Stormwater overflows don’t only occur during the summer

Rain storms in southeast Michigan in late November added 79 million gallons to the 2014 sewage overflow totals.

While many people breathed a sigh of relief that the storms that occurred on November 23 and 24, 2014 came down in the form of rain rather than snow, it did create additional environmental problems we probably would not have seen if it had been snow. Specifically, nearly 79 million gallons of sewage overflowed into Lake St. Clair alone from that storm, due to the massive amount of stormwater that resulted from these storms. Stormwater is water from any form of precipitation, rain, melting snow, hail or sleet that runs off rather than soaking into the ground.

In comparison, estimates indicate that nearly 10 billion gallons of sewer overflows poured into all of southeast Michigan’s waterways from the unprecedented flooding experienced across southern Oakland and Macomb counties on August 11-12, 2014. To put this massive amount into context, the U.S. Geological Survey suggests trying to visualize 20 million 50-gallon bathtubs filled with water.

There are two types of sewer overflows:

  • A combined sewer system means that both stormwater and sewage from homes and businesses are combined and sent to the wastewater treatment facility before discharging into a local body of surface water. Combined sewer overflows (also known as CSOs) result when the collection system is unable to handle a larger-than-normal volume of stormwater and wastewater, such as during a large precipitation event. The result is partially-treated sewage being directly discharged to surface water bodies such as rivers and lakes. Communities must obtain a permit with limits on how much partially (chlorine added and solids removed) treated sewage can be discharged.
  • Sanitary sewer overflows (also known as SSOs) result when wastewater collection systems which transport human waste are unintentionally discharged to surface water before the waste has been adequately treated. Normally, a discharge of raw sewage is illegal. However, sanitary sewer overflows can also occur during wet weather events, such as was experienced last August, when the system was not able to handle the amount of water coming into it. When this occurs, the system malfunctions and raw sewage overflows into lakes, rivers, road and basements.

Many factors can contribute to a sewage overflow. First, the sheer volume of rain in a short period of time can cause a sewage overflow. During the August storm, southeast Michigan received 5.5 inches of rain in three hours resulting in massive overflows and flooded streets and basements. A few years earlier, another 5.5 inch rain event occurred but that happened over three days and resulted in no flooded basements or streets and significantly less sewage discharges. Second, the amount of impervious surface in an area can contribute to a sewage overflow. A high percentage of impervious surfaces (sidewalks, roads, driveways, rooftops, etc) reduce the amount of infiltration into the ground and results in more runoff going into the system.

A third factor contributing to sewage overflows is the age of the infrastructure in Southeast Michigan. Southeast Michigan Council of Governments (SEMCOG) estimates that “60 to 70 percent of the existing sewer system was built before 1970 which makes it at the end or beyond its useful life.” Combining some or all of these factors leads to more frequent overflows in the region.

On a positive note, the state and region have done a lot to address some of the issues, which have resulted in fewer gallons overflowing than in the past. Statistics from the Macomb County Health Department, which compiles overflow amounts in Lake St. Clair, show that even with the catastrophic August storm and resulting overflows, 2014 does not hold the record for the highest number of gallons discharged. In 2011, overflows totaling 5.7 billion gallons were recorded while through mid-December, 2014, 3.72 billion gallons were discharged.

Many communities and states are focusing their efforts for stormwater management in the urban and rural areas rather than wastewater plant discharges. While this change is more difficult, it can have greater impact. Wastewater plant discharges come from only a few sources while stormwater pollution sources are every driveway, sidewalk, street, yard, rooftop and parking lot.

When the next large storm is predicted, a snowfall rather than rain may be preferable since snow can melt slowly and not overflow our sewage systems.

For more information on what you can do to help manage stormwater in your home landscape, read Part 1 and Part 2 series on “Managing stormwater in urban and rural areas.”

For more information on landscaping for water quality, read the Michigan State University Extension article on the introductory resource, “Landscaping for Water Quality: Garden Designs for Homeowners.”

Consult the Michigan Department of Environmental Quality (MDEQ) web site for the latest information on combined and sanitary sewer overflows and records by year of discharges.

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