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• The value of hydraulic tomography with periodic signals is evaluated. • Transient signals with short periods provide better resolution. • Resolution is improved by the right combination of periods. • Optimal periods are linked to hydraulic properties. • The choice of periods is crucial for optimal results. An accurate assessment of the hydraulic properties of aquifers is required to represent groundwater flow and solute transport. This study investigates periodic hydraulic tomography performed between wells to obtain accurate images of hydraulic properties. Tomographic experiments with different period and amplitude of sinusoidal test flow, hydraulic properties and well configurations were simulated with a numerical flow model. An l -curve analysis of the obtained heads and sensitivities identified the optimal parameter resolution and served as a basis for comparing the experiments. The results show that the transient phase of signals with short periods provides the most information about the resolution of the aquifer. The resolution could be further improved if tests with different periods were properly combined in the analysis. The study concludes that periodic tomography provides valuable insight into the spatial resolution of hydraulic conductivity and its vertical anisotropy and specific storage, but the choice of signal characteristics is critical. [ABSTRACT FROM AUTHOR]

Characterizing groundwater flow parameters is crucial for understanding complex aquifer systems, and inverse techniques play a fundamental role in modeling hydrogeological parameters and assessing their uncertainties. Nonetheless, the use of a forward model in these methods can be highly time-consuming, especially with an increasing number of model parameters. To address this issue, we propose a surrogate model based on a U-Net architecture that replaces the transient groundwater flow model, reducing runtime and enabling a fast quantification of uncertainties related to key parameters, including heterogeneous hydraulic conductivity, boundary conditions, specific storage, and pumping rate. The surrogate is trained using limited evaluations of the forward model to learn the physical relationship between hydraulic conductivity fields and transient hydraulic heads measured on-site. The physical principles of the studied problem, including boundary conditions, specific storage, and source terms, are also mapped and introduced as inputs to the model to enhance its understanding of the governing equation of transient groundwater flow. To speed up learning using image–image regression, the previously predicted transient hydraulic heads also serve as an input to predict the transient heads at the current time step. Once the model is trained, we use a spectral geostatistical method to solve the inverse problem, a pumping test of 12 h, using the surrogate model in place of the forward model. Our study demonstrates that the trained U-Net accurately reproduces the state variables corresponding to a specific parameter field, and in terms of computational demand, using U-Net as a surrogate model reduces the required computational time by approximately an order of magnitude for the defined problem. The proposed approach offers an efficient and accurate method for groundwater flow parameter characterization and uncertainty quantification in complex aquifer systems. [Display omitted] • A U-Net surrogate approach is developed for transient groundwater inverse problems. • Uncertainties are rapidly evaluated using a geostatistical spectral method. • Uncertain parameters include log(K), Q, Ss, and boundary conditions • Integrate time into image–image regression for capturing time-varying processes. • The surrogate model significantly reduces computation times for inverse problems. [ABSTRACT FROM AUTHOR]

• Spatially variable, scale-dependent dispersivities are jointly estimated with hydraulic conductivity. • Joint inversion of tracer and ERT data improves aquifer characterization and transport predictions. • Spatially variable dispersivities outperform uniform dispersivities in transport prediction when adding ERT data. The hydraulic conductivity (K) has been recognized as one of the controlling parameters that significantly impact the transport behavior in groundwater systems. To improve the predictive ability of a transport model, concentration measurements are commonly used in inverse modeling to characterize the K distribution. In addition to K , the spread of the solute plume also depends on dispersion coefficients. The evolution of the plume morphology can only be satisfactorily reproduced with appropriate dispersivities. Dispersivity values or distributions rely on the resolution of the inferred K distribution. Dispersivities were typically treated as spatially uniform in inverse modeling and jointly estimated with K. Although employing spatially variable dispersivities may hold the potential to better capture the solute transport behavior, it may significantly increase the number of unknown parameters for inverse modeling. Solving such an ill-posed inversion problem is challenging, especially when the amount of intrusive borehole-based measurements is limited. As a non-intrusive, cost-effective, and high sampling density method, time-lapse geophysical technique (e.g. , electrical resistivity tomography, ERT) has not yet drawn much attention as a useful source of information for dispersivity estimation. In this study, we developed a joint hydrogeophysical inversion framework to simultaneously estimate spatially variable K and dispersivities by integrating tracer-concentration and ERT-derived potential data. We evaluated the proposed framework by conducting numerical tracer experiments in a highly heterogeneous aquifer. Results show that using limited tracer concentration data alone failed to resolve the fine structure in K field. In this case, there was little difference in the prediction performance of solute transport by considering spatially uniform or variable dispersivities. In contrast, the K distribution was estimated at a significantly higher resolution when integrating tracer-concentration and ERT data, contributing to a remarkable improvement in transport modeling. Powered by the sufficient amount of ERT measurements, jointly estimating spatially variable dispersivities further enhanced the reproduction of tracer plumes and breakthrough curves. [ABSTRACT FROM AUTHOR]

