Swamp-Assisted Remediation By Natural Attenuation: Rocky Mount, NC
A former surface impoundment for electroplating sludge at a metals manufacturing facility resulted in an elongated plume of dissolved chlorinated hydrocarbon compounds, including 1,1,1-TCA, 1,1-DCA, and 1,1-DCE, extending from the former surface impoundment to a downgradient swamp approximately ½ mile away, which serves as a reservoir of water that trickles into a series of receiving creeks/streams. Multiple rounds of groundwater and surface water sampling in and around the swamp had demonstrated that the swamp was the discharge zone of groundwater flowing from the site and that groundwater contaminant concentrations, which were as much as 500-times the groundwater and surface water standards directly upgradient of the swamp, were being attenuated to levels below both groundwater and surface water standards through the swamp discharge process.
Under the requirements of the facility U.S. EPA, Part B HSWA permit, the precise geochemical mechanism through which the groundwater contaminant concentrations were being eliminated during discharge to the swamp needed to be established in order for “swamp-assisted remediation by natural attenuation” to be defined as the Final Remedy for site remediation.
Buildup of organic detritus and sediment over the years resulted in a swamp-bed of dark muck, rich in organic matter but depleted in dissolved oxygen (DO). Discharge of contaminated groundwater into the swamp from the surficial sandy aquifer is channeled by an underlying clay aquitard. Through a multi-level well-point system installed by Piedmont Geologic, the process of reductive dechlorination of chlorinated hydrocarbons in groundwater through the swamp sediments was defined. With increasing vertical travel distance through the swamp bed, ratios of parent to daughter compounds steadily decrease, with a final dramatic decrease in both parent/daughter ratio and contaminant concentrations through the organic muck swamp-bed layer. By this matter, the swamp muck serves as a “filter” for removal of contaminants from groundwater.
Swamp-assisted remediation by natural attenuation is specified as the Final Remedy for site groundwater contamination in the facility Part B HSWA permit. The finding has resulted in a Client savings of hundreds of thousands of dollars that would have been required for installation and operation of an active groundwater remediation system.