The rapid expansion of hydraulic fracturing to retrieve once-inaccessible reservoirs of oil and gas could put pressure on already-stressed water resources from the suburbs of Fort Worth to western Colorado, according to a new report from a nonprofit group that advises investors about companies’ environmental risks.

“Given projected sharp increases” in the production of oil and gas by the technique commonly known as fracking, the report from the group Ceres said, “and the intense nature of local water demands, competition and conflicts over water should be a growing concern for companies, policy makers and investors.”

The overall amount of water used for fracking, even in states like Colorado and Texas that have been through severe droughts in recent years, is still small: in many cases 1 percent or even as little as a tenth of 1 percent of overall consumption, far less than agricultural or municipal uses.

But those figures mask more significant local effects, the report’s author, Monika Freyman, said in an interview. “You have to look at a county-by-county scale to capture the intense and short-term impact on water supplies,” she said.