• An analytical model is developed for inter-aquifer, abandoned-well leakage. • A well injected to the confined aquifer is considered in the leakage model. • A thinner confined aquifer will lead to a higher abandoned well leakage rate. A non-plugged abandoned well may cause unwanted flows and solute transport between aquifers. This research provides a novel analytical model to calculate the leakage rate in an abandoned well between a confined aquifer and an unconfined aquifer under steady-state flow conditions. The abandoned well is treated as a passive preferential flow channel connecting a confined aquifer receiving injection from a separate well and an overlying unconfined aquifer; an impermeable aquiclude separates the two aquifers. A closed-form analytical solution for the steady-state leakage rate through an abandoned well is derived. The concerned abandoned well in this study fully penetrates the receiving unconfined aquifer but only penetrates the ceiling of the injecting confined aquifer with a very small screen length (with respect to the confined aquifer thickness). The injection well is sealed from the receiving unconfined aquifer and fully penetrating in the injecting confined aquifer. The solution developed in this study is compared with a previous solution by Avci (1994) (Avci, 1994. Evaluation of flow leakage through abandoned wells and boreholes. Water Resources Research, 30 (9), 2565–2578). Compared to Avci (1994), this research has two different assumptions: (1) The Avci solution assumed that the leakage effect occurs between two confined aquifers, and leakage concerned in this research occurs between an injecting confined aquifer and a receiving unconfined aquifer. (2) The abandoned well in the Avci (1994) solution fully penetrates both the receiving and injecting confined aquifers, while the abandoned well in this research fully penetrates the receiving unconfined aquifer but only penetrates the ceiling of the injecting confined aquifer with a minor screen length. In both this study and the Avci (1994), the injecting well is isolated from the receiving aquifer and fully penetrating in the injecting aquifer. The results show that the two solutions have similarities in thin confined aquifers but are much different in relatively thick confined aquifers (with thickness greater than 10 m). Using the Sobol method for global sensitivity analysis, we tested controls of the hydraulic conductivity ratio between confined and unconfined aquifers, the transmissivity of the confined aquifer over the injection rate per unit screen length, and the aquifer thickness on model performance. The results show that the model is most sensitive to the transmissivity of the confined aquifer over the injection rate per unit screen length. This research can be applied in multiple applications, such as modeling the leakage effect among two aquifers involving stormwater recharge and using a monitoring well to detect the existence of abandoned wells when the abandoned wells are not visible on the surface or not documented, among others. [ABSTRACT FROM AUTHOR]

