

Perhaps of more significant consideration is an understanding of the types of pollutant generating activities that commonly occur in association with various PSH operations. Six common operations and a subset of related activities that can contribute to stormwater quality problems at a site are discussed below.

types of pollutants associated with pshs
Figure 4:

Stormwater pollutants associated with common operations at potential stormwater hotspots. Source: Scheuler et al., 2004

Vehicle operations

Nearly all PSHs devote some portion of the site to vehicle operations such as maintenance, repair, recycling, fueling, washing or long-term parking. Vehicle operations can be a significant source of trace metals, oil, grease, and hydrocarbons, and are the first operations inspected during a hotspot source investigation. Vehicle maintenance and repair operations often produce waste oil, fluids and other hazardous products, particularly if work areas are connected to the storm drain system. Routing protective rooftop runoff through a fueling area has become a common practice in Minnesota; simple re-routing of runoff away from a potential fuel wash-off location could eliminate this from the hotspot list.

Outdoor materials

Most PSH sites handle some kind of material that can create stormwater problems if not properly handled or stored. The first step is to inventory the type and hazard level of materials at the site. Next, it is important to examine loading and unloading areas to see if materials are exposed to rainfall and/or are connected to the storm drain system. Third, any materials stored outdoors that could potentially be exposed to rainfall or runoff should be investigated. Public and private road salt and sand storage areas are of particular concern for this category.

Waste management

Every business generates waste as part of its daily operations, most of which is temporarily stored at the site pending disposal. The third common hotspot operation involves the way waste products are stored and disposed of at the site in relation to the storm drain system. In some sites, simple practices such as dumpster management can reduce pollutants, whereas other sites may require more sophisticated spill prevention and response plans.

File:Example of dumpster management at psh.png
Figure 5:

Dumpster leaking directly into a storm sewer.

Physical plant practices

The fourth hotspot operation relates to practices used to clean, maintain or repair the physical plant, which includes the building, outdoor work areas and parking lots. Routine cleaning and maintenance practices can cause runoff of sediment, nutrients, paints, and solvents from the site. Sanding, painting, power-washing, resealing or resurfacing roofs or parking lots always deserves particular scrutiny, especially when performed near storm drains.

Turf and landscaping

The fifth common hotspot operation involves practices used to maintain turf or landscaping at the site. Many commercial, institutional and municipal sites hire contractors to maintain turf and landscaping, apply fertilizers or pesticides, and provide irrigation. Current landscaping practices should be thoroughly evaluated at each site to determine whether they are generating runoff of nutrients, pesticides, organic carbon, or are producing non-target irrigation flows.

Unique hotspot operations

Some operations simply resist neat classification, and this last category includes unique sites known to generate specific pollutants. Examples include swimming pools, construction operations, golf courses, fairgrounds/racetracks, marinas, hobby farms, and restaurants.

A particular PSH of concern is associated with salt storage and the environmental threats that result from our need as a state for safe winter roads. Water quality problems from very soluble Na, Cl and cyanide have been documented as resulting from stored salt piles. MPCA does not regulate the storage of salt unless the storage becomes a documented contamination problem. Instead, the state encourages all public and private entities storing salt to follow the Salt institute's recommended BMPs, which include such things as covering, impervious pads and drainage routing. MS4 communities are asked by MPCA to include a salt management component in their municipal pollution prevention programs.

Stormwater management design at PSHs

Understanding the types of future operations expected to occur on a site helps designers develop a more thoughtful stormwater management and pollution prevention plan for a given site. This approach provides more flexibility in terms of what stormwater treatment approaches are appropriate for different portions of a site. Runoff management at PSHs should also be linked to the pollutant(s) of greatest concern in the subwatershed. Similarly, understanding the pollutants potentially generated by a site operation provides designers with important information on proper selection, siting, design, and maintenance of the nonstructural (e.g., source control or pollution prevention) and structural practices that will be most effective at the PSH site.

example of pollution prevention practices
Figure 6:

Examples of three common pollution prevention practices include wash water containment, secondary containment of outdoor storage and covered loading areas.

