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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]

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]

Summary: The HydroGeoSphere model is further developed and used to investigate the effects of viscosity, capillarity and grid spacing on thermal variable-density flow. Under saturated and unsaturated flow conditions, the flow dynamics significantly depends on the viscosity assumption (constant vs. variable), where downwelling regions (constant viscosity) become upwelling regions (temperature-dependent variable viscosity). Capillarity does not change the location of downwelling and upwelling regions. Capillarity can significantly alter the flow dynamics in the way that the water table acts as a “lid” to flow, and it diverts a thermal plume laterally. Significance of capillarity increases with increasing soil moisture. Thermal convective flow is highly sensitive to spatial discretization. While the flow dynamics remains to be a function of grid level, spatial discretization Δx =Δz =1m appears to be appropriate to simulate unsaturated variable-density flow and heat transfer in porous media because estimated errors have asymptotically reached a minimum. [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]

Summary: In this paper, we investigated the tide-induced seawater–groundwater circulation in shallow beach aquifers using the finite element model MARUN. The numerical solutions were generalized using a dimensionless formulation. From a dimensionless tidal period and a dimensionless beach slope of 10%, we obtained results that apply to a wide range of beach permeabilities from 10−4 m/s to 10−3 m/s, beach slopes from 3.16% to 31.6%, tidal amplitude (0.3m-2m) and period (diurnal or semidiurnal). The numerical simulations demonstrated the following: The maximum Darcy velocity always occurs at the intersection of the watertable and the beach surface. The offshore beach groundwater is almost stagnant compared with the onshore groundwater flow, which may explain the previous observations that the major portion of the seaward groundwater seepage usually occurs in the shallow part of the submerged beach. The outflow from the seepage face accounts for 41–97% (average 55%) of the outflow from the intertidal zone. The amount of seawater infiltrating into the intertidal zone in a tidal cycle increases with the beach permeability and decreases when the inland recharge increases, and ranges from 35.5m3 yr−1 m−1 to 505.8m3 yr−1 m−1 for all the cases considered. Smaller beach slopes, smaller inland freshwater recharges, and/or greater beach permeability lead to larger saltwater plumes in the intertidal zone of the beach. The results are in line with the existing results of field observations and numerical simulations by previous researchers. [Copyright &y& Elsevier]

Abstract: We used two stream tracer studies to address issues related to using the transient storage model (TSM) in simulating solute transport in streams containing riffle-pools and step-pools. The injection durations were short such that one may consider the storage zones to consist mainly of surface water storage zones. We found that the TSM was not able to reproduce the high portion (near the peak) of the breakthrough curve in the first subreach (85m) regardless of the functional form of the objective function. We attributed this to the presence of a large pool 35m downstream of the injection location that caused segregation of solute flow that was completely mixed across the stream upstream of the pool. The model was able to closely fit the breakthrough curves at x=185 and 323m, most likely because the signature of the pool diminished downstream. The fitting method did not always ensure that the parameter values represented physical reality, and we believe that physical measurements should be made and incorporated into the parameter estimation procedure utilizing Bayesian estimation. Our findings indicate that the representative values of TSM parameters in a reach do not generally fall between the representative values of its subreaches. Hence, the expression ‘reach-averaged’ should be used cautiously. The results indicated that it is more reliable to extrapolate (going downstream) than to interpolate. We believe this is due to local physical controls that could not be accounted for with parameters estimated far downstream from the controls. [Copyright &y& Elsevier]

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]

• 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]