Effect Of Aquifer Heterogeneity Research Articles

Effect of Aquifer Heterogeneity on the Transport and Activity of Functional Dechlorinating Bacteria under an Electric Field

Effect of Aquifer Heterogeneity on the Transport and Activity of Functional Dechlorinating Bacteria under an Electric Field

Influence of nonuniform recharge on groundwater flow in heterogeneous aquifers

<abstract><p>The composition of soils in aquifers is typically not homogeneous, and soil layers may be cracked or displaced due to geological activities. This heterogeneity in soil distribution within aquifers affects groundwater flow and water level variations. In the present study, we established a two-dimensional (2D) mathematical model that considers the influence of surface recharge on groundwater flow in heterogeneous sloping aquifers. By considering temporal variations in surface recharge, slope angle and aquifer heterogeneity, the simulated results are expected to better reflect real conditions in natural aquifers. The effects of aquifer heterogeneity on groundwater flow and water levels are particularly significant in sloping aquifers. The study's findings indicate that even when the soil composition remains constant, variations in groundwater level and flow may be considerable, depending on factors such as soil alignment, slope angle of the aquifer's base layer and the direction of water flow.</p></abstract>

Development of a robust ensemble meta-model for prediction of salinity time series under uncertainty (case study: Talar aquifer)

The aim of this study is to develop an accurate and reliable numerical model of the coastal Talar aquifer threatened by seawater intrusion by developing an ensemble meta-model (MM). In comparison with previous methodologies, the developed model has the following superiority: (1) Its performance is enhanced by developing ensemble MMs using four different meta-modelling frameworks, i.e., artificial neural network, support vector regression, radial basis function, genetic programing and evolutionary polynomial regression; (2) The accuracy of different MMs based on 16 integration of four meta-modeling frameworks is compared; and (3) the effect of aquifer heterogeneity on the MM. The performance of the proposed MM was assessed using an illustrative case aquifer subject to seawater intrusion. The obtained results indicate that the ensemble MM that combines all four meta-modeling frameworks outperformed the GP and ANN models, with a correlation coefficient of 0.98. Moreover, the proposed MM using nonlinear-learning ensemble of SVR-EPR achieves a better and robust forecasting performance. Therefore, it can be considered as an accurate and robust simulator to predict salinity levels under different abstraction patterns in variable density flow. The result of uncertainty analyses reveals that robustness value and pumping rate are inversely proportional and scenarios with a robustness measure of about 12% are more reliable.

Fine Characterization of the Effects of Aquifer Heterogeneity on Solute Transport: A Numerical Sandbox Experiment

Hydraulic conductivity (K) and the specific storage coefficient (S) are among the most important hydrogeological parameters of an aquifer. Traditionally, the hydrogeological parameters of a field aquifer system are mainly determined through a range of experiments that are both time-consuming and of poor operability. To accurately characterize aquifer heterogeneity, a synthetic sandbox is constructed using VSAFT2 (Variably Saturated Flow and Transport utilizing the Modified Method of Characteristics, in 2D) as a reference aquifer system by incorporating multilevel a priori geologic information into the sandbox configuration. The spatial distribution of the field of hydraulic conductivity (i.e., K) is inversely obtained by hydraulic tomography (HT). Then HT is compared with traditional kriging-estimated method in the fine characterization of aquifer heterogeneity, and the optimal K field is eventually selected to predict the solute transport. The influence of the number of pumping cycles on the accuracy of heterogeneity characterization is also discussed. The results show that the accuracy of the inversely obtained K field is improved with the increased number of pumping cycles. When incorporating multilevel a priori geological information, HT can characterize aquifer heterogeneity more finely than traditional kriging, and there is also a very good fitting of solute transport between the optimally estimated K field and the reference K field. Our study highlights the importance of the fine characterization of aquifer heterogeneity for the prediction of solute transport.

The effects of aquifer heterogeneity on the 3D numerical simulation of soil and groundwater contamination at a chlor-alkali site in China

The simulation of groundwater flow and solute transport at contaminated sites often neglects the important influence that aquifer heterogeneity can have on the sub-surface distribution of contaminants. In this paper, the method of transition probability for geological statistics (T-PROGS) included in the Groundwater Model System (GMS) was applied to a chlor-alkali-contaminated site that was sampled with 68 soil borings and 15 groundwater monitoring wells. A 3-D groundwater numerical model and solute transport model was developed that was constrained by soil and groundwater data from the site. The spatial distribution of chloroethylene concentrations was simulated for a number of times using the levels measured in the field as a baseline. The results of these simulations showed that shapes and distribution of contaminant plumes are irregular both vertically and horizontally. The solute-transport simulations indicated that much of the contamination will preferentially move in groundwater through silt and fine-sands whereas flow is largely blocked in clays. Consequently, fine sand and silts become the most seriously polluted zones at the site, whereas, areas underlain by clays are largely uncontaminated. Heterogeneous lithologies beneath a site increase the complexity of coupling simulations of soil and groundwater.

