Thirteen families in the heart of the gas-rich Marcellus Shale of northeastern Pennsylvania say their water wells have been contaminated by hazardous fluids blasted deep underground by fracking, a process used to free the gas.

A lawsuit filed in Susquehanna County says a faulty well drilled by Southwestern Energy Co. of Houston, Tex., leaked fracking fluid into the surrounding groundwater. The suit claims the well’s cement casing was defective. It also cites spills of industrial waste, diesel fuel and other hazardous substances.

The suit, filed Monday, also says a child has been sickened, showing neurological symptoms “consistent with toxic exposure to heavy metals.” The plaintiffs seek monetary damages, an environmental cleanup and medical monitoring.

A Southwestern spokesman said the well in question is mechanically sound and that there’s no evidence Southwestern’s drilling operations have harmed water supplies. He declined comment on the suit.

The suit was first reported by The Times-Tribune of Scranton.