Countermeasures against saltwater intrusion (SWI) are critical to prevent coastal groundwater deterioration. Among different measures to prevent SWI, subsurface dams have shown to be an effective ...approach, but it is likely to produce residual saltwater behind the dam in a landward aquifer. This study investigated the influences of subsurface dam design and aquifer properties on the dynamic characteristics of residual saltwater in a field‐scale aquifer and for the first time revealed the desalinization mechanism of residual saltwater behind the dams from the point of mixing zone. It was found that the low‐concentration mixing zone (LCMZ) (for the area between 10% and 50% of seawater salinity) was a major channel for the saltwater to flow over the dam to the ocean boundary while the residual salt was continuously dispersed to the LCMZ from the high‐concentration mixing zone (HCMZ) (for the area between 50% and 90% of seawater salinity) under high‐concentration gradients. Moreover, we developed two formulas of the reduction rate of saltwater wedge length (RSWL*) and the removal rate of total residual salt mass (RTSM*) to evaluate the desalination effectiveness of high‐ and low‐concentration residual saltwater, respectively. The results showed that it took much longer time for a taller dam and a dam at a closer position to the sea boundary to desalinize the high‐concentration residual saltwater in the upstream aquifer, more than 50 years for the cases of dam height beyond 16 m. On the contrary, only a slightly shorter time was needed to remove the low‐concentration saltwater behind the dams with the decrease of the distance from the sea boundary. Aquifer properties including the hydraulic gradient, hydraulic conductivity, and dispersivity strongly altered the desalinization time of the residual saltwater. The dispersivity was found to be the most critical factor influencing the removal effectiveness of saltwater retained in the landward aquifer. Increase of dispersivity from 1 to 3 m can dramatically reduce the desalinization time from more than 30 to 4 years.
Key Points
The saltwater desalinization mechanism is firstly revealed from the point of mixing zone
Desalinization timescale and effectiveness of residual saltwater are quantified in field scale
Dam height and dispersivity are found to be the key factors controlling the desalinization
•Solute penetration with asymmetric matrix properties.•Closed-form transient solutions for conservative solute penetration.•Closed-form steady-state solutions for reactive solute penetration.•A ...dimensionless number defined for quantifying maximal penetration depth.
Solute and/or heat penetration into a fracture-matrix system is an important subject in subsurface transport. Solute and/or heat partitions among the fracture and its surrounding rock matrixes are based on coupled transport processes that are closely related to transport properties of the media. The penetration processes in an asymmetric fracture-matrix system are more complicated than that in a symmetric fracture-matrix system, due to different matrix properties. Accurate quantification of plume distribution in such a system is the basis of remediation design and risk assessment of a contaminated fracture-matrix system. Closed-form analytical and semi-analytical solutions (involving integrations) are obtained in this study to quantify the influences of asymmetric matrix properties on solute penetration processes in a fracture-matrix system. Advection, matrix diffusion, sorption, source decay and aqueous phase decay are considered. Numerical simulations are performed using HydroGeoSphere to test the solutions. Well matched curves of the numerical results and the closed-form solutions and semi-analytical solutions are obtained. The matrix porosity and retardation factor appear to significantly affect the spatiotemporal distribution of solute in a fracture-matrix system. Penetration depth in the matrix is a linear attenuation function of the horizontal distance from the source. And a new dimensionless number called the Z number (or the matrix penetration number) is defined to quantify the maximal penetration depth into the matrix. The solutions obtained can act as an effective tool for assessment of solute and/or heat transport in a fracture-matrix system. The solutions are also applicable to advection-dominated solute and/or heat transport in a thin aquifer bounded by two different aquitards.
Biofilm‐induced dynamic evolution of streambed permeability commonly concurs with the transition from connection to disconnection between surface water and groundwater in arid regions. However, in ...most previous studies, static streambeds were assumed to examine the evolution of disconnection, or despite dynamic streambeds being considered, the feedbacks between nutrients transport and microbial growth were ignored. In this study, we developed an innovative coupled variably saturated flow, microbial growth, biogeochemical reactions, and bioclogging model. We applied this model to investigate the feedbacks between nutrients transport and microbial growth and their controls on infiltration evolution. Our results showed that a new clogging layer can naturally develop due to these feedbacks and does not require prior clogging. The development of the new clogging layer promotes the occurrence of disconnection. These results illustrate that as bioclogging is a dominant process, previous static disconnection conditions cannot be used as criteria to predict whether disconnection can occur in a stream‐aquifer system. Furthermore, different from the previous assumption of constant specific microbial growth rates, biomass growth, and streambed permeability evolution are self‐limiting. Accordingly, due to initial low growth rate, infiltration increases when the water table declines, and it then decreases and reaches a minimum while a stable biofilm is developed. These trends coincide with the infiltration variations reported in previous field investigations. After reaching the minimum, infiltration increases again with decline of the water table until achieving a constant at the moment of disconnection. This stage was missing in previous studies because constant specific growth rates were assumed.
