The Future of PFAS Groundwater Remediation:
Enhanced Natural Attenuation

By Scott Wilson
President and CEO, REGENESIS

If history is any guide, the threat of “Forever Chemicals” (PFAS) spreading from industrial facilities, airports, and military bases will largely be remedied by allowing natural processes to take their course. With the enormous cost of operating inefficient pump and treat systems (P&T) and the attendant generation of hazardous waste, there is every reason to believe that natural attenuation will become the “go-to” remediation strategy for this suite of contaminants as well.

Natural Attenuation Accepted For Over 20 Years by US EPA

We first saw natural attenuation displace P&T for hydrocarbon treatment in the mid-1990s after the Air Force Center for Environmental Excellence (AFCEE) developed a protocol for natural attenuation of fuel hydrocarbons in 1994. Then, in 1996, AFCEE published its protocol for natural attenuation of chlorinated solvents ushering in this passive approach to remediation and significantly reducing the need for costly P&T systems. These concepts and processes were further adapted by the US EPA and state regulatory agencies throughout the early 2000s and were broadened to incorporate a wide range of contaminants including recalcitrant or non-degradable compounds such as metals.

Adding Substrates Accelerates Natural Attenuation

Over the past two decades, it has become common practice to enhance natural attenuation processes when a site evaluation indicates passive natural attenuation alone will not quite stop a contaminant plume from impacting a receptor. In these cases, we enhance the natural attenuation by adding substrates to the soils and/or aquifer waters to stimulate in situ contaminant retardation or degradation, ultimately resulting in plume stagnation and elimination of environmental risk to down-gradient receptors.

Potential to Save Billions with More Sustainable Approach

Well, history is once again repeating itself as researchers publish protocols for the natural attenuation of PFAS. It only makes sense. This approach will eliminate the environmental risk of PFAS while avoiding the huge carbon footprint associated with inefficient pump and treat systems and the required incineration of spent GAC. By avoiding P&T at many sites, responsible parties will save billions of dollars that can be used for other initiatives.

Remediation Defined

The Interstate Technology and Regulatory Council (ITRC) defines remediation as “the process used to reduce or eliminate the risk for humans and the environment that may result from exposure to harmful chemicals.” ¹ It follows that if the potential for environmental or human health exposure is eliminated (e.g., ingestion of PFAS-contaminated water), then the risk is also eliminated.

Natural Attenuation Defined

According to the EPA, Monitored Natural Attenuation (MNA) is a remedial approach that:

. . . relies on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remedial objectives within a time frame that is reasonable compared to other methods. The “natural attenuation processes” act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil and groundwater. These in-situ processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization, transformation, or destruction of contaminants.² It is important to note that all of these processes contribute to natural attenuation and do not rely solely on one process to mitigate risk.

The EPA further states that “Natural attenuation processes may reduce the potential risk posed by site contaminants in three ways:

1. Transformation of contaminant(s) to a less toxic form through destructive processes such as biodegradation or abiotic transformation;
2. Reduction of contaminant concentrations whereby potential exposure levels may be reduced; and,
3. Reduction of contaminant mobility and bioavailability through sorption onto the soil or rock matrix. ²

The decision to implement MNA should include comprehensive site characterization, risk assessment where appropriate, and measures to control sources.”

MNA is one of the most common remediation approaches used for groundwater remediation today and has been widely applied to non-degrading contaminants like metals and radionuclides.^3,4 Recently, groundwater remediation experts have suggested the use of MNA for managing PFAS plumes.^5,6 The PFAS MNA approach is currently being studied under the Department of Defense’s (DoD’s) Environmental Security Technology Certification Program (ESTCP Project ER21-5198).

Aquifer Foc Controls Natural Attenuation of PFAS

In the subsurface aquifer environment, PFAS are present sorbed to the soil/aquifer matrix and dissolved within pore water. An equilibrium exists between these two phases. Without intervention, as in MNA, the distribution between these phases is controlled largely by the amount of organic carbon in the soil/aquifer matrix, referred to as the soil fraction organic carbon (Foc). Soils with a naturally high Foc will sorb more PFAS than soils with a naturally low Foc and will naturally impede (i.e., retard) PFAS movement in groundwater.

PFAS have not been shown to degrade at any appreciable rate within the natural environment and their dilution to acceptable parts-per-trillion concentrations most often requires massive quantities of groundwater to be impacted, resulting in miles-long, very dilute, PFAS plumes.