• We solve low-velocity non-Darcian flow to a confined and partially penetrating well in a leaky aquifer system. • An analysis is conducted to examine the temporal and spatial changes in confined groundwater head differences between Darcian and non-Darcian flow conditions. • Non-Darcian flow in the aquitard needs to be considered under small confined aquifer hydraulic conductivity and large threshold pressure gradient of the aquitard. Previous studies on the flow dynamics of leaky aquifer systems have primarily relied on Darcy's law, assuming fully penetrating wells in confined aquifers. However, when dealing with aquitards dominated by clay, the flow behavior often deviates from Darcy's law. Moreover, in the majority of cases, this entails partially penetrating well pumping. This paper introduces a mathematical model for non-Darcian flow in a confined and partially penetrating well system within a leaky aquifer system. The model is based on the low-velocity non-Darcian flow equation, incorporating the threshold pressure gradient. In the confined aquifer, the flow exhibits two-dimensional Darcian flow behavior, while in the aquitard, the flow is characterized by one-dimensional non-Darcian flow in the vertical direction. In the shallow aquifer, the flow is one-dimensional Darcian flow in the radial direction. The mathematical model is solved using the finite difference method, and the obtained results are compared with calculations based on traditional Darcian flow theory. The findings indicate that the confined groundwater head difference, as predicted by the Darcian and non-Darcian flow theories, diminishes radially as the distance from the pumping well increases and vertically as one moves away from the top of the confined aquifer. As pumping progresses, the confined groundwater head difference initially rises and subsequently stabilizes gradually. Moreover, the magnitude of the confined groundwater head difference is more pronounced when the threshold pressure gradient, vertical hydraulic conductivity of the aquitard, pumping rate, hydraulic conductivity and specific yield of shallow aquifer are larger, or when the hydraulic conductivity and specific storage of the confined aquifer are smaller. The effect of well screen length on the confined groundwater head difference is minimal. [ABSTRACT FROM AUTHOR]

• Transient tidal overheight represented by phase-averaged tidal boundary condition. • Planar beach model successfully predicts overheight for complex coastal morphology. • Boundary condition performance is sensitive to coastal morphology representation. • Phase-resolved simulations and observed data can be replicated reasonably well. • Considerable reduction of coastal groundwater flow simulation runtimes achieved. Ocean tides cause water table overheight near the seaward boundary of coastal aquifers which can have a large impact on groundwater flows and saltwater intrusion. Despite this, regional-scale coastal groundwater flow models often neglect the effects of ocean tides or alternatively assume that the influences of ocean tides and sea level are constant over time. These simplifications are often required because simulation of the phase-resolved tidal signal at a coastal boundary is computationally demanding. Our objective was to derive a phase-averaged tidal boundary condition for application along gently sloping coastlines that enables simulation of real, complex tidal signals combined with sea-level changes due to meteorological effects. The boundary condition extends an existing analytical solution of tidal overheight by including an empirical correction function for conditions where the assumptions of the existing solution are violated. The correction function was developed by conducting a parametric study on an idealized two-dimensional cross-sectional coastal aquifer model and considering the effects of parameters including horizontal hydraulic conductivity, vertical anisotropy, specific yield, aquifer thickness, and beach slope. The performance of the new phase-averaged tidal boundary condition was assessed using a three-dimensional groundwater flow model of the island of Spiekeroog, Northwest Germany, which comprises complex coastal morphology with non-planar beach slopes. Simulations applying the new boundary condition were compared to simulations that adopted a phase-resolved tidal boundary condition and to observed groundwater levels. Accounting for time-varying changes in the tidal signal and sea level was found to be critical to simulate observed data and to adequately reproduce the transient tidal overheight simulated by the phase-resolved model. The performance of the new phase-averaged boundary condition in the regional-scale model varied depending on how the coastal morphology was represented in the boundary condition with local morphological features increasingly important for lower intertidal vertical infiltration capacity (low isotropic hydraulic conductivity or high vertical anisotropy) or specific yield. In conclusion, the new boundary condition presented overcomes current limitations in simulating transient tidal overheight in regional-scale groundwater models. [ABSTRACT FROM AUTHOR]