Key Points
Feedbacks between nutrients transport and microbial growth within a streambed are critical controls on infiltration during disconnection
A clogging layer can naturally develop without a requirement of prior clogging and promotes the occurrence of disconnection
Biomass growth‐induced dynamic evolution of streambed permeability is a self‐limiting process
Horizontal drilling has become an appealing technology for water resource exploration or aquifer remediation in recent decades, due to decreasing operational cost and many technical advantages over ...vertical wells. However, many previous studies on flow into horizontal wells were based on the Uniform Flux Boundary Condition (UFBC), which does not reflect the physical processes of flow inside the well accurately. In this study, we investigated transient flow into a horizontal well in an anisotropic confined aquifer laterally bounded by two constant‐head boundaries. Three types of boundary conditions were employed to treat the horizontal well, including UFBC, Uniform‐Head Boundary Condition (UHBC), and Mixed‐Type Boundary Condition (MTBC). The MTBC model considered both kinematic and frictional effects inside the horizontal well, in which the kinematic effect referred to the accelerational and fluid‐inflow effects. A new solution of UFBC was derived by superimposing the point sink/source solutions along the axis of a horizontal well with a uniform flux distribution. New solutions of UHBC and MTBC were obtained by a hybrid analytical‐numerical method, and an iterative method was proposed to determine the well discretization required for achieving sufficiently accurate results. This study showed that the differences among the UFBC, UHBC, and MTBC solutions were obvious near the well screen, decreased with distance from the well, and became negligible near the constant‐head boundary. The relationship between the flow rate and the drawdown was nonlinear for the MTBC solution, while it was linear for the UFBC and UHBC solutions.
Key Points
We developed new solutions of flow into the horizontal well
UFBC, UHBC, and MTBC solutions were obvious near the well screen
The influence of the frictional effect is the greatest comparing with the other effects
•Numerical model of SWPP test with skin effect and groundwater velocity was developed.•The effects of groundwater velocity and skin properties on BTCs were analyzed.•The model of SWPP test provide ...insights to estimate regional groundwater velocity.•The skin effects on SWPP test are significant and should be considered.
Single-well push-pull (SWPP) test is one of the most important ways to estimate flow and transport parameters, e.g. porosity, dispersivity, regional groundwater flow velocity. Usually the wellbore is surrounded by a finite-thickness skin, such as a gravel pack. The positive skin has a smaller hydraulic conductivity than the aquifer formation zone, and the negative skin has a greater one. In this study, a numerical model of a SWPP test considering skin effects was established using the finite-element COMSOL Multiphysics program to estimate aquifer parameters. Several important results were obtained. Firstly, regional groundwater velocity affects the types of breakthrough curves (BTCs) through changing the flow pattern. Secondly, a positive (or negative) skin leads to a slower (or faster) tracer transport process, and a smaller ratio between the hydraulic conductivity of the skin and that of the aquifer formation zone results in greater solute plume retardation in the skin zone. Thirdly, a larger thickness of the positive skin leads to a higher tracer concentration around the well, and the opposite is true if the skin is negative. Besides, the model of Leap and Kaplan (1988) underestimates the regional groundwater velocity for the second type of BTCs, and a larger longitudinal dispersivity can lead to a greater error, where the second type of BTCs has a rising limb at early stage followed by a falling limb at late stage. The general conclusion is that the estimations of groundwater velocity from SWPP tests are also affected by skin effects.
To remove contaminants from a layered heterogeneous porous system where the flow direction is parallel to the horizontal layering, the flushing front may advance faster in one layer than the other, ...resulting in a significant vertical concentration gradient across the layer interface. This gradient leads to mass exchange between the layers due to the vertical dispersive transport. Such a mass exchange phenomenon can greatly alter the mass (and heat if the temperature is a concern) distribution in a multi-layer porous media system but has never been investigated before in a quantitative manner. In this study, high-resolution finite-element numerical models have been employed to investigate how transport properties affect contaminant transport during flushing, using a two-layer system as an example. The results showed that the porosity and retardation factor play similar roles in affecting mass flux across the interface. Increasing the porosity (or retardation factor) of one layer with a faster flushing velocity would decrease the total mass flux across the interface of the layers, while increasing the porosity (or retardation factor) of the layer with a slower flushing velocity played an adverse influence. Furthermore, increasing the transverse dispersivity of any layer increased the mass flux across the interface of the two layers. However, changes in the transverse dispersivity did not affect the spatial range (or gap along the flow direction) in which significant vertical mass flux occurs. This study has important implications for managing contaminant remediation in layered aquifers.
•New solutions of flow into a partially penetrating well were developed.•Kinematic and friction effects were considered for intra-wellbore flow.•Intra-wellbore losses were significant in the early ...pumping stage.
