New York City officials want to ban natural-gas drilling within a mile of six major upstate reservoirs for fear that the city’s drinking water could become contaminated. Extracting gas from the Marcellus Shale rock layer, as some state regulators and lawmakers are pushing to do, would require shooting millions of gallons of water and unidentified chemicals underground to break up the rock. Similar drilling in other states has caused more than 1,000 wastewater spills that have tainted drinking-water supplies. The pristine water that quenches the thirst of 9 million New Yorkers requires no filtration; if it became tainted, a water-treatment facility could cost nearly $10 billion — approximately the same amount that the state estimates it could earn from natural-gas development in the next 10 years.