Background/Objectives: Sound conceptual site models (CSMs) are essential for developing a comprehensive understanding of natural attenuation processes and preferential groundwater flow pathways at contaminated sites. However, as groundwater remediation projects are commonly challenged by inherent geologic complexity in the subsurface, the development of CSMs and a quantification of the associated uncertainties often yield results that are less accurate than desired. In this study, we leverage PRISM™, an integrated approach that harnesses sequence stratigraphy, facies analysis, and subsurface geophysics, to better understand the subsurface geology and accurately characterize preferential flow pathways within a complex groundwater remediation site.

Approach/Activities: In this presentation, high resolution site characterization (HRSC) tools, including membrane interface probe/hydraulic profiling tool (MIP/HPT) borings and the analysis of grab and monitoring well groundwater data are used to characterize a stable contamination plume. HRSC methods and environmental visualization system (EVS) software are used to build a dynamic CSM, refine mass flux/discharge estimates, and evaluate contaminant discharge zones through a focused remedial approach, while PRISM™ is used to refine the stratigraphy and accurately predict contaminant migration pathways at the site.

Results/Lessons Learned: Correlation of existing MIP/HPT borings and complementary wells logs from the site reveals a previously undetected horst and graben system that induced the development of structurally controlled incised valleys. In light of this high-resolution stratigraphic interpretation, re-examination of the subsurface heterogeneity and impacted groundwater data shows significant structural control on plume concentrations. Contaminants appear to be primarily moving through highly-connected channel bars within grabens, which can be explained by the fact that grabens are flanked on both sides by normal faults (i.e., natural barriers to groundwater flow). Thus, using PRISM™ to define the subsurface heterogeneity at complex geologic sites is crucial for accurately predicting contaminant flow pathways and developing effective remedial strategies.

Ryan Samuels, Geologist/Stratigrapher, AECOM
Ryan Samuels is a Geologist/Stratigrapher in AECOM’s Arlington, Virginia office. He holds a MS degree in Sequence Stratigraphy from Texas A&M University-College Station, and a BA degree in Geology from Franklin and Marshall College. His education and career have focused on using sedimentological and stratigraphic information from cores and well-logs for sequence stratigraphic analysis and clastic reservoir characterization. Prior to joining AECOM, Ryan was in the petroleum industry, conducting sequence stratigraphic analysis in the Denver Basin, Powder River Basin, and Gulf of Mexico. But now, as a lead stratigrapher at AECOM, he is successfully applying his technical skills to develop robust conceptual site models at complex geologic sites.

Background/Objectives: Per- and poly-fluoroalkyl substances (PFAS) have been widely used and released into the environment over the last 60 years. Among the broad category of PFAS, the occurrence and environmental impact of perfluoroalkyl acids (PFAAs), such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) have been studied and characterized globally. PFAS have chemical structures involving the extremely strong carbonfluorine (C-F) covalent bond, representing a class of chemicals that are extremely challenging to remediate. The current trends for treating PFAS impacted water rely on mass transfer technologies (e.g., granular activated carbon [GAC], ion exchange resin [IX-R], reverse osmosis [RO]) that do not destroy PFAS but concentrate PFAS on the selected media. The spent media generated will require off-site incineration in the case of GAC and single use IX-R or on-site regeneration for reusable IX-R. Emerging separation technologies such as ozone fractionation (OF) and ex-situ foam fractionation (FF) will also generate a PFAS laden waste. Advances are being made in destructive technologies, such as electrochemical oxidation (EO), sono-chemistry and plasma, with all producing promising results in the laboratory. This presentation will detail the latest developments, including optimisation modifications and proof of demonstration results using a pilot scale EO reactor to treat various real world PFAS impacted liquids.

