Polychlorinated biphenyls (PCBs) are one of the most frequently reported contaminants associated with sediments in the country and rank second after mercury for the basis for fish consumption advisories. Recent in situ studies have demonstrated the feasibility of PCB bioavailability reduction using activated carbon (AC) as an amendment. A desirable goal was to ultimately reduce the inventory of legacy PCBs in sediments while also reducing bioavailability to the food chain. The objective of this project was to test the efficacy of in situ application of AC amended with PCB degrading microorganisms to reduce the total mass and bioavailability of PCBs in sediments.
The potential of a remedial strategy for the sequestration and degradation of PCBs by micro-organisms in sediments was initially developed under funding by both SERDP and the NIEHS-SBIR. The lab-based work showed that the potential for enhanced dechlorination activity and subsequently complete PCB mineralization with bacterial populations delivered on granular activated carbon. In this ESTCP project, biofilms of anaerobic halorespiring Dehalobium chlorocoercia (DF1) and aerobic Burkholderia xenovorans (LB400) were added concurrently with the commercially-available GAC product SediMite as a delivery system to contaminated sediments.
Mesocosm studies were conducted first to find the the optimal loading cell titer and carbon loading rates. The research team conducted a pilot-scale field application of bioamended AC in Abraham’s Creek (Marine Corp Base Quantico) with the following objectives: 1) demonstrate the scalability of growing PCB respiring microorganisms for field application, 2) develop and test the application of PCB halorespiring and degrading bacteria using pelleted AC as a delivery system, 3) assess the benefits of bioamended AC treatment on concentrations of PCBs in sediments and porewater, 4) assess the fate of the bioamendment over time, and 5) evaluate the impact of treatment on the indigenous microbial populations.
Field treatments at Abraham’s Creek consisted of four plots of 400 m2 each: (1) a control plot with no treatment, (2) a plot receiving only an AC agglomerate (SediMiteTM) and (3,4) two plots that were treated with with SediMiteTM bioamended with PCB- degrading bacteria. The three plots receiving SediMite were calibrated to receive an equivalent target dose of 1.5 mg/kg AC. Monitoring of the plots occurred pre-treatment, and then post-treatment at 140- and 409-days post-application. Each monitoring event included five, 5-cm bulk sediment cores for total organic carbon, black carbon, bulk-sediment congeners and total PCBs, and cell numbers of DF1 and LB400. Passive samplers also were deployed to provide a measure of freely-dissolved PCB congeners.
Efficacy of the method for placing the GAC + bacteria was demonstrated. Collection trays placed out during application on the plots receiving SediMite or SediMite with bacteria had roughly equivalent GAC mass (1.7 – 2.4 kg/m2) delivered within the target range of 1.5 ± 50% kg/m2. These data provided validation of the effectiveness of the placement method. Post-monitoring of the black carbon levels indicated heterogeneity within and between the test plots. After 409 days, the relative mean distribution of black carbon was Plot 4 (7.0%) > Plot 2 (2.1%) > Plot 3 (0.7%) > Plot 1 (0.6%). Bacterial population cell numbers were highest in Plots 3 and 4, as would be expected through the first 140 days. The titer of the bioamendments decreased but were still detectable after 409 days, but the cell number counts for both DF1 and LB400 were similar to that of the control plot (Plot 1), but not to the plot receiving only SediMite.
The overall trend at the demonstration site was apparent reductions in the PCB concentrations in the test plots, relative to the untreated- and SediMite-only plots. In Plot 4, the apparent total PCB concentration in the top 7.5 cm declined by 28% at 140 days, and 52% after 409 days. In treatment-equivalent Plot 3, the apparent PCB percent decrease was 17% after 140 days and 30% after 409 days. The apparent reductions were not statistically different from the baseline PCB concentrations at Day 140 but were for both treatment plots after 409 days.
For Plot 4, aqueous concentrations of tri- to nona-chlorobiphenyl (tri+PCBs) PCB congeners by as much as 95% in 409 days. Co-planer congeners were reduced by up to 80% in sediment and were undetectable in the porewater. For Plot 3, the tri+PCBs decreased by 84%, but there was no difference between the baseline and Day 409 levels of the co-planer congeners. Over the same time period (409 days), the di- to deca-chlorobiphenyl-summed total PCBs declines were measured in all treatment plots: Plot 1 (45%), Plot 2 (32%), Plot 3 (73%) and Plot 4 (76%). The declines observed in Plots 1 and 2 were substantial, but not statistically different (p = 0.05).
Taken in total, the data for the application of PCB respiring microorganisms on GAC as a delivery mechanism are encouraging, but not definitive. The small sample size (n=5) could be overcome by additional definitive sampling with either a greater number of samples and/or composite sampling. Prior to implementing a full-scale site remedy, it would be recommended to conduct a site-specific pilot scale study, drawing from the important lessons learned in this project.
In situ treatment of PCBs using an AC agglomerate as a delivery system for bioamendments is particularly well-suited for environmentally sensitive sites where there is a need to reduce exposure of the aquatic food web to sediment-bound PCBs with minimal disruption to the environment. The net cost for the full remediation of the 7.8 acre site using bioamended SediMite application was estimated at $1.8M compared to $4M for an isolation cap and $25M for full excavation and disposal off-site. The annual average maintenance costs for bioamended AC is estimated to be in the range of costs for Monitored Natural Attenuation or capping which are estimated at about $100k/year for the first 5 years.
The effectiveness of bioamended AC for reducing concentrations of total and soluble PCBs was affected by the homogeneity of the application. The ventrui horn induction system device used in this study was appropriate for application in water margin areas and difficult to access areas such as below piers. For large areas, a boat mounted belt spreader or land based telebelt were required to evenly distribute the bioamendments and obtain consistent maximum effectiveness.
May, H.D. and K.R. Sowers. 2016. Dehalobium chlorocoercia-From Discovery to Application. In: F. Loffler and L. Adrian (eds.). Organohalide Respiring Bacteria. Springer, New York, pp. 563-586. ISBN: 978-3-662-49873-6; ISBN: 978-3-662-49875-0 (eBook).
Payne, R.P., U. Ghosh, H.D. May, C.W. Marshall and K.R. Sowers. 2019. A Pilot-Scale Field Study: In Situ Treatment of PCB-Impacted Sediments with Bioamended Activated Carbon. Environmental Science and Technology, 53(5):2626-2634.
Payne, R.P., U. Ghosh, H.D. May, C.W. Marshall, and K.R. Sowers. 2017. Mesocosm Studies on the Efficacy of Bioamended Activated Carbon for Treating PCB-impacted Sediment. Environmental Science and Technology, 51(18):10691-10699.
Sowers, K.R. and H. D. May. 2013. In situ Treatment of PCBs by Anaerobic Microbial Dechlorination in Aquatic Sediment: Are We There Yet? Current Opinion in Biotechnology, 24(3):482-488.