• An analytical model is developed to handle horizontal aquifer/aquitard heterogeneity. • A high- K aquitard window tends to dampen tide-induced aquifer head fluctuations. • Aquifer head fluctuations may be dampened by a large-size high- S s aquitard window. • Sandbox experiments validated the effects of a high- K aquitard window. Low-permeability aquitards are important sedimentary units that protect the underlying confined aquifers from contamination by seawater and anthropogenic sources in coastal leaky aquifer systems. Therefore, the integrity of the aquitard is of crucial importance. However, the aquitard integrity may be vulnerable to horizontal heterogeneities in the form of aquitard windows. In this study, an innovative analytical model was developed to describe groundwater flow in a coastal leaky aquifer system with horizontal heterogeneities in hydraulic properties of either the aquitard or the confined aquifer. Consequently, the effects of aquitard windows, which manifest as horizontally local heterogeneities in hydraulic conductivity and/or specific storage, on the tidal wave propagation can be considered in the analysis. Analytical solutions were derived for tidal-head or constant-flux inland boundary conditions. The derivation process avoided previous methodologies that involved the assembly of large-size matrices. The newly developed analytical model can cover multiple existing analytical models that are associated with the assumptions of homogeneity in hydraulic properties of the aquifer and the aquitard, infinite horizontal extent of the coastal leaky aquifer system, impermeability of the aquitard, or ignorance of aquitard storativity. Analytical investigations revealed that high-permeability aquitard windows acting as preferential-flow conduits tended to dampen the tide-induced head propagation within the underlying confined aquifer. Moreover, an aquitard with a higher storativity (but the same permeability) and of considerable size can result in an apparent propagation bias. The newly developed analytical model in this study was further corroborated through sandbox experiments. Both piezometer measurements and parameter estimates indicated the non-negligible effects of high-permeability aquitard windows on the tidal wave propagation. The sandbox investigation also highlighted the importance of geostatistical inverse modeling in characterizing local heterogeneities within the confined aquifer and in detecting the location and size of aquitard windows, when measurement data of tide-induced aquifer head fluctuations were provided. [ABSTRACT FROM AUTHOR]

A deep learning technique called Physics Informed Neural Networks (PINNs) is adapted to study steady groundwater flow in unconfined aquifers. This technique utilizes information from underlying physics represented in the form of partial differential equations (PDEs) alongside data obtained from physical observations. In this work, we consider the Dupuit–Boussinesq equation, which is based on the Dupuit–Forchheimer approximation, as well as a recent, more complete model derived by Di Nucci (2018) as underlying models. We then train PINNs on data obtained from steady-state analytical solutions and laboratory based experiments. Using PINNs, we predict phreatic surface profiles given different input flow conditions and recover estimates for the hydraulic conductivity from the experimental observations. We show that PINNs can eliminate the inherent inability of the Dupuit–Boussinesq equation to predict flows with seepage faces. Moreover, the inclusion of physics information from the Di Nucci and Dupuit–Boussinesq models constrains the solution space and produces better predictions than training on data alone. PINNs based predictions are robust and show a little effect from added noise in the training data. Furthermore, we compare the PINNs solutions obtained via the Di Nucci and Dupuit–Boussinesq flow models to examine the effects of higher order flow terms that are included in the Di Nucci formulation but are neglected by the Dupuit–Boussinesq approximation. Lastly, we discuss the effectiveness of using PINNs for examining groundwater flow. • PINNs predict hydraulic conductivity and water table heights using experimental data. • PINNs eliminate inability of Dupuit–Boussinesq equation when predicting seepage face. • Inclusion of physics improves PINNs predictions compared to plain neural networks. [ABSTRACT FROM AUTHOR]

Hydraulic conductivity and its anisotropy are fundamental aquifer properties for groundwater flow and transport modeling. Current in-well or direct-push field measurement techniques allow for relatively quick determination of general conductivity profiles with depth. However, capabilities for identifying local scale conductivities in the horizontal and vertical directions are very limited. Here, we develop the theoretical basis for estimating horizontal and vertical conductivities from different types of steady-state single-well/probe injection tests under saturated conditions and in the absence of a well skin. We explore existing solutions and a recent semi-analytical solution approach to the flow problem under the assumption that the aquifer is locally homogeneous. The methods are based on the collection of an additional piece of information in the form of a second injection (or recirculation) test at a same location, or in the form of an additional head or flow observation along the well/probe. Results are represented in dimensionless charts for partial validation against approximate solutions and for practical application to test interpretation. The charts further allow for optimization of a test configuration to maximize sensitivity to anisotropy ratio. The two methods most sensitive to anisotropy are found to be (1) subsequent injection from a lateral screen and from the bottom of an otherwise cased borehole, and (2) single injection from a lateral screen with an additional head observation along the casing. Results may also be relevant for attributing consistent divergences in conductivity measurements from different testing methods applied at a same site or location to the potential effects of anisotropy. Some practical aspects are discussed and references are made to existing methods, which appear easily compatible with the proposed procedures. [ABSTRACT FROM AUTHOR]

