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Intertidal aquifers are hotspots of biogeochemical cycling where nutrients and contaminants are processed prior to discharge to the ocean. The nature of the dynamic subsurface mixing zone is a critical control on mitigating reactions. Simulation of density‐dependent, variably saturated flow and salt transport incorporating realistic representations of aquifer heterogeneity was conducted within a Monte Carlo framework to investigate influence of nonuniform permeability on intertidal groundwater flow and salt transport dynamics. Results show that heterogeneity coupled with tides creates transient preferential flow paths within the intertidal zone, evolving multiple circulation cells and fingering‐type salinity distributions. Due to heterogeneity, strain‐dominated (intense mixing) and vorticity‐dominated (low mixing) flow regions coexist at small spatial scales, and their spatial extent reaches peaks at high tide and low tide. Such topological characteristics reveal complex tempo‐spatial mixing patterns for intertidal flow with localized areas of high and low mixing intensities, which have implications for intertidal biogeochemical processing. Plain Language Summary: In marine coastal environments, groundwater‐seawater mixing and exchange have been identified as critical factors affecting biogeochemical cycles and nutrient loads from aquifers to coastal waters. Our results for the first time demonstrate a dynamic response of intertidal flow topology and preferential flow paths to tides in the presence of aquifer heterogeneity. Heterogeneity coupled with tides results in different types of mixing than homogeneous or nontidal systems, which is important for geochemical reactions in the beach. These results highlight the importance of considering geologic heterogeneity in studies of groundwater flow and solute transport processes in tidally influenced beach aquifers. Key Points: Heterogeneity coupled with tides creates transient preferential flows within the intertidal zone, altering salinity distributionStrain‐dominated and vorticity‐dominated intertidal flow regions coexist at small spatial scales due to geologic heterogeneityHeterogeneity increases spatial variability of mixing of pore water flowing through the intertidal zone [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Most existing numerical research on tide‐induced groundwater dynamics assumes a constant surface water salinity on the seaward boundary (constant salinity case). Few studies have investigated the influence of tidally varying salinity on shallow groundwater dynamics in coastal aquifers (tidal salinity case). We compiled field observations of tidally varying salinity in multiple estuaries across the eastern coast of China and a tidal creek in North Inlet‐Winyah Bay, United States. Numerical simulations were then conducted to explore the effect of tidally varying salinity on groundwater flow and salt transport in an idealized creek marsh aquifer. Results showed that the upper saline plume and classical saltwater wedge appeared in all cases, but the salinity in the saltwater wedge was diluted in the tidal salinity cases. Notably, groundwater transit times were shorter in the tidal salinity case than in the constant salinity case, especially under the creek bottom. Quantitative analyses indicated that tidally varying salinity significantly enhanced surface water‐groundwater exchange, increasing submarine groundwater discharge by 10% and the total inflow of surface water across the water‐sediment interface by 7%. As the density of groundwater differs from that of the overlying surface water, fingered saltwater flow formed in sediments under the creek bottom, leading to some small local water circulation cells. These small cells reduced groundwater transit times and almost doubled the water exchange rate. Coupling the density‐dependent flow to a simplified nitrogen reaction network revealed that the tidally varying salinity may have the potential to influence nitrogen biogeochemical transformations that modify nitrogen loads prior to discharge. Key Points: A large salinity gradient causes unstable density fingering in sediments under the creek bottomUnstable flow enhances local water circulation and reduces groundwater transit timesTidally varying salinity enhances surface water‐groundwater exchange and has the potential to modify nitrogen loads prior to discharge [ABSTRACT FROM AUTHOR]

A numerical study was undertaken to investigate subsurface release and fate of benzene and toluene in a tidally influenced beach. The simulations were conducted by using a numerical model BIOMARUN, which coupled a multi-Monod kinetic model BIOB to a density-dependent variably saturated groundwater flow model MARUN. The fate and transport of the contaminant plume were characterized by computing its centroid trajectory, spreading area and percentage of biodegradation in beach saturated and unsaturated zone, respectively. Key factors likely affecting this process were investigated, including tide amplitude, capillarity and hydraulic conductivity. It was found that aerobic biodegradation was the major fate of the contaminant plume in the beach. Tidal action twisted the centroid of the contaminant plume during its migration in the beach, which increased the residence time of the plume in the beach. High tidal range significantly altered the spatial distribution of the contaminant biodegradation in the beach. In contrast, the capillary fringe had impacts on the percentage of benzene biodegraded in the saturated and unsaturated zone of the beach. The increase in capillary fringe enhanced the percentage of the contaminant biodegraded in the unsaturated zone, up to 40%, which is comparable to that in the saturated zone. Hydraulic conductivity seemed to have large impacts on the biodegradation rate of the contaminant in the beach. Higher hydraulic conductivity induced faster contaminant biodegradation in the beach. [ABSTRACT FROM AUTHOR]

Numerical simulations were conducted to quantify bank storage in a variably saturated, homogenous, and anisotropic aquifer abutting a stream during rising stream stage. Seepage faces and bank slopes ranging from 1/3 to 100/3 were simulated. The initial conditions were assumed steady-state flow with water draining toward the stream. Then, the stream level rose at a constant rate to the specified elevation of the water table given by the landward boundary condition and stayed there until the system reached a new steady state. This represents a highly simplified version of a real world hydrograph. For the specific examples considered, the following conclusions can be made. The volume of surface water entering the bank increased with the rate of stream level rise, became negligible when the rate of rise was slow, and approached a positive constant when the rate was large. Also, the volume decreased with the dimensionless parameter M (the product of the anisotropy ratio and the square of the domain’s aspect ratio). When M was large (>10), bank storage was small because most pore space was initially saturated with ground water due to the presence of a significant seepage face. When M was small, the seepage face became insignificant and capillarity began to play a role. The weaker the capillary effect, the easier for surface water to enter the bank. The effect of the capillary forces on the volume of surface water entering the bank was significant and could not be neglected. [ABSTRACT FROM AUTHOR]

