21 July 2012

Urban Stormwater and the US EPA

Urban stormwater was one of the focus subjects of my MS program in Civil Engineering - Hydrology at Colorado State University. Part of that focus was because of my own advisor, who came to academia from a long career in private practice as a civil engineer with his own focus on stormwater pollution abatement. His experience, including growing up in Colorado and his time in professional engineering, was highly informative to my own interests in the combination of atmospheric science and meteorology with hydrologic science and engineering. In much of the US West, cities are relatively young enough that scientists, engineers and civic leaders came to an adequate understanding of stormwater problems at an early stage of development and could help shape the course of construction from that time forward.

The result was the Municipal Separate Storm Sewer System (MS4 in the engineering and policy parlance), "separate" because it was constructed on its own and apart from the sanitary sewer system. Streets and storm drains in developed communities are the largest sources of stormwater in cities and cover very large areas of the urban environment. With that coverage comes numerous impacts on the receiving waters (streams and rivers, lakes, a bay or estuary, and sometime the oceans) to which the urban runoff is discharged during storm events. Since that stormwater comes from such a large and distributed area, the pollution is considered "non-point source." This is in contrast to sanitary sewer systems that collect pollutants and contaminants from "point sources" (homes, offices, businesses) for treatment before discharge, urban practices that can be traced back more than 150 years. Now that I think of it again, that would be a good topic for another post on the history of water quality and human health... remind me of that if I haven't posted it in the next couple weeks, would you?

From City of Philadelphia, Combined Sewer Overflow Program
Anyway, in the United States our principal policy and regulatory body for urban stormwater runoff standards is the Environmental Protection Agency (EPA). The EPA was established in 1970 in a flurry of laws including the Clean Air Act and Clean Water Act, as well as the omnibus National Environmental Policy Act (NEPA) that codified the process of Environmental Assessment. The 1972 Clean Water Act legislation established the National Pollutant Discharge Elimination System (NPDES), a program administered by the EPA originally to address the treatment of point-source wastewater before discharge to receiving waters throughout the country and later expanded to include non-point-source municipal stormwater discharges. The federal EPA enacts minimum standards for the quality of discharges and the health of the receiving waters, and various state-level environmental protection agencies throughout the country each have their own authority to enable more exacting and strict standards, according to their own development and environmental situations. For example, cities and counties in the interior western US are much more likely to have criteria for urban stormwater management that are separate from their standards for sanitary sewers, because they have addressed the disparate problems of these two issues early in their developmental history. Some urban communities with exemplary stormwater standards in the US West include Denver CO, Maricopa County AZ (including the Phoenix metro area), and Clark County NV (including the Las Vegas metro area). Much of this work is now known as "green infrastructure."

From City of Philadelphia, Combined Sewer Overflow Program
In much of the eastern US and in older cities on the west coast, however, there has persisted a different approach to stormwater as a legacy of long-term, historical development and the now-outdated idea that urban runoff is a nuisance, to be disposed of as quickly as possible. Just as home wastewater is carried away in the interest of personal and public health, urban planners saw those same sewer systems as the ideal conveyance for nuisance stormwater in order to protect people and property. The result was a proliferation of "combined sewers" that handled both sanitary and stormwater discharges. Many cities in the eastern US still have these, even as some (like Philadelphia, as shown in these figures) are beginning to build MS4s. In the portions of the cities with older systems, both sanitary and storm sewers are routed through the municipal wastewater treatment process before discharge to receiving waters. At least that is the idea in relatively dry periods, with only small storm events contributing to the system.

In large events, however, these systems become overwhelmed with stormwater and, mixed with raw sewage, the excess that cannot be handled by the treatment works is discharged (untreated) to the receiving waters. This is known as a "combined sewer overflow" (CSO) event and was simply accepted as a consequence of system design. A certain level of pollution from CSO events was considered "acceptable" and built into these systems, despite its impacts on the health of the receiving waters. It was often rationalized that the drinking water treatment facilities in downstream communities would keep them safe, especially as almost all of out water in the US is treated to minimum drinking water standards no matter the intended use. Otherwise, the pollution remained out of sight and was eventually discharged to the ocean, where it was considered an infinitesimal source of contamination in such a large ecosystem.

Storm drain stencil, from Squidoo.com
We know now that things are different. The impacts of development on these natural systems, for a long time perceived as negligible, are now coming to be recognized by a slow-growing minority as unsustainable and ultimately limiting to the collective health of humans and our environment. In the US, the EPA maintains an extensive (though underfunded) grant program to help urban communities throughout the country to maintain and upgrade their stormwater systems as part of municipal efforts at meeting NPDES standards. Earlier this week, the EPA announced nearly US$1 million in grants to 17 communities in 16 states for such stormwater improvements.

Some of these grants are going to communities that maintain MS4s and are looking to improve the management of that stormwater with efforts at pre-treatment, sometimes called "best management practices" (BMPs), as well as education and other non-structural approaches. Some non-structural efforts include school and public education programs, establishment and review or improvement of development standards, hazardous waste reclamation programs, and the storm-drain stencils as shown above that improve public awareness of connections between their built and natural environments. The EPA also assists in the dissemination among communities of knowledge on both structural and non-structural measures with a BMP Database that includes design and performance information on a wide variety of constructed stormwater management measures. When you see trees and rain gardens and runoff-fed plantings in densely developed cities, grass and gravel islands and permeable pavement in suburban parking lots, or a small grass swale or pond in a city park, there is a good chance that their design and maintenance were informed by a growing legacy of use elsewhere and experiences passed along through such collections as the BMP Database, and a near-certainty that such measures conform to the NPDES program.

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