Updates - Congaree River

*October 13, 2023

photo of completion of cofferdam removal on Congaree River

On October 5, DHEC visited the Area 2 Cofferdam to visually inspect that tar-like material and sediment has been removed to the extent practicable. After the final site inspection, DHEC sent a letter to Dominion Energy stating that the removal efforts were complete and that the Area 2 Cofferdam could begin process of deconstruction. In all, more than 35,000 tons of material was removed from the Area 1 and Area 2 Cofferdams and sent to a landfill.

deconstruction of cofferdam on congaree river

Workers have started the process of removing the Area 2 Cofferdam. The pumps have been removed and the concrete armoring is being stripped off.  Once that is complete, workers will begin in the center of the cofferdam road and work their way both ways removing the rock until they get to the river bank.  

restoration grass in area 1 of Congaree River cleanup

Restoration of the shoreline near the Area 1 cofferdam is taking shape. The banks have been armored to prevent erosion, the area beyond that has been hydroseeded, and soon trees will be planted to aid in the restoration process.

image of material being run through a landfill screen searching for artifacts

Once the material has been excavated from the cofferdam it is being transported to a landfill.  At the landfill, the tar-like material and sediment is being run through a screen to sort out the material by particle size. The larger screened particles are scanned with metal detectors and any items detected are looked at by archeologists to determine if they may be a Civil War artifact or have other historical significance. They have been able to identify a fair amount of Civil War artifacts and other items through this process. This work will be ongoing for the next few months.

cofferdam in Congaree River armoring being removed

In mid-August, workers began removing the Area 1 Cofferdam from the Congaree River. In the image above, only a small section of the concrete fabriform armoring is left. Following removal of the armoring, workers began in the middle of the cofferdam wall and started removing rock going both south and north. In the image below from August 25, workers have removed most of the rock from the Area 1 Cofferdam except a small section of the northern leg.

cofferdam in Congaree River being removed

As the Area 1 Cofferdam was being removed, some of the rock from it has been utilized in the construction of the Area 2 Cofferdam. The Area 2 Cofferdam covers a much smaller area than the Area 1 Cofferdam at about one-fifth the size. The image below shows the Area 2 Cofferdam.  In the next few days, the Area 2 Cofferdam will be armored with the concrete fabriform to strengthen it against overtopping events.

cofferdam in Congaree River being armored with concrete

Congaree - Coal Tar Removed

Workers have completed the removal of coal tar and sediment from the Area 1 Cofferdam. Over 26,000 cubic yards of material has been removed. DHEC completed a final inspection within the Area 1 Cofferdam on August 7, 2023, and determined that all coal tar and sediment has been removed to the extent practicable as envisioned in the Modified Removal Action Work Plan.

Congaree River - Water Cannons removing remaining coal tar and sediment

Once the coal tar and sediment was excavated, workers used high pressure water cannons to remove any remaining coal tar and sediment until until they reached the granite river bottom. The water used during pressure washing was maintained within the cofferdam and pumped up to a water treatment system to be processed before going to the City of Columbia Sewer system.


After all material was removed and cleaned with pressure washing, workers placed rock along the river's edge to stabilize the banks from erosion. In the next few weeks, the Area 1 Cofferdam will be removed. The large sand bags will be removed first and then the pumps will be moved from the cofferdam. River water will begin to infiltrate into the cofferdam. Workers will remove the concrete fabriform armoring from the cofferdam. In the last step, workers will start from the middle of the cofferdam and remove the rock until they complete the removal at the shore line.

As this is taking place, workers will begin to construct the Area 2 Cofferdam. Some of the material from the Area 1 Cofferdam will be utilized in the construction of the Area 2 Cofferdam. The Area 2 Cofferdam is much smaller and only a fifth of the size of the first cofferdam. Depending on weather conditions, the Area 2 Cofferdam will be completed during this working season.

Aerial Image Cell 1 Removal

Workers have made great progress in removing the coal tar and sediment from Cell 1 of the Area 1 Cofferdam. They have removed over 500 truckloads and 10,000+ tons of material from Cell 1. Removal was completed in Cell 1 on June 16, 2023. Workers  pressure washed the bottom of the area down to bedrock to get as much coal tar removed as possible.

Cell 1 Removal from Berm

The Unexploded Ordnance team completed clearing of Cell 2 of the Area 1 Cofferdam on June 17, 2023. They have recovered a fair amount of Civil War era artifacts that have been turned over to the archaeologists for preservation.

Cell 1 Removal from Bridge

Now that Cell 2 of the Area 1 Cofferdam is cleared, workers will start removal operations in it. Removal operations in Cell 2 should take about 2-4 weeks depending on weather conditions. On June 22 the Congaree River levels overtopped the cofferdam due to heavy rain in Columbia and upstate South Carolina. The cofferdam remained overtopped until June 26, 2023. Dewatering is already underway and may delay removal efforts by a few days.

Congaree River cleanup landfill

The coal tar and sediments that are being removed are being carried to a landfill in Richland County. The landfill has allowed Dominion Energy to set up a special area for the material to be staged so that the archaeologists can sift through and locate any artifacts that may have not been detected during the Unexploded Ordnance team's clearing of the area. They have identified several small Civil War artifacts to date. Once the material has been investigated at the landfill it will be disposed of properly there.

view of cofferdam in Congaree River from bridge 4

As you can see from the picture above, workers have made huge progress in sealing and dewatering the cofferdam. The Northern Cell sealing is complete and should be cleared for any Civil War artifacts or munitions by June. The Southern Cell sealing efforts are 80% complete and clearing of artifacts in it will begin in June.   

