David Mano and his neighbors on Valleyview Drive said there wasn't much of a mystery in July when the water from their kitchen taps suddenly turned smelly and cloudy. They immediately suspected the underground drilling that Sunoco Pipeline LP was conducting beneath their Chester County neighborhood.

"I walked down the street to find where they're drilling, and see there's water coming out of the drill hole," said Mano, who lives in an enclave of about three dozen 60-year-old Exton homes with private water wells. "That's our spring draining itself out."

Sunoco's construction of the Mariner East 2 pipeline set off alarms in West Whiteland Township, where the company halted drilling beneath Valleyview Drive after the private wells went bad on Independence Day. As Mano learned more about Sunoco's massive project to deliver Marcellus Shale gas liquids such as propane across Pennsylvania to a terminal in Marcus Hook, he discovered his experience was not isolated.

"At the very beginning, the battle was all about our well," he said. "But once we started seeing how big this battle is across the state, we realized how many other people have been affected."

The latest landowners to experience a rude intrusion from the Mariner East system live less than two miles from Mano, on a quiet cul-de-sac called Lisa Drive. Last month sinkholes developed along Sunoco's pipeline easement, less than 10 feet from some Lisa Drive houses, exposing the original Mariner East 1 pipeline. The sinkholes are about 200 feet from the Amtrak line, also used by SEPTA's Regional Rail.

The Pennsylvania Public Utility Commission, which is responsible for enforcing federal pipeline safety rules, said the subsidence could have "catastrophic results" and ordered Sunoco to halt pipeline flows until the company thoroughly studied and stabilized the sinkholes.

On March 30, Sunoco's parent company, Energy Transfer Partners (ETP), offered to temporarily relocate five Lisa Drive households for six weeks while it conducts subsurface studies and comes up with a remediation plan. Residents declined to comment; some have sued and others say they have obtained lawyers.

Operation of the existing pipeline is on hold indefinitely, and much of the remaining construction of the $5 billion Mariner East 2 expansion is stalled. Sunoco says 95 percent of the project is completed.

Mariner East's critics suspect the common thread between the sinkholes and the fouled water wells is Sunoco's construction method. Sunoco is using horizontal directional drilling (HDD) to bore an underground pathway for the pipeline in much of densely populated Delaware and Chester Counties. HDD allows it to pass under streams, roadways, and neighborhoods without causing surface disturbances associated with a conventional open-cut trench.

But Sunoco's effort to inflict less disturbance has backfired miserably in Chester County's Great Valley, which is underlain by several fault lines and a fragile geologic limestone formation called karst. Karst is characterized by cavities and sinkholes where the limestone rock has dissolved.

"This is an unstable area where they should never have put a pipeline in the first place," said State Sen. Andy Dinniman (D., Chester), who, along with State Sen. John Rafferty (R., Montgomery), has introduced legislation to increase pipeline oversight.

Horizontal drilling involves the use of a steerable drill bit that chews its way through the earth, aided by drilling fluid that carries back the rock cuttings from the drill bit. The drilling mud also seals the bore's interior.

But drilling fluid can leak through underground fractures and faults, causing "inadvertent returns," as it did when Sunoco began horizontal drilling in November near Lisa Drive. Though drilling mud is composed of water and nontoxic bentonite clay, it can harm aquatic life if enough of it leaks into a stream.

And horizontal drilling can disturb underground aquifers, as it did on Valleyview Drive.

Sunoco agreed to connect 34 households on Valleyview Drive and nearby Township Line Road to Aqua Pennsylvania's public water system, and gave each resident $60,000 to cover the costs of future water bills. Everybody accepted the offer except Mano, who feared that by accepting the money, he would release Sunoco of all future liability. He said his well water cleared up after Sunoco's drilling paused.

Sunoco said that inadvertent returns of drilling mud during horizontal drilling are "not unexpected" and that its permits from the Pennsylvania Department of Environmental Protection (DEP) include contingency plans for responding to leaks and spills.

"We have experienced inadvertent returns on 37 percent of our HDDs — which is below the industry average of 50 percent," ETP spokeswoman Lisa Dillinger said in an email. "We have worked in coordination with the DEP to ensure that the environment is protected and will continue to do so."

Sunoco/ETP and state environmental regulators say horizontal drilling can be conducted safely in karst formations.

