Sipping Coal Tar, a Cup at Time – Up Until Now

For many years, black and thick coal tar has been slowly collecting in a bedrock depression that forms a natural catch basin 50 feet below the grass of a U.S. municipal park. And as the accumulating weight of the underground "pond" is now forcing the tar into ever-smaller fissures of bedrock, the obvious response is to get the gunk out of there ASAP.

Which is exactly what the area utility company, its contracted engineers and the administrators of a mid-sized city want to do. So far it has been slow going; they are employing two top-head-drive piston pumps to pull up a mere cup of tar each, twice an hour. That's a quart in 60 minutes, or only six gallons a day. It takes nine days to fill a 55 gallon drum.

But there's good reason behind the serum-slow production. And the engineers say that volume is about to ramp up in a hurry.

The utility company, like virtually all utilities, for decades burned coal to manufacture gas. And like others, the company removed the byproduct coal tar to holding tanks, in this case near a local river. Eventually those tanks began leaking and over time the barely-liquid tar made its way down to bedrock, rode the rock surface slope under the river, then finally pooled in the depression, a natural bowl against a berm.

The flow was slow but relentless, and eventually the weight of the underground pond began forcing the tar into bedrock cracks, posing a threat to deep groundwater. And because of the tar's high viscosity, shallow groundwater contaminated by lighter oil also has collected in the underground pool above the tar.

The simple solution is pumping, but the pumping is not simple – engineers know it's tricky when it's sticky. The site's two-phase recovery goal is to remove both tar and oil but not the groundwater. The tactical challenge is to bring up the products without roiling the formation. Too much water per stroke significantly increases costs and the negative eco impacts of disposal.