Approach/Activities: The electrode was fabricated and used to mineralize C4~C12 PFAAs and their precursors with evidence of complete defluorination and desulfurization. PFAS are destroyed via direct electron transfer on “nonreactive” anodes and hydroxyl radicals generated under room temperature and atmospheric pressure with relatively low energy consumption. This EO reactor was developed and demonstrated using different types of PFAS impacted liquids, including: 1) IX-R regenerant; 2) soil washing effluent; 3) source zone groundwater; 4) FF concentrate; and, 5) untreated wastewater. Pre- and post-treated samples were analysed for over 20 different PFAS using liquid chromatography (LC – MS/MS). The same samples were also tested for total oxidisable precursor (TOP) assay, total organic carbon (TOC), perchlorate, chloride, fluoride, sulphate, metals and volatile organic compounds (VOCs).

Results: The EO pilot demonstration treated PFAS from low to high concentration ranges in water and PFAS concentrates in liquid waste. For instance results for the treatment of IX-R still bottoms indicated 100% removal of measurable PFAS and 300% + of fluoride concentration as an end product, indicating the successful mineralization of unquantified PFAA precursors as well as measurable PFAAs. It was also discovered that this technology was capable of simultaneously decomposing high concentrations of total organic carbon (8,000 parts per million [ppm]). This presentation will discuss the results of all testing conducted to date and the challenges faced during the development of the technology.

Shangtao Liang, Environmental Scientist, AECOM
Shangtao Liang, Ph.D., is an Environmental Scientist at AECOM with a focus on PFAS treatment technology development. She received her Ph.D. degree from the University of Georgia, and in her thesis research she developed electrochemical technology for the remediation of emerging contaminants, including PFAS, in groundwater and soil. She received her MS in Soil Science from the North Carolina State University and BS in Environmental Science from the China Agricultural University.

Per- and polyfluoroalkyl substances (PFAS) are a family of anthropogenic chemicals that contain one or more of the perfluoroalkyl moiety (CnF2n+1–). “PFAS precursors” are short for “the precursor substances to perfluoroalkyl acids (PFAA)”, and are polyfluorinated. The “precursors” have the potential to form PFAAs as a result of abiotic or biotic transformations that may occur under industrial, environmental or metabolic conditions. This presentation highlights the findings of recent environmental biodegradability studies, in particular, AFFF-derived PFAS. (1) Biodegradability of polyfluoroalkyl chemicals (or precursors) is mainly owing to its non-fluorinated functionality, whose breakdown precedes the breakdown of the perfluorinated carbons if the later occurs. Defluorination and breakdown of C-F bonds happen to a range of fluorotelomer precursors in aerobic systems. In contrast, anaerobic transformation potential for most precursors is mostly unknown, and reductive defluorination has not been found to be occurring to result in defluorination. (2) Many fluorotelomer precursors may degrade to a common intermediate of fluorotelomer sulfonate, the further degradation of which is often the rate-limiting step. Biodegradability of 6:2 fluorotelomer sulfonate varies widely from no degradation in certain surface soils to rapid breakdown in a Gordonia sp. culture that desulfonates the fluorinated sulfonates. (3) Six AFFF-derived PFAS, synthesized from electrochemical fluorination, were found to degrade following similar pathways to produce PFOS or PFOA. The ones with amide group next to the perfluoroalkyl chain (PFOA precursors) tend to degrade faster than those with sulfonamide groups (PFOS precursors). The hydrophilic functional group has a strong influence on transformation kinetics. Presence of fluorinated amines as an impurity and their transformation can confound study findings of the main precursors. In conclusion, the laboratory studies can provide useful insights into environmental monitoring efforts in the field and ecotoxicological and risk assessment of PFAS. Some discussion will be made regarding the disconnects between the findings in laboratory research and the field observations regarding the fate of significant precursors and transformation products.

Jinxia Liu, Associate Professor, Environmental Engineering, McGill University and Chwang-Seto Faculty Scholar
Dr. Jinxia Liu is an Associate Professor in environmental engineering at McGill University and Chwang-Seto Faculty Scholar. She has an extensive portfolio of PFAS research include developing PFAS analytical methods, elucidating sorption mechanisms, assessing precursor biotransformation and microbial defluorination, and developing PFAS remediation technologies. She also studies the water quality issue and micropollutants for future wastewater treatment processes that are decentralized, source separated, and recover energy and nutrients.