The search continues for whatever is polluting Santa Rosa Creek on B Street near downtown.

After years of questions, city public works officials this week ripped up a part of parking lot at the corner of Ross and B streets to see if there was a leaking underground fuel tank that might be contaminating the soil and nearby waterways.

After digging a six-foot deep, L-shaped trench and sampling the soil, they came up empty, failing to find any tanks or contamination.

"We sampled the heck out of the soil and it's squeaky clean," said Andy Rodgers, president of the Sebastopol-based environmental consulting firm ECON.

That's good news for the city, which would have been on the hook for the costly clean-up of the site. The city paid about $35,000 for the work, which has been sought by regulators with the North Coast Regional Water Quality Control Board since 2007.

But it leaves unsolved the mystery of what's causing the pollution. Water quality officials have been wondering what's going on beneath B Street for years.

The area is home to some of the city's oldest buildings, many of which had heating oil tanks in their basements. Several gas stations and garages once occupied the area, which is one block east of Highway 101.

At least five other sites along B Street have been identified by the board as possibly being the source of pollution reaching the creek through the city's storm drains.

One of the most significant sites is a former Pacific Gas & Electric Company property on First Street along Santa Rosa Creek near the B Street intersection.

A manufactured gas plant operated on the site from 1876 to 1924, turning coal into gas for heating and cooking. After natural gas came to the area, the plant was dismantled and the property was later used as a natural gas distribution center. The utility sold the property in 1987, but remains involved in the clean-up.

Left behind in the soil was a large volume of coal tar, a by-product of the coal gas process. Between 2009 and 2011, the utility removed 55,000 gallons of coal tar from the site. It used large electric probes to heat up the earth and liquefy the tar and clean the soil, said PG&E spokeswoman Brittany McKannay.

In addition, 200 tons of contaminated soil was removed from the site. Efforts currently are under way to stabilize the soil with cement to prevent migration of remaining contaminants, she said. Work is expected to be done this summer.

Other sites include 416 B Street, the site of a former Greyhound Bus Depot, and the site of the new Luther Burbank Savings building, which for years housed Traverso's deli but before that was the site of a Phillips 66 gas station.

The Empire Building, at 37 Old Courthouse Square, has also been identified as having an underground heating oil tank. Bank of America has been identified by the board as responsible for the clean-up. Whether that tank or one of several others in the area is responsible for contamination has been the source of significant debate and legal action.

Though the source of the leak was not identified, the digging in the city's parking lot was worthwhile because the water board can now cross the site off its list of potential culprits and focus on other sites, Rodgers said.

While the city is pleased its property is clean, it will continue to work with the water board to help find the source of the leaks, said David Vandeveer, a city engineer.

"We don't argue with the regional water quality board, we cooperate with them because we want the creek to be clean, too," Vandeveer said.