Pennsylvania is known by anglers and outdoor enthusiasts for its thousands of miles of streams and rivers that lace the rugged Allegheny Mountains, rolling farmlands, and cities such as Pittsburgh and Philadelphia. Many of those waterways supply drinking water for millions.

But a study by the U.S. Geological Survey found that many of those waterways are also tainted by so-called forever chemicals. The USGS sampled streams across Pennsylvania and found that 76% of them contained the presence of at least one compound from the per- and polyfluoroalkyl substances (PFAS) family, according the first study of its kind looking at the problem across a single state and associating possible sources.

PFAS are a group of 12,000 man-made chemicals that have been used widely to make a range of products such as nonstick cookware, stain-resistant furniture, the linings of fast-food boxes, and firefighting foam.

High concentrations can lead to adverse health risks in people, according to the U.S. Environmental Protection Agency. The USGS says that the chemicals’ persistence in the environment and prevalence make them a top concern for water quality. Some health effects associated with PFAS include decreased fertility, testicular and kidney cancers, high cholesterol, autoimmune and thyroid problems, alterations in hormone functioning, and developmental effects.

The two highest concentrations of per- and polyfluoroalkyl substances (PFAS) were found in the Philadelphia region: Neshaminy Creek in Bucks County and Valley Creek in Chester County.