The Tuolumne County solid waste division is seeking bids for a contractor to drill six more monitoring wells at the old Big Oak Flat Landfill outside Groveland and they want the work completed by May 16.

The landfill was a burn dump from 1967 to 1975. Then it was a solid waste landfill until May 2001. It was capped and closed in 2002. There are already at least eight monitoring wells there, according to county and state records, with five of them drilled in June 2014.

Now the Central Valley Regional Water Quality Control Board has directed the county to put in three sets of two wells each by May 16. The county request for bids was put out at 3 p.m. Friday.

From the county perspective, the extra monitoring wells are overkill for trace amounts of elements that have been present at the landfill since it closed, especially since the old landfill is situated over fractured rock that characterizes much of Tuolumne County.

No outside wells at risk

Wendy Wyels, an environmental program manager with the Central Valley Regional Water Quality Control Board, said Monday that concentrations of volatile organic compounds and inorganic compounds at the old landfill are low, and she is not aware of any public or private wells that are at risk from the landfill.

Wyels also said compounds detected at the landfill include dichlorofluromethane, 1,1-dichloroethane, chloride, sodium, potassium, sulfate, magnesium and bicarbonate.

The release from the landfill is a violation of state Waste Discharge Requirements, Wyels said.

“Tuolumne County is working to define the extent of the plume and determine the best way to remediate it,” Wyels said.

The Central Valley Regional Water Quality Control Board is not considering enforcement actions at this time, Wyels said.

Local knowledge

Dan Hembrick, a geologist and specialist with Tuolumne County’s solid waste division, said Monday the county has to monitor the Big Oak Flat Landfill site for at least 30 years.

“These things that are there, they’re associated with landfill gases,” Hembrick said Monday. “They’ve steadily gone down over time. These things were there when the landfill closed. The last report we had 1,1-DCH just above the trace amount, and dichlorofluromethane, they were both in trace amounts.”

Hembrick said county solid waste staff had to tell state water quality people that monitoring results they were seeing were not new releases, and the state ultimately rescinded a notice of violation.

“Long story short, they said they are concerned this stuff is migrating further downgrade and now they want six more wells to see if the stuff is going any further,” Hembrick said. “I’ll be shocked if it is. We’re in a fractured bedrock environment and it’s hard to determine where those fractures go. But there is no groundwater aquifer up here.”

Wyels said there are specific state regulations regarding landfills, known as Title 27 of the California Code of Regulations. The Water Board adopts permits for each landfill, which then implement Title 27.

One of the requirements of Title 27 is that there not be a release of waste from a landfill to groundwater, but if there is, then the extent must be determined and then the release must be cleaned up and prevented from happening again.

According to Feb. 26 correspondence from an engineering geologist with the state’s Waste Discharge Requirements Compliance and Enforcement Unit to Tuolumne County’s Department of Public Works, the county owns and maintains the Big Oak Flat Landfill, which is regulated by Waste Discharge Requirements Order R5-2013-0031.

The site consists of a single, five-acre closed unlined Class III waste management unit, about two miles south of Groveland on Merrell Road.

The county has already proposed in a December 2015 work plan to further assess landfill gas impacts within the landfill to better evaluate potential collection and treatment options.

County staff are required to upload data and documents for remediation at the landfill to GeoTracker, a State Water Resources Control Board website for tracking regulated facilities in California.