The most cost effective approach to managing stormwater at potential hotspot sites is to employ a variety of non-structural pollution prevention, and source control measures. To do this effectively, it is necessary to have a thorough understanding of a site and the respective areas of the site where specific operations will occur. Hogland, et al. (2003) suggest most of the following principles for design:

  • Develop detailed mapping of the different areas of the site along with associated planned activities and the preliminary drainage design.
  • Separate hotspot activity areas from non-hotspot activity areas, if possible.
  • Prevent or confine drips and spills.
  • Enclose or cover pollutant generating activity areas and regularly provide cleanup of these areas.
  • Provide spill prevention and clean-up equipment at strategic locations on site.
  • Provide pre-treatment and spill containment measures such as catch basins and inserts, oil-water separators, etc.
  • Strategically locate slopes and separation berms to prevent co-mingling of dirty and clean runoff.
  • Retain and reuse stormwater for irrigation, wash down water, or other onsite uses.
  • Maintain equipment to minimize leaks.
  • Train and educate employees, management and customers.

Meeting the design intent of the non-structural practices above typically involves simple and low-cost measures to address routine operations at a site. For example, the non-structural design components for a vehicle maintenance operation might involve the use of drip pans under vehicles, tarps covering disabled vehicles, dry cleanup methods for spills, proper disposal of used fluids, and covering and secondary containment for any outdoor storage areas.

Each of these practices also requires employee training and strong management commitment. In most cases, these practices save time and money, reduce liability and do not greatly interfere with normal operations. A more complete summary of 15 basic pollution prevention practices applied at PSH operations is provided in Table 1 (Schueler et al., 2004).

Table 1

Pollution prevention practices for PSH operations (Source: Scheuler et a., 2004).

PSH operation Profile sheet* Pollution prevention practices
Vehicle maintenance and repair H-1 Drip pans, tarps, dry clean-up methods for spills, cover outdoor storage areas, secondary containment, discharge washwater to sanitary system,proper disposal of used fluids, disconnect storm drains, automatic shutoff nozzles, signs, employee training, spill response plans
Vehicle fueling H-2
Vehicle washing H-3
Vehicle storage H-4
Loading and unloading H-5 Cover loading areas, secondary containment, storm drain disconnection or treatment, inventory control, dry cleaning methods, employee training
Outdoor storage H-6
Spill prevention and response H-7 Inventory materials, employee training, spill planning, spill clean up materials, dumpster management, disconnect from storm drain or treat. Liquid separation/containment
Dumpster management H-8
Building repair and remodeling H-9 Temporary covers/tarps, contractor training, proper cleanup and disposal procedures, keep wash and rinse-water from storm drain, dry cleaning methods
Building maintenance H-10
parking lot maintenance H-11
Turf management H-12 Integrated pest management, reduce non-target irrigation, careful applications, proper disposal of landscaping waste, avoid leaf blowing and hosing to storm drain
Landscaping/grounds care H-13
Swimming pool discharges H-14 Varies, depending on the unique hotspot operation
Other unique hotspots H-15

* Due to the volume of material, the reader is referred to Scheuler et al. (2004) to see the profile sheets. Each profile sheet explains how the practice influences water quality and lists the type of PSH operation where it is normally applied. The sheets also identify the primary people at the hotspot operation that need to be trained in pollution prevention. Next, each sheet reviews important feasibility and implementation considerations and summarizes available cost data. Each profile sheet concludes with a directory of the best available internet resources and training materials for the pollution prevention practice.

It should be noted that the profile sheets developed by Scheuler et al. (2004) are written primarily from the perspective that the site(s) in question is an existing site and pollution prevention measures are recommended as a retrofit approach. Designers of new sites, however, can still use the guidance effectively.

Wright et al. (2004) provide a detailed description of the rapid field assessment protocol for identifying PSHs and the appropriate pollution prevention practices for the activities causing pollution. The protocol is known as the Unified Subwatershed and Site reconnaissance (USSR) and the PSH assessment is called a Hotspot Site Investigation. These methods are not directly applicable to greenfield development or redevelopment situations; however, they have significant application for NPDES Phase II communities that are working towards compliance with Minimum Control Measures 1, 2, 3, and 6 (public education and outreach, public participation/involvement, illicit discharge detection and elimination, and pollution prevention/good housekeeping, respectively),

After considering the non-structural elements to incorporate into a site based on its layout and proposed operations, designers need to assess what structural practices will be most appropriate given site constraints while providing the greatest pollutant loading reductions for targeted pollutants. Table 2 presents representative pollutant removal data for common PSH pollutants of concern as a function of practice group. Details on BMP design and performance occur in design and fact sheets in Chapter 12.

