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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]

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: Conservative solute tracer experiments were conducted in Indian Creek, a small urban stream located in Philadelphia, Pennsylvania, USA. Estimated flow rates were between 46Ls−1 and 81Ls−1, average stream width was 5.5m and average stream depth was 0.2m. Given these dimensions, most researchers would think it reasonable to assume that the stream is completely mixed vertically and horizontally. However, we found that the stream was not vertically completely mixed in a 1.0m deep, 30m long pool. The limited mixing was demonstrated by the vertical stratification of a tracer cloud which was completely mixed both laterally and vertically across the stream prior to entering the pool. We suggest that the cause of limited mixing is due to a balance between groundwater inflow and transverse dispersion at the cross-section. We show that the unsupported assumption of complete mixing may result in a wide range, and thus increased uncertainty, of the values of stream flow and longitudinal dispersion coefficient estimated from these data. We conclude that the assumption of complete mixing and one-dimensional modeling must be checked against actual field conditions, even in small streams. [Copyright &y& Elsevier]

Scaling analyses on geophysical measurements of electrical conductivity, gamma radiation, and magnetic fields, at the Oak Ridge Reservation were conducted. The electrical conductivity and magnetic data exhibited multifractality in the north-south and east-west directions. The radiation data were observed to be non-scaling; a variogram with a sill was found to be more appropriate. The scaling of the EC and magnetic was generally within a range smaller than the maximum distance selected, as periodicity dominated at the larger distances. The electrical conductivity had anisotropy in the scaling of their variograms. But the magnetic data appear to have an isotropic scaling. The underlying statistics of the fields were near Gaussian for the electrical conductivity, but essentially Gaussian for the magnetic data. In environmental hydrogeology, knowledge of the spatial distribution of the intrinsic permeability, K, is very helpful in understanding the transport and spreading of contaminant plumes. Our previous studies have shown that the subsurface permeability, K, is multifractal. Detailed measurement of K is costly. Hence, large data sets of value collected both on a fine scale and over large distances are rare. In this study, we hypothesize that geophysical data could be used indirectly as a surrogate measurement for K, for obtaining statistical information on scale limited K data, and perhaps, directly at sites where K and electrical conductivity are correlated. [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]

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]