As communities add thousands of new BMPs, they need to keep objectives and secondary effects in sight.

By Marshall Taylor and Jaime Henkels

Since November 1990, when EPA promulgated the first phase of the National Pollutant Discharge Elimination System (NPDES) stormwater regulations (Phase I), a large amount of data and experience has been obtained on the impact of runoff and the effectiveness of runoff controls. The Phase I program directly impacted 173 cities, 47 counties, and approximately 40 additional local and state governmental departments (USEPA, 1999[a]). To date, more than 250 Phase I local government permits have been issued (Figure 1). Those permits address the operations of 800-plus governmental units.

Figure 1. Required Permits

Number of local government permits required under NPDES Phase I and Phase II stormwater rules.

As a result of Phase I and the growing attention to stormwater issues, tens of thousands of postconstruction runoff control best management practices (BMPs) have been installed. In the engineering staffs of many municipalities and state departments of transportation, there has been a noticeable shift from designs that take a relatively narrow "drainage" view to those that provide for runoff control features such as in right-of-way detention, filtering, and biological treatment. In some cases the design shifts have been small: for example, placing an emphasis on the use of grass swales instead of concrete or rock-lined drains. Still, the net impact has been to raise awareness and to train many planners and engineers in some of the principles of improved stormwater management. Many Phase I cities and counties have established stormwater utilities charged with meeting the Phase I permit requirements. In carrying out that charge, those utilities have significantly increased local government awareness and understanding of stormwater issues.

EPA understood the limitations of the available information on the impacts and sources of urban nonpoint-source pollution when it designed the Phase I program. While requiring some immediate actions, such as prohibiting nonstormwater discharges into stormwater systems, EPA used Phase I as a mechanism to collect data and increase the state of knowledge about the problems. Specifically, Phase I permittees were required to:

• identify nonpoint sources,
• develop meteorological data,
• estimate volumes and quality of stormwater discharges,
• prepare lists of receiving water bodies and compile information on the impacts of stormwater runoff on those water bodies,
• characterize discharge quality and the areas producing that discharge,
• develop estimates of annual pollutant loads and event mean concentrations,
• develop management programs that would reduce pollutants in runoff,
• monitor the implementation of the management program and its impact on water quality.

Drainage problem or nonstructural BMP? A shift in thought about drainage and stormwater retention will help meet Phase II objectives.

Today’s planners and engineers are benefiting from the data and experience generated by the Phase I program. A number of studies have used the information coming from Phase I, including studies summarized in previous issues of this magazine, to help us better understand the impacts of stormwater runoff and to design structural and nonstructural means of reducing or mitigating those impacts.

EPA, under a consent decree, promulgated the NPDES Phase II rule in 1999. Phase II expands federally mandated stormwater controls to about 3,500 local governments. It further expands controls during construction activities to sites equal to or greater than 1 ac. (down from 5 ac. in Phase I). This part of the program is estimated to affect about 110,000 construction sites per year (USEPA, 1999[a]).

The final size of the program is likely to be significantly larger than what was federally mandated. The Phase II rule provides designated NPDES state programs with the authority to add communities to the program. Also, in compliance with the Clean Water Act, Phase II provides for citizen petitioning for community designation. In North Carolina, the Division of Water Quality has proposed state rules that would designate 20 counties, which were not federally designated as "urban counties." The state rules would require these countries to obtain Phase II permits (North Carolina Division of Water Quality, 2001). Additionally, some citizen and environmental groups are preparing to petition for the designation of additional communities along impaired and threatened waterways.

Many states, recognizing the need to address their current lists of impaired waters, or 303(d) lists, and to meet antidegradation water-quality goals, will likely adopt a similar broadening of the program. Finally, for more than 18% of the states’ nearly 22,000 impaired waters, urban runoff and other nonagricultural nonpoint-source pollution are listed as the leading sources of impairment (USEPA, 2001). Many of the local governments around those impaired waters will eventually be added to the Phase II program. Although there is a large amount of overlap between runoff-impaired waters and the Phase I and II designated communities, total maximum daily load (TMDL) implementation plans will likely add hundreds of communities to the Phase II program during this decade.