Flow to a partially penetrating vertical well is made more complex by intra-wellbore losses. These are caused not only by the frictional effect, but also by the kinematic effect, which consists of the accelerational and fluid inflow effects inside a wellbore. Existing models of flow to a partially penetrating vertical well assume either a uniform-flux boundary condition (UFBC) or a uniform-head boundary condition (UHBC) for treating the flow into the wellbore. Neither approach considers intra-wellbore losses. In this study a new general solution, named the mixed-type boundary condition (MTBC) solution, is introduced to include intra-wellbore losses. It is developed from the existing solutions using a hybrid analytical-numerical method. The MTBC solution is capable of modeling various types of aquifer tests (constant-head tests, constant-rate tests, and slug tests) for partially or fully penetrating vertical wells in confined aquifers. Results show that intra-wellbore losses (both frictional and kinematic) can be significant in the early pumping stage. At later pumping times the UHBC solution is adequate because the difference between the MTBC and UHBC solutions becomes negligible.
Accurate estimation of slope stability based on numerous candidate estimation methods is difficult as different results may be yielded. It becomes even more challenging when only limited data of ...geotechnical parameters (e.g., shear strength parameters) are available to evaluate slope reliability. Based on the Bayesian sequential updating technology, a hybrid framework for slope reliability was proposed in this study, through which prior knowledge, multiple estimation methods, and corresponding model uncertainties could be integrated to estimate slope reliability using a small amount of geotechnical data. Three slope examples with various stratigraphic configurations and soil properties were used to illustrate the accuracy and efficiency of the proposed framework, during which the Bishop’s simplified method, the upper bound limit analysis method, and the finite element method were adopted. The results showed that with results of direct Monte Carlo simulation based on each method as the benchmark, a compromised mean of the factor of safety (
μ
FS
), and conservative standard deviation of the factor of safety (
σ
FS
) and failure probability (
P
f
) were yielded through the proposed framework. When the sample size of geotechnical parameters was greater than a threshold, the estimated
μ
FS
was stable, while the
σ
FS
and
P
f
synchronously varied within a small range with the increase in sample size. Demonstrations of the three examples indicated that the proposed hybrid framework can provide reliable and accurate estimations of slope reliability. The proposed framework may serve as a promising vehicle for slope/landslide engineering including failure and preventative mechanisms, movement prediction, and back analysis of geotechnical parameters in a probabilistic context, and big data analysis of geological and geotechnical problems as well.
•Streambed storage can affect the head variations of a riparian zone significantly.•Neglecting streambed storage causes high estimation errors of streambed transience.•Trust region reflective ...algorithm improved parameter estimation accuracy.•The new method generates good estimations on application to synthetic/field data.
Determination of streambed hydraulic conductivity (Ksb) and its variations over time are critical for quantifying exchange fluxes between surface water and groundwater. Previous studies have used flood wave response techniques (FRT) to determine Ksb or its transience, excluding the streambed storage effect, thus the estimated Ksb may not be reliable. In this study, after improving a previous FRT by substituting the gradient descent algorithm with an advanced and robust optimization method, trust region reflective algorithm, we have analyzed the influence of streambed storage effect on the estimation of time-dependent Ksb through successive inversion of flood wave responses. The study shows that the streambed storage effect is not important for identifying the transience in the hydraulic properties of a relatively thin streambed (e.g., several or dozens of centimeters). But significant under-estimation (1–2 orders of magnitude) can be seen because of neglecting the streambed storage effect. It is particularly true for a stream-confined aquifer system if the streambed is composed of fine deposits (e.g., silt, clay) with a thicker thickness greater than 1 m corresponding to low permeability but high storativity, and the aquifer is characterized by relatively low permeability (e.g., roughly 0.1–10 m d−1) and relatively low storativity (roughly 1E-5 m−1) (e.g., dense fine sands). The new method in which streambed storage effect is incorporated, can provide more reliable estimations of time-dependent Ksb for a stream-confined aquifer system, and it has been successfully applied to estimate the time-dependent Ksb of a reach of the Arkansas River, Kansas, USA.
Natural rock often suffers from cyclic wetting–drying involving different water types, and the resulting deterioration may differ from laboratory tests using distilled water or salt solutions. An ...inappropriate estimation of this deterioration effect may lead to fatal geological hazards and engineering failures. A multiscale study is conducted to investigate the physical and mechanical features of sandstone in Three Gorges Reservoir region (TGR sandstone) subjected to cyclic wetting–drying of Yangtze River water. During this study, three types of water, i.e., Yangtze River water, ionized water having similar ion compositions as the Yangtze River water, and distilled water, are used for comparison. The results show that the multiscale physical properties including mineral compositions (especially calcite and albite), micro-pore parameters, computed tomography values, and macro-mechanical parameters (i.e., Young’s modulus, uniaxial compression strength and tensile strength) are remarkably altered during the cyclic wetting–drying process. Significant correlations are found between these numerous multiscale properties. The results indicate that changes of mineral compositions and microstructure are the primary reasons for the deterioration of sandstone strength. The deterioration effect of distilled water on TGR sandstone is the least, while the effect of ionized water is the greatest, and that of river water being intermediate. These differences are ascribed to different chemical interactions, together with possible microorganism effects for river water, as microorganisms in river water potentially weaken the deterioration of cyclic wetting–drying of river water. In situ water is recommended for studying how rock properties are affected by water–rock interactions in real settings.
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