The Alaska Department of Environmental Conservation is investigating the cause of a recent oil spill near the Tesoro Refinery in Nikiski.

On Dec. 18, the facility operator noticed blackened snow near the wastewater tank at the Tesoro facility during a nightly inspection, according to a Dec. 27 situation report from the Department of Environmental Conservation. The company estimated that approximately 165 gallons of a crude oil and water mixture had spilled, according to the report.

About 55 gallons were spilled into the secondary containment area around the wastewater tank and 110 gallons outside the containment area, according to the report.

The company shut down all its pipelines running to and from the wastewater tank and emptied the leaking pipeline as well as other lines in the area. Tesoro’s response contractor began cleaning up and has removed pooled oily water, oiled snow and contaminated soils, according to the report. The oil had permeated at least down to a valve box buried 7.5 feet below the ground surface — approximately 3.5 feet of oily water was found in the valve box on Dec. 22, according to the report.

Right now, the company is focusing on cleanup, said Jade Gamble, specialist for the Cook Inlet/Kodiak Unit Environmental Program. The cause of the leak is still under investigation.

“They’re just continuing along,” she said. “Right now, our priority is just chipping (the oil) out.”

The facility is staffed 24/7, so the cleanup continued through the holiday and the workers are still investigating the cause, Gamble said. There’s no clear timeline for concluding the investigation, she said.

“Part of the line is subsurface, and they’re working on excavating that,” she said.

Though the wastewater tank is about 1,000 feet from Cook Inlet, the oil did not reach the inlet and there were no observed impacts to wildlife, according to the report.

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