The early May Congaree River flooding event was a good test of the sealed cofferdam and provided a good practice for workers to stabilize the site and move pumps from the cofferdam’s edge in a limited timeframe. The flooding event overtopped the cofferdam by around 10 feet for several days and within 5 days workers had the interior of the Northern Cell dry once again.   

coal tar in Congaree River

This picture is a good representation of the layers of coal tar that can be seen since the Northern Cell has been dewatered. Removal of coal tar in the Northern Cell will begin by June 1. Dominion Energy will be monitoring any odors from the coal tar once it is disturbed and will be spraying the material in loaded trucks with a solution that will trap the odor and limit its ability to affect the neighborhood during transport.   

wastewater treatment plant

The wastewater treatment plant at the site has been constructed and is operational. It will be utilized to treat any water that comes into contact with the coal tar during removal activities or water used to pressure wash the bedrock bottom of the river. Once treated, the water will be disposed of in the POTW. 

Year two of work is underway at the Congaree River Sediment Site.

Workers have mobilized once again to the Congaree River Sediment Site. For the first few weeks, they will be installing a berm in the center of the Area 1 Cofferdam which will aid in dewatering behind the cofferdam. They will also be working on sealing any areas on the inside of the cofferdam where water may be seeping through. By mid-May, removal activities will begin. As material is removed, it will be examined for any Civil War artifacts or munitions. In order to maximize production, the site will operate six days a week this year, but no trucks will be hauling material on Saturdays. Weather permitting, the Area 1 cofferdam is expected to be completed this summer.

Year one of work was a huge success and concluded on Oct. 7, 2022. The river cooperated and stayed at comfortably low levels throughout the working season. 

During year one, Dominion Energy:

  • Completed the rock cofferdam for Area 1
  • Armored the rock cofferdam for Area 1 with a concrete injected liner
  • Conducted unexploded ordnance evaluations for Area 1 and Area 2
  • Began leak testing and sealing
aerial view of cofferdam

The cofferdam was designed to handle expected overtopping from flooding. So far during the fall and winter season, there have been four periods where the Congaree River levels were above the height of the cofferdam. The cofferdam has withstood these events with minimum issues.

flooded, muddy congaree river

Year two operations will begin in mid-April. During year two, Dominion Energy plans to:

  • Seal leaks in the Area 1 cofferdam 
  • Build an internal berm across Area 1 
  • Dewater Area 1 
  • Remove the sediment and coal tar from within Area 1 
  • Remove the Area 1 cofferdam
  • If time permits, build Area 2 cofferdam and begin Area 2 removal activities

Over the last month, Dominion Energy has:

  • Completed the rock cofferdam for Area 1
  • Started armoring the cofferdam with a concrete injected liner to provide additional strength in the event of an overtopping

Next, Dominion Energy will:

  • Drain the entrapped water
  • Relocate mussels from the dammed area 
 Finished Cofferdam Aerial Image


Armoring cofferdam in process


up close shot of the cofferdam being armored

Over the last few weeks, the contractors have been installing the Area 1 cofferdam. Working from both the north end and the south end of Area 1, they were able to meet in the middle to form the footprint for the Area 1 cofferdam. They will continue to raise the level of the cofferdam and conduct other related activities over the next week. In a couple of weeks, they will start a process to armor the cofferdam so it will be able to be overtopped in the event of flooding conditions in the river. They will also be relocating mussels from the area blocked off by the cofferdam.

Congaree Update - Cofferdam Connected


Congaree Update - Cofferdam Connected2

Landside setup operations have been completed. The Unexploded Ordnance Team from Tetra Tech has completed clearing the Area 1 and Area 2 cofferdam footprint from any Civil War era munitions. Cofferdam construction for Area 1 began on June 20, 2022. Cofferdam construction for Area 1 is expected to take about 5 weeks followed by 4 weeks for the cofferdam to be reinforced and armored.  

Cofferdam A with Bulldozer

cofferdam and dump truck

Cofferdam with Excavator

Congaree - Cofferdam PM

Landside setup operations continue at the site.  Installing fencing around the site, completing the access roads for the Area 1 cofferdam, installing security cameras, and setting up staging areas for cofferdam stone have been top priorities.  

The Unexploded Ordnance Diving Team from Tetra Tech arrived this week and will begin clearing the Area 1 cofferdam footprint from any Civil War era munitions.  If the weather is good, this process should take about two weeks.  Once the cofferdam footprint is cleared, cofferdam construction will begin.  

Area 1 Access Road


Boat setting buoys


Sign with Partially Constructed Fence Behind

On May 16, 2022, the contractor for Dominion Energy (Entact) mobilized off the end of Senate Street to set-up an operations area and begin installing:

  • Onsite routes for truck traffic; 
  • Utility poles for light and video surveillance;
  • Temporary office trailers; and
  • Fencing around the construction area.

Security of the site will be enforced 24 hours per day/7 days a week for the duration of the cleanup.

In early June, the contractor plans to begin stockpiling stone for the coffer dam construction. 

Congaree Sign

Congaree Land Side

Congaree - Sign with Site Behind

Congaree - Site Prep from Bridge