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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]

Subsurface heterogeneity could influence groundwater flow with implications on structure and productivity of aquifer ecosystems. Here we investigate effects of multifractal heterogeneity on topology of groundwater flow through MODFLOW simulations within a Monte Carlo framework. The results show that heterogeneity leads to focused groundwater advection and the creation of hotspots of bending‐type vortex flow in the fields. We demonstrate for the first time that the vortex structures characterized by Q criterion are 3‐D distributed and greatly deform surrounding pore‐water flow. The structures exhibit scale‐invariant features in multifractal fields and in stationary fields below the correlation scale, indicating that such vortex flow might be widely present with no characteristic scale. Complex spatial patterns of kinetic energy dissipation rate are identified for pore water flowing through heterogeneous porous media and correlate strongly with preferential flows. These findings are important for understanding solute fate and transport in aquifer systems. Plain Language Summary: The findings of this study provide insight into influences of heterogeneity on groundwater flow and solute transport processes in aquifers. Our results reveal that multifractal heterogeneity leads to focused regions of groundwater advection and creates hotspots of bending‐type vortex flow that are 3‐D and greatly deform surrounding pore water. It has strong implications for groundwater contamination and associated implementation of effective bioremediation strategies. This is because solutes/contaminants would be spun around these structures and therefore have less interaction with the soil matrix in the inner part of the vortices. We also demonstrate that the structures of the vortex flow are scale invariant in multifractal K fields and below the correlation scale of stationary K fields. It indicates that these vortex flow structures may be widely present in the field with no characteristic scale. Substantial energy dissipation along the high‐velocity zones also has strong implications for understanding activity, diversity, and the distribution of subterranean microorganisms. Key Points: Deformation and rotation of pore‐water flow occur near high vorticity zones, formed as blobs and correlated strongly to preferential flowsVorticity‐dominated blobs exhibit scale invariance in both multifractal K fields and up to the correlation scale in variogram K fieldsHeterogeneity dissipates substantial kinetic energy within large pore‐water velocity zones [ABSTRACT FROM AUTHOR]

Summary A numerical study, based on a density-dependent variably saturated groundwater flow model MARUN, was conducted to investigate subsurface flow and salt transport in bare saline aquifers subjected to evaporation, which was simulated using the bulk aerodynamic formulation. As evaporation was assumed to depend on the pore moisture, the evaporation flux evolved gradually causing a gradual increase in the pore salinity. This is in contrast to prior studies where the high salinity was imposed instantaneously on the ground surface. Key factors likely affecting subsurface hydrodynamics were investigated, including saturated hydraulic conductivity, capillary drive, relative humidity in the air, and surrounding groundwater replenishment. The simulations showed two temporal regimes where the first consists of rapid evaporation for a duration of hours followed by slow evaporation, until evaporation ceases. In the absence of surrounding groundwater replenishment, evaporation-induced density gradient generated an upward water flow initially, and then the flow decreased at which time a high density salt “finger” formed and propagated downwards. Capillary properties and atmospheric condition had significant impacts on subsurface moisture distribution and salt migration in response to the evaporation. The results also suggested that the presence of subsurface water replenishment to the evaporation zone tended to produce a steady evaporation rate at the ground surface. [ABSTRACT FROM AUTHOR]

A numerical study was undertaken to investigate the effects of waves on groundwater flow and associated inland-released solute transport based on tracer experiments in a laboratory beach. The MARUN model was used to simulate the density-dependent groundwater flow and subsurface solute transport in the saturated and unsaturated regions of the beach subjected to waves. The Computational Fluid Dynamics (CFD) software, Fluent, was used to simulate waves, which were the seaward boundary condition for MARUN. A no-wave case was also simulated for comparison. Simulation results matched the observed water table and concentration at numerous locations. The results revealed that waves generated seawater-groundwater circulations in the swash and surf zones of the beach, which induced a large seawater-groundwater exchange across the beach face. In comparison to the no-wave case, waves significantly increased the residence time and spreading of inland-applied solutes in the beach. Waves also altered solute pathways and shifted the solute discharge zone further seaward. Residence Time Maps (RTM) revealed that the wave-induced residence time of the inland-applied solutes was largest near the solute exit zone to the sea. Sensitivity analyses suggested that the change in the permeability in the beach altered solute transport properties in a nonlinear way. Due to the slow movement of solutes in the unsaturated zone, the mass of the solute in the unsaturated zone, which reached up to 10% of the total mass in some cases, constituted a continuous slow release of solutes to the saturated zone of the beach. This means of control was not addressed in prior studies. [ABSTRACT FROM AUTHOR]

