Two-layer Aquifer Research Articles

Vertical Flux in a Two-Layer Aquifer System Absent of Aquitard

Vertical Flux in a Two-Layer Aquifer System Absent of Aquitard

Groundwater responses to recharge and flood in riparian zones of layered aquifers: An analytical model

A riparian zone is an important element in a river-aquifer system, controlling water exchange and other chemical and biological processes between a river and an aquifer. Complex groundwater flow patterns may occur due to aquifer heterogeneity within a riparian zone. The purpose of this study is to investigate the impacts of layered heterogeneity on water exchange in the riparian zone using a mathematical model for groundwater flow in a two-layer aquifer that is recharged by precipitation and floods. A semi-analytical solution is derived for the hydraulic head, lateral discharge, and fluxes between the layers. Results demonstrate that the hydraulic conductivity difference between the two layers enhances lateral flow in the higher permeable layer and, more importantly, generates vertical flow between the two layers. The vertical flow induced by the recharge event is downward while it could be upward or downward induced by the flood event, which is determined by the contrast in permeabilities of the two layers. Using an equivalent hydraulic conductivity approach underestimates discharge of the two-layer aquifer due to recharge or flood. The analytical solution closely matched the observed hydraulic heads in riparian zone well of White Clay Creek and provided reasonable estimates of aquifer parameters. The present solution provides a valuable basis for further study of chemical and biological processes in riparian zone.

Modeling of convective-diffusion substance transfer and proposals as for development of an automatic system for monitoring and prevention of pollution of groundwater

At the enterprise being studied, due to losses, compounds of chromium accumulated on the soil floor of the workshop and gradually migrated in the zone of aeration of groundwater (aquifer). The level of groundwater in the area of the electric workshop is in the depth of around 10 m (as measured by the Pravoberezhna Expedition), that is, the ways of migration of chromium compounds in the area of aeration was about 10 m). The aquifer layer is located in flood-fed glacial deposits represented by sands of heter-ogenous grains. The sands are overlayed by loamy sand and loam with inclusion of gravel of basement crystalline rock. The filtration coefficient of sands varies from 1 m to 5 m per day, and of loams, from 0.1 m to 1 m per day. For this reason, the loam can only cautiously considered as low-permeable soil overlaying the aquafer. Therefore, it was more sound to consider a scheme of two-layer aquifer, in which the upper layer (loam) is about 1.5-m-thick, and the thickness of the lower layer (sand) is 5 to 15 m. Hexavalent chromium pollution using a model of convective-diffusive flow in an aquifer at a given research location was studied. The flow was defined as linear. The model is formu-lated as a differential equation and comprises the following variables: (C) – concentration of chromium compounds; (t) – time of observation; (Д) – coefficient of filtering (convective) dif-fusion; (Vn) – actual speed of filtration of the flow; (V) – existing speed of filtration of the flow; (na ) – active porosity of the soils forming the aquifer; (K) – filtering coefficient of the aquifer; (I) – hydraulic slope of the flow in the natural environment. Real input data and values of variables were used. Function С (х, t) was calculated and plotted, with its envelope curves determining the maximum concentration of the substance. Proposals are given as for development of an automated monitoring system and avoid-ance of the pollutant.

Application of geoelectrical resistivity for delineating crystalline basement aquifers in Basiri, Ado-Ekiti, Southwestern Nigeria

Delineating crystalline basement aquifers with adequate capacity to store and transmit sufficient groundwater is quite challenging as substantial spatial variability predominantly marked the basement aquifers. Electrical resistivity is a non-invasive geophysical technique for delineating weathered and fractured zones that permit groundwater accumulation in the basement rocks. This study uses two-dimensional (2D) electrical resistivity imaging (ERI) to delineate the weathering profile developed above the crystalline basement rocks and thus assesses the groundwater potential of the aquifer systems in Basiri, Ado-Ekiti, southwestern Nigeria. The study evaluated the reliability of the 2D ERI images by computing the depth of investigation (DOI) index to predict the depth extent at which the 2D resistivity models are no longer reliable. This approach can be used to minimise the mismatch between the resistivity models and subsurface targets, particularly in complex and subtle hydrogeological conditions as in the crystalline basement complex. This study indicates that the weathering profile developed into a two-layered aquifer system—a low yield shallow saprolite aquifer overlying a deeper fractured bedrock (saprock) aquifer with greater capacity to support adequate drawdown for high yield.

