The current boom in U.S. natural gas production from glassy shale rock formations is poised to usher in an era of energy independence and could bridge the gap between today’s fossil-fuel age and a clean-energy future. But that future may be swamped in a legacy of wastewater, a new study suggests.

Natural gas production is soaring thanks to hydraulic fracturing, or fracking, a technique that shoots several million gallons of water laced with chemicals and sand deep underground to break apart chunks of the glassy rock, freeing trapped gas to escape through cracks and fissures into wells.

An average of 10 percent of this water flows back to the surface within a few weeks of the frack job. The rest is absorbed by the surrounding rock and mixes with briny groundwater, explained Radisav Vidic, a civil and environmental engineer at the University of Pittsburgh.