One-tenth of an inch of rain over an hour is all it takes for the region’s sewers to overflow.

That’s according to the Allegheny County Sanitary Authority (ALCOSAN), which has already issued two Combined Sewer Overflow (CSO) alerts this season.  The Authority is hoping that as the river recreation season takes hold more recreational users will take note.

The Allegheny County Health Department issued its second combined sewer overflow (CSO) alert of the season Thursday.

The advisories notify swimmers, boaters and other river-goers if the water has been contaminated by raw sewage after heavy rainfall clogs waste treatment facilities. The length of the advisories depends on the time it takes for the sewer systems to return to normal levels.

The CSO alerts do not prohibit recreational river activity, but advise the public to reduce water contact, especially those with weak immune systems or open cuts and sores.