It is often receiving water designation or watershed classification that will drive the criteria and associated practices that are acceptable for use. However, by virtue of being a PSH there are a set of general guidelines to always consider when designing structural stormwater management systems. The following should be carefully considered by designers when specifying and siting practices at PSHs.

  • Convey and treat the mostly clean runoff separately from the dirty runoff.
  • Infiltrate only the mostly clean water.
  • Pre-treatment, pre-treatment, pre-treatment. This includes oversizing sediment trapping features such as forebays and sedimentation chambers; incorporating appropriate proprietary and nonproprietary practices for spill control purposes and treatment redundancy; oversizing pre-treatment features for infiltration facilities such as swales, filter strips, and level spreaders; and ensuring full site stabilization before bringing practices online.
  • Consider closed systems with liners, under-drains, or comparable safeguards against infiltration for practices that manage dirty waters.
  • Locate practices offline and minimize offsite run-on with appropriate diversions.
  • Establish rigorous maintenance and inspection schedules for practices receiving the dirty waters.
  • For ponds and wetlands, over-design by between 10-25% the allowable storage volume for sediment accumulation over time if sediment is a problem.

Table 2

Percent removal of key pollutants by practice group.

Practice Total nitrogen (%) Metals (%)1 Bacteria (%) Hydrocarbons (%)
Detention ponds 25 60 702 802
Wet ponds 30 60 70 802
Stormwater wetlands 30 40 752 852
Filtering practices and bioretention 35 65 452 802
Infiltration practices3 50 802 NA NA
Vegetated swales and grass channels4 802 60 NA NA

1 Average of zinc and copper. Only zinc for infiltration. 2 Based on fewer than five data points (e.g. independent monitoring studies) 3 Includes porous pavement as primarily a volume reduction BMP 4 Higher removal rates for dry swales NA: data not available Removals represent median values from Winer (2000)

Infiltration practices are the practice group that requires the most scrutiny prior to implementation at a PSH. A conservative approach would avoid the use of infiltration practices at a PSH; however, with appropriate site and conveyance design it is possible for the designer to incorporate infiltration into many sites to treat areas sufficiently separated from pollutant generating activities. Most other practice groups should be acceptable for use in treating PSH runoff, so long as appropriate design modifications are incorporated. Most design modifications are simple and in the form of enhanced pre-treatment, over-design, or design redundancies. Others are added features that limit the likelihood of ground water recharge. For example, practice groups such as bioretention, ponds and wetlands that receive runoff from pollutant generating activities should be designed with the necessary features to minimize the chance of ground water contamination. This includes using impermeable liners. The use of ponds and wetlands without liners should also be avoided where water tables are shallow and the practice would likely intercept the water table.

Importance of plan review at proposed PSHs

Ultimately, the level of safeguards that are in place when providing stormwater management at PSHs should be related to the expected review process. Communities that can allocate adequate and qualified staff to effectively review all stormwater management plans for proposed PSHs can arguably provide designers with great flexibility as to how to meet the management criteria required at a site. In these cases, designers should have most of the accepted stormwater treatment practices at their disposal for implementation. However, for communities that don’t have the resources to provide the necessary level of site and stormwater management plan review, a more conservative approach to allowable treatment practices should be taken.

In many cases, industrial PSHs will be covered by the NPDES industrial stormwater permit or by some other federal/state permitting program related to the materials they store or handle on site. Communities are encouraged to focus their attention on the unregulated PSH sites.

NPDES industrial stormwater requirements

Industrial PSHs that are regulated under NPDES stormwater permits must prepare stormwater pollution prevention plans or SWPPPs, and implement source control practices at the facility. These plans must include spill response and prevention, employee training, and implementation of pollution prevention practices to reduce exposure of products to rainfall or runoff. In some cases, stormwater treatment practices may need to be installed at the site to remove pollutants from runoff. Permitted industrial PSHs should be regularly inspected to determine if they are complying with the SWPPP, or even possess a permit. However, the MPCA does not inspect any facilities because of the staffing cut-backs it has experienced. In lieu of this, communities could conduct their own site visits as part of its local stormwater program. The storm drain system should also be investigated to determine if an industrial PSH is generating illicit discharges of sewage or other pollutants. Methods to detect and correct illicit discharges are described in Brown et al. (2004).

