WASHINGTON — 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, according to a new report.The study, published this week 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.The study was published as the Energy Information Administration issued a forecast that natural gas production would continue to rise, and gas would overtake coal as the United States main source of fuel for power plants. The fact that the domestic boom in oil and gas is driven by fracking has made discussions of its impact extremely fraught.Nagel and her colleagues tested samples of surface water and groundwater from Garfield County, Colo., which, with its approximately 10,000 wells, is a center of oil and gas development driven by fracking. The research team gathered multiple water samples at five natural gas sites where spills of fracking wastewater had occurred over the last six years, Nagel said.The team tested for the presence of four different classes of endocrine-disrupting chemicals. Out of 39 water samples collected at five drilling sites, 89 percent showed estrogenic properties, 41 percent were anti-estrogenic, 12 percent were androgenic and 46 percent were anti-androgenic, according to the report. The samples were not tested for specific fracking chemicals or for concentrations of chemicals.Water from control sites in Colorado and Missouri where there is no fracking showed some EDC activity, but the levels were lower than in the water samples from the Garfield County sites, according to the study.The team also tested water samples from the Colorado River. These samples showed the presence of more EDC activity than the control samples, the researchers found.In another part of the study, researchers conducted laboratory analysis of 12 fracking chemicals that are used in Colorado to extract oil and gas. They found that the chemicals were endocrine disrupters that could interfere with human sex hormones.Exposure to EDCs is particularly risky for fetuses, babies and young children, scientists said. Last year, the World Health Organization issued a report raising the alarm on EDCs, saying that endocrine-related illnesses were on the rise worldwide.Mark Salley, spokesman for the Colorado Department of Public Health and Environment, said his agency had not yet had time to review the study.But Katie Brown, a spokeswoman with the industry advocacy group Energy In Depth, dismissed the study as inflammatory. In a statement on the groups web site, Brown said most fracturing operations use few of the chemicals on any one job and noted that EDCs can come from numerous sources.Most previous studies of water contamination in areas with oil and gas production have focused on the presence of methane, the principal component of natural gas, or on salts and hazardous chemicals associated with energy operations.In July, a team of researchers led by a University of Texas at Arlington professor found higher concentrations of heavy metals, such as arsenic, near natural gas wells in the Barnett Shale. Some exceeded EPA levels for drinking water.That study also looked for, but did not find, benzene and other volatile organic compounds that can accompany oil and gas production.The UTA study did not cite a particular reason that the heavy metals were more prevalent near gas wells. It said the metals could show up in water wells because of falling water tables, vibrations that loosen metal-containing scales that accumulate on pipes over time, or surface spills and accidents.Also this year, Duke University researchers found higher levels of methane, ethane and propane in water wells that were closer to gas wells in the northeastern United States. While a number of factors could have been involved, distance to gas wells was, by far, the most significant factor influencing gases in the drinking water we sampled, a Duke researcher said at the time.Duke researchers have also measured gas levels at private water wells in the Barnett Shale. Those results have not yet been released, but two Parker County homeowners whose water was tested have told the Star-Telegram they have seen their own results and they show high levels of methane.Another ongoing water study, the most specific but also the most limited, involves a tracer added to fracking fluid at a single natural gas well in the Marcellus Shale in Pennsylvania. That continuing study, by the U.S. Department of Energy, has not to date found the tracer in nearby groundwater monitoring wells. Staff writer Jim Fuquay contributed to this report.