Flow to a partially penetrating well with variable discharge in an anisotropic two-layer aquifer system

A general analytical model for the problem of variable discharge of groundwater from a well in an anisotropic two-layered aquifer system is developed by taking account of the interface flow between the two layers. The well of infinitesimal radius partially penetrates the lower layer and is pumped at a variable discharge. The horizontal as well as vertical flows in both layers are included, and importantly, the effects of constant-head (Case 1) and no-flux (Case 2) boundaries at the top of the upper layer are considered. Laplace domain solutions for drawdown in dimensionless form are derived with the help of Hankel transform, and are inverted to the time domain numerically. The drawdown characteristics induced by an exponentially decayed rate of pumping are discussed, and sensitivity analysis is also performed to explore the influence of different parameters on drawdown characteristics. The results show that the drawdown around a partially penetrating well in the lower pumped layer induced by an exponential decreasing pumping rate usually contains three stages. The effect of anisotropy and well configuration have great influences on the drawdown distribution near the pumping well, and the horizontal flow and storativity of the upper (unpumped) layer cannot be ignored in determining the upper aquifer drawdown. The sensitivity analysis also illustrates that the dimensionless drawdown in the lower pumped layer is very sensitive to well configuration parameters all the time. It is only sensitive to the hydraulic parameters of the lower aquifer for Case 1 and to the hydraulic parameters of the lower layer and the specific storage of the upper layer at late times. The dimensionless drawdown of the upper layer is not sensitive to the variable well discharge parameters over the whole pumping period, and is not sensitive to the storage parameters of both layers for Case 1. However, the drawdown in the upper layer is sensitive to all the hydraulic parameters and well configuration parameters for Case 2 at late times.

Groundwater flow to a general well configuration in an unconfined aquifer overlying a fractured bedrock

Unconfined aquifers sometimes overlie fractured bedrocks, consisting of unconfined-fractured aquifer systems. Pumping induced hydrodynamics in such aquifer systems has not received much attention as in other types of aquifer systems. This paper aims to obtain semi-analytical solutions of flow to a general well configuration in such an unconfined-fractured two-layer aquifer system. The examples of general well configuration, specifically discussed here, include: 1) a horizontal well, 2) a vertical well, 3) a vertical well with one horizontal collector, and 4) a vertical well with two horizontal collectors which are perpendicular with each other. The methodology is general and can be extended to other types of wells with minor modifications of solutions developed in this study. The point sink/source solution is first obtained using double infinite Fourier and Laplace transformations (where sink is for pumping and source is for injection). The line sink/source solution is then obtained via integrating the point sink/source along the desired direction and following the principle of superposition. The de Hoog numerical inverse Laplace transform and Gaussian Quadrature are used to obtain time-domain dimensionless drawdown solutions. The instantaneous drainage of water table and the inter-porosity flow of the underlying fractured aquifer are taken into account. The effects of the length of horizontal well on dimensionless drawdown and dimensionless drawdown derivative, and scaled sensitivity of the hydraulic parameters of the underlying fractured aquifer are explored. The dimensionless drawdowns and drawdown derivatives of a vertical pumping well, a vertical well with a single horizontal collector and a vertical well with two perpendicular horizontal collectors are also compared. The scaled sensitivity of dimensionless drawdown to the hydraulic parameters of the underlying fractured aquifer of these different pumping well structures is explored. The results of this study can be used to evaluate the hydraulic parameters using the drawdown data collected during a pumping test performed in a general well configuration in a unconfined-fractured two-layer aquifer system. Furthermore, the presented solutions can be utilized to design the pumping well to minimize drawdown in low-permeability or thin aquifers. By eliminating the inter-porosity flow term from the underlying fractured aquifer, the solutions are applicable to groundwater flow to a pumping well in leaky aquifers.

