CSO Advisory FAQ
A Combine Sewer Overflow (CSO) Advisory is an alert that is issued when the sewers carrying sewage and storm water have filled beyond capacity and are discharging their excess flow into rivers and streams. This usually occurs during or right after a heavy rainfall.
The Allegheny County Sanitary Authority (ALCOSAN) issues these alerts because river water that is contaminated with sewage can be dangerous, especially to people with open wounds or those with weakened immune systems. During a CSO Advisory, it's best to try and minimize contact with the contaminated water.
How can I find out when a CSO Advisory is declared or canceled?
Visit ALCOSAN’s Sewer Overflow Advisories page for more information.
You can also sign up to receive CSO alerts by email or text.
A daily river water quality report is also issued throughout the recreational boating season (May 1st through October 31st) to let boaters know whether a Combined Sewer Overflow (CSO) alert has been declared.
If you see an orange flag with the black letters "C-S-O" around a marina, dock, or other riverfront location, this means an alert is currently in effect.
When is a CSO Advisory declared?
Sometimes in very wet weather, the combined sewer system fills beyond it capacity to carry sewage and storm water. When this happens, the excess flow is discharged into surrounding bodies of water, polluting rivers and streams.
Does an alert prohibit river recreational activity?
No, but during an alert, people are urged to minimize water contact on river outings. Anyone with a weakened immune system or with open cuts or sores is especially vulnerable to infection from exposure to these contaminated waters.
Is tap water safe during a CSO Advisory?
Water treatment plants are able to respond to changes in river water quality and are equipped to continue producing safe drinking water during a CSO Advisory.
Are alerts canceled immediately after it stops raining?
No, not immediately. Once it stops raining, combined sewers continue to overflow until the excess sewage and storm water is drained out of the sewer system. An alert is usually canceled 24 to 36 hours after sewer overflows end.
How often are alerts declared?
Since CSO Advisories were introduced in 1995, an average of 11 alerts per year have been declared. They can vary in length and have ranged anywhere from a single day to 45 days. An average alert lasts about seven days.