An independent review has concluded that plans to allow gas drilling inside New York’s main watershed could lead to the contamination of water supplies. In a preliminary report obtained by the investigative website ProPublica, an environmental engineering firm hired by New York City says the threat of contamination “cannot be eliminated.” The assessment comes one week after state regulators proposed to allow gas drilling inside the Marcellus Shale, which supplies drinking water to some 15 million people, including nine million New Yorkers.