The Koch brothers finally took their towering piles of tar-sands oil refinery waste away from Detroit.

But where did they send the stuff? That’s a bit of a mystery.

Huge piles of petroleum coke started building up along the city’s riverfront after a refinery began processing tar-sands oil from Canada in November. Koch Carbon, an affiliate of Koch Industries, peered into the dark mass and saw, ka-ching, opportunity, so it bought up all the waste.

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The material has little commercial value in the U.S., where burning it would likely violate clean air laws unless expensive emissions-control equipment were used. But it can be sold for a decent-enough price in other countries with laxer air pollution laws. Indeed, we told you in June that ships were hauling some of the waste back to a power plant in Canada — but not enough of it to keep the piles from growing.

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Detroit ordered the petcoke piles to be removed by Aug. 9. That order was ignored, so the mayor’s office issued another order, saying they had to be removed by Aug. 27. That deadline was also not met. But this week, The Columbus Dispatch reported that the waste had finally been removed from the riverfront:

A four-story mound of black, gritty refinery waste that recently was ordered off the banks of the Detroit River likely was moved to Ohio. Where? Those who know aren’t saying. …

Brad Wurfel, spokesman for the Michigan Department of Environmental Quality, said he was told that the pile was shipped to Ohio. But he said he didn’t know where. And Koch Carbon isn’t talking.

If you happen to notice an enormous pile of black waste in your neighborhood, do let us know, won’t you?