The Horizontal Reactive Media Treatment Well (HRX Well®) for Passive In‐Situ Remediation

AbstractA new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.

Effects of source size, monitoring distance and aquifer heterogeneity on contaminant mass discharge and plume spread uncertainty

Estimating the movement of dissolved contaminants in heterogeneous aquifers is very important for the monitoring design, risk assessment, and remediation of contaminated aquifers. This work explored the influence of source size, monitoring distance, and aquifer heterogeneity on the accuracy of contaminant mass discharge (CMD) estimation using leaching surface approach as well as on the plume spread uncertainty in a 2-D heterogeneous aquifer. The interaction among source size, monitoring distance, and aquifer heterogeneity regarding the accuracy of CMD estimation and the plume spread uncertainty at downstream of the contaminated aquifer was extensively investigated. The transient leaking of a contaminated aquifer in a saturated heterogeneous aquifer under steady-state flow conditions was simulated. The effect of aquifer heterogeneity on the CMD uncertainty was evaluated through the expected values and variance. The results showed that the CMD estimation error varied from underestimation in the mildly heterogeneous aquifer, over accurate estimation in the medium heterogeneous aquifer to overestimation in the highly heterogeneous aquifer. Additionally, the results illustrated that the mean and variance of the transverse spatial extent of the peak concentrations for the plume at the control plane were very sensitive to the aquifer heterogeneity and detectable concentrations of contaminants.

Capture and release zones of permeable reactive barriers under the influence of advective–dispersive transport in the aquifer

The problem of permeable reactive barrier (PRB) capture and release behavior is investigated by means of an approximate analytical approach exploring the invariance of steady-state solutions of the advection–dispersion equation to conformal mapping. PRB configurations considered are doubly-symmetric funnel-and-gate as well as less frequent drain-and-gate systems. The effect of aquifer heterogeneity on contaminant plume spreading is hereby incorporated through an effective transverse macro-dispersion coefficient, which has to be known. Results are normalized and graphically represented in terms of a relative capture efficiency M of contaminant mass or groundwater passing a control plane (transect) at a sufficient distance up-stream of a PRB as to comply with underlying assumptions. Factors of safety FS are given as the ratios of required capture width under advective–dispersive and purely advective transport for achieving equal capture efficiency M. It is found that M also applies to the release behavior down-stream of a PRB, i.e., it describes the spreading and dilution of PRB treated groundwater possibly containing incompletely remediated contamination and/or remediation reaction products. Hypothetical examples are given to demonstrate results.

The effect of vertical aquifer heterogeneity on the results of the simulation of contaminant migration in lateral groundwater flows

This study provides a solution for groundwater contamination problems. High anthopogenic loading on the water intake area makes it difficult to predict the migration of a contaminant to water supply wells, despite the known source location. Predictions were made using finite-difference grid models. The study provides an evaluation of the effects of the aquifer heterogeneity on the simulation results.

Effect of Aquifer Heterogeneity on Non-pumped, Reactive Well Networks for Removing Pollutants in Groundwater

This purpose of this study was to evaluate the impact of heterogeneity in aquifer hydraulic conductivity on configurations of non-pumped wells filled with reactive media for removing contaminant plumes in groundwater. Among one homogeneous and three heterogeneous simulated aquifers, 2-16 wells were necessary to contain a plume, with no clear relationship between degree of heterogeneity and number of wells. Generally, heterogeneous aquifers with initial plumes having broad rather than narrow downgradient margins required more wells and showed greater tendency for plumes to move around wells. Cleanup time increased up to 181 % with degree of heterogeneity in hydraulic conductivity.

Effect of permeability variations on solute transport in highly heterogeneous porous media