Precise information about the spatial distribution of hydraulic conductivity (K) in an aquifer is essential for the reliable modeling of groundwater flow and transport processes. In this study, we present results of a new inversion procedure for induced polarization (IP) data that incorporates petrophysical relations between electrical and hydraulic parameters, and therefore allows for the direct computation of K. This novel approach was successfully implemented for the Bolstern aquifer analog by performing synthetic IP experiments with a combined surface and cross-borehole setup. From these data, the distribution of K was retrieved with high accuracy and resolution, showing a similar quality compared to images achieved by hydraulic tomography. To further improve the quantitative estimates of K , we use synthetic pumping test data to inform two novel calibration strategies for the IP inversion results. Both calibrations are especially helpful for correcting a possible bias of the IP inversion, e.g., due to resolution limitations and/or to bias in the underlying petrophysical relations. The simulation of tracer experiments on the retrieved tomograms highlights the accuracy of the inversion results, as well as the significant role of the proposed calibrations. • New IP inversion procedure for direct computation of hydraulic conductivity. • IP inversion results have similar quality as hydraulic tomography. • Hydraulic calibration of IP results to improve transport simulations. [ABSTRACT FROM AUTHOR]

The Horizontal Reactive Media Treatment (HRX) well is a novel technology for in situ treatment of contaminated groundwater. The most significant technical performance risk associated with field-scale implementation of the HRX well is the potential for water to bypass the well untreated. Therefore, the groundwater capture simulation plays a very important role in the design of the HRX well. We build a model based on a cell-based smoothed radial point interpolation method (CS-RPIM) for simulating groundwater flow captured by the HRX well in both 3D test pit pilot scale and field scale. This method is compared with the finite element method and measured values. Capture width and groundwater velocity obtained by CS-RPIM are consistent with the measured values. We further develop MATLAB software modules to analyze the effects of the hydraulic conductivities, dimension of well and other parameters on the groundwater capture zone, velocity and residence time in the reactive media. The results show that increase of well diameter has a significant effect on enlargement of the capture zone and the extension of media treatment time. When well diameter increases by 100%, horizontal and vertical capture width increase by 57% and 50% respectively, and media treatment time can be extended by 68%. • A realistic 3D model was built for simulating groundwater captured by the HRX well. • The simulation results outperform FEM or the average capture width formula results. • MATLAB software modules are developed for hydraulic capture zone simulation. • Impact of various factors on the capture zone, groundwater flow velocity and treatment time are analyzed. [ABSTRACT FROM AUTHOR]

Abstract: We consider common bank filtration systems and develop an explicit analytic solution representing steady, two-dimensional, groundwater flow to a horizontal well near a river in an unconfined aquifer. For the boundary-value problem investigated, we find that a unique solution exists for all negative well discharges. For positive discharges, a maximum value exists which corresponds to the formation of a cusp on the free surface. For positive discharges less than the maximum, the solution is not unique, consisting of two alternate configurations of the free surface. One solution includes a stable free surface on a single-valued physical plane, while the alternate solution includes a looped free surface lying on two sheets of a Riemann surface. Imposing a stability condition on the free surface results in a unique solution to the problem. We use the solution to investigate the behavior of the free surface under varying well discharges to identify stable pumping rates and predict well yield. In particular, we examine the well yield and the stability of the free surface when the head in the horizontal well is maintained at the top of the well screen. This condition is shown to produce a stable free surface for a wide range of well radii; the stability is independent of the hydraulic conductivity of the aquifer, the location of the well, or the presence of a skin resistance at the well. [Copyright &y& Elsevier]

