The public parking lot at the southeast corner of South Washington and Church streets in Sonora will be closed and gated-off until early October to clean up underground soil contamination from a former service station that shut down more than 20 years ago.

Stantec Consulting Services Inc. was contracted by Chevron Corp. to complete the project, which involves the removal and replacement of a large chunk of gasoline-tainted soil underneath the paved lot. Work began Sept. 16 and is expected to be finished Oct. 4.

"Under the direction of the Central Valley Regional Water Quality Control Board, we're doing the onsite excavation as a final step," said Chevron spokeswoman Marielle Boortz.

Unocal owned the site before that former gasoline company was acquired by Chevron in August 2005.

Testing and monitoring of soil and groundwater at and around the site has been conducted ever since the contamination was detected shortly after the former station was dismantled in 1993.

The contamination stems from the removal of leaky gas tanks at the former Unocal station, which operated from the early 1940s to the early 1990s.

Benzenes and other cancer causing chemicals in gasoline were found in the soil at elevated levels.

The problem was so bad that the assessed value of the 4,500-square-foot property plummeted from more than $83,000 to $500 in December 1993.

The company couldn't sell the property because of the contamination and has been leasing it to the city of Sonora for as little as $100 per year. The city paved the lot and made 17 new parking spaces in 1998.

Excavation occurred at the site in 2005 for more testing.

"We've had an investigation and now we're excavating the impacted soil and any liquid that was there," said Boortz.

Boortz said the regional board has required the company to do a year's worth of soil and groundwater monitoring at the site to confirm the clean-up is finally complete. Chevron can then apply to close the decades-old case.

Stantec plans to remove a maximum of 200 cubic yards, or an area approximately 10-feet-wide, 30-feet-long and 18-feet-deep. Boortz said construction crews will then fill it in with non-contaminated soil.

According to the work plan, excavated soil will be transferred to a different company for analysis and disposal.

Chevron is covering all costs associated with the work, though Boortz declined to provide a total dollar amount. Before the work began, the plans were submitted for review and approved by both the city and county.

Boortz said the fencing around the work site is there for the protection of construction workers and passers-by. The green screening wrapped around the fence blocking the view is there to prevent drivers from becoming distracted, she added.

"Our plan is to revert it back to a parking lot when we're done," she said. "We'll be doing paving and striping and then give it back to the city of Sonora."