A Cost Comparison of Pump-and-Treat and In Situ Colloidal Activated Carbon for PFAS Plume Management

Abstract

This article compares the full installation and operational costs of a hydraulic containment (“pump-and-treat,” P&T) system with those of a hypothetical contemporary in situ colloidal activated carbon (CAC) barrier. Worked examples are provided using public domain data from the FT-02 fire-fighting training site on the former Wurtsmith Air Force Base in Oscoda, MI. The projected CAC costs are approximately a third of the P&T costs over projection periods of 15–100 years ($ 7.2 M vs. $ 19 M at 30 years; 38%). Hydraulic containment and CAC remediation systems prevent the spread of per- and polyfluoroalkyl substance (PFAS)-contaminated groundwater. Hydraulic containment works by extracting contaminated groundwater, removing the contamination using activated carbon or other means, and re-injecting the cleaned groundwater. The arrangement of extraction and injection wells and the related groundwater pumping rates create a hydraulic barrier that captures and contains the PFAS plume. In situ CAC barriers work as passive underground filters. Micron-scale particles of activated carbon are injected into contaminated aquifers using drilling equipment or injection wells. Once injected, the carbon particles attach to the soil. An in situ permeable barrier is installed across a contaminant plume through the injection of a number of points at suitable spacings. Groundwater flows through the barrier zone unimpeded while PFAS contamination is captured and contained by the activated carbon. The longevity of the barrier is determined by the quantity of carbon emplaced relative to the contaminant flux. Barriers are typically designed to last for years (decades), after which time, carbon re-applications can be made, if required. Principal differences between the approaches are the scale of operation and maintenance requirements, and, for P&T, the bringing of PFAS-impacted water above ground to treat. This generates a filtration medium that is contaminated with PFAS, and which therefore requires handling as a PFAS waste with attendant liability. Hydraulic containment is also an active technology (requiring external energy input) and requires the operation and maintenance of pumping and filtration equipment. In situ CAC barriers are passive (powered by natural groundwater flow) and have no operation and maintenance requirements. They do not bring PFAS-impacted material above ground and do not generate waste. Performance data of the installed P&T hydraulic containment system were analyzed to estimate the time to remedial completion using the system alone. Data extrapolation supported by statistical analysis indicates clean-up targets will not be reached within 100 years of pumping. It is not realistic for P&T to be regarded as a means of aquifer clean-up as the aquifer will remain contaminated for the realistic future. Comparison is made between P&T and CAC on their common basis as containment approaches. The goal is to reduce the exposure of down-gradient receptors to PFAS.