A state environmental judge has permitted Sunoco Pipeline LP to resume horizontal drilling at 16 of the 55 locations where underground work was temporarily halted in July on the company's contentious Mariner East 2 project.

Judge Bernard Labuskes Jr., in an order posted Friday by the Environmental Hearing Board, allowed Sunoco to restart drilling at 16 of 17 locations on the 350-mile pipeline route where the company said that underground work was incomplete, leaving boreholes unstable and drill bits suspended in the hole.

Two of the locations where drilling can resume are in Chester County, and two are in Delaware County, including one under Chester Creek, where nontoxic bentonite drilling fluid seeped outside the well bore during the drilling process.

But underground work is still halted under an Exton neighborhood where drilling in July contaminated the private water wells of 14 homeowners, prompting Sunoco to scramble to extend public water service to an enclave of homes using private wells near Shoen Road in West Whiteland Township.

The decision to partially lift the drilling ban grew out of settlement negotiations Thursday between Sunoco and the Clean Air Council, the environmental nonprofit group that has attempted to block the pipeline's permits. Alex Bomstein, a Clean Air Council attorney, said he was unable to comment on the ongoing settlement negotiations.

The judge's order doesn't affect his July 25 decision to halt most of the underground drilling on the Mariner East 2 pipeline. That decision is the subject of a Harrisburg hearing that is set to start next Wednesday, rescheduled from Aug. 7.

The hearings concern the Clean Air Council's effort to temporarily block permits granted in February by the Department of Environmental Protection that allowed Sunoco to start building the pipeline. At that time, Labuskes declined the organization's appeal, but he ordered the halt to horizontal drilling after the organization documented spills associated with the practice.

Sunoco, which is owned by Energy Transfer Partners LP of Texas, is building the cross-state pipeline to deliver gas liquids such as propane from the Marcellus Shale formation to its terminal in Marcus Hook. The Mariner East 2 pipeline largely follows the same route as the smaller Mariner East 1 pipeline, which went into service more than two years ago.

Most of the pipeline is being installed in conventional open trenches, which aren't affected by the order and where work continues.

The company decided to use the horizontal drilling method to install the new pipeline beneath large streams, rivers, and highways, and under densely developed areas, to reduce the nuisance caused by cutting an open trench. But the technique seems to be causing more public and regulatory disturbances than anticipated.

With horizontal directional drilling, the company creates a 24-inch diameter lateral pilot hole as far as a mile in length. The pilot hole is then reamed with a larger drill bit to enlarge it to 30 inches in diameter, allowing the coated steel pipeline to be pulled through it. The pipe is then sealed in place with cement.

Sunoco, in its filing with the Environmental Hearing Board, argued that termination of horizontal drilling before an individual hole is completed can cause more potential environmental risk than finishing the drilling.

David Runte, Sunoco's senior director of engineering, said in an affidavit that incomplete drilling creates a risk of collapse of the borehole wall, and increases the likelihood of leaks when drilling is resumed.

Drilling equipment can also be lost if the drilled hole collapses, he said, requiring a new hole to be drilled.

The 16 locations where drilling can resume include a 3,557-foot section beneath Eagleview Boulevard in Exton, Chester County; a 3,657-foot section under Ship Road in West Whiteland Township, Chester County; a 2,844-foot section beneath Commerce Drive in Chester Township, Delaware County; and a 4,979-foot section in Delaware County that crosses under Chester Creek, three railroad tracks, and four roadways.

The pipeline installation under Chester Creek has created significant challenges for Sunoco, including six separate losses of drilling fluids that the company blamed on cracks in the bedrock, created during the previous installation of a sewer line. The company said it introduced grout to fill the cracks, but the grout seal will degrade and create the risk of future drilling fluids leaks unless the drilling is completed soon.