The Pennsylvania Department of Environmental Protection (DEP) on Friday released the first four years of data from its mechanical integrity assessment program and said an analysis shows that oil and natural gas wells are being operated in an environmentally safe manner.

Adopted in 2011 and implemented about three years later, the program requires unconventional and conventional operators to inspect the mechanical integrity of wells on a quarterly basis and file annual reports with the DEP. The operators are required to inspect the structural soundness of their wells to ensure that gas migration is prevented, leaks are managed and groundwater is protected.

An analysis, including audits and independent site verification of data submitted in 2014, showed that less than 1% of operators indicated the types of integrity problems, such as gas outside surface casing, which could allow gas to move beyond the well footprint. A poorly constructed well is typically the cause of environmental impacts.

According to DEP, about 30% of the wells analyzed had gas present outside production casing. But the agency said in some instances, it occurred because of an approved well design that allowed for engineered vents. “In a properly designed and operated well,” DEP added, “gas is kept to the well footprint, and won’t flow into a water supply.”

DEP also said operators have consistently cooperated with the program’s inspection requirements. Since it began, 99% of unconventional wells have been in compliance, while 50-60% of conventional wells have complied. District offices are “actively working” to improve the number of conventional operators that are complying.

The agency, which bills the effort as the nation’s “most rigorous” routine well integrity assessment program, said the data has provided district offices “with useful information for identifying potential concerns at wells and more effectively assessing whether operator intervention is necessary to protect groundwater.”

Marcellus Shale Coalition President David Spigelmyer said the data shows that natural gas development and strong environmental protection “are not mutually exclusive.” He said “our industry is laser-focused on leveraging continuously improving technologies, world-class engineering solutions and best practices aimed at safeguarding and enhancing our environment, including groundwater protection, as well as public health.”