Officials are still working to figure out just how a toxic industrial solvent contaminated five SLO wells in late December.

The chemical, known as trichloroethene or trichloroethylene (TCE), is a probable carcinogen and was discovered in amounts above Federal Environmental Protection Agency standards in the area of Buckley Road, bounded by Davenport Creek Road, Evans Road, and Thread Lane Dec. 24, according to SLO County’s Environmental Health Services Department.

According to the department, staff responded to a complaint in October from a well owner who said their water smelled like chemicals. The water was tested and TCE was discovered. A later test of 10 other nearby wells revealed TCE amounts above the EPA’s levels in four additional wells.

According to the department, an investigation into the contamination is being conducted in concert with the Central Coast Regional Water Quality Board. So far, there’s been no answer to the question of how the chemical got into the water.

“The water board has an open case in this area and is responsible for investigating what is believed to be historical contamination but has not identified a specific property or source of contamination,” a statement on the county’s website read. “The water board has asked other property owners to conduct assessments in past years and more recently has asked the airport to conduct a review of their property as well.”

While the board’s staff could find no property that recently used TCE, it noted that the state’s historic drought might be responsible for higher concentrations of the chemical in the wells.

“According to the water board staff, it is possible that the drought may be contributing to the increase in TCE concentrations due to a decreased dilution effect,” the county’s website stated. “Meaning TCE may represent a higher proportion of the water content in use since groundwater levels are historically low.”

Property owners in the impacted area were notified by letter, and the board is offering to pay for well testing for 40 wells located in the immediate area where the contamination was discovered. The water board will hold a public meeting in January to discuss its investigations, but has not yet set a date.