CHEYENNE, Wyo. (AP) — A new study by the U.S. Geological Survey says more data is needed to be able to say for sure if a link exists between unconventional oil and gas development and degraded water quality.

The study published in the American Geophysical Union journal Water Resources Research finds no evidence hydraulic fracturing is polluting nearby surface water.

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However, the researchers say existing data to investigate long-term water quality trends is adequate in just 16 percent of U.S. watersheds with unconventional oil and gas resources.

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Some environmentalists blame fracking for causing pollution. Fracking employs pressurized water mixed with sand and chemicals to break open oil and gas deposits underground.

The researchers say insufficient water-quality data from the years before fracking became commonplace impede long-term analysis of watersheds with oil and gas development.

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