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

Topic Area: 7j. PFAS Conceptual Site Model Approaches

Formatted Title:

Using Sequence Stratigraphic Understanding to Inform PFAS Migration: 40,000 Acres of South Central Tennessee

Background/Objectives:

One of the most critical factors to account for in a site assessment is the media that transport and store groundwater. An incomplete understanding of geologic heterogeneity can lead to ineffective remediation, which is often overlooked during characterization and remedial design in sedimentary deposits. Defining geological constraints by the depositional environment provides a more accurate picture of the subsurface framework, which is essential for identifying surface water-groundwater interactions, preferential contaminant migration pathways, and developing effective remediation strategies. This case study focuses on a Tennessee site almost 40,000 acres in size, where the geology consists of weathered, eroded, and folded marine carbonate deposits. The objectives of this work were to develop a hydrogeologic CSM to understand groundwater pathways and areas of highest mass flux.

Approach/Activities:

Geologic interpretation using sequence stratigraphy and depositional environments has been used by the oil and gas industry and academia since the 1970s to refine the understanding of subsurface heterogeneity. The power of this approach in characterizing and predicting aquifer heterogeneity has been tested in the present case study. By integrating regional tectonic history, observation of core lithology and geophysical logs including image logs, the aquifer heterogeneity for the site has been analyzed in detail both at the basin scale and bed-scale. This understanding has been then tested by observed hydrologic and chemical data to define preferential contaminant migration pathways, producing a geologically-defensible conceptual site model.

Results/Lessons Learned:

Results from the updated stratigraphic model revealed complicated PFAS migration pathways due to complex weathered and folded bedrock geology. Analysis showed that PFAS vertically migrated from on-Base surface water features that recharged groundwater. Once in the subsurface, PFAS preferentially migrated to the high permeability proximal submarine fan deposits of the Fort Payne Formation that are now weathered and folded. These weathered pathways lead to gravel-rich rubble zones focused within bedrock depressions. These pathways can be cut into by modern rivers allowing for springs or encounter karstic bedrock allowing for further downgradient migration. The underlying Chattanooga Shale was determined to be a vertical barrier to groundwater flow. Together this showed that PFAS is largely transported in folded proximal submarine fan deposits within the outcrop of the aquitard shale. Stratigraphic analysis paired with detailed hydrologic mapping refined the highest mass flux hydrogeologic pathways vertically and laterally to provide the client with mass flux “choke points” for targeted future investigative activities.

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David Parse