MADISON, Wis. (AP) — Madison water utility officials say they will step up testing for toxic chemicals that are spreading from contaminated soil and groundwater at Truax Air National Guard Base on the city's north side.

The State Journal reports that fluorinated compounds from military firefighting foam have soaked into soil and shallow groundwater on the base. Last year the chemicals turned up in low levels at a city well nearly a mile away.

The Air National Guard has known about the contamination for at least three years but hasn't monitored its spread, so authorities aren't sure if levels in the well water will rise to more dangerous levels. Fluorinated compounds have been linked to serious health problems.

Madison Water Utility spokeswoman Amy Barrilleaux says the collection of monthly samples will begin in January.