The hydrogeochemical and isotopic investigations of the two-layered Shiraz aquifer in the northwest of Maharlou saline lake, south of Iran

Maharlou saline lake is the outlet of Shiraz closed basin in southern Iran, surrounded by several disconnected alluvial fresh water aquifers. These aquifers in the west and northwest of the lake are recharged by karstic anticlines such as Kaftarak in the north and Barmshour in the south. Here groundwater salinity varies along the depth so that better quality water is located below brackish or saline waters. The aim of this study is to investigate the reason for the salinity anomaly and the origin of the fresher groundwater in lower depth. Hence, the change in groundwater salinity along depth has been investigated by means of a set of geoelectrical, hydrogeological, hydrogeochemical, and environmental isotopes data. The interpretation of geoelectrical profiles and hydrogeological data indicates that the aquifer in the southeast of Shiraz plain is a two-layer aquifer separated by a fine-grained (silt and clay) layer with an approximate thickness of 40 m at the depth of about 100–120 m. Hydrgeochemistry showed that the shallow aquifer is recharged by Kaftarak karstic anticline and is affected by the saline lake water. The lake water fraction varies in different parts from zero for shallow aquifer close to the karstic anticlines to ∼70 percent in the margin of the lake. The deep aquifer is protected from the intrusion of saline lake water due to the presence of the above-mentioned confining layer with lake water fraction of zero. The stable isotopes signatures also indicate that the ‘fresh’ groundwater belonging to the deep aquifer is not subject to severe evaporation or mixing which is typical of the karstic water of the area. It is concluded that the characteristics of the deep aquifer are similar to those of the karstic carbonate aquifer. This karstic aquifer is most probably the Barmshour carbonated anticline buried under the shallow aquifer in the southern part. It may also be the extension of the Kaftarak anticline in the northern part.

Hydrologic and hydrogeologic analyses of an alluvial aquifer underlying Kushabhadra-Bhargavi River basin, Odisha, India

This paper deals with the analysis of groundwater condition in an alluvial aquifer system underlying Kushabhadra-Bhargavi River basin of Odisha, India. The rainfall data and river-stage data of the Kushabhadra River were analyzed for the periods of 1995–2009 and 1991–2010, respectively. Using the available lithologic data, geologic profiles along North-South and East-West sections were prepared and stratigraphy analysis was performed to characterize aquifers and confining layers present in the river basin. The results of stratigraphic analyses indicated that a two-layered aquifer system consisting of an unconfined aquifer and a confined aquifer exists in the study area. The thickness of unconfined aquifer varies from 3.4 to 46.5 m, whereas that of confined aquifer varies from 3.1 to 80.3 m over the basin with an interconnecting confining layer of thickness ranging from 2.1 to 60.0 m. The rainfall-groundwater dynamics and hydraulic connectivity were also investigated for gaining insights into groundwater characteristics. The analysis of groundwater levels indicated that the correlation among the 14 sites is better for most pairs of the sites (r = 0.50 to 0.96) in case of pre-monsoon season’s data and annual data as compared to monsoon and post-monsoon season’s data. This indicates good hydraulic connectivity among the observed sites in the study area. The significant seasonal groundwater fluctuations in the study area indicate appreciable recharge to the aquifer during the monsoon season. The findings obtained and insights gained from this study can be helpful for the water managers and decision makers to understand groundwater dynamics for the efficient planning and management of vital groundwater resources in the region. It is recommended that groundwater monitoring should be continued at more sites to understand long-term spatio-temporal characteristics of groundwater in the study area.

Implications of matrix diffusion on 1,4-dioxane persistence at contaminated groundwater sites

Management of groundwater sites impacted by 1,4-dioxane can be challenging due to its migration potential and perceived recalcitrance. This study examined the extent to which 1,4-dioxane's persistence was subject to diffusion of mass into and out of lower-permeability zones relative to co-released chlorinated solvents. Two different release scenarios were evaluated within a two-layer aquifer system using an analytical modeling approach. The first scenario simulated a 1,4-dioxane and 1,1,1-TCA source zone where spent solvent was released. The period when 1,4-dioxane was actively loading the low-permeability layer within the source zone was estimated to be <3years due to its high effective solubility. While this was approximately an order-of-magnitude shorter than the loading period for 1,1,1-TCA, the mass of 1,4-dioxane stored within the low-permeability zone at the end of the simulation period (26kg) was larger than that predicted for 1,1,1-TCA (17kg). Even 80years after release, the aqueous 1,4-dioxane concentration was still several orders-of-magnitude higher than potentially-applicable criteria. Within the downgradient plume, diffusion contributed to higher concentrations and enhanced penetration of 1,4-dioxane into the low-permeability zones relative to 1,1,1-TCA. In the second scenario, elevated 1,4-dioxane concentrations were predicted at a site impacted by migration of a weak source from an upgradient site. Plume cutoff was beneficial because it could be implemented in time to prevent further loading of the low-permeability zone at the downgradient site. Overall, this study documented that 1,4-dioxane within transmissive portions of the source zone is quickly depleted due to characteristics that favor both diffusion-based storage and groundwater transport, leaving little mass to treat using conventional means. Furthermore, the results highlight the differences between 1,4-dioxane and chlorinated solvent source zones, suggesting that back diffusion of 1,4-dioxane mass may be serving as the dominant long-term “secondary source” at many contaminated sites that must be managed using alternative approaches.

