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Starting in January, Madison’s water utility will step up its testing for toxic chemicals that are spreading from highly contaminated soil and groundwater at Truax Air National Guard Base on the city’s North Side.

Fluorinated compounds, known as PFAS, from military firefighting foam have soaked into soil and shallow groundwater on the base, and last year the chemicals turned up in low levels nearly a mile away at Madison Water Utility’s Well 15 on East Washington Avenue.

The Air National Guard has known about the contamination for at least three years but hasn’t monitored its spread, so there’s no way to know if or when the levels in Well 15 waters will rise to more dangerous levels.

To keep track of the concentrations being drawn into the well, the Madison Water Utility will begin taking one sample monthly at the well next month at a cost of $250 to $700 per sample, said spokeswoman Amy Barrilleaux.

Evidence has mounted since the 1990s linking PFAS compounds to cancer and an array of other serious health problems in children and adults.