ASHEVILLE, N.C. (AP) — North Carolina’s environmental agency has ordered Duke Energy to install monitoring wells in a residential neighborhood outside Asheville to determine whether toxic chemicals from the company’s coal ash pits are contaminating homeowners’ drinking water.

During tests at five homes last fall, traces of thallium were detected in one of the drinking wells. Although below state or federal drinking water standards, authorities say more testing is needed to determine both the source and the severity of pollution they believe is leaching from coal ash pits nearly a quarter mile away.