계절양수가 하천건천화에 미치는 영향

ABSTRACT Visual MODFLOW was used for quantifying stream-aquifer interactions caused by seasonal groundwater pumping. Ahypothetical conceptual model was assumed to represent a stream-aquifer system commonly found in Korea. The modelconsidered a two-layered aquifer with the upper alluvium and the lower bedrock and a stream showing seasonal waterlevel fluctuations. Our results show that seasonal variation of the stream depletion rate (SDR) as well as the groundwaterdepletion depends on the stream depletion factor (SDF), which is determined by aquifer parameters and the distance fromthe pumping well to the stream. For pumping wells with large SDF, groundwater was considerably depleted for a longtime of years and the streamflow decreased throughout the whole year. The impacts of return flow were also examined byrecalculating SDR with an assumed ratio of immediate irrigation return flow to the stream. Return flow over 50% ofpumping rate could increase the streamflow during the period of seasonal pumping. The model also showed that SDR wasaffected by both the conductance between the aquifer and the stream bed and screen depths of the pumping well. Ourresults can be used for preliminary assessment of water budget analysis aimed to plan an integrated management of waterresources in riparian areas threatened by heavy pumping.Key words: Stream Depletion Rate (SDR), Stream Depletion Factor (SDF), Stream-aquifer interaction, Seasonal pump-ing, Visual MODFLOW

Functioning and precipitation-displacement modelling of rainfall-induced deep-seated landslides subject to creep deformation

We propose an approach to study the hydro-mechanical behaviour and evolution of rainfall-induced deep-seated landslides subjected to creep deformation by combining signal processing and modelling. The method is applied to the Sechilienne landslide in the French Alps, where precipitation and displacement have been monitored for 20 years. Wavelet analysis is first applied on precipitation and recharge as inputs and then on displacement time-series decomposed into trend and detrended signals as outputs. Results show that the detrended displacement is better linked to the recharge signal than to the total precipitation signal. The infra-annual detrended displacement is generated by high precipitation events, whereas annual and multi-annual variations are rather linked to recharge variations and thus to groundwater processes. This leads to conceptualise the system into a two-layer aquifer constituted of a perched aquifer (reactive aquifer responsible of high-frequency displacements) and a deep aquifer (inertial aquifer responsible of low-frequency displacements). In a second step, a new lumped model (GLIDE) coupling groundwater and a creep deformation model is applied to simulate displacement on three extensometer stations. The application of the GLIDE model gives good performance, validating most of the preliminary functioning hypotheses. Our results show that groundwater fluctuations can explain the displacement periodic variations as well as the long-term creep exponential trend. In the case of deep-seated landslides, this displacement trend is interpreted as the consequence of the weakening of the rock mechanical properties due to repeated actions of the groundwater pressure.

Groundwater flow in hillslopes: Analytical solutions by the theory of holomorphic functions and hydraulic theory

