Opening the Floodgates

Regulation drives a surge in innovative solutions for stormwater treatment

• By Dan Cobb, PE, Vaikko Allen
• Jan 01, 2003

Non-point source pollution has emerged as the leading cause of pollution threatening the health of watersheds around the country. Typical urban runoff transports pollutants such as trash, hydrocarbons (including motor oil and gas), sediment and countless human-made (anthropogenic) chemicals to our lakes, rivers and oceans. The magnitude of this problem is becoming more widely understood as pollutant transport modeling is improving. For example, a recent article published by the National Academy of Sciences attributes 85 percent of anthropogenic oil pollution in North American oceans to consumption related spills. A primary subset of this source class is leakage at automobile refueling stations and improperly maintained cars and trucks.

As a result of widespread public education efforts, an informed base of concerned citizens and community groups who are aware of the health-related dangers associated with stormwater runoff is developing. Over the past 30 years, federal, state and local regulations have become increasingly stringent about the need to capture and treat non-point source pollution. Most of the local and state regulations have spawned from the Clean Water Act, passed in 1972, which was designed to eliminate discharge of pollutants into the nation's bodies of water.

Stormwater Discharge Regulations
As authorized by the Clean Water Act, the National Pollutant Discharge Elimination System (NPDES) permit program protects the water quality of the nation's water bodies by regulating point sources that discharge pollutants into U.S. waters. Although stormwater runoff is considered to be a non-point pollution problem on a watershed scale, discharges from individual sites that are conveyed by pipes or ditches, or are otherwise concentrated, are regulated as point sources. This distinction means that most stormwater discharges require coverage by an NPDES permit. NPDES Phase II regulations become effective in March 2003, and will extend coverage to include all construction activity disturbing between one acre and five acres.

In addition, NPDES Phase II regulations will require between 3,000 and 4,000 urbanized municipalities to develop comprehensive stormwater management plans to help ensure that water bodies within their jurisdiction are not adversely impacted by watershed development. The Clean Water Act requires that "non-attainment" waters in each state -- those bodies of water that fail to meet quality standards associated with their designated uses -- be identified and that Total Maximum Daily Loads (TMDLs) be developed for them. Municipalities commonly incorporate the TMDL promulgation process into their citywide Phase II stormwater management programs. The result is a set of specific numeric standards regulating the discharge of all pollutants contributing to non-attainment.

To date, over 22,000 U.S. water bodies have been identified as impaired and over 5,200 TMDLs have been developed.

In many urban watersheds with impaired waters, regulating new development is insufficient to restore water quality to its target level. In these cases, additional treatment must be provided by retrofitting existing sites with best management practices (BMPs). As development pressure drives up land values and as BMPs and the general public are forced to share the same space, traditional land-based BMPs, such as stormwater ponds, become impractical.

Stormwater management philosophy is continually evolving as new information becomes available. Now, more than ever, stormwater management is a multidisciplinary effort drawing on the expertise of urban planners, biologists, civil engineers, hydrologists and many other professionals. New methods of tracing pollutant sources and better monitoring technologies have made the connection between stormwater quality and receiving water quality indisputable.

The result is that stormwater managers strive to develop treatment, storage and infiltration technologies into their designs that mimic the predevelopment water balance. The most common technologies or BMPs typically mimic or utilize natural systems, such as localized depressional storage, infiltrating soils and gravitational settling. Detention and retention basins, artificial wetlands, swales and rain gardens are frequently incorporated into site designs where land is available, but increasingly their use is precluded by space considerations.

Hazards of Traditional BMPs
When properly designed and maintained, land-based BMPs are relatively effective and require minimal up-front investment. The failure of these systems is often an insidious process, and the results can be catastrophic. Natural systems that are not maintained (and in practice, few are) can become problematic because some pollutants have a tendency to accumulate in concentrations that far exceed healthy levels for most biota. The result is low species diversity and high concentrations of heavy metals and anthropogenic chemicals in plant and animal tissue. Frogs, birds and fish that can adapt to the polluted environment may themselves become vectors for pollutants or cause other environmental problems. Sediments may also become highly contaminated and cause toxic releases if disturbed during extreme weather events or other highly turbulent activities. In order for pollutants and excess nutrients to be completely removed from the environment, aquatic plants must be harvested and removed and sediments must be periodically excavated.