Industrial NPDES stormwater permits are an important regulatory tool at many PSH operations. Significant penalties can be imposed for non-compliance. State and federal regulators are still grappling with the administration of industrial stormwater permits, and they remain an imperfect tool for several reasons. First, the permit system allows potential hotspot operators to prepare and implement their own pollution prevention plans and to keep them on site rather than sending them to MPCA. If a particular plan is weak or is only a paper exercise, the Agency might never know until it is too late. Second, very few trained state or federal-level inspectors are available to inspect and enforce the thousands of industrial sites covered by the permit program. Third, although communities usually have the best understanding of how the local stormwater network works, they lack direct authority to inspect or enforce regulated PSHs, although they can refer them to state agencies for enforcement. Communities can also address these sites through other programs, such as zoning, stormwater utility or conditional use permits, and can address potential problems whenever new construction at the facility occurs. All three problems can be overcome if the locality works with industry and state regulatory agencies to share hotspot inspection and enforcement responsibilities as part of industrial permitting or MS4 programs. Portland (OR) recently negotiated such an agreement to expand the reach of its hotspot inspection program (Pronold, 2000).

From the regulated community standpoint, the lack of a viable, well funded state industrial stormwater permit program has resulted in uncertainty over regulatory status and frustration over paying an annual fee with no return. An industrial stormwater permit holder could conceivably have several different programs that address stormwater management, including local MS4 authority with its set of stormwater controls. Also, most industry handling polluting materials likely comes under the authority of related regulations, such as hazardous waste spill prevention or chemical storage laws, each of which could have a stormwater component.

Most industrial stormwater permit issues will not be solved until a viable state and federal regulatory program exists. Until this happens, communities and industrial permit holders are urged to work together to define problems and solutions within the SWPPP framework.

Guidance on infiltration of runoff from PSHs

Preventing or minimizing the likelihood of contaminated runoff from leaving a PSH site is the core objective of stormwater management at these sites. Introduction of contaminated runoff to the ground water is probably the greatest concern in developing effective stormwater management plans at PSHs. This is for three primary reasons:

  • Ground water contamination is hard to detect immediately and therefore can persist over long periods of time prior to any mitigation;
  • There is an immediate public health threat associated with ground water contamination in areas where ground water is the primary drinking water source, which is most of Minnesota; and
  • Mitigation, when needed, is often difficult and is usually very expensive.

This section focuses on these issues and presents a potential approach for establishing design guidelines for infiltration based on the six common operational areas presented in Table 13.6 plus a seventh area that addresses major transportation routes (e.g., highways). Figure 13.12 serves as a frame of reference for revisiting these areas.

Potential for PSH impact on ground water

source water protection areas
Figure 7:

January 2004 location of Source Water Protection Areas in Minnesota. The map includes vulnerability designations for Drinking Water Source Management Areas, ranging from very low to very high vulnerability, and designation of Source Water Assessment areas as vulnerable or not vulnerable.

Caution should be exercised when dealing with the introduction of stormwater runoff into the ground via infiltration systems or even low impact development-type techniques that encourage infiltration naturally. This issue gets particularly important when the infiltration occurs within a defined drinking water source area[1]. It is important to note that the map illustrating Source Water Protection areas shows only the public systems covered under the Minnesota Department of Health program. There are thousands of additional private and domestic wells that could be impacted by PSHs and not subject to any special protections against stormwater runoff.

Infiltration guidance at PSHs

Table 13.10 provides potential infiltration guidelines associated with each of the seven operational areas. Infiltration at PSHs relies on overall site design and facility operations management. Good design and committed, well-trained facility staff should make infiltration possible for certain areas of the site. Where uncertainty is present, designers should avoid infiltration practices. The Minnesota Department of Health recommends that infiltration should not be used within the one-year wellhead protection area and limited in vulnerable wells for the 10-year wellhead protection area.