Consequently, PFAS’ sorption onto soil is viewed as a principal controlling factor for a PFAS groundwater plume’s natural attenuation. For this reason, the aquifer’s ability to sorb PFAS will be the most important hydrogeological factor evaluated in future PFAS risk assessments to demonstrate the viability of MNA to remediate a project site. And what many of these risk assessments will find is that the natural soil Foc is too low for MNA to meet regulatory requirements. The aquifer’s ability to sorb PFAS however can be enhanced by orders of magnitude enabling the enhanced natural attenuation process to achieve regulatory goals without the time and cost associated with P&T.

What is Enhanced Natural Attenuation?

According to the ITRC, “Enhanced Attenuation is any type of intervention that might be implemented in a source-plume system to increase the magnitude of attenuation by natural processes beyond that which occurs without intervention. Enhanced attenuation is the result of applying an enhancement that sustainably manipulates a natural attenuation process, leading to an increased reduction in mass flux of contaminants.”⁸

 Enhancing PFAS Natural Attenuation — Colloidal Activated Carbon

The most highly effective and sustainable method of enhancing the natural attenuation of PFAS is to increase the aquifer’s ability to sorb PFAS. This enhancement has been widely demonstrated through the use of aqueous colloidal activated carbon (CAC) suspensions.⁹ These fluids, composed of micron-scale activated carbon particles, readily flow into contaminated soils and aquifer matrices, permanently coating the solid surfaces with a thin layer of highly sorptive activated carbon particles, while having little impact on hydraulic conductivity. The amount of colloidal activated carbon added to a given volume of an aquifer, and the way it is applied allows for the engineering of the contaminant retention capacity of the aquifer- enhancing the natural attenuation of the PFAS.¹¹

Understanding the In-Situ Treatment of PFAS Using PlumeStop Colloidal Activated Carbon

Proven Performance — PlumeStop

PlumeStop^® is a patented, commercially available CAC technology specifically developed for use in the enhanced natural attenuation of polluted aquifer settings and has been successfully used to remediate hundreds of project sites since its release in 2014. Application of PlumeStop involves the creation of PlumeStop treated aquifer zones where the contaminant flowing through is filtered out of solution and bound to the aquifer solid materials. This can be accomplished in the form of permeable reactive barriers (PRBs) to cut off contaminant plume migration beyond a boundary (i.e., a property line or a sensitive receptor such as a stream or potable well) or in contaminant source areas to reduce the flux of contaminants moving down-gradient.

PlumeStop Permanently Coats the Aquifer Matrix — Increasing Contaminant Sorption

PlumeStop, a black-colored, water-like suspension of CAC particles, is simply poured down wells or injected into the subsurface under low pressure. This particular formulation of CAC allows PlumeStop to travel through the aquifer under a low-pressure application without clogging or fracturing the aquifer. Upon application, PlumeStop permanently coats the aquifer matrix with a thin layer (1-2 μm) of activated carbon particles, creating a massive surface area for PFAS sorption within the PlumeStop treatment zone. PFAS dissolved in groundwater migrating through a PlumeStop-treated zone are rapidly sorbed into the PlumeStop particles and removed from the groundwater.

Immediate Results and Engineered to Work for Decades

Standard PlumeStop treatment is engineered for decades of PFAS removal from groundwater, with effects equal to increasing the natural soil Foc by >1,000x. This PFAS-enhanced natural attenuation approach performs immediately, is highly sustainable, requires no permanent mechanical installation, and provides remediation practitioners the necessary mechanism to apply MNA for PFAS remediation. PlumeStop has been used for enhanced attenuation of PFAS since 2016 with long-term, sustained PFAS removal from groundwater.

A single PlumeStop application will treat PFAS for many decades according to an independent modeling expert.^10,11 If the source of the PFAS is addressed simultaneously, a single application of PlumeStop may treat the PFAS forever.

A Proven, Sustainable, Cost-Effective, Solution

Enhanced natural attenuation will play a large role in the future of PFAS groundwater treatment. Remediation practitioners now can engineer the capacity of soils and aquifer materials to sorb PFAS. By simply flooding a portion of the contaminated subsurface with a CAC suspension the capacity of the aquifer to sorb PFAS from groundwater is increased by orders of magnitude, effectively enhancing the natural attenuation process. The use of PlumeStop, a commercially available CAC technology, has already been proven effective on PFAS remediation projects at large industrial complexes, airports, and military bases.^12,13 With the looming cost of treating the thousands of PFAS sites emerging around the world, this low-cost, effective, and environmentally sustainable approach will undoubtedly play a key role in the future of PFAS remediation. It only makes sense.

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