The challenge of urban stormwater management is to make the built environment function reasonably similar to the undeveloped watershed.

This discussion leads to a logical conclusion that this decade will see an order of magnitude increase in the rate of installation of structural and nonstructural BMPs. The data gathered during the past 10 years should provide us with a strong technical foundation on which to plan, design, construct, and maintain stormwater controls. Before we develop and begin implementing our Phase II program, however, we should carefully examine what our goals might be and consider the secondary impacts that our actions might produce.

Phase II Objectives

The overall objective of the Phase II stormwater rule is simply to provide a mechanism by which the nation can reach the "fishable and swimmable" goals of the 1972 Clean Water Act. That objective can be restated pragmatically as:

1. eliminating listings for the water bodies on each state’s 303(d) list solely or substantially as a result of stormwater runoff, and

2. providing a regulatory and technical program that will prevent degradation of water quality from stormwater runoff to a degree that additional water bodies become impaired.

While there is much discussion about the most appropriate mechanisms to obtain those objectives, it is important for designated and potential Phase II municipalities to remember those are the two objectives. They are objectives around which public consensus can be constructed. Complying with the Clean Water Act requires meeting the first objective. Developing and implementing a stormwater program that meets the second objective will both reduce a municipality’s long-term costs and provide a healthier and more stable environment for community development.

Although they seem simple, meeting these objectives can be quite complex. For example, Brian Bledsoe and Chester Watson point out that an increase in watershed imperviousness of only 10-20% could result in geomorphic instability in a watershed’s streams (Bledsoe and Watson, 2000). Resulting increased sediment transport and stream entrenchment can cause dramatic changes in the hydrological and ecological function in a watershed. For areas where the imperviousness has surpassed those levels, it can be very difficult to find artificial means (i.e., structural BMPs) to restore the watershed to a stable and desirable regime.

Overall, urbanization creates significant changes in surface-water and groundwater hydrology, water quality, and ecology. To accomplish the objectives of the Phase II program, we must find ways to effectively and cost-efficiently restore some of the lost function.

The Center for Watershed Protection in Ellicott City, MD, (www.cwp.org) states the challenge illustrated in Figure 2. To the extent practicable, we need to use planning and engineering to make our highly altered watersheds function as they did before substantial development. To the extent that we succeed, we will correct, or mitigate, many of the water quantity and quality problems prevalent in our watersheds. Success requires two things:

1. Think about our efforts at the watershed scale.
2. Understand the primary functions and secondary impacts, both positive and negative, of the BMPs we employ.

BMP Functions and Impacts

Stormwater detention ponds can have a mix of beneficial and negative impacts upon the local stream ecology.

Space does not allow an examination of the likely hydrological and ecological impacts of all the potential BMPs. In addition, the ability of any BMP to effectively perform its intended functions always depends on the specific conditions at the implementation site. The remainder of this article will document some of the hydrological, ecological, and human environment impacts of some of the common structural BMPs, and then present a more detailed look at potential impacts of a wet detention pond designed to control runoff from a small watershed area.

Infiltration Systems. Infiltration systems are designed to reduce the quantity of stormwater runoff from a site and to improve the quality of the runoff through filtering. The systems reduce the runoff during storms and increase groundwater recharge. They help to restore some of the natural infiltration capacity lost from paving and soil compaction in developed areas. One benefit of infiltration systems is that they increase soil interflow and groundwater flow and extend the base flow of adjacent streams. This can result in cooler stream temperatures and a beneficial impact on downstream aquatic life. The lengthened flow path, through a soil medium and particularly through deep-rooted vegetation, can create a significant reduction in the dissolved nutrients delivered to surface waters. Nutrients and other pollutants attached to soil particles are effectively filtered from the runoff. Infiltration systems also can provide effective bacteria filtering. Infiltration systems should be used as near the source of runoff as possible and might not be appropriate where there is concern about groundwater pollution or a rise in the local groundwater level. Infiltration systems can be designed to be unobtrusive in the landscape. In general, they have little impact on the human environment and present no unavoidable health or safety risks.

