Numerical Study of Solute Transport in Heterogeneous Beach Aquifers Subjected to Tides

Abstract

A numerical study, based on a variably saturated groundwater flow model within a Monto Carlo framework, was conducted to investigate flow and solute transport in a heterogeneous beach aquifer subjected to tides. The numerical simulations were conducted based on our previous tracer experiments performed in a laboratory beach. Heterogeneity is assumed to be multifractal generated using the Universal Multifractal model. Our results show that heterogeneity greatly alters temporal and spatial evolution of the tracer plume migrating in the beach. The spreading coefficient of the plume shows very dynamic response to tides; it increases as the tidally driven recirculation cell overlaps with the plume and decreases as the recirculation cell moves far from the plume with tides. Descriptive statistics suggests that heterogeneity enhances spreading of the plume in the beach in an ensemble sense along with significant spatial and temporal variation. Due to heterogeneity, high spots of the pore water velocity are formed within the recirculation cell, creating transient preferential flow paths in the beach in response to tides. Contours of the Okubo‐Weiss (OW) parameter show that coupling with tides, heterogeneity creates vorticity‐dominated flow regions and also expands strain‐dominated flow regions more downward, indicating complex local‐scale mixing in the beach, compared to corresponding homogeneous case. Geologic heterogeneity also alters the spatial extent of the recirculating cell and induces highly variable transit time along the recirculating flow paths. The results provide insights into effects of geologic heterogeneity on seawater‐groundwater mixing and associated solute transport processes in tidally influenced coastal aquifers.

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

“…Numerical simulations of water flow toward the root layer were conducted with randomized heterogeneous fields; a simple linear correlation was assumed between height of the capillary fringe and saturated K. Their results revealed that an increase in heterogeneity at the root-soil interface leads to high root water uptake rates. While a number of previous studies have explored the effects of geologic heterogeneity on flow and transport processes in coastal aquifer systems, the studies only considered spatial variability in K (Geng et al, 2020b;Heiss, Michael, & Koneshloo, 2020;Lu et al, 2013;Michael et al, 2016;Pool et al, 2015;Sebben et al, 2015). In coastal aquifers, due to oceanic forcing (e.g., tides and waves), variably saturated conditions are ubiquitously observed in nearshore beach systems; therefore, it is important to consider heterogeneity of sediment capillarity when investigating coastal flow and transport processes.…”

Impact of Hydrostratigraphic Continuity on Brine‐to‐Freshwater Interface Dynamics: Implications From a Two‐Dimensional Parametric Study in an Arid and Endorheic Basin

“…provide an extensive analysis on the impact of geologic heterogeneity on seawater circulation, while Kreyns et al (2020) document that freshwater discharge can extend further offshore in heterogeneous volcanic aquifers when compared to homogeneous counterparts. Geng et al (2020) use simulations to investigate the impact of subsurface heterogeneity on tidally influenced circulation, thereby confirming the importance of coupling heterogeneity with influences on an aquifer's hydrologic dynamics in order to further constrain geologic impacts on solute transport. further detail heterogeneity's influence on variable solute transport and decreased travel time through an aquifer in response to relatively deeper groundwater pumping.…”

“…For the purpose of this study, the e-folding time serves as a characteristic of the response rate, with the amount of time corresponding to the relative speed of response to a perturbation in recharge. We additionally assess flow topology using the Okubo-manuscript submitted to Water Resources Research Weiss (OW) method to provide a mechanistic explanation for the simulation results (de Barros et al, 2012;Geng et al, 2020). e-folding time in response to inflow perturbations is larger by at least a factor of three for the HSF model when compared to the homogeneous model (Figure 8).…”

Impact of Hydrostratigraphic Continuity in Heterogeneity on Brine-to-Freshwater Interface Dynamics; Implications from a 2-D Parametric Study in an Arid and Endorheic Basin

“…For the purpose of this study, the e-folding time serves as a characteristic of the response rate, with the amount of time corresponding to the relative speed of response to a perturbation in recharge. We additionally assess flow topology using the Okubo-manuscript submitted to Water Resources Research Weiss (OW) method to provide a mechanistic explanation for the simulation results (de Barros et al, 2012;Geng et al, 2020).…”

“…Density-dependent numerical simulations of brine-tofreshwater interfaces in arid basins have either been homogeneous (Vásquez et al, 2013;Tejeda et al, 2003) or simply layered models of local geology that underestimate basin-scale heterogeneity and produce unrealistic results of the modeled interface (Marazuela et al, 2018). Previous geostatistical modeling documented the influence of subsurface heterogeneity on seawater circulation in coastal aquifers Geng et al, 2020;Kreyns et al, 2020).…”