A study in the Marcellus Shale region of western Pennsylvania has shown that even after being treated, wastewater from hydraulic fracturing operations left significant contamination in a waterway downstream of treatment plants.

Researchers from Penn State University, Colorado State University, and Dartmouth College studied sediments from Conemaugh River Lake — a dammed reservoir east of Pittsburgh — and found that they were contaminated with endocrine-disrupting chemicals called nonylphenol ethoxylates; polycyclic aromatic hydrocarbons, which are carcinogens; and elevated levels of radium.

The study, published in the journal Environmental Science & Technology, said that the highest concentrations of these pollutants were found in lake sediment layers deposited five to ten years ago during a peak period of fracking wastewater disposal. The high radium levels were found as far as 12 miles downstream of treatment plants.

Hydraulic fracturing now accounts for one-half of U.S. oil production and two-thirds of natural gas production. During the fracking process, drillers pump chemicals and water into shale formations under high pressure to free oil and gas deposits. The process produces wastewater that is returned to the surface, and the study said that the wastewater, laced with various pollutants, is then released back into creeks and rivers with only limited treatment. The researchers found that peak concentrations of radium, alkaline earth metals, and organic chemicals were all found in the same sediment layer in Conemaugh River Lake.

The scientists said that while the potential risks associated with these contaminants are unknown, state regulators should enact tighter regulations governing the treatment of fracking wastewater in order to protect human health and the environment.