WASHINGTON—The U.S. Environmental Protection Agency, as part of its review of a natural-gas drilling procedure, is looking at the radioactivity of wastewater used in the process.

Environmental Protection Agency Administrator Lisa Jackson, speaking at a congressional hearing Thursday, defended her agency's efforts to study the safety of natural-gas drilling and left the door open to further regulatory action on the issue. The process, known as hydraulic fracturing, is used to extract hard-to-reach natural-gas pockets in the...