What seems like a clear case of an energy company polluting people's drinking wells in North Carolina is actually much more complicated, thanks to the state's rock formations.

Last week, North Carolina's Department of Environment and Natural Resources (DENR) sent another round of letters to residents living near the 32 ponds where Duke Energy, the largest electric power company in the country, dumps highly toxic waste known as coal ash produced at its 14 power plants. DENR found that 152 of the 163 private water wells it tested failed to meet state groundwater standards, according to the Associated Press. That's a 93 percent contamination rate.

The tests for contaminates are being conducted on all water wells within 1,000 feet of Duke's coal ash storage ponds, thanks to a 2014 state law passed after a breech at one of the storage ponds sent 39,000 tons of toxic ash into a major river basin — one of the worst coal ash spills in history. Coal ash is what's left over when coal is burned for electricity. It contains toxins such as arsenic, lead, mercury, cadmium, hexavalent chromium, and selenium, all of which can cause an array of illnesses in humans.

The most recent tests conducted by DENR found high levels of lead, vanadium, and hexavalent chromium in the contaminated wells. While Duke told AP it is providing bottled water to "about half a dozen" of the affected residents, Duke Energy spokeswoman Erin Culbert told VICE News that some of the toxic metals may be forming naturally in the groundwater.

"The substances identified in neighbor wells are naturally occurring and also found in coal, and that's why we're completing more studies to determine their origin," Culbert said.

That might sound hard to believe, especially given Duke's egregious history in North Carolina. But it turns out that rock formations in the western part of the state, where Duke's dirtiest coal-fired power plants are located, are naturally conducive to some of the metals found in the well tests.

Western North Carolina is in the middle of what geologists call the Blue Ridge ultramafic rock belt, which extends from the Appalachian Mountains in Virginia down into Georgia. Within this belt are high concentrations of serpentine and amphibolite rocks, which contain manganese and iron, the elements needed to oxidize chromium-3 — which is non-toxic — into hexavalent chromium, a carcinogen. While hexavalent chromium is most often produced by anthropogenic pollution, especially through the combustion of coal, oil, and natural gas, it is closely associated with rocks found in the Blue Ridge, according to the US Geological Survey.

Vanadium, another prevalent toxin in the well tests, is naturally present at high levels in mafic and ultramafic rocks. Pete Harrison, an attorney for the Waterkeeper Alliance, the watchdog group suing Duke Energy over its coal ash dumping, claimed that Duke Energy was the number one producer of vanadium in the state.

"There's still a lot of data to be gathered and analysis to be done, but I think Duke's position that this is all naturally occurring and their refusal to acknowledge the possibility that this stuff could be leaking out of their ash ponds looks more unreasonable over time," he told VICE News.

Dr. Laurie McNeill, a leading researcher at the Utah Water Research Laboratory who has studied the presence of hexavalent chromium in drinking water, told VICE News that it is difficult to discern whether the toxins found in the wells were a result of Duke's coal ash dumping.

"The fact is, it could be naturally occurring," McNeill said. "It is certainly suspicious when you look at the correlation [with Duke's past pollution], but correlation does not imply causation."

McNeill said that in order to build the case that contamination is the result of Duke's pollution, wells should be tested for toxins most commonly associated with coal ash. She also said tests should clarify which of the tested wells were upstream, because hexavalent chromium found upstream would indicate it was naturally occurring and not a result of coal ash "leaching" into the groundwater.

Those answers may come soon: DENR plans to continue testing wells near Duke's coal ash ponds in the coming months, and Harrison told VICE News that the Waterkeeper Alliance is exploring new ways to "fingerprint" contaminates like hexavalent chromium and vanadium back to coal ash.

In the meantime, residents are advised not to drink the water.