Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7j. PFAS Conceptual Site Model Approaches

Formatted Title:

Application of Monitored Natural Attenuation to PFAS Plumes in Groundwater: A Stratigraphic Approach

Background/Objectives:

Historically, a simplifying assumption of relatively homogeneous and isotropic permeability distribution in aquifers has been used in the design and implementation of remedies for sites with groundwater contaminated with dissolved plumes of chlorinated solvents. However, many remedies have fallen short of their predicted outcomes due to the inherent heterogeneity in the permeability architecture in aquifers coupled with matrix diffusion effects when fine-grained strata act as secondary long-term sources of contamination. While the heterogeneity and diffusion effects have hampered cleanup efforts for chlorinated solvents and prolonged cleanup timeframes, these effects can also restrict the migration of and/or contain PFAS in groundwater naturally, forming a potential basis for application of monitored natural attenuation (MNA) as a long-term site management strategy. There is precedent for application of monitored natural attenuation to non-biodegradable contaminants in groundwater (e.g., metals), and given the magnitude of PFAS contamination in groundwater and limited financial resources it is likely that MNA will be necessary component of remediation approaches.

Approach/Activities:

In order to assess the potential for application of MNA to PFAS sites, the aquifer heterogeneity including the nature and distribution of fine-grained strata with potential to sequester PFAS must be well understood. Fortunately, a great body of research and knowledge has evolved over the past half century to address permeability heterogeneity in oil and gas reservoirs to maximize production, and the tools of sequence stratigraphy and facies models have been developed. We present an overview of application of these techniques to provide a predictive framework for understanding aquifer heterogeneity and the feasibility of MNA to address PFAS contamination in aquifers.

Results/Lessons Learned:

Rather than demonstrating that biological degradation processes are active and ongoing, application of MNA to PFAS sites will require demonstrating that geologic materials are actively retaining PFAS in the saturated zone. The organization of fine-grained units in clastic (non-bedrock or karst) aquifers is governed by the depositional environments in which the strata were laid down. Some stratigraphic features which are likely to favor physical retention and containment of PFAS in groundwater systems include:

A high degree of interbedding of coarse- and fine-grained strata at multiple scales
Isolation of permeable sand bodies such as point bar deposits in relatively low-permeability strata
Facies changes from coarse- to fine-grained strata in the direction of groundwater flow (stratigraphic “dead ends”)
Gradational / transitional contacts between coarse-grained and fine-grained depositional elements as opposed to sharp contacts
Degree of sorting of permeable facies (retention of PFAS and restriction of K)
Nature and geochemical aspects of fine-grained strata including geometry, organic content, etc.

We present an overview of the importance of an understanding of depositional environments and facies models for assessing the above-mentioned stratigraphic features and as a framework for assessing the potential for application of MNA to PFAS plumes in groundwater.

Menu

Leo Giannetta