Portions of Gas Works Park will be closed on the weekdays starting next week so city crews can investigate the extent of underground pollution...

Portions of Gas Works Park will be closed on the weekdays starting next week so city crews can investigate the extent of underground pollution left over from decades of converting coal and oil into gas.

The crews will be taking core samples from the meadow north of the children’s play barn to check how much buried coal tar still remains, and where.

The sampling will help city officials and Puget Sound Energy (PSE) figure out how the underground tar may contribute to contamination of the Lake Union shoreline.

Through the first half of the 20th century, the gas plant at Gas Works left behind an underground soup of carcinogenic chemicals that officials have been investigating on and off for the past 30 years.

In 1977, city officials hauled away 3,000 pounds of tar. In 2000 and 2001, the city burned off an underground plume of benzene and trucked in thousands of pounds of soil to cap 5.8 acres of the park.

Even so, thick, molasses-like tar has burbled up a few times each year, erupting onto the grass in baseball- or shovel-sized hunks. The tar is quickly cleaned up by city employees, said Colleen Browne, Gas Works project manager for Seattle Parks and Recreation.

While the tar contains toxic substances that should not be inhaled or eaten — particularly polycyclic aromatic hydrocarbons — city officials said the greatest risk is to the lake’s ecology, not human health.

With the city, state and PSE working toward a long-term plan to clean up about 40 acres of lake bottom around the park, officials want to learn more about the tar so they can keep it from migrating to the lake after they complete the cleanup in the water.

“We don’t want to have those sediments recontaminated over time,” said Sarah McKearnan, a strategic adviser with Seattle Public Utilities.

The drilling will take place on weekdays between 7 a.m. and 10 p.m. beginning Monday. The northeast corner of the park, including the play barn, will remain closed during that time. City officials expect the work to end Sept. 27.

It will take at least a month for researchers to interpret the results from the testing.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com