Summary: The submarine outlet of a coastal confined aquifer is usually covered by less-permeable material such as silt and fine sand which forms a sloping or almost flat seabed. Previous analytical studies of the tide-induced head fluctuation usually assumed a vertical interface between the outlet-capping and the seawater and neglected the effects of the outlet-capping’s slope. Here we conducted a series of numerical simulations to investigate the effect of the slope of the outlet-capping on the tide-induced head fluctuations in a coastal confined aquifer. The numerical simulations demonstrated that when the hydraulic diffusivity, slope and/or the outlet-capping leakance are large, the tidal loading effect is relatively weak and the leakance dominates. In this case the analytical solution applies. In general, the joint actions of the outlet-capping leakance and the tidal loading effects result in complicated, 2-dimensional flow in the aquifer near the shoreline. For aquifers with small hydraulic diffusivity, small slope and/or small outlet-capping leakance, the outlet acts approximately as a no-flow boundary condition and the tidal loading dominates. In this case, negative phase shifts (time–advance) may occur near the coastline. The study provides potential guide to infer the aquifer’s submarine structure and parameter from tidal head fluctuation observations in inland wells. [Copyright &y& Elsevier]

Summary: This paper investigated the tide-induced head fluctuation in a coastal confined aquifer with an infinitely-extending semipermeable roof. The roof is overlain by an unconfined aquifer terminating at the coastline. Previous analytical studies ignored the effect of the elastic storage of the semipermeable roof. Our model incorporated this effect. The analytical solution indicated that the increase of the elastic storage of the offshore semipermeable roof may significantly enhance the tidal head fluctuation in the confined aquifer at the coastline. A threshold leakance exists in the sense that when the leakance is less than the threshold, the head fluctuation at the coastline will become insensitive to leakance and the roof can be regarded as impermeable. The threshold strongly depends on the roof’s elastic storage and decreases as the roof’s elastic storage increases, indicating that the roof’s leakage effect is significantly enhanced by the increase of the roof’s elastic storage. [Copyright &y& Elsevier]

Summary: This paper considers a leaky aquifer system comprising an unconfined aquifer, a confined aquifer, and a confining layer (the aquifer’s roof). The unconfined aquifer terminates at the coastline, the confined aquifer and its roof extend under the sea for a certain distance. The aquifer’s submarine outlet is covered by a thin layer of sediment (referred to as the “outlet-capping” hereafter) with properties dissimilar to the aquifer. Previous studies either neglected the leakage through the aquifer’s roof or the outlet-capping. This paper considers both effects and gives an analytical solution to describe the tide-induced head fluctuation in the aquifer system. For aquifers with zero or infinite offshore length, or with impermeable roof or outlet-capping, or without the outlet-capping, existing solutions in the literature are obtained. We defined the threshold value for the offshore aquifer length, which was found to be less than 5km for typical coastal aquifer systems. For an aquifer with its offshore length greater than the threshold, the inland head fluctuation becomes insensitive to variations of this length and the outlet-capping’s leakance. The leakage through the offshore roof decreases the threshold value. When the offshore confined aquifer is short, the inland head fluctuation increases with the outlet-capping’s leakance. In this case negative phase shift occurs near the coastline only for aquifers with small leakages through the offshore roof and the outlet-capping. In general, the combined actions of the tidal loading, the leakage and length of the offshore roof, and the leakage of the outlet-capping lead to complex, non-monotonic dependency of the inland head fluctuation on the aquifer parameters. [Copyright &y& Elsevier]

Coastal beach aquifers are biogeochemically active systems that mediate chemical and material fluxes across the land-sea interface. This paper provides a review of major physical stressors and geologic features that influence flow and solute fate and transport in coastal beach aquifers. We outline current understanding of the interactions between these factors and their associated impacts on water and geochemical fluxes within and across these aquifers. The physical processes that control flow, transport, and the formation and distribution of reactive zones in beach aquifers (e.g., tides, waves, density gradients, precipitation, episodic ocean events, and evaporation) operate across overlapping temporal and spatial scales, and present challenges for measuring and modeling physical flow and biogeochemical processes in coastal groundwater systems. Geologic heterogeneity introduces further complexity by modifying flowpaths, mixing patterns, and rates of biotransformation. Interactions between these physical stressors and geological controls are likely to evolve with changes in sea level, climate variability, human settlement, coastal erosion, and other natural and anthropogenic stresses, providing avenues for scientific exploration into the future role of beach aquifers as chemical mediators between the land and ocean. [ABSTRACT FROM AUTHOR]

Abstract: Submarine springs discharge offshore groundwater from confined aquifers extending under the sea. The effects of these springs on the propagation of tidal oscillations in coastal confined aquifers are not known. This paper presents an approximate analytical solution of tidal head fluctuations in a confined aquifer with one submarine spring. The aquifer is assumed to extend in all directions infinitely. The spring is represented by a permeable round column on the seabed, which penetrates completely the impermeable layer overlying the confined aquifer. The error of the approximate solution is negligible if the distance from the spring to the coastline is much greater than the radius of the permeable column representing the spring. Through a hypothetical example, we demonstrate that it is possible to identify the spring’s location using tidal signals observed from inland wells. Tidal groundwater head fluctuations from three inland observation wells at least are needed to determine the 5 model parameters, including the location (2 parameters), the radius of the permeable column representing the spring, the diffusivity of the aquifer, and the tidal loading efficiency of the system. [Copyright &y& Elsevier]