Formatted Title
DNAPL Containment Remedy at a Large Site with a Complicated CSM
Background/Objectives
At a 187-acre remedial site in an industrial portion of central New Jersey, which has been undergoing remediation since 2010, supplemental investigation activities identified a 3.5-acre area impacted with chlorotoluene DNAPL outside the established site-wide remedial measures. The dense non-aqueous phase liquid (DNAPL) impacts were attributed to sediments present in historic alignments of drainage ditches which conveyed wastewater from the former industrial operations at the site to an adjacent river. The alignments of the drainage ditches had been significantly altered numerous times over the past 60 years due to the placement of dredge spoils and heterogeneous fill material by third parties. This resulted in DNAPL within the former ditch alignments and adjacent areas which was buried under 10 to 20 feet of fill material with varying hydrogeologic properties. Supplemental remedial actions were performed at the site with the objective of remediating the DNAPL and associated soil and groundwater impacts in accordance with the New Jersey Site Remediation Program requirements.
Approach/Activities
A multi-phase supplemental investigation of this area of concern (AOC) was performed from 2015-2018 to refine the extent of DNAPL contamination and update the site’s conceptual site model (CSM). Investigation activities included the use of sonic drilling combined with dual tube sampling technology, as the fill material contained large industrial debris that prevented the use of more conventional investigation technologies. From 2020-2022, remedial construction activities were implemented, including excavation and disposal/on-site consolidation of 1,800 cubic yards of accessible DNAPL-impacted media, installation of a 1,800 linear feet of steel sheet pile barrier wall, construction of 4.1 acres of a low-permeability surface cap, installation of 40 recovery/injection/ piezometer wells, in-place mitigation/creation of 8.5 acres of freshwater wetlands, and capping of 18 acres of adjacent portions of the site with overlapping areas of concern. A post-construction performance monitoring plan, including a bench-scale in situ treatability study, was performed to verify the effectiveness of the remedial actions and transition the AOC to long term monitoring. A long-term operations and maintenance plan, which includes routine passive DNAPL monitoring/recovery, is currently being performed.
Results/Lessons Learned
Significant industrial debris (i.e., concrete/steel bridge decking, a rail tanker car) present in the overlaying fill material caused disruptions to the remedial construction activities, but were ultimately overcome. The post-construction performance monitoring results indicate the remedial actions are effectively containing the DNAPL source material and creating conditions for groundwater impacts outside the containment area to naturally attenuate. The remedial design and hydrogeologic properties of the AOC resulted in the horizontal and vertical containment of the source material without having to incorporate a groundwater extraction system to facilitate hydraulic control. . A treatability study performed to evaluate if injections of remedial amendments could enhance in-situ anerobic biological degradation of the DNAPL concluded that the evaluated reagents did not have a significant effect and therefore a supplemental injection program was not implemented as originally planned.