Need some natural gas? To generate more of it—and more income—energy companies are resorting to creative measures, eking out every last bit from the gas wells they drill. But environmentalists and public-health advocates warn that one such process, called hydraulic fracturing, or fracking, can also taint nearby water supplies. Advances in horizontal drilling technologies have allowed energy producers to reach gas packed in dense rock formations that happen to coexist with the sources of drinking water for a sizeable segment of the U.S. population. This fall, the Environmental Protection Agency is releasing a plan to study the health hazards of the practice.