Formatted Title

PFAS in Karst: Using Process-Based Characterization to Guide Remedial Design

Background/Objectives

Karst aquifers are widely recognized as the most difficult medium to characterize and remediate, particularly regarding recalcitrant contaminants. The challenges faced when characterizing and remediating contaminants in karst terrane include:

• High integration between the groundwater and surface water systems.
• Concentration of flow at primary discharge locations and exposure points with springs integrating contamination present within the aquifer.
• High heterogeneity and anisotropy that contribute to complex contaminant distributions.
• High stormwater loading potential due to infrastructure engineered to leverage natural plumbing – routing stormwater directly to sinkholes.
• The potential for rapid, long-distance transport of contaminants emanating from source areas, increasing offsite risk.

Examples of additional contaminant-specific challenges PFAS present in karst terrane include:

• Turbulent flow that can lead to enhanced foaming along air-water interfaces and transport of PFAS-laden foams through conduit networks.
• Transport of PFAS-laden sediment and microplastics.

Approach/Activities

Considering the unique challenges presented by karst terrane, various characterization methods are being adapted to identify and quantify primary PFAS mass flux pathways. Such characterization tools include both conservative and non-conservative fluorescent tracer dyes, sediment tracers, and tracing approaches to investigate air-water interface interactions. These techniques are being used to better understand PFAS mass flux in karst terranes, support improved processes-based conceptual site models, and highlight remedial implications.

Results/Lessons Learned

Several case studies will be discussed. In one example, PFAS-impacted stormwater runoff from an airfield and other sources flows into “swallow holes” and other channel features in an area underlain by karstic limestone.  This stormwater has impacted portions of the aquifer with PFAS detected in the single ng/L range in both shallow and deep nearby domestic wells.  PFAS-optimized passive samplers placed in four nearby large springs (which are considered integrative discharge points for the aquifer) detected PFAS in all four springs at total PFAS concentrations from 4 to 52 ng/L.  PFAS was also detected in two domestic wells 3.5 miles from the suspected source areas.  These data highlight the potential for heterogeneous and long travel distances for PFAS. They also call attention to the importance of springs in karst terrane as primary discharge locations where convergence of flow integrates contamination present within an aquifer and completes exposure pathways to biota and surface water resources. Insights gained from these examples will contribute to the fundamental understanding of PFAS transport phenomena in karst and provide a characterization framework to assess karst-specific PFAS mass flux challenges.

Additionally, accurate definition of source terms, understanding primary mass flux pathways and their transiency, and delineating primary exposure pathways such as major springs will be critical when designing remedial systems. Due to these unique challenges, standard remedial approaches may not provide the necessary effectiveness to meet the clean-up criteria currently proposed. Rather, specialized remedial systems designed to target the site-specific primary mass flux pathways identified throughout the characterization process are likely to be required.