The effect of aquifer heterogeneity on flow and solute transport in two-dimensional isotropic porous media was analyzed using the Monte Carlo method. The two-dimensional logarithmic permeability (ln K) was assumed to be a non-stationary random field with its increments being a truncated fractional Lévy motion (fLm). The permeability fields were generated using the modified successive random additions (SRA) algorithm code SRA3DC [ http://www.iamg.org/CGEditor/index.htm]. The velocity and concentration fields were computed respectively for two-dimensional flow and transport with a pulse input using the finite difference codes of MODFLOW 2000 and MT3DMS. Two fLm control parameters, namely the width parameter ( C) and the Lévy index ( α), were varied systematically to examine their effect on the resulting permeability, flow velocity and concentration fields. We also computed the first- and second-spatial moments, the dilution index, as well as the breakthrough curves at different control planes with the corresponding concentration fields. In addition, the derived breakthrough curves were fitted using the continuous time random walk (CTRW) and the traditional advection-dispersion equation (ADE). Results indicated that larger C and smaller α both led to more heterogeneous permeability and velocity fields. The Lévy-stable distribution of increments in ln K resulted in a Lévy-stable distribution of increments in logarithm of the velocity (ln v). Both larger C and smaller α created sharper leading edges and wider tailing edges of solute plumes. Furthermore, a relatively larger amount of solute still remained in the domain after a relatively longer time transport for smaller α values. The dilution indices were smaller than unity and increased as C increased and α decreased. The solute plume and its second-spatial moments increased as C increased and α decreased, while the first-spatial moments of the solute plume were independent of C and α values. The longitudinal macrodispersivity was scale-dependent and increased as a power law function of time. Increasing C and decreasing α both resulted in an increase in longitudinal macrodispersivity. The transport in such highly heterogeneous media was slightly non-Gaussian with its derived breakthrough curves being slightly better fitted by the CTRW than the ADE, especially in the early arrivals and late-time tails.

The effect of aquifer heterogeneity on natural attenuation rate of BTEX

Monitored natural attenuation can be a viable option for remediation of groundwater contamination by BTEX compounds. Under the field conditions, the rate of contaminant mass attenuation through natural processes, such as biodegradation, to a large extent affected by the groundwater flow regime, which is primarily controlled by the aquifer heterogeneity. Numerical simulation techniques were used to describe quantitatively the relationship between biodegradation rate of BTEX and aquifer heterogeneity. Different levels of aquifer heterogeneity were described by random hydraulic conductivity fields (K) having different statistical parameters, the coefficient of variation (CV) and the correlation length (h). The Turning Bands Algorithm was used to generate such K fields. Visual MODFLOW/RT3D was used to simulate the fate and transport of dissolved BTEX plume within heterogeneous aquifers. The multispecies reactive transport approach described BTEX degradation using multiple terminal electron-accepting processes. First-order biodegradation rate constants were calculated from simulated BTEX plumes in heterogeneous flow fields. The results showed that aquifer heterogeneity significantly affected biodegradation rate; it decreased with increasing CV when h was in the range of up to 12 m, whereas it increased with increasing CV when h was greater than about 12 m. For well characterized aquifers, this finding could be of great value in assessing the effectiveness of natural attenuation during feasibility studies at BTEX contaminated sites.

Full‐scale demonstration of in situ cometabolic biodegradation of trichloroethylene in groundwater 1. Dynamics of a recirculating well system

Recirculating well systems provide an engine for the in situ treatment of subsurface contaminants. Although numerous recirculating wells have been installed in the field, for such systems, there is a paucity of comprehensive monitoring data and models constrained to data appearing in the research literature. Here we present an extensive data set combined with detailed inverse and simulation analyses for a two‐well groundwater recirculation system used for in situ bioremediation at Edwards Air Force Base in southern California. The “conveyor belt” flow system, which was established for in situ treatment of trichloroethylene (TCE) in two bioactive zones, was created by pumping water upward in one well and downward in another well, each well being screened in both the upper and lower aquifers. A bromide tracer test was conducted and extensively monitored for 60 days. Combined inverse analysis was conducted on hydraulic heads from 38 monitoring wells, 32 bromide concentration histories, and a constraint on the degree of recirculation that was based on TCE concentration data. Four different formulations involving alternative weighting schemes used in a nonlinear weighted least squares simulation‐regression analysis were explored. The best formulation provided parameter estimates with tight bounds on estimated covariances, suggesting that the model provides a reasonable description of the hydrogeologic system. Our investigation indicates the geometry of the recirculation zone and the degree of recirculation under two different sets of operating conditions. Surprisingly, our analysis suggests that the effects of aquifer heterogeneity are not significant at this site under the conditions of forced recirculation. Furthermore, anomalous flow through an open monitoring well created significant vertical short‐circuiting between the generally insulated aquifers. Flow through this small open conduit was equivalent to as much as 33% of the flow through the pumping wells. Using the model as a guide, we treated the aquifer system and bioactive zones as an equivalent mixed reactor to develop simple expressions relating effluent concentrations to influent concentrations. We demonstrate how these expressions are useful in predicting the removal of TCE that had undergone in situ bioremediation in the recirculatory treatment well system. The finite element model developed in this work serves as the foundation for a reactive transport simulator that we developed to analyze bioremediation which occurred during a 444 day experiment [Gