A preliminary report from a consultant hired by New York City warns that “nearly every activity” associated with natural gas drilling, as proposed for southern New York, could potentially harm the city’s drinking water supply and that while the risk can be reduced with strict regulations, “the likelihood of water quality impairment … cannot be eliminated.”

That assessment contrasts sharply with the picture presented by an environmental review released by state officials last week. Aside from clauses that ban some waste pits and promise additional consideration for drilling within 1,000 feet of the city’s reservoirs and water infrastructure north of New York City, the environmental review does little to respond to New York City’s long-standing concerns that the watershed deserves special environmental consideration and instead paves the way for drilling to proceed throughout the watershed.

Marc LaVorgna, a spokesman for the New York City mayor’s office, said Mayor Michael Bloomberg will withhold judgment until he sees the final version of the report the city commissioned from Hazen and Sawyer, a New York City-based environmental engineering firm. The full report isn’t expected to be delivered until December, after the public comment period for the state environmental review has ended.

LaVorgna emphasized the Bloomberg administration has invested heavily in the city’s water system and would not rule out a protracted fight to protect it. Bloomberg, who has generally supported energy development, told WNYC radio that “if this has the danger of polluting, we will fight it.”

New York City depends almost entirely on water delivered from other areas.

New York is one of four major cities in the United States with a special permit from the U.S. Environmental Protection Agency allowing its drinking water to go unfiltered. That pristine water comes from a network of reservoirs and rivers spread across five counties. Those reservoirs — which all lie west of the Hudson River — supply 90 percent of the drinking water for 9 million downstate residents. If the EPA were to rescind the city’s special permit, New York City would have to build a treatment facility costing up to $30 billion. Hazen and Sawyer’s early findings were summarized at a city meeting last week and posted on the city Department of Environmental Protection’s Web site.

The consultants found that drilling “introduces hazardous chemicals into the watershed” and that “the well bore, which acts as a conduit between geologic formations, can allow previously isolated contaminants to flow into shallow groundwater or surface water.” It said the disturbance from hydraulic fracturing could damage the tunnels or aqueducts that bring the water to the city. Hydraulic fracturing shoots millions of gallons of water, sand and chemicals underground with such force that it breaks rock and releases pockets of gas.

A spokesman for the state DEC did not return repeated calls for comment.

Abrahm Lustgarten is a reporter for ProPublica, an independent investigative newsroom.