“When we removed the tanks, there was just one little hole in the diesel tank and there was no product in the groundwater underneath the thing, so we’re kind of all scratching our heads, the EPA (Environmental Protection Agency) included,” said Chris Singleton of Singleton Environmental, which is working on the cleanup.

The corrosion-caused hole is about the size of a BB, roughly 3 millimeters on each side, Singleton said. It’s the only deficiency he could find in the tank, with the other three tanks on the site — all holding gasoline — appearing “brand new.”

Contamination around the site doesn’t square with the amount of fuel that’s been discovered leaking out of the riverbanks, Singleton said. Given the roughly 500 gallons of petroleum recovered so far, he expected to find much worse contamination in the tank’s immediate vicinity.

“There might be some further investigation to see if there’s any more tanks in the area,” he said. “What we saw around the tanks just wasn’t indicative of what we’re seeing entering the river.”

At this point, it’s hard to say exactly what happened. Possibilities include that some of the contamination came from the property across the street, which used to house a gas station. Or the contamination could have traveled through ditches and culverts, accounting for the lack of oil in the immediate vicinity of the tank. The cleanup is just getting to a stage where Singleton can start investigating the cause. Over the coming months, he plans to take a hard look at where the contamination came from and the path it took to arrive where it did.

“It’s kind of like a ‘to be continued,’” he said.

The issue first arose on April 3, when the Eastern Band of Cherokee Indians’ Office of Environment and Natural Resources discovered petroleum product along the banks of the Oconaluftee River, presumably due to leakage from underground fuel tanks at the Cherokee Mini-Mart.

The store proceeded to hire Singleton Environmental to deal with the cleanup and Mountain Environmental to protect the river from contamination.

Singleton’s company dug out all four tanks — three for gasoline and one for diesel — and, after inspecting them, sent them to a scrapyard in Asheville. Soil surrounding the tanks was also removed, excavated to 18 inches below the water table, said Jeremy Hyatt, the EBCI’s director of natural resources and construction.

Oil booms were set up along the riverbank to trap fuel that’s still leaking out of the soil, to date catching about 500 gallons of diesel. The tank had a capacity of 4,000 gallons, but it’s uncertain how much of that leaked. Once the leak was detected, the Mini-Mart had the remaining fuel pumped out, injecting the tank with water to test its ability to hold liquid.

The booms are apparently doing their job.

“There’s nothing getting past the booms we have out here,” Singleton said.

According to Hyatt, water samples taken in early June revealed 0 percent contamination in the river water.

“This is great news and shows that our mitigation efforts are really paying off,” he said.

But that doesn’t necessarily mean that the site is home free. Singleton is just starting on the next phase of the cleanup — mapping the oil plume in the soil to determine just how far underground contamination has spread. That process could take another two or three months and will reveal whether contamination has worked its way into the groundwater.

Though it’s impossible to say for sure at this point, he predicts the oil plume could extend anywhere between 200 and 500 feet horizontally, with the possibility that it’s reached underground culverts in the area, carrying it further. Contamination would have to penetrate 10 to 12 feet below ground to reach the groundwater.

To find out whether that’s happened, he’ll install some groundwater monitoring wells and, based on the results, install additional ones to pinpoint the limits of the plume.

The Mini-Mart is paying for all the cleanup efforts, with oversight from the EBCI, but they will likely get some reimbursement from the N.C. Commercial Trust Fund, which provides reimbursement to companies for cleaning up soil and groundwater contamination resulting from underground tank leaks. To date, cleanup costs have exceeded $120,000, Singleton said.

It’s a significant spill, but not unheard of, he said. In fact, this is the third location in that half-mile stretch of U.S. 441 where such a spill has happened in the past decade.

“When you put single-walled tanks that close to the river, it’s something that can happen,” he said. “Newer tanks are better designed to handle the conditions than these older single-walled tanks are.”

The corroded diesel tank dates back to the mid-1980s, Singleton said, and had a single-walled construction. It’s hard to say how long the tank was designed to last, he said, because back then manufacturers weren’t required to put warranties on their tanks. These days, they are, with most modern double-walled fiberglass tanks guaranteed for 50 years. All tanks are required to undergo yearly inspections, a regulation with which the Mini-Mart has complied.

“They were inspected annually by the tribe and by the EPA,” Singleton said. “They had very good records.”