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Bedrock slows search for source of toxic TCE in groundwater near SLO airport

How experts are searching for TCE near the SLO airport

Jon Rohrer, principal hydrogeologist with Roux Associates, describes the process for TCE testing near Buckley Road and the airport in San Luis Obispo.
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Jon Rohrer, principal hydrogeologist with Roux Associates, describes the process for TCE testing near Buckley Road and the airport in San Luis Obispo.

San Luis Obispo County is continuing its investigation into a toxic solvent contaminating 12 wells along Buckley Road near the San Luis Obispo Regional Airport, but work has slowed because drillers encountered hard bedrock.

“The first exploratory boring that was drilled encountered several unexpected ‘hardpan cemented layers’ of soil that were very difficult to drill through, and these cemented layers were present at shallower depths than the water table and bedrock,” said Dean Thomas, a geologist with the Central Coast Regional Water Quality Control Board.

The cemented layers were present at depths shallower than 45 feet and did not allow drillers to reach groundwater.

The county switched to a different drilling method and was able to penetrate the hardpan layers and bedrock to a depth of 115 feet. However, preliminary results from the water testing, which were due Aug. 10, have not been released.

“We will provide an update once groundwater samples are available from the sonic drilling investigation along Buckley Road,” Thomas said.

Sonic drilling uses vibrations in order to hammer its bit through materials, as opposed to traditional drilling methods.

The county began its investigation in July, when the 12 wells along Buckley Road were found to have levels of the solvent trichloroethylene, a known carcinogen, that exceeded state standards. In all, 67 wells in the Buckley Road area were sampled.

Also as part of its investigation, the county has collected more than 100 passive shallow soil gas samples. Soil gas sampling is a technique for detecting TCE by the vapors it gives off.

Passive soil gas samples are being used to identify potential areas where TCE may have impacted soil and groundwater at various locations on the airport property. Nineteen soil gas samples have been tested, and all tested negative, Thomas said.

“There’s no smoking gun yet,” he said.

Soil gas samples are obtained by inserting a probe containing granulated charcoal 3 feet into the ground and allowing it to collect vapor that emanates from the soil.

State water officials consider the airport a likely source for the contamination because TCE is used as a chlorinated industrial solvent. However, the county disagrees, saying there is no record of chemical being used at the airport.

“To date, we have yet to find anything to indicate that the source of TCE found in the groundwater along Buckley Road is coming from airport property,” said Guy Savage, assistant county administrative officer. “Regardless, we will continue searching until we are satisfied that airport property is not the source.”

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