COLUMBIA — Toxic tar was discovered by a kayaker on the bottom of the Congaree River in 2010. Nearly a decade later, work could finally begin to remove it.

The saga surrounding what to do about the remnant from a former coal plant that produced gas to fuel street lights more than 100 years ago has concerned environmentalists and held up plans for riverfront development in Columbia.

Virginia-based Dominion Energy, which inherited the coal tar after completing its purchase of SCANA this year, anticipates filing in the spring for what Congaree Riverkeeper Bill Stangler calls the hardest of the necessary permits for cleanup — one that would allow partial damming of the river.

"Whenever you talk about building a dam in a river, that water has to go somewhere," said Brice McKoy, Columbia branch chief for the Army Corps of Engineers Regulatory Division.

The latest plan calls for installing temporary barriers, called cofferdams, around the 3-acre work sites, pushing the water away while excavation occurs. The work could start May 2021 and last until 2024. Previous cost estimates for cleanup have ranged from $9 million to $18 million, though those numbers could change.

The plan won't remove all the tar, which can contain cancer-causing chemicals and can irritate skin, but it could eliminate 70 to 75 percent from the areas of the river between the Gervais and Blossom street bridges, where most recreation and public access happens, Stangler said. The coal tar would be loaded onto dump trucks and hauled to a landfill. Most of the coal tar is on the Columbia side of the river.

In order to win the Corps' permission, Dominion must show the temporary dam won’t cause major flooding on the West Columbia and Cayce side of the Congaree while work is occurring.

The dam is going through a series of studies.

Dominion is awaiting comments from the Corps on an analysis of how the dam could affect flooding during heavy downpours, according to its latest filing with the S.C. Department of Health and Environmental Control. Still on the list are engineering studies determining stability, height and width of the dam as well as dam leakage estimates.

Dominion is also considering a different type of cofferdam that could reduce the size of the work area and eliminate adverse effects on the river's western shoreline. Dominion anticipates winning approval for the dam in about a year. Floodplain approvals also are needed from the Federal Emergency Management Agency. 

"It gets very, very technical," McKoy said.

Dominion also plans to begin work early next year on determining how large a work site is needed on the riverbank, according to the DHEC filing.

Not knowing how much land will be needed for these operations has held up further development of Columbia's riverfront, said Charlie Thompson, who manages a roughly 60-acre swath of land owned by the Guignard family between Huger Street and the Congaree.

The Guignards and the city of Columbia have long-laid plans for a riverfront park edged by shops and restaurants, Thompson said, similar to the type of development seen across the river in West Columbia.

Thompson is worried that delays in getting tar removal allow time for the economy to dip or for momentum around the public portion of the project to falter. Richland County is already discussing cuts to its penny tax project list, a program slated to pay for some of the infrastructure in the riverfront area.

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Stangler had expected the permit filing to take place this fall.

"This is a complex project with several moving parts, and we will keep working to make sure this effort continues to move forward," Stangler said.

Dominion stated it is committed to completing the cleanup project.

These projects are not rare in the Carolinas.

"Over the last 28 years, SCANA subsidiaries, which are now part of Dominion Energy, have worked diligently to clean up manufactured gas plant sites in Charleston, Columbia, Florence and Sumter, S.C., and Concord, Gastonia, Statesville, Durham and Raleigh, N.C.," company spokeswoman Aimée Murray said in an email.

Utilities have been cleaning up some coal tar sites for a quarter-century. Duke Energy, South Carolina's other large investor-owned utility, has conducted cleanup at locations in Greenville, Spartanburg and Anderson.

McKoy said there have been other water-based removal efforts in the state but "not in a very dynamic river system like this" where there can be a lot of changes in water velocity and flow.

In Charleston, SCANA dug up soil along the Cooper River contaminated with coal tar on the site that now houses the S.C. Aquarium. In the river itself, the company installed a stone and fabric cap. A similar solution was proposed by SCANA for the Congaree in 2017 until Congaree Riverkeeper sued to force a return to removal efforts.

The coal tar came from an old SCE&G coal gasification plant, leaking into the river in the early to mid-20th century.

Coal tar can contain benzene, a cancer-causing material, and while there has been no evidence at the Congaree site, in other parts of the country, tumors have been found on fish exposed to coal-tar polluted water.

Jessica Holdman is a business reporter for The Post & Courier covering Columbia. Prior to moving to South Carolina, she reported on business in North Dakota for The Bismarck Tribune and has previously written for The Spokesman-Review in Spokane, Wash.