Merced County officials want to discontinue their cleanup efforts of the toxic plume that sits under the county road yard in South Merced.
But regional water officials say not quite yet.
The Central Valley Regional Water Quality Control Board at the end of March denied a county request to wrap up the environmental remediation project at the public works yard at 715 Martin Luther King Jr. Way.
The source of the plume has been attributed to commingled discharge from several leaky underground fuel storage tanks at the county yard and the Leonard Truck Repair site across the street at 625 Martin Luther King Jr. Way. Most, if not all, of the tanks are believed to have been removed.
For several years, cleanup efforts at the Leonard Truck Repair site and the county yard have been sucking up the plume of toxic hydrocarbons that extends under both businesses and a handful of local residences.
County officials now claim they've removed all petroleum-based fuel products that leaked from tanks located at the county yard. They argue that cleaning up the remaining plume shouldn't be their responsibility.
"The samples that we're getting out of the ground are the same as the stuff from across the street," said Richard Schwarz, assistant director of the Merced County Department of Public Works. "We've only recently started seeing that pattern. We're pulling stuff out of the ground that is exactly the same stuff that's coming from Leonard Truck Repair."
It's plausible that the remaining plume consists almost completely of fuel spilled at the Leonard Truck Repair site and is spreading "down-gradient" underneath the county yard.
And there's a way to test the theory, said Lonnie Wass, spokesman for the regional water board. "If the plume looks the same up-gradient as it does down- gradient, then we know it all came from one source. It's like a fingerprint."
However, the regional water board needs to see more evidence, Wass said. "We do not believe the numbers presented by Merced County yet show that. We don't believe they've presented enough data," Wass said.
The property owners of Leonard Truck Repair couldn't be reached for comment.
Part of the reason the regional water board didn't approve the county's request is that environmental remediation efforts at the county yard have been frequently interrupted by mechanical failures.
According to the most recent regional water board report: "The operation of the soil vapor extraction and air sparge system (which injects air into the ground in order to push hydrocarbons to the surface where they can be captured and burned off) have been plagued with installation and operational problems since its startup and may only now be operating as designed. The remediation equipment has not been continuously operated at optimum conditions for sufficient time to adequately measure performance."
The county will continue remediation for probably the next six months, Schwarz said. "I know it will carry us through the end of the calendar year. And once all the data are collected and analyzed, then the regional water board will make the determination of what they want to do."
Is the plume dangerous?
In the past, county officials and the regional water board have publicly stated that the plume poses no immediate danger to people working or living in the area.
However, the regional water board recently recommended reducing air flow at certain sparging injection sites to mitigate a "potential risk" to residents living along Sixth Street near Martin Luther King Jr. Way. The regional board also recommended suspending several air injection sites located next to the residence at 509 Martin Luther King Jr. Way, which was singled out as particularly vulnerable.
According to the regional water board, air sparging techniques over the past year may have inadvertently exposed residential areas to the toxic substances.
At the same time, petroleum-based product was found floating in a monitoring well near K Street between Eighth and Seventh streets. The regional water board called the discovery "alarming."
The health-risk assessment conducted by the county in 2009, which concluded the plume posed no significant threat, didn't take into account indirect effects from air sparging, according to the regional water board.
Reporter Joshua Emerson Smith can be reached at (209) 385-2486 or firstname.lastname@example.org.