Great Lakes Environmental Assessment and Mapping Project

Combined sewer overflows

US cities with combined sewers. Source: US EPA
US cities with combined sewers. Source: US EPA


Combined sewers frequently overflow during heavy storms because they are not designed to handle the large volumes of stormwater runoff. These overflows result in the discharge of untreated sewage into surface waters. In addition, stormwater may contribute pollutants such as oil, grease and metals from vehicles, fecal bacteria from pet and wildlife waste, and pesticides and lawn chemicals that accumulate on diffuse surfaces during dry periods.


Mapping CSOs as a Great Lakes stressor

  • We obtained data on CSO outfalls for US cities within 10 km of the Great Lakes from an EPA report.1
  • We used a combination of state agency reports and personal communications to determine CSO discharge volumes.
  • When discharge volume data were not available, we estimated discharge based on the number of outfalls present using a log-linear regression between discharge volume and number of outfalls (Adj R2 = 0.59).
  • Information on CSO contributions from communities and estimated discharge volumes for Canadian cities was obtained from MacDonald and Podolsky, 2009.2


We included data from 303 Chicago area CSO outfalls. Although Chicago’s 369 outfalls normally discharge away from Lake Michigan, heavy and sustained rainfall can cause 303 of Chicago’s outfalls to empty into the lake.


Although our data contain a number of uncertainties, we were able to estimate CSO discharge volume for 49 cities3 bordering the Great Lakes in millions of gallons per year, averaged over time from 2005 to 2008.


CSO discharge volumes were propagated by assuming their influence declined to 10% at 3.5 km from an outfall and were negligible beyond 7 km.

Spatial distribution of CSOs as a stressor in the Laurentian Great Lakes. (Inset: Southern Lake Michigan).


Data Sources: 

1. U.S. Environmental Protection Agency. 2001. Report to Congress: Implementation and Enforcement of the Combined Sewer Overflow Policy. Washington D.C. EPA 833-R-02-003
2. MacDonald E. and L. Podolsky. 2009. Flushing out the Truth: Sewage Dumping in the Great Lakes. Ecojustice. Toronto, Ontario, Canada.
3. CSO discharge volumes for Buffalo, NY, a recent addition, are not included in the propagated stressor layer map. We are in the process of updating our stressor layers as information becomes available. 


Additional Resources: