PAVILLION, Wyo. — It has been more than four decades since the first well was drilled in the natural gas field beneath this stretch of slow rolling alfalfa and sugar beet farms. But for some who live here, in the shadows of the Wind River Mountains, the drilling rigs have brought more than jobs and industry.

For the last few years, a small group of farmers and landowners scattered across this rural Wyoming basin have complained that their water wells have been contaminated with chemicals from a controversial drilling technique known as hydraulic fracturing, or fracking.

A draft report by the Environmental Protection Agency, issued in December, appeared to confirm their concerns, linking chemicals in local groundwater to gas drilling.

But here on the front lines of the battle over fracking, which has become an increasingly popular technique to extract previously unobtainable reserves of oil and gas, no conclusion is yet definitive.