Filtration Systems. Filtration systems use a natural or artificial filtering material to trap particles and absorb specific pollutants. These systems are particularly useful in dense industrial and commercial areas. Because of their cost, they are usually designed to be effective only for low flow rates and provide little or no filtration for larger runoff events. They are often used for pretreatment of runoff delivered to a detention pond or an infiltration system. Filtration systems generally have a direct beneficial impact on water quality but have very limited usefulness in restoring a watershed’s hydrologic function. Except in situations where specific toxic materials are effectively filtered from the runoff, filtration systems have not been shown to have significant ecological benefit. Filtration systems that remove sand and larger particles from runoff but allow small particles to pass through the system can have a negative ecological impact. Filtration systems require a diligent maintenance program in order to provide their design functions.

Bioretention Areas. This category of BMP includes large areas set aside for bioretention, vegetated swales, riparian buffers, grass filter strips, rain gardens, depressed road center strips, parking lot islands, and similar areas. These reconstructed "natural areas" provide many of the hydrological and ecological benefits of infiltration systems. In addition, they provide terrestrial and aquatic habitat. If well designed and properly maintained, bioretention areas are aesthetically pleasing and can contribute to property values. The Center for Watershed Protection and the Chesapeake Bay Local Assistance Department have compiled a number of case studies that effectively document the beneficial impacts and development cost savings related to development and preservation of bioretention areas (Center for Watershed Protection, 2002[a]).

Detention Ponds. Wet and dry detention BMPs have received the most study in terms of their impacts on both watershed hydrologic function and water-quality impacts on receiving waters. Although design criteria may be selected based on, for example, reduction of downstream peak flow and stream energy during high-frequency storm events, detention BMPs can have implications beyond their designed functions (Center for Watershed Protection, 2002[b]). Detention ponds can affect these parameters:

Stream Temperature: Change in stream temperature is a function of pond detention time and surface area. Increases in water temperature affect the composition of the macroinvertebrate communities at the bottom of a stream’s food chain, with repercussions felt throughout. Detrial processing, respiration, bacterial growth, and aquatic community reproduction are all affected by variations in stream temperature. Detention ponds that provide for increased infiltration can mitigate rises in stream temperature by increasing the sustained delivery of cooler groundwater base flows.

Dissolved Oxygen: As a result of increased detention time and increases in water temperature, dissolved oxygen content generally decreases in detention ponds. There is a tradeoff in the impact on downstream waters. First, consumption of oxygen that might have occurred in the receiving stream might occur instead within the detention pond. This can serve to limit the effect of high oxygen demands by geographically limiting the area where those demands are met and can decrease the deposition of oxygen-demanding sediments in the receiving waters. However, without incorporation of aeration facilities in the detention pond or incorporation of reaeration components in the pond’s outlet structures, a detrimental zone of sustained low oxygen can be introduced downstream of the detention pond.

Primary Productivity: Water detention in nutrient-rich wet ponds promotes the growth of algae. Increased alga growth might have multiple impacts on the aquatic system and on the effectiveness of the BMP’s design functions.

Amphibian Community: Extended detention ponds can provide amphibian habitat if habitat requirements are addressed during the design. Wet ponds provide the moisture necessary for frogs and salamanders. Off-stream dry ponds provide poor amphibian habitat because water is available sporadically. In one Maryland study, the green frog and bull frog preferred wet detention ponds, and the Fowler’s toad showed preference for short detention ponds with nearby stream habitat (Bascietto and Adams, 1983). Amphibian diversity can be achieved by providing a range of habitats with shallow pools, gentle slopes, dense emergent vegetation, and adjacent forest and stream habitats.

Metal and Pesticide Bioaccumulation: Detention ponds promote sediment deposition. Metals and pesticides attached to soil particles can accumulate in the sediment of detention ponds. These pollutants can be reintroduced into the food chain if they are ingested by macroinvertebrates. Metals and pesticides bioaccumulate up the food chain, leading to concerns of contaminated fish. Depending on the upstream nonpoint sources, it might be necessary to manage harvesting of fish from detention ponds for human consumption to avoid potential human health risks.

