PHILADELPHIA — A preliminary study by researchers with the Academy of Natural Sciences asserts that rivers and streams could be at risk of pollution because of a boom in drilling on the Marcellus Shale natural gas formation, even without spills or accidents.

Researchers at the academy compared watersheds where there was little or no drilling to those watersheds where there was drilling and found significant changes, according to published reports.

It has not been peer-reviewed or published in any scientific journal.

The study found that water conductivity, a barometer of contamination by salts that are found in drilling waste water, was nearly twice as high in streams nearby high-density drilling.

The aim of the research was not to look at drilling accidents or other irregularities, specifically. Instead, it focused on if, and how so, drilling could pose any potential harm.

David Velinsky, vice president of the academy's Patrick Center for Environmental Research, said the early findings suggested that "there is indeed a threshold at which drilling — regardless of how it is practiced — will have a significant impact on an ecosystem."