"Large portions of the commonwealth are underlain by karst geology like that found in the southeastern portion of the state, and Pa. DEP, local governments, and other regulatory agencies regularly permit activities in these areas and address the unique issues associated with such geologic formations," Neil Shader, DEP's spokesman, said in an email.

"Commercial, residential, and industrial structures, pipelines and other utilities are installed throughout such areas and have existed for many decades," Shader said.

Richard B. Kuprewicz, a respected engineer and pipeline expert from Redmond, Wash., agrees that horizontal directional drilling can be viable in a karst terrain, and said there is nothing to preclude the operation of a pipeline carrying highly volatile liquids such as propane through karst. "But you better know what the hell you're doing."

In reviewing Sunoco's applications for stream-crossing permits in 2016, DEP said that use of HDD in Exton within 1,500 feet of three public water-well sites operated by Aqua Pennsylvania presented "additional risks" of leaks during drilling. It asked Sunoco to provide details on how it would reduce the risk.

In its response, Sunoco said it had met with Aqua and prepared an HDD monitoring program. It also said it would reevaluate the pipeline's depth where it passes close to Aqua's Hillside wells on Swedesford Road, to address the utility's concerns about "potential turbidity increases" during drilling.

DEP approved Sunoco's permits in February 2017, including horizontal drilling across most of the Great Valley.

"This has been the area that has been the epicenter of problems with HDD drilling, and DEP identified it in advance," said Alex Bomstein, an attorney for the Clean Air Council, which has led a coalition of environmental groups that are challenging the Mariner East permits. "DEP identified it, but permitted it anyway."

Just a few days after Sunoco halted drilling when the Valleyview Drive water wells were fouled, Aqua's chief environmental officer, Christopher S. Crockett, asked Sunoco/ETP to evaluate alternative pipe installation techniques for Mariner East around the Hillside wells.

"Based on aquifer testing jointly performed by ETP and Aqua, it appears that HDD will have adverse impacts on our public water supply wells at this location, resulting in permanent loss of these wells," Crockett said in a July 17 email.

The email correspondence was included in an application ETP submitted in October to abandon HDD on Mariner East sections under North Pottstown Pike and under Swedesford Road, near the  Exton Square Mall. ETP proposed to switch to open-trench construction and auger drilling, an underground drilling method that involves short, straight-shot boring under roadways and does not use drilling mud.

Aqua's request to abandon HDD near its wells had been in the works for many months, based upon tests done in February 2017, and was not a reaction to the Valleyview Drive mess, Aqua spokeswoman Donna Alston said in an email.

"Aqua took proactive measures to establish the need for alternative piping installation methods, such as open-cut pipe trenching, prior to the discovery of the contaminated private wells in the locations you reference," she said in an email. Crockett sent the July 17 email at ETP's request to support the pipeline company's application to switch to open-trench cutting, she said.

Pipeline opponents say ETP's abandonment of HDD plans near Aqua's wells is an acknowledgment that horizontal drilling is unsuitable in the area.

"They should have never done HDD in such a densely populated area," said Cheryl Wardle, a resident on Shoen Road, whose front yard faces a now-idled Sunoco site where it conducted the horizontal drilling that ran under Valleyview Drive.  "We're the guinea pigs."

She and her husband, Bill, who accepted $8,000 from Sunoco in 2015 for a pipeline easement across their rear yard, fear that the project has diminished the value of their home. "We couldn't sell now," she said, pointing to the ugly 25-foot-high fabric barrier that shields the drill site from their house.

DEP's Shader said the agency will conduct a new hearing on Mariner's permits since the company's proposal to change construction methods in West Whiteland is a significant modification. "Those modifications are currently being reviewed by DEP, and information on the hearing will be made available once the details have been ironed out," he said in an email Wednesday.

A hearing may provide pipeline opponents with a new opportunity to express frustration with Sunoco, DEP, and Gov. Wolf, whose administration signed off on Mariner East as a major economic development project.

"I feel frightened," said Virginia Kerslake, who lives on Shoen Road across from the Sunoco drill site. "I feel powerless. I feel like our governor hasn't done anything for us. I feel like we are basically just collateral."

Kuprewicz, the pipeline expert whose work has been cited by Sunoco in the past, suggests a delay might help Sunoco to regroup and to heal from self-inflicted wounds, such as a $12.6 million fine it agreed to pay in January after DEP cited it for "egregious and willful violations."