Macroinvertebrate and Insect Community: When macroinvertebrate communities are compared above and below detention ponds, population densities remain similar; however, species diversity decreases downstream of the detention pond. Pollution-sensitive macroinvertebrate species are often lost in a reach downstream of detention ponds. This often can be attributed to characteristics of the pond discharges, such as water temperature, dissolved oxygen, and nutrient concentrations. Typically a detention BMP induces higher local insect populations. In some areas the insect population might be both a nuisance and a risk to public health. A number of pond management techniques are available to mitigate this impact. The treatment of detention ponds with larval insecticides should be avoided.

Fish Community: Creating detention ponds can also create barriers to fish movement, preventing migration and recolonization in upstream disturbed areas. A warm-water fish community might develop within the detention pond, and the introduction of exotic species might be a concern. Potential migration of these species to other areas of the watershed should be considered.

Bacteria: Detention ponds can provide a significant die-off of bacteria associated with human and animal contamination of water. They also might attract geese and other waterfowl and wildlife that are important contributors to bacterial contamination.

In addition to the these hydrological and ecological impacts, detention ponds have an obvious impact on the landscape. In many cases, the ponds contribute to a pleasant human environment. If poorly designed or maintained, however, they can become a nuisance and a risk to public health and safety.

All of these items should be considered before site selection and design of a detention BMP. Table 1 is an annotated example of a worksheet that can be used to aid the consideration of the many effects of a BMP. The purpose of the worksheet is simply to encourage a watershed-scale review and assessment of secondary effects of specific BMP design and management plans. It separates effects into those associated with delivery of BMP discharges to receiving waters and those that occur as a result of installation of the BMP itself.

Conclusions

The next two decades will see the design and installation of many BMPs for stormwater control and treatment. Many local governments will be tempted to take a narrow view of what it means to comply with NPDES Phase II, simply searching for cost-effective means of implementing EPA’s six minimum implementation measures (USEPA, 1999[b]). A wiser strategy will be to focus on the two objectives stated above.

Meeting the goals of the Clean Water Act and the NPDES Phase II stormwater rule will require careful planning, design, and maintenance of a wide range of nonstructural and structural BMPs. Although continued research is needed into the pollutant removal mechanisms and efficiencies and into the secondary impacts of BMPs, adequate progress has been made to allow us to proceed with the task effectively and cost-efficiently.

References

Bascietto, J.J. and L.W. Adams. "Frogs and Toads of Stormwater Management Basins in Columbia, MD." The Bulletin of the Herpetological Society of Maryland, 19(2). 1983.

Bledsoe, Brian and Chester Watson. "Observed Thresholds of Stream Ecosystem Degradation in Urbanizing Areas: A Process-Based Geomorphic Explanation." Watershed Management 2000: Science and Engineering Technology for the New Millennium (M. Flug and D. Frevert, Eds.). Americans Society of Civil Engineers. 2000.

Center for Watershed Protection. "Better Site Design, An Assessment of the Better Site Design Principles for Communities Implementing Virginia’s Chesapeake Bay Preservation Act." 2000(a).

Center for Watershed Protection. "The Environmental Impacts of Stormwater Ponds, Watershed Protection Techniques." Articles 79 and 83. 2000(b).

North Carolina Division of Water Quality. "North Carolina Municipal NPDES Phase II Stormwater Program Strategy." April 11, 2001.

USEPA. "Preliminary Data Summary of Urban Stormwater Best Management Practices." EPA-821-R-99-012. August 1999(a).

USEPA. 40 CFR Parts 9, 122, 123, and 124. Federal Register, 64 (235). December 8, 1999(b).

USEPA. "The National Costs of the Total Maximum Daily Load Program" (Draft Report). EPA 841-D-01-003. August 1, 2001.

Marshall Taylor is water resources project manager and Jaime Henkels is an environmental specialist with HDR Engineering Inc. of the Carolinas in Charlotte, NC.

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