239-3 Doing Your Homework: Using Sequence Stratigraphy and Facies Analysis to Evaluate Preferential Contaminant Migration Pathways at a Former DOD Installation – Upper Peninsula Michigan

Session: Federal PFAS Remediation: Successes and Challenges

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Background: This study focuses on a Department of Defense site where the influence of geologic heterogeneity on per- and polyfluoroalkyl substances (PFAS) migration is examined. The site, located in northern Michigan, presents a complex setting characterized by glacial deposits (e.g., outwash fans, lacustrine sediments) over highly undulatory crystalline bedrock which significantly impact groundwater flow and contaminant migration.

Approach: A common role of an environmental geologist is to predict subsurface strata heterogeneity. The typical borehole diameter is four inches wide which is significantly smaller than the area of a typical cleanup site. This makes the task of evaluating groundwater flow and contaminant migration in that heterogeneity difficult. To account for sedimentary heterogeneity and develop a comprehensive hydrogeologic conceptual site model (CSM), we reinterpreted the subsurface with the principles of sequence stratigraphy and facies analysis. This approach is particularly valuable for sites requiring numerical groundwater models and characterizing PFAS fate and transport, as it enhances the prediction and validation of geologic conditions. This process involved integrating geophysical logs, borehole descriptions, depositional models, and insights from regional publications. To address data gaps at the local scale, regional models and conceptual understandings guided interpretations ensuring a comprehensive and defensible understanding of the geology.

The refined CSM developed a comprehensive three-dimensional geologic understanding by mapping depositional environments both vertically and laterally. This provided a geologically defensible framework that addressed previous ambiguities related to deposit geometry, extent and grainsize variations. These factors had previously hindered the site understanding of groundwater flow and contaminant migration.

Results: Our findings highlight the significant role of glacial deposit heterogeneity on a complicated bedrock surface in influencing the transport of PFAS. Specific coarse-grained fluvial and outwash sediments acting as primary pathways for groundwater flow and finer-grained outburst flood and lacustrine sediments that serve as zones of storage were identified and correlated. By mapping these deposits based on their depositional processes rather than just grainsize, we have generated maps of how subsurface pathways are connected or separated by high and low-permeability sediments to identify data gaps and inform RI field activities. This CSM is guiding future investigations by ensuring a more effective approach to managing groundwater flow, contaminant migration and identification of exposure pathways.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7816

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