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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. 

Nitrous oxide (N2O) is a potent greenhouse gas that also contributes to ozone depletion. Recent studies have identified river corridors as significant sources of N2O emissions. Surface water-groundwater (hyporheic) interactions along river corridors induce flow and reactive nitrogen transport through riparian sediments, thereby generating N2O. Despite the prevalence of these processes, the controlling influence of physical and geochemical parameters on N2O emissions from coupled aerobic and anaerobic reactive transport processes in heterogeneous riparian sediments is not yet fully understood. This study presents an integrated framework that combines a flow and multi-component reactive transport model (RTM) with an uncertainty quantification and sensitivity analysis tool to determine which physical and geochemical parameters have the greatest impact on N2O emissions from riparian sediments. The framework involves the development of thousands of RTMs, followed by global sensitivity and responsive surface analyses. Results indicate that characterizing the denitrification reaction rate constant and permeability of intermediate-permeability sediments (e.g., sandy gravel) are crucial in describing coupled nitrification-denitrification reactions and the magnitude of N2O emissions. This study provides valuable insights into the factors that influence N2O emissions from riparian sediments and can help in developing strategies to control N2O emissions from river corridors