Formatted Title
Combined Remedies to Expedite Remediation of a Carbon Tetrachloride Source Zone at an Active Grain Elevator Facility
Background/Objectives
Background/Objectives. The site is located in Kansas City, Kansas, and currently operates as an active grain elevator facility. The site entered into the Voluntary Cleanup and Property Redevelopment Program (VCPRP) in 2000 following groundwater and soil detections of grain fumigant constituents of concern (COCs), including carbon tetrachloride, in the vicinity of a former fumigant aboveground storage tank (AST). Following source area investigation and groundwater plume delineation activities, dual-phase vacuum extraction (DPVE) was implemented in 2007 for the removal of COCs in source area soils and groundwater. Groundwater is encountered at the site approximately 7 to 8 feet below ground surface (bgs). Lithology within the targeted source zone generally consists of well sorted, loose, silty-sand to depths ranging from approximately 13 to 17 feet bgs, underlain by silty clay.
After approximately 6 years of DPVE operation, resulting in the removal of over 9,000 pounds of total VOCs, a subset of source area extraction wells continued to exhibit elevated COC groundwater concentrations. Additional high-resolution investigation was conducted to assess the nature and extent of residual COC mass in the source area and provide data required for the evaluation of alternatives that could expedite source area remediation. The investigation results indicated significant sorbed-phase COC mass, generally limited to the shallow, sandy interval of an area bound by the DPVE wells exhibiting elevated COC concentrations. Light non-aqueous phase liquid (LNAPL) heavily impacted with the site COCs was also identified. Surfactant enhanced extraction (SEE) was subsequently identified as the optimal source zone remedial alternative because of the technology’s ability to quickly and efficiently remove a concentrated, but relatively isolated and shallow zone of contaminant mass with relatively low surfactant application concentrations. A combined enhanced anaerobic biodegradation (EAB) and in situ chemical reduction (ISCR) amendment was subsequently delivered to the shallow zone to further reduce residual mass following SEE.
Approach/Activities
Approach/Activities. Burns & McDonnell conducted a SEE pilot study in 2015 and full-scale SEE in 2016 and 217. The pilot study and full-scale SEE included single-well “push-pull” and multi-well “point-to-point” delivery and extraction techniques using a solution consisting of 1 to 2 percent (by volume) Ivey-sol® 106 biodegradable surfactant. In general, the solution was gravity fed into each well to achieve a targeted injection radius, then allowed to remain in the formation for a prescribed residence time period. The surfactant solution, groundwater, and liberated COCs were subsequently extracted using the DPVE system. An EAB/ISCR amendment formulation, Provectus Environmental Products™ Provect-IR60 (Provect-IR60), consisting of approximately 60% ZVI and 40% carbon source (organic components), was delivered to the saturated, shallow, silty-sand subsurface zone in 2020 using direct-push injection technology to address the residual COC mass. In total, approximately 24,000 pounds of Provect-IR60 was applied at a total of 59 direct-push injection locations within the targeted treatment area.
Results/Lessons Learned
Results/Lessons Learned. Prior to DPVE, carbon tetrachloride concentrations of over 100,000 µg/L were observed within the most impacted portion of the source area. Based on a comparison of COC concentrations before/after implementation of each technology, this combined remedy has resulted in carbon tetrachloride groundwater concentration reductions of up to 99 percent. Semi-annual performance monitoring groundwater results indicate conditions within the treatment area continue to be supportive of anaerobic biodegradation and/or abiotic degradation of chlorinated VOCs (cVOCs) following EAB/ISCR. Ten of the 14 monitoring wells sampled within the target remediation area are now below the regulatory cleanup standard for carbon tetrachloride of 5 µg/L, with the remaining wells demonstrating COC concentrations up to 99 percent lower than the historical highs. Additionally, Burns & McDonnell has successfully reduced the site monitoring program by approximately 50 percent.