Abstract: An existing capillarity correction for free surface groundwater flow as modelled by the Boussinesq equation is re-investigated. Existing solutions, based on the shallow flow expansion, have considered only the zeroth-order approximation. Here, a second-order capillarity correction to tide-induced watertable fluctuations in a coastal aquifer adjacent to a sloping beach is derived. A new definition of the capillarity correction is proposed for small capillary fringes, and a simplified solution is derived. Comparisons of the two models show that the simplified model can be used in most cases. The significant effects of higher-order capillarity corrections on tidal fluctuations in a sloping beach are also demonstrated. [Copyright &y& Elsevier]

• Finite particle method (FPM) simulates groundwater solute transport. • FPM outperforms smoothed particle hydrodynamics (SPH) with irregular particles. • FPM improves SPH by using a modified kernel gradient for concentration gradient. When a smoothed particle hydrodynamics (SPH) method, a Lagrangian and meshfree numerical scheme, is used to solve the advection-dispersion equation (ADE), SPH solutions may not be accurate when particles are irregularly distributed, i.e., the distance between two neighbor particles varies irregularly in a simulation domain. Particle irregularity may be caused by nonuniform groundwater flow in a heterogeneous field of hydraulic conductivity. This study explores for the first time whether the Finite Particle Method (FPM) can provide more accurate ADE solutions than SPH does for irregularly distributed particles. FPM is similar to SPH in theory, but uses a modified kernel gradient to construct a SPH approximation of solute concentration gradients. Performance of SPH and FPM with irregularly distributed particles is evaluated by using two numerical cases. The first case considers only diffusive transport, and has an analytical solution for the evaluation. The second case considers both advection and dispersion, and uses a numerical solution as a reference for the evaluation. For each of the two cases, several numerical experiments are conducted using multiple sets of irregularly distributed particles with different levels of particle irregularity due to different levels of heterogeneity of hydraulic conductivity. Numerical results indicate that, for the numerical experiments of this study, FPM outperforms SPH to yield more accurate ADE solutions. However, FPM solutions are still subject to numerical errors, and the errors increase when the level of heterogeneity of hydraulic conductivity increases. Further improvement of FPM is warranted in a future study. [ABSTRACT FROM AUTHOR]

To consider the non-Darcian nature of groundwater flow and the permeability reduction owing to well and aquifer clogging, an Izbash's law-based analytical model with a time-dependent hydraulic conductivity function is proposed for constant-rate recharge and constant-head recharge. By means of linearization approximations, variable substitutions and the Laplace transform, approximate analytical solutions for the model are derived. Comparison of developed approximate analytical solutions with numerical solutions indicates that the approximate solutions are highly accurate except during the short period of initial recharge stage. The proposed model provides a theoretical basis for managed aquifer recharge involving clogging-related permeability reduction and non-Darcian flow. [ABSTRACT FROM AUTHOR]

• Variations in the support/observation scale of hydraulic conductivity induce changes in the information content of the associated Darcy fluxes. • Propagation of information about conductivity onto Darcy flux through the flow and mass balance equation can be quantified via information theory. • Flux data at large scales provide more information about fluxes at small scales than what can be observed with reference to conductivities. We rest on an Information Theory perspective and assess (i) the average information content and (i) the information shared between Darcy flux fields associated with fields of hydraulic conductivity (K) characterized by differing support (or measurement) scale. We treat hydraulic conductivity as a spatial random field, characterized by a given distribution and correlation structure. The latter is modeled through a truncated power law variogram (TPV), which explicitly takes into account a characteristic length scale of the support volume of K through a lower cutoff scale. We then frame our study in a numerical Monte Carlo context where groundwater flow is evaluated across a collection of realizations of hydraulic conductivity characterized by different values of the TPV parameters and subject to uniform in the mean flow. We quantify information through the Shannon entropy of the probability mass functions of the Darcy flux components as well as the mutual information and the multivariate mutual information respectively shared by pairs and triplets of Darcy flux components related to hydraulic conductivity fields evaluated at diverse scales and associated with various levels of heterogeneity. Partitioning of multivariate mutual information according to unique, redundant and synergetic contributions is also quantified. We found consistent trends (i) in the variation of the average information content with respect to the size of lower cutoff scale and (ii) in the way information is shared between pairs and triplets of Darcy flux components associated with diverse support scales of the underlying conductivities. [ABSTRACT FROM AUTHOR]

