CUMBERLAND CITY (AP) — Coal ash is leaking from a dump complex at the largest coal-fired plant owned by the Tennessee Valley Authority and the utility is monitoring whether it’s affecting public water supplies.

CEO Jeff Lyash acknowledged the contamination during a media tour Tuesday of the Cumberland Fossil Plant in Stewart County, Tennessee, the Knoxville News Sentinel reported. Coal ash is waste left from burning coal to produce electricity.

TVA reports say coal ash is escaping through holes in dirt walls and toxins are leaking into groundwater monitoring wells. The wells are dug to determine what contamination may be present at a storage site, according to the newspaper.

“None of us should be surprised trace contaminates are showing up around our landfills,” the CEO said. “The right thing to do is to monitor that, characterize it, see if any of that material is migrating (into water supplies). … We don’t see any evidence there is any migration.”

Coal ash wastewater from the plant is released into the Cumberland River, according to TVA permits. The river is a source for two public water suppliers and a nearby city.

Plans to build a new coal ash dump on undeveloped land at the plant are underway, according to the TVA. There are also plans to build new lined water basins to better handle and treat water contaminated by coal ash, said Lyash, TVA Vice President Scott Turnbow and TVA staffer Danny Stephens.