The biggest reason for the decline in greenhouse gas emissions over the past couple years is the replacement of dirty coal with natural gas to generate electricity. But this will not last if the fracking that has unlocked so much natural gas leads to the release of underground methane, a powerful greenhouse gas. Among all the other pollutants we’re seeing in the water supply of areas with a high frequency of fracking, we’re starting to see lots of methane out there.

Mike and Nancy Leighton’s problems began on May 19, just as Mike was settling in to watch the Preakness Stakes. A neighbor in Leroy Township, Pa., called Mike and told him to check the water well located just outside his front door.

Down the road, Ted and Gale Franklin’s water well had gone dry. When water started coming out later that week, the liquid was “black as coal,” according to Gale.

Since then, both families have been dealing with methane-contaminated water supplies, as well as dozens of mysterious, flammable gas puddles bubbling up on their properties.

Pennsylvania’s Department of Environmental Protection blames a nearby hydraulic fracturing, or fracking, operation. It says methane gas has leaked out of the well, which is operated by Chesapeake Energy, and into the Leightons’ and Franklins’ water supplies.

The danger goes beyond contaminated water. In a letter to both families detailing test results and preliminary findings, state regulators wrote that “there is a physical danger of fire or explosion due to the migration of natural gas water wells.” Chesapeake has installed ventilation systems at the two water wells, but the letter warns, “it is not possible to completely eliminate the hazards of having natural gas in your water supply by simply venting your well.”

In addition to the contaminated water and the possibility of explosions (and seismic activity, actually), you have the impact of the release of methane. I don’t know that this is totally being measured at this point, but leaking wells can easily act as a way to flow methane to the surface along with the natural gas. Geologist David Yoxtheimer calls this a “methane gas express elevator.” The margins of reductions of greenhouse gas emissions from fracking will drop significantly if this comes to pass. And the more sloppily frackers build their wells, the more of a problem this becomes.