• A general mathematical model describing non-Darcian flow in a confined aquifer for a single well circulation system was developed. • This study provided an extension of the analytical solution based on Darcy's Law to non-Darcian flow conditions for a single well circulation system. • An approximate analytical solution for non-Darcian flow in the Laplace domain and the steady-state analytical solution in the time domain were obtained. • The characteristics of non-Darcian flow and the influences of different parameters on drawdown were investigated. An analytical model is proposed in this study to describe transient drawdown induced by non-Darcian flow in a confined aquifer with a single well circulation groundwater heat pump system. The Izbash equation and a linearization method are employed to describe non-Darcian flow in the horizontal direction of a confined aquifer and to approximate the nonlinear term in the governing equation, respectively. By applying a combination of the Laplace and Fourier cosine transforms, an approximate analytical solution in the Laplace domain is obtained, which is numerically inverted to obtain transient drawdown in the time domain using the Stehfest algorithm method. The results of the derived analytical solution for the special case of Darcian flow (m = 1) correspond well with the existing solution derived using Darcy's law. The steady-state analytical solution in the time domain is obtained by applying the Fourier cosine transform. Moreover, the sensitivity analysis is performed to investigate the influence of selected parameters, such as the power index m , the radial hydraulic conductivity K r , the aquifer specific storage S , and the length of the sealed section d 2 , on drawdown. The results show that each parameter has its influence period on drawdown, and that drawdown is more sensitive to the power index m compared to other parameters. [ABSTRACT FROM AUTHOR]

• Represents local heterogeneity at micro-scale level with an average error variance less than 0.02. • The FSM shows consistent results at well node locations with changing grid dimensions. • ROM is computationally efficient and replicates FSM with desired accuracy. • Both FSM and ROM shows good mesh convergence. Groundwater, being a vital component of the natural water resource system, needs continuous monitoring and dynamic management strategies. That said, we require computationally inexpensive groundwater flow models for repetitive solutions with desirable accuracy under budgetary limitation(s). Natural aquifer systems inherit strong heterogeneity at local scales. In this work, we have proposed ordinary kriging-based sequential algorithm for generating replicates of randomly distributed heterogeneous hydraulic conductivity field (Monte Carlo method-based algorithm) conditioned by field values from sampled locations in an irregular-unstructured grid system. Finite Volume method-based groundwater models often encounter difficulties with the representation of point source/sink terms operating within the domain. In this paper, we have proposed an irregular-unstructured grid Finite Volume discretization technique for overcoming the singularity of point source/sink term to yield a consistent output with different grid dimensions. Furthermore, full-system groundwater models often come with a substantial computational burden. Hence, reduction in model order cuts down the computational expenses (in terms of CPU time and usage) to a significant level. We have also put forth a model order reduction methodology for three different illustrative pumping tests. The proposed framework for the model order reduction projects the governing groundwater flow equation onto a set of identified patterns or orthonormal basis functions, applying the Galerkin Projection method to compute a vector of time-dependent coefficients. We have performed pattern identification by Singular Value Decomposition (SVD) of snapshots of full-system model simulation data at selected time instants within the pumping test time domain. The numerical results of the proposed reduced-order models show a good approximation of the full-system models at a comparatively lesser computational time. The accuracy and efficiency of the models attempt to ensure their potential applicability for identifying groundwater dynamics. [ABSTRACT FROM AUTHOR]