Seven years after San Joaquin County started extracting flammable gases from the Corral Hollow Sanitary Landfill, state regulators are still not convinced the county has an adequate plan to clean up groundwater pollution underneath the former city dump.

The area around the landfill on the southwestern corner of Interstate 580 and Corral Hollow Road once was a remote pit five miles from town, but now is just outside city limits at the southern boundary of the proposed Tracy Hills development.

Central Valley Regional Water Quality Control Board wants but has yet to receive a report that would show just how much groundwater is contaminated and how deep the contamination goes. The report would also suggest how the county will clean up that water.

The landfill took in Tracy’s garbage from the mid-1950s until 1995. County officials previously estimated that it holds about 1.9 million cubic yards of trash — enough to fill more than 10 million large trash bags. In 2000, when the county installed the gas extraction network, some of the air samples contained 30 percent to 40 percent methane, which is odorless and colorless in its natural state but is flammable and can cause asphyxiation if breathed continuously.

In March, the Central Valley Regional Water Quality Control Board gave a notice of violation to the county, stating that the county’s reports on cleanup of the site were incomplete. This came three months after the state told the county that methane and chemicals continued to seep into the air and water despite the county’s efforts to capture and burn these gases.

Wendy Wyels, environmental program manager for the regional board’s Sacramento office, said the county has pulled gases from the 45-acre site. But groundwater reports provided by the country show that there has been little progress since 2001, when the state issued an order to clean up the site.

At the time, the county’s solid waste division had already installed a network of underground pipes designed to pull methane and volatile gases out of the landfill.

“Their monitoring shows that hasn’t totally addressed the problem,” Wyels said, “but they haven’t said how they will address the problem.”

Michael Carroll, head of the county’s solid waste division, did not return phone calls seeking comment this week. Wyels said the latest that she heard was that the county was drilling around the landfill to see where it would find contaminated water.

“What they’re doing now is trying to find the edge of the (contamination) plume and see how far it extends,” she said. The state knows that contamination from the landfill gas goes at least 60 feet deep.

To reach reporter Bob Brownne, e-mail brownne@tracypress.com.