Three 2-D steady Darcian flows in an aquifer with a subjacent confining layer of a non-constant slope or a bedding inconformity are studied by two models: a potential theory (conformal mappings, the inverse boundary-value problem method, and the theory of R-linear conjugation) and hydraulic approximation. First, flow over a corner, whose vertex is either a stagnation point or point of infinite Darcian velocity, is analysed as a transition from one “normal” regime upstream to another downstream. The hodograph domain is a circular triangle, which is mapped onto a complex potential strip via an auxiliary half-plane. Parametric equations (backwater curves) for the phreatic surface are obtained. For the same flow problem, a depth-averaged 1-D nonlinear ODE for the thickness of the saturated zone (a generalized Dupuit–Fawer model) is numerically solved showing a perfect match with the potential (2-D) solution. Second, a non-planar aquifuge boundary is reconstructed as a streamline, along which an additional “control” boundary condition holds in the form of pore pressure as a function of an auxiliary variable (a relation between the hydraulic head and vertical Cartesian coordinate). The free surface is found in terms of Cauchy’s integrals for the Zhukovskii function, with explicit integrations for selected “controls”. Third, a confined flow in a two-layered aquifer having a lens-type semi-circular inclusion in the subjacent stratum and incident velocity parallel to the interface between two aquifers is examined. The conjugation conditions along all four boundaries, across which the hydraulic conductivity jumps, are exactly met. The three velocity fields are explicitly presented, with examination of the flow net, including separatrices (“capture zone” boundaries), demarcating suction/barriering of the lens, and evaluation of the lens-induced cross-flow (commingling) between the two strata.

Air and water flows in a large sand box with a two-layer aquifer system

The pumping of water from a sand box with a finer layer on the top was studied theoretically and experimentally. The sand box, saturated at its lower portion and initially in hydrostatic equilibrium, was pumped at constant rate. The results show that significant negative air pressure can be generated in the vadose zone during pumping. The negative air pressure increases quickly in the earlier stage of pumping, reaches a maximum, and then gradually becomes zero. The initial water-table depth has a significant effect on the generated negative air pressure. The shallower the initial water table, the larger the vacuum, and the longer the time to reach the maximum vacuum. A transient, three-dimensional model was constructed using TOUGH2-MP to simulate the air-water two-phase flow processes in the sand box. The reasonable match between the numerical solutions and the experimental data indicates that the numerical model can reproduce the dynamic process of air and water flows. The study has implications in pumping test analyses. If the air pressure in the two-layer system is ignored, the drawdown in the system will be underestimated, especially when the upper layer has low permeability and the initial water table is close to the interface of the two layers.

Computation of Moments to Examine Plume Evolution in a Two-layer Aquifer

This paper presents statistical behavior of contaminant concentration distribution in a hypothetical two-layer aquifer. The aquifer consists of two layers, connected through patchy pervious interfaces through which exchange of concentration takes place. To obtain time evolution of contaminant plume, an alternating direction implicit (ADI) scheme is adopted to solve coupled advection-diffusion equations. The effect of rain infiltration, boundary reaction, and evolution of contaminant plume are examined from the mean concentration distribution, calculated from four central moments using the Hermite polynomials. By applying Edgeworth asymptotic series, the centroid displacement for contaminant cloud and deviations of mean concentration from Gaussianity are examined by means of skewness and other parameters. Model results show that the effect of the boundary reaction plays a significant role in plume dilution in contrast to rain infiltration, and their combined effect cause negatively skewed deviations from Gaussianity. The dilution of contaminant concentration has been reported using breakthrough curve analysis.

Age structure and recharge conditions of a coastal aquifer (northern Germany) investigated with 39Ar, 14C, 3H, He isotopes and Ne

A tide-influenced two-layer aquifer system in northern Germany was investigated using environmental dating tracers (3H, 39Ar, 14C), the noble gas isotopes 3He, 4He and Ne. The study area is a marshland at the River Ems estuary, exposed to regular flooding until AD 1000. The construction of dykes, artificial land drainage and groundwater abstraction define the hydraulic gradient. The aquifer depicts a pronounced age stratification with depth. Tritium concentrations above 0.03 TU are found only in the top 30 m. Two tritium-free samples between 20 and 30 m depth show 39Ar ages of 130 and 250 years. Below a clay layer–about 50 m below surface level (mbsl)–all analysed samples are 39Ar free and, thus, older than 900 years. The initial 14C activities were about 70 pmC. Resulting 14C ages increase with depth and increase up to 9,000 years, in agreement with minimal 39Ar ages. Concentrations of radiogenic 4He correlate with 14C ages. Samples in a mid-depth range (20–70 mbsl) show significant gas loss. The gas loss is assigned to recharge in a methane producing environment. Deduced 4He ages were used to assign this water to a infiltration period of about AD 1000.

