An estimated 4,700 gallons of hydrochloric acid (HCL) that were spilled on Wednesday at a Chief Oil & Gas LLC drilling operation in Leroy Township in Bradford County, PA, had been remediated almost entirely by Thursday, officials said.

The HCL release, discovered by personnel on site, occurred around 1 p.m. on Wednesday at the Yoder well site in northeastern Pennsylvania. According to Chief, most of the spilled acid remained on the well pad.

Preliminary blame was being placed on a valve malfunction. “It appears that a valve on the back of the tanker containing the HCL was found partially open, which resulted in the release,” Chief said.

Once the acid was released, it “breached containment and flowed off the well pad,” said a Pennsylvania Department of Environmental Protection (DEP) spokesman. “Some of the acid was collected in a sedimentation pond while the remainder flowed through a field and some reached a small tributary to Towanda Creek causing a minor fish kill.”

Dams were constructed in the tributary before any acid reached the creek, the DEP spokesman said.

“DEP and the Bradford County emergency medical service were notified and response measures were implemented,” the producer said. “The majority of the release, around 4,000 gallons, was held to the initial containment area on the pad site. All pad sites are lined with a thick plastic so any inadvertent release of fluids can be remediated at the pad site.”

About 700-800 gallons of HCL “left the initial containment area and traveled into a sediment pond, which is designed as an additional safety measure to contain any runoff from the pad site.” An estimated 50 gallons of HCL migrated from the pond but “remained localized to a small plunge pool next to the sediment pond.”

No drilling or hydraulic fracturing operations were taking place when the release occurred, according to Chief. Landowners in the area were notified immediately, “however, there was never an issue of safety.”

In addition to DEP’s investigation, Chief said it is investigating the incident, as is oilfield services contractor FTS International.

“There is no evidence that any runoff entered Towanda Creek and all pH readings have been normal,” Chief said. “DEP and the Fish and Boat Commission have been on site and, along with Chief, are continuing to monitor. There were a few dead minnows observed, localized in the small plunge pool, but there was no evidence of HCL and normal pH readings and live fish were noted further downstream in the tributary that leads to Towanda Creek. And again, no evidence of any runoff into Towanda Creek. Chief and DEP will continue to take readings and monitor.”