Ponds pose health and safety issues to human populations, too, as they become storage areas for debris, oil and trash carried by stormwater runoff. Often, vegetation and other biological development in ponds is controlled through treatment of fertilizer, pesticides and herbicides. Whether or not there is visible trash or an oil sheen, the health and safety risks associated with ponds have led institutional sites, such as schools and churches, to begin discouraging the use of ponds due to liability issues.

Traditional stormwater treatment systems, such as ponds, require that valuable acreage from new developments be set aside for stormwater treatment. This leaves less buildable land available for developers. At the local level, stormwater management planning is often a sticking point for permitting, as landowners and planners work to help protect existing developed property from flooding and other potential risks.

New Options Reduce Risks
In many ways, new proprietary BMPs are able to meet new stricter requirements by mimicking those natural systems in place before development. All major pathways for precipitation, excluding evapo-transpiration, can be approximated by manufactured technologies that have very little impact on the buildability of a site. Chambers and large diameter pipes can be installed below paved areas to provide detention and infiltration. Hydrodynamic separators and catch basin inserts can intercept the majority of sediment, oil, grease and trash as they convey water off the site. Filter technologies may target metals, nutrients and trace chemicals.

In more urbanized areas where land is scarce and values are high, regulators and developers are increasingly turning to proprietary, manufactured solutions to help remove pollutants and manage flows. These manufactured BMPs can be implemented with far less land than a pond requires, sometimes opening up land that was previously considered undevelopable. They can be designed more easily amidst site constraints, such as existing utilities, vehicle traffic areas, right of ways, buildings and other structures. All of these features maximize land values and encourage dense development -- which could prove a valuable tool in battling the sprawl that has begun to plague many metropolitan areas.

Because structural BMPs are typically installed below grade, they present little public health risk. The danger of accidental drowning associated with urban ponds is all but eliminated. Trapped pollutants are isolated out of sight and out of contact with the rest of the environment. In areas where petroleum product spills are likely, BMPs can be designed to capture and store significant volumes. Perhaps most significantly, these systems can typically be easily maintained to help ensure that the pollutants are removed from the environment.

Manufactured systems are produced under controlled conditions making consistency in performance and product quality much more reliable than site-constructed land-based systems.

Demonstrating the Results
Engineers specifying these systems are anxious to see data to support the performance claims of proprietary BMPs. Most of these BMPs are designed to achieve annual net removal of 80 percent of total suspended solids (TSS), which is the default standard recommended by the U.S. Environmental Protection Agency (EPA). Demonstrating this removal efficiency has proven especially challenging in the field, where accurate sampling is difficult and expensive, and variations in methodology make comparisons a true "apples to oranges" undertaking. The transferability of data is further complicated by variations between regions and site use -- even if an accurate result is gained from one site, there is no guarantee that result applies to any other site, unless there is a sufficient model for performance prediction that can adjust for variations between sites. As the "one size fits all" 80 percent TSS reduction standard is replaced by more watershed specific performance goals, BMP design flexibility becomes more important.

In order to integrate urban stormwater BMPs into a watershed-wide pollutant load-reduction plan, detailed information about the catchment contributing runoff to the BMP and about the performance of the BMP itself is required. Although this information is typically sought through field-testing, which can be an inefficient and inaccurate process, it is probably best collected in a laboratory setting. In the lab, parameters that affect performance, like pollutant load variations, BMP scale and flow rate can be isolated and analyzed. A rigorous field-testing is then needed to validate performance estimates that are based on that laboratory data and site information. Site owners and contractors also bear some of the responsibility for meeting long-term goals by ensuring that the systems are installed as designed and maintained regularly.

Meeting the Goals
Non-point source pollution is not an inevitable consequence of development. We can limit the effect of new urban development on the natural environment by using low-impact development techniques that preserve the functioning of natural systems. In addition, existing developments that do not fully address stormwater quality issues can be retrofitted to reduce the overall impact of urban development. The ability of affected communities to do this by successfully designing and implementing the stormwater pollution prevention plans required by Phase II NPDES depends on the coordination of scientists, engineers, BMP providers, contractors and site owners.

Structural BMPs will play an increasingly important role in these plans as land values continue to skyrocket and concerns about health and safety and aesthetics increase. We look forward to meeting that demand with innovative products tailored to the diverse water quality goals of Phase II communities.

This article originally appeared in the 01/01/2003 issue of Environmental Protection.