The hydraulics of a gravel beach on Eleanor Island in Prince William Sound (PWS), Alaska, was investigated by measurements of the water table and by numerical simulations using the software SUTRA. Sediment samples were obtained from six wells at the approximate three depths: 0.30, 0.60, and 0.90 m, and they were sieved in the lab to generate the grain size distribution, and subsequently the saturated hydraulic conductivity, K0. The results indicated that the beach consisted of two layers: an upper layer with a high K0 and a lower layer with a low K0. In the landward portion of the intertidal zone, K0 was around 10-2 m/s and 10-5 m/s for the upper and lower layers, respectively. In the seaward portion of the intertidal zone, the K0 values were 10-4 m/s and 10-6 m/s in the upper and lower layers, respectively. The agreement of the model with the observed water table data was very good. A decrease in the slope of the beach in the seaward direction caused water to flow out from the lower layer and propagate seaward within the upper layer. This ensured that the water table seaward of the exit point was replenished and remained above the interface of the layers. The results suggest that the porosity of the beach decreases from 0.33 at the surface to around 0.05 at 1.0 m deep. The absence of oil on this beach is probably because the beach water table remained above the layers' interface during low tides. Using the calibrated model, it was found that the flow from land into the beach is around 6 m3/day (per meter parallel to the shoreline) and that the flow out of the lower layer varied with time reaching a maximum of 13 m3/(day·m) at low tide. The beach exchanges up to 7 m3/(day·m) with the sea. [ABSTRACT FROM AUTHOR]

Summary: Experiments were conducted to investigate the flushing of saltwater out of a laboratory aquifer (or beach) by freshwater propagating seaward. After a steady state distribution was achieved with a seaward hydraulic gradient, freshwater was introduced while keeping the total head constant at each boundary. This caused the propagation of freshwater seaward. Two initial uniform concentrations were used: Case 1: 2.0g/L (low salinity case) and Case 2: 34.0g/L (high salinity case). The observed salinity and pressure data were closely reproduced using the MARUN () numerical code. The results indicated that buoyancy plays an important role for Case 2 but is negligible for Case 1. The results also indicated that the flow in the offshore beach aquifer (submerged portion of beach) was negligible especially for Case 2. For this case, the pressure increased with time until reaching a peak and then decreased (i.e., humps were formed). This was not observed in the low salinity case. Investigations revealed that the increase in pressure is due to a combination of remnant high salinity and a rise in the water table at that location. Numerical investigations revealed that for the same difference in total head, the seaward flow of freshwater increases with a decrease in the seawater salinity. The increase, however, was nonlinear as a function of seawater density. For example, the discharge in the high salinity case was 20% lower than that in the low salinity case. [Copyright &y& Elsevier]

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]

Biostimulation by nutrient application is a viable technology for restoring oil-contaminated beaches. Maximizing the nutrient residence time is key for achieving a rapid cost-effective cleanup. We considered the nutrient injection strategy through a perforated pipe at the high tide line and we simulated numerically beach hydraulics, which allowed us to estimate the optimal injection flow rate of nutrient solution. Our results indicate that the optimal application is one that starts following the falling high tide and lasts for half tidal cycle. The saturated wet-front of the nutrient solution on the beach surface would move seaward with the same speed of the falling tide keeping a constant distance with the tide line. The numerical results were generalized to beaches of wide ranges of hydraulic and tidal properties using a novel dimensionless formulation for water flow and solute transport in porous media. Nomographs were presented to provide the flow rate based on four parameters: The beach slope and hydraulic conductivity, and tidal amplitude and period. [Copyright &y& Elsevier]

• Beach stratification and inland recharge affect the oil persistence in PWS beaches. • Inland recharge prevents oil from infiltrating into lower low-permeability layer. • High seaward flow in upper high-permeability layer causes washout of oil to the sea. • Depth (and slope) of bedrock greatly alters hydrodynamics in the lower beach layer. A combined field and numerical study of water flow and solute transport was conducted along a clean transect of a tidal gravel beach in Smith Island, Prince William Sound (PWS) that was heavily polluted after the Exxon Valdez oil spill. Water table and pore water salinity were directly measured in the field. The numerical model MARUN, validated with the field observations, was used to simulate subsurface flow and dissolved salt transport processes. Our study identified a two-layer hydraulic structure in the beach, a high-permeability upper layer and a low-permeability layer at the bottom. Numerical simulations indicated that the density gradient between the saltwater and freshwater strongly affects solute transport in the intertidal zone of the beach. A sensitivity study indicated that depth to and slope of the bedrock had major effects on solute transport in the lower layer, but minor effects in the upper layer. A deep bedrock caused the water particles travelling in the lower layer to penetrate deeper in the beach. Our study revealed high terrestrial freshwater recharge to the transect which promotes the removal of oil in two-layer beaches by maintaining the water table at or above the interface of the two layers. The study presented in this paper has strong implications on the oil persistence and associated removal strategies along the Alaskan coastline polluted with oil. Findings from this work in relation to oiled beaches include: 1) oil tends to persist at locations of small freshwater recharge and 2) Prior to oil arriving to the shoreline, one could minimize oil penetration into the beach by releasing water onto the beach at the high tide line during low tides. [ABSTRACT FROM AUTHOR]