SCE&G’s (now Dominion Energy) investigation activities of the Congaree River consisted of 5 phases of preliminary surveys and sampling from September 2010 through March 2012. A total of 244 sediment and soil samples were collected to determine the depth and extent of the tar-like material (TLM). Of the samples collected, 40 were sent to a laboratory to be further analyzed. The analytical results indicated the presence of some volatile organic compounds (VOCs) in the samples. The dominant group of semi-volatile organic compounds (SVOCs) detected were polynuclear aromatic hydrocarbons (PAHs). The sediment layer containing TLM varies from approximately less than 1 inch in thickness to at least 1 foot. Water samples from the river were also collected and have been analyzed. The water samples have shown no tar-related chemicals of concern.
The map below shows the sampling points and grid lines used in the assessment of the affected area of the Congaree River.
In April 2017, DHEC completed a surface water sampling event in the Congaree River to determine if TLM constituents were being released into water. A total of 14 surface water samples were collected near the eastern bank from the Gervais Street Bridge to the Blossom Street Bridge. There were no detections in the samples for any TLM-related constituents.
Following this event, DHEC requested SCE&G to conduct future surface water sampling on a semi-annual basis along this stretch of the river. SCE&G has conducted 3 additional surface water sampling events. To date, no TLM-related constituents have been detected.
Aquatic Macroinvertebrate Bio-assessment
In June 2017, staff of the Aquatic Biology Section within DHEC’s Bureau of Water conducted an aquatic macroinvertebrate bio-assessment of the Congaree River. The goal of the study was to determine if sediment contaminated with coal tar was having an adverse impact to the indigenous invertebrate fauna near the sediment plume. Two stations were established to evaluate the potential effects of the contamination on the biotic health of the river. A control site was located immediately upstream of the Gervais Street Bridge and a test site was established at the Blossom Street Bridge, directly downstream of the areas with highest volumes of TLM material.
The results of the June 2017 study indicated that the aquatic macroinvertebrate community at the Blossom Street Bridge was comparable to the upriver control, both receiving a bio-classification score of 4.5 (Excellent) on the Carolina Bio-condition Scale. The community structure at both sites showed that there was a diverse and balanced community of invertebrates, with the presence of numerous pollution sensitive species. The EPT index, which quantifies the number of pollution sensitive mayflies, stoneflies, and caddisflies, respectively, was 22 at both the control and test site. These values were similar to those recorded in the recent past by DHEC on the lower Broad River and other locations on the Congaree River.