A study of tainted drinking water in areas where natural gas is produced from shale shows that the contamination is most likely caused by leaky wells rather than the process of hydraulic fracturing used to release the gas from the rock.

The study looked at seven cases in Pennsylvania and one in Texas where water wells had been contaminated by methane and other hydrocarbon gases. Both states have extensive deposits of gas-bearing shale that have been exploited in recent years as part of a surge in domestic energy production. Some environmental groups have suggested that hydraulic fracturing, or fracking, could cause the gas to migrate into drinking water aquifers.

Shale-gas producers commonly drill a deep vertical well that is then extended horizontally in several directions into the rock, like spokes from a hub. In fracking, water and chemicals are injected at high pressures into these spokes, creating fissures and releasing the natural gas trapped within.

But in their analysis, published Monday in The Proceedings of the National Academy of Sciences, the researchers found no evidence that fractured shale led to water contamination. Instead, they said cement used to seal the outside of the vertical wells, or steel tubing used to line them, was at fault, leading to gas leaking up the wells and into aquifers.