The state is investigating an oil spill reported at Hilcorp’s shipping terminal on the west side of Cook Inlet.

The spill occurred at the Drift River Terminal, which Hilcorp subsidiary the Cook Inlet Pipeline Company operates. Over-pressurization of 20-inch fill line caused the spill, according to a report issued Tuesday from the Alaska Department of Environmental Conservation.

The situational report lists two dates. Cook Inlet Pipeline Company inspectors first discovered the spill on July 2, when an inspector found approximately 14 gallons of crude oil inside a valve box and one on the ground. Later in the month on July 28, an employee was inspecting the area above the buried 20-inch fill line and found an oil-stained area approximately 24 feet by 24 feet by 30 feet, according to the DEC report.

The volume of oil in the ground above the buried pipeline is unknown at present. DEC investigators initially went to the facility July 21 to investigate the first spill report and plan to return to monitor cleanup activity associated with spill that was discovered on July 28.

“ADEC representatives will be present for any testing or activities involving the 20-inch line,” the report states.

The DEC ‘s Tuesday report initially stated that here had been two spills. However, there was only one spill, said Hilcorp spokesperson Lori Nelson in an email.

“The spill volumes referenced in the report were the result of a single event,” Nelson wrote.

The over-pressurization occurred while workers at the terminal were emptying two tanks to prepare for an inspection. Workers pumped down two tanks to a 270,000-barrel crude oil storage tank, called Tank 3, on June 25. At the same time, crude oil from the facility’s crude oil transmission pipeline was being moved into the same tank through the 20-inch fill line. When the transfer was finished for the day, the controller at the Kenai Gas Field closed the fill valve, but at that time, the pump drawing down from the two tanks was still operating.

“Closing the fill valve to Tank 3 closed off the flow path of the oil from the two tanks into Tank 3,” the DEC report states. “Due to the operations being conducted simultaneously, the closure of the valve to Tank 3, and the continued throughput from the two tanks, the 20-inch fill line was overpressurized.”

Cook Inlet Pipeline Company has shut down the 20-inch fill line and isolated it, according to the report.

The oil inside the valve box from the first spill discovered has been recovered and returned to storage tanks, and the gallon on the ground has been excavated. The company also reviewed and changed its procedures on removing liquids from the two tanks for the future, according to the report.

The Drift River Terminal is located on the west side of Cook Inlet just west of Kalgin Island, adjacent to Rust Slough and the Drift River, which drain into Redoubt Bay.

The report notes that bears, moose and nesting waterfowl may be in the area, but no impacts to wildlife have been reported.

Reach Elizabeth Earl at elizabeth.earl@peninsulaclarion.com.