Flooding appears to be the worst problem left behind by Hurricane Irene, and especially in big cities, those floodwaters are likely to be full of raw sewage discharged by overwhelmed city sewer systems.
Believe it or not, this is by design. Many older cities in the Northeast, including Philadelphia, New York, and Boston, have what are called combined sewer systems, which carry both sewage and stormwater to water treatment facilities. When they fill up from heavy rains -- like the more than eight inches dropped in many places across the Northeast and Mid-Atlantic by Irene -- these "combined sewage overflows" are designed to dump untreated wastewater (meaning human sewage) into local waterways.
That means flooded communities have to worry about more than just rising water after Irene. The floods could bring significant health risks too. A study published last year in Environmental Health Perspectives found an 11 percent increase in pediatric emergency room visits for acute gastrointestinal illnesses after heavy rainfall events. Another study found high concentrations of human adenovirus, which can cause upper respiratory tract infections, in wastewater discharged into a Michigan River after heavy rainfalls.
Recreational activity, such as wading, swimming, or boating in contaminated rivers, increases the health risks from combined sewer overflows, but untreated wastewater can also pose a risk to drinking water. In 1993, more than 400,000 people in Wisconsin were sickened during a Cryptosporidium outbreak that coincided with heavy rains and record high flows in the Milwaukee River. That’s why many Northeastern communities are telling residents to boil drinking water in Irene’s wake, in order to kill possible pathogens.
In the U.S. each year, 85 billion gallons of untreated sewage and wastewater enters the nation’s lakes, streams, and coastal waters as a result of combined sewer overflows. If you filled Olympic-sized swimming pools with all that sewage and laid them out end to end, they would circle the earth 1.6 times.
Wastewater dumped into rivers and streams is often the product of aging infrastructure (some cities still use sewer system components that are over 100 years old) and increased urban runoff from industrial, commercial, and residential development, which has led to more impervious concrete (meaning rainwater can’t soak into it) and less green space.
Though retrofitting an aging sewage system isn’t cheap, increasing "green infrastructure" can help bring down the costs and avoid flooding in heavy rains (although Irene was still beyond what most systems could handle). Philadelphia, which the New York Times said was probably the big city hit hardest by Irene, is doing just that. Earlier this year, Philadelphia received federal approval for a 25-year, $2.4 billion initiative to reduce combined sewer overflows by transforming one-third of its impervious paved areas into green acres.
As I learned in my research for a two-page infographic in OnEarth’s latest issue (see "INFOGRAPHIC: The Philadelphia Story"), green tools, such as rain gardens, porous pavement, and rooftop gardens can reduce the annual amount of rainfall running into city sewers by 80 to 90 percent. Plants and trees can absorb rainwater in their roots or allow it to evaporate from their leaves and branches, while porous pavement will filter rainwater into underground stone reservoirs that provide temporary storage before the water enters the soil underneath.
Though green infrastructure will go far in stemming run-off from most heavy rain events (which may become more common in the coming years due to climate change), larger storms, such as Hurricane Irene, will still overwhelm the system. However, when you start talking about green improvements on a regional scale -- urban sprawl around Philadelphia and even further afield in the Delaware River basin in New York and New Jersey can have a big impact on places like downtown Philly -- "the preservation of natural green infrastructure can make a huge difference even for very large storms," says Larry Levine, senior attorney with NRDC’s water program.
Philadelphia expects green infrastructure improvements within the city limits to soak up about an inch of rainwater, the equivalent (by current definitions, at least) of a five-year storm event; the Federal Emergency Management Agency said Irene represents a 100-year flood for Philadelphia’s Schuylkill River. Still, green infrastructure is a step in the right direction as floods become more routine.
Image: Flooding on the Schuylkill River in Philadelphia, by Dave Levitan