Formatted Title
Colloidal Activated Carbon Barrier Long-Term Performance: Ten-Year Review Supported by Aspect Ratio Analysis, Flux Measurement and Modelling
Background/Objectives
Background/Objectives. The migration of organic contaminants in groundwater can be mitigated through injection of activated carbon reagents into the subsurface. The approach offers significant design flexibility in strategies ranging from source area containment to barrier configurations. Contaminant flux retardation within the treatment zone may be supplemented with biodegradation. This can extend the time to breakthrough owing to bioregeneration of the sorption sites on the carbon. The barrier longevity may be extended indefinitely if the degradation rate is sufficient. An understanding of the in-barrier degradation rate is therefore important for performance calibration and management. Injectable colloidal activated carbon (CAC) barriers are increasingly used in plume management. Their purpose can be to retard plume migration per se as in the case of non-degradable species such as PFAS, or to increase residence time of degradable species in a treatment zone. Historic solvent releases on a site in the eastern US have resulted in a solvent plume extending 1,700 feet (500 m). The plume principally comprises TCE at concentrations exceeding 10,000 µg/L. Migration of the plume has been mitigated by a sequence of colloidal activated carbon (CAC) barriers co-applied with an electron donor to combine retardation and biodegradation. The full-scale application was preceded by pilot study in 2014, conducted in an area of the plume spatially separate from the full-scale barrier application that followed. This has allowed long-term performance without subsequent intervention to be evaluated.
Approach/Activities
Approach/Activities. Barrier longevity is principally determined by carbon dose, contaminant flux, and regeneration of sorption sites such as through bio or ISCR. The contribution of regeneration can be determined if flux and carbon dose are known and contaminant concentrations within the barrier monitored. TCE flux was initially calculated at the design stage based on estimated seepage velocity. For post-application performance evaluation, the estimate was refined using desk-top approaches of modelling and aspect ratio analysis – tools that had subsequently become available. The refinements were then confirmed by direct measurement using passive flux devices. Barrier performance was determined through measurement of arrival time of the advancing plume front at a monitoring well located within the barrier. The contribution of biodegradation was estimated through comparing measured performance to the projected performance at the known flux and carbon dose.
Results/Lessons Learned
Results/Lessons Learned. Modelling, aspect ratio analysis and passive flux data indicated contaminant flux to be approaching an order of magnitude higher than the original estimate made without the help of these tools (140 mg/m2/day vs.1,200 mg/m2/day). This difference has a significant implication to project design – the higher flux would require a correspondingly higher carbon dose to secure equivalent performance. At the carbon dose applied, the higher flux (as measured) would reduce the projected barrier longevity from 50 years as designed to approximately 5 years. The advancing TCE front within the barrier would be projected to arrive at the mid-barrier well after approximately 3 to 4 years. The observed arrival time was approaching 8 years. This is approximately double the sorption-only projection. The difference is consistent with a contribution from biodegradation enhancing performance. This indicates a synergy between retardation and degradation. A single donor application would not be expected to contain 12,000 µg/L for 8 years, or indeed within a 10-foot footprint at a seepage velocity of 500 ft/year, and plume retardation / barrier longevity was enhanced significantly beyond that projected for sorption alone. This analysis suggests that at the conditions as understood at the design stage, the projected barrier longevity would be in excess of 50 years based on sorption only, with extension possible through periodic bio and possibly ISCR regenerative supplements. This study provides a demonstration of tools and processes that may be helpful in long-term monitoring and management of CAC barriers. The study further underlines the importance of dependable flux estimation / quantification within the design process.