CHARLOTTE, N.C. -- Thallium used to be an ingredient in rat poison, until the United States banned it in 1975.

It doesn't take as high a concentration as other things to make you sick, says Catawba Riverkeeper Sam Perkins.

So... Why are we talking about it today? Because state regulators found thallium in surface water near one of Duke Energy's coal ash ponds last month -- specifically at the Cliffside Steam Station on the Cleveland-Rutherford County line.

The big problem is, when you have coal ash being stored in an unlined facility, then rainwater, storm water will steep that coal ash and carry a lot of those metals out into groundwater and surface water, says Perkins.

The report from the North Carolina Department of Environment and Natural Resources, NCDENR for short, says the amount of thallium found in water at the foot of one of Duke s coal ash dams was higher than EPA standards for drinking water. Another test closer to the Broad River found thallium at lower levels, and NCDENR said no drinking water downstream had been affected.

In previous stories NBC Charlotte has shown you water flowing from the foot of these dams that hold these ponds back.

I think that's the first thing to understand is that these coal ash ponds are not ponds dug in the ground like we would think of a pond at a park, says Perkins. These are piled up like mashed potatoes holding back a toxic gravy. And they're unlined.

Last month, the Riverkeeper in Asheville sampled water in other spots upstream on the Broad River. In one spot, he found arsenic at 25 times the level acceptable for drinking water. NCDENR says it did test for other toxic metals like arsenic-- but those results haven't come back from the state lab just yet.

When you see thallium and you see it above what the state standard is at the coal ash, then you've got to be concerned about the arsenic, manganese, cobalt and a lot of the other metals that cause problems, says Perkins.

This isn't just happening at Cliffside -- NCDENR tested water around all of duke's coal ash ponds in North Carolina last month. That testing found a higher level of thallium near one of Duke's ponds at its power plant south of Asheville, but since there's not a drinking water source downstream, the EPA standards are less strict.

The dangers of coal ash were highlighted in February, after a spill sent millions of gallons of coal ash into the Dan River in Rockingham County. Duke says it wants to clean up its ponds, but also says customers like you and me would have to pay for it.

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