Research

Transport and mixing in porous media

Heterogeneity and transient forcing, such as tides, can generate complex transport processes in porous media, including time-dependent spreading and chaotic mixing. This work combines numerical flow and transport simulations with particle tracking and stochastic transport theory to uncover the mechanisms underlying these behaviours and develop quantitative descriptions of their temporal evolution. The broader aim is to shed new light on fundamental transport dynamics driven by land–ocean interactions, achieve more accurate representations of subsurface transport processes, and provide insights relevant to subsurface contamination and seaward nutrient export.

Chaotic advection in a heterogeneous aquifer under transient forcing
Chaotic advection in a heterogeneous aquifer under transient forcing. (a) The Poincaré section reveals a complex Lagrangian structure containing stochastic regions. (b) The corresponding finite-time Lyapunov exponents for points A–C in (a) remain positive, providing quantitative evidence of chaotic advection. Tajima & Dentz (2025; AWR).
Pathlines illustrating chaotic mixing in a three-dimensional heterogeneous coastal aquifer under transient forcing
Pathlines illustrating chaotic mixing in a three-dimensional heterogeneous coastal aquifer under transient forcing. Red and blue pathlines represent contaminant particles originating from the land and sea boundaries, respectively. The pathlines are projected orthogonally onto the x-z plane, with the x-axis aligned with the mean flow. White contours denote normalized salinity. The trajectories form confined periodic and chaotic orbits within the freshwater–saltwater transition zone, perpetually trapping land- and sea-derived particles and enhancing intermixing between them. Tajima & Dentz (2025; WRR).
Related publications
Tajima, S. & Dentz, M. (2025). Tides and heterogeneity drive trapping and chaotic mixing in coastal aquifers. Water Resources Research, 61(11), e2025WR040620. Link
Tajima, S. & Dentz, M. (2025). Transient forcing in heterogeneous aquifers drives solute containment and chaotic mixing. Advances in Water Resources, 203, 105021. Link
Tajima, S., Dentz, M., Liu, J., & Tokunaga, T. (2024). Time-dependent dispersion coefficients for the evolution of displacement fronts in heterogeneous porous media. Advances in Water Resources, 189, 104714. Link

Storm-surge impacts on coastal surface–subsurface systems

Storm surges affect coastal surface–subsurface systems through wave overwash and subsequent vertical seawater intrusion, as well as groundwater flooding driven by the subsurface propagation of a hydraulic head rise. Using integrated surface–subsurface numerical simulations, this work investigates the temporal dynamics of these processes and the influence of hydrogeological settings on these dynamics. The goal is to advance mechanistic understanding of hydrological processes driven by storm surges and improve coastal hazard assessments, thereby contributing to the resilience of coastal regions.

Schematic of storm–surge impacts on the coastal surface–subsurface continuum
Schematic of storm–surge impacts on the coastal surface–subsurface continuum. Rising sea levels during a storm surge often lead to wave overwash, resulting in seawater inundation of the land surface and vertical seawater intrusion (SWI) into the subsurface. The elevated sea level also increases the hydraulic head in the sea, driving landward lateral SWI. In addition, the groundwater table rises in response to the higher sea head, sometimes causing groundwater flooding. Tajima et al. (2026; WRR).
Temporal evolution of surge-driven groundwater flooding under different hydrogeological settings
Temporal evolution of surge-driven groundwater flooding under different hydrogeological settings. Atolls are characterized by a dual-aquifer configuration, where a Holocene sediment aquifer overlies a more permeable Pleistocene limestone aquifer, with the unconformity known as the Thurber discontinuity. Swamp water depth varies with (a) the elevation of the Thurber discontinuity and (b) the hydraulic conductivity of the Pleistocene aquifer, demonstrating how the dual-aquifer configuration controls the magnitude and persistence of surge-driven groundwater flooding (surge duration shown in red). Tajima et al. (2023; WRR).
Related publications
Tajima, S., Brunner, P., Liu, J., Delottier, H., & Tokunaga, T. (2023). Groundwater flooding on atolls caused by storm surges: Effects of the dual-aquifer configuration. Water Resources Research, 59(10), e2023WR034762. Link
Tajima, S., Therrien, R., & Brunner, P. (2026). Climate change alters post-surge recovery of coastal aquifers. Water Resources Research, 62(3), e2025WR042142. Link
Tajima, S., Therrien, R., Purtschert, R., & Brunner, P. (2026). Groundwater contribution to cyclone-driven compound flooding. Preprint at ESS Open Archive. Link

Island groundwater resources under climate change

Groundwater is a vital source of freshwater on small islands, yet its availability is highly sensitive to climate change through sea-level rise and shifts in the intensity and frequency of extreme events such as storm surges. This work uses integrated surface–subsurface simulations to assess climate change impacts on freshwater availability under different climate scenarios, while developing new tools to quantify these impacts with limited data and computational resources. It also seeks to bring this process-based hydrogeological understanding into dialogue with diverse knowledge systems through collaboration with a broad range of stakeholders, thereby supporting transdisciplinary research that can inform sustainable groundwater management and climate adaptation.

Schematic of hydrogeological setting of a Pacific Island, and projected climate–change impacts and trends
Schematic of hydrogeological setting of a Pacific Island, and projected climate–change impacts and trends. Rainfall infiltrates through the unsaturated zone to replenish fresh groundwater, which overlies denser saline groundwater, with a transition zone in which salinity gradually increases. Fresh groundwater resources can decline through two main mechanisms: decreased recharge and lateral or vertical seawater intrusion (SWI). Climate change can drive these mechanisms through changes in both climate averages and the frequencies and intensities of climate extremes and variability. Illustration by Lisa Vogel. Tajima et al. (2026; Preprint).
Effects of sea level rise (SLR) on freshwater lens volume
Effects of sea level rise (SLR) on freshwater lens volume. (a) Schematic of freshwater lens contraction and volume reduction induced by SLR. The shoreline retreats inland by ΔR in response to cumulative SLR (ΔH), with the extent of retreat controlled by the coastal slope (tanφ). (b) Relationship between cumulative SLR (ΔH) and the ratio of post- to pre-SLR freshwater lens volume (V'/V) for different coastal slopes (tanφ). Tajima et al. (2026; Preprint).
Related publications
Tajima, S., Liu, J., & Tokunaga, T. (2022). Estimating freshwater lens volume in highly permeable aquifers. Hydrological Research Letters, 16(1), 12-17. Link
Tajima, S., Therrien, R., & Brunner, P. (2026). Climate change alters post-surge recovery of coastal aquifers. Water Resources Research, 62(3), e2025WR042142. Link
Tajima, S., Therrien, R., Purtschert, R., & Brunner, P. (2026). Boundary-integral solutions for freshwater lens volume in arbitrarily shaped islands. Preprint at ESS Open Archive. Link
Tajima, S., Purtschert, R., Savard, M.–C., Rosier, M., Kotra, K. K., Therrien, R., Coulombe, C., Masse–Dufresne, J., Richard, F., Lucas–Picher, P., Müller, C., Barbecot, F., Bertone, L., Corcho Albarado, J. A., Tigona, R., Nauka, L. N., & Brunner, P. (2026). Groundwater security in the Pacific Islands: Bridging knowledge systems for climate adaptation. Preprint at ESS Open Archive. Link