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Per- and polyfluoroalkyl substances (PFAS) are surface-active contaminants, which are detected in groundwater globally, presenting serious health concerns. The vadose zone and surface water are recognized as primary sources of PFAS contamination. Previous studies have explored PFAS transport and retention mechanisms in the vadose zone, revealing that adsorption at interfaces and soil/sediment heterogeneity significantly influences PFAS retention. However, our understanding of how surface water−groundwater interactions along river corridors impact PFAS transport remains limited. To analyze PFAS transport during surface water−groundwater interactions, we performed saturated−unsaturated flow and reactive transport simulations in heterogeneous riparian sediments. Incorporating uncertainty quantification and sensitivity analysis, we identified key physical and geochemical sediment properties influencing PFAS transport. Our models considered aqueous-phase transport and adsorption both at the air−water interface (AWI) and the solid-phase surface. We tested different cases of heterogeneous sediments with varying volume proportions of higher permeability sediments, conducting 2000 simulations for each case, followed by global sensitivity and response surface analyses. Results indicate that sediment porosities, which are correlated to permeabilities, are crucial for PFAS transport in riparian sediments during river stage fluctuations. High-permeable sediment (e.g., sandy gravel, sand) is the preferential path for the PFAS transport, and low-permeable sediment (e.g., silt, clay) is where PFAS is retained. Additionally, the results show that adsorption at interfaces (AWI and solid phase) has a small impact on PFAS retention in riparian environments. This study offers insights into factors influencing PFAS transport in riparian sediments, potentially aiding the development of strategies to reduce the risk of PFAS contamination in groundwater from surface water. 

Groundwater-surface water interaction (hyporheic exchange) is critical in numerous hydrogeochemical processes; however, hyporheic exchange is difficult to characterize due to the various spatial (e.g., sedimentary architecture) and temporal (e.g., stage fluctuations) variables that influence it. This interdisciplinary study brings forth novel insights by integrating various methodologies including geophysical surveys, physical and chemical sediment characterization, and water chemistry analysis to explore the interplay of the numerous facets governing hyporheic zone processes within a compound bar deposit. The findings reveal distinct sedimentary facies and geochemical zones within the compound bar, driven by the sedimentary architecture. Cross-bar channel fills are identified as critical structures influencing hydrogeochemical dynamics, acting as baffles to groundwater flow and modulating nutrient transformations. Geophysical imaging and hydrogeochemical analyses highlight the complex interplay between sediment characteristics and subsurface hydraulic connectivity, emphasizing the role of sediment heterogeneity in controlling hyporheic exchange and solute mixing. The study concludes that sediment heterogeneity, particularly the presence of cross-bar channel fills, plays a pivotal role in the hydrogeochemical dynamics of the hyporheic zone. These structures significantly influence hyporheic flow paths, solute residence times, and nutrient cycling, underscoring the necessity to consider the fine-scale sedimentary architecture in models of hyporheic exchange. The findings contribute to a deeper understanding of riverine ecosystem processes, offering insights that can inform management strategies for water quality and ecological integrity.