Closure to “Stream Depletion in a Two-Layer Leaky Aquifer System” by Bruce Hunt

Closure to “Stream Depletion in a Two-Layer Leaky Aquifer System” by Bruce Hunt

Discussion of “Stream Depletion in a Two-Layer Leaky Aquifer System” by Bruce Hunt

Discussion of “Stream Depletion in a Two-Layer Leaky Aquifer System” by Bruce Hunt

Minimal advective travel time along arbitrary streamlines of porous media flows: The Fermat–Leibnitz–Bernoulli problem revisited

Travel time of marked fluid particles along arbitrary streamlines in arbitrary porous streamtubes is estimated from below based on the Cauchy–Bunyakovskii (Schwartz) and Jensen inequalities. In homogeneous media the estimate is strict and expressed through the length of the streamline, hydraulic conductivity, porosity and the head fall. The minimum is attained at streamlines of unidirectional flow. The bounds for heterogeneous soils, non-Darcian flows and unsaturated media are also written. If such bounds are attained the corresponding trajectories become brachistochrones . For example, in a two-layered aquifer and seepage perpendicular to the layers there is a unique conductivity–porosity ratio which makes a broken streamline brachistocronic . Similarly, if conductivities of two layers are fixed there is a unique incident angle between flow in one medium and the interface which makes a refracted streamline brachistocronic .

A field study of the coupled effects of aquifer stratification, fluid density, and groundwater fluctuations on dispersivity assessments

A number of experimental studies have tackled the issue of solute transport parameter assessments either in the laboratory or in the field. But yet, the behavior of a plume in the field under density driven forces, is not well known due to possible development of instabilities. Some field tracer tests on the fate of plumes denser than native groundwater such as those encountered under waste disposal facilities, have pointed out the processes of sinking and splitting at the early stage of migration. The process of dispersion was widely investigated, but the range of dispersivity values obtained from either experimental tests, or numerical and theoretical calculations is still very large, even for the same type of aquifers. These discrepancies were considered to be essentially caused by soil heterogeneities and scale effects. In the meantime, studies on the influence of sinking and fingering have remained more scarce. The objective of the work is to analyze how transport parameters such as dispersivities can be affected by unstable conditions, which lead to plume sinking and fingering. A series of tracer tests were carried out to study under natural conditions, the transport of a dense chloride solution injected in a shallow two-layered aquifer. Two types of experiments were performed: in the first type, source injection was such that the plume could travel downward from one layer to the other of higher pore velocity, and in the second one, the migration took place only in the faster layer. The results suggest some new insights in the processes occurring at the early stages of a dense plume migration moving in a stratified aquifer under groundwater fluctuations, which can be summarized through the following points: (i) Above a stability criterion threshold, a fingering process and a multi modal plume transport take place, but local dispersivities can be cautiously derived, using breakthrough curves matching. (ii) When water table is subject to some cycling or rising, the plume can be significantly distorted in the transverse direction, leading to unusual values of the ratio between longitudinal and transverse dispersivities. (iii) Under stable conditions, for example in the case of straightforward injection in the faster aquifer layer, longitudinal dispersivity is greater than the transverse component as usually encountered, and the obtained transport parameters are closed to macro dispersivity values, which reach their asymptotic limit at very short distances. (iv) The classical scale effect about the varying dispersivity at short distances could be a process mainly due to the distance required for a plume stabilization.

The effective resistance to vertical flow in Dupuit models

Analytic solutions are developed for two-dimensional flow in the vertical plane in a stratified aquifer of two strata. Vertical flow in the aquifer is created by a discontinuous impermeable boundary separating the two aquifer layers. The solutions are compared to Dupuit solutions for a two-layer aquifer system in which the two layers are separated by a fictitious leaky layer; the resistivity of the fictitious leaky layer represents the average vertical resistance to flow in the aquifer. By comparison of the exact and Dupuit solutions, the appropriate characteristic length of the leaky system (the leakage factor) is evaluated, such that the discharges in the Dupuit and exact solutions are identical and the heads converge a short distance from the discontinuity in the impermeable boundary. It is shown, for this flow case, that the effective leakage factor is not a function of the flow field or the hydraulic conductivity of the two layers and that the effective resistivity is about 20% of the total resistance to vertical flow of the aquifer. The results provide guidance for selecting the resistance to vertical flow in numerical or analytical Dupuit models with coarse vertical discretization.