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Water samples collected at Colorado sites where hydraulic fracturing was used to extract natural gas show the presence of chemicals that have been linked to infertility, birth defects and cancer, scientists reported Monday.

The study, published in the journal Endocrinology, also found elevated levels of the hormone-disrupting chemicals in the Colorado River, where wastewater released during accidental spills at nearby wells could wind up.

Tests of water from sites with no fracking activity also revealed the activity of so-called endocrine-disrupting chemicals, or EDCs. But the levels from these control sites were lower than in places with direct links to fracking, the study found.

“With fracking on the rise, populations may face greater health risks from increased endocrine-disrupting chemical exposure,” said senior author Susan Nagel, who investigates the health effects of estrogen at the University of Missouri School of Medicine.

Fracking involves injecting millions of gallons of chemical-laced water and sand deep underground to crack shale formations and unlock oil and gas. The process is exempt from some regulations that are part of the Safe Drinking Water Act, and energy companies do not have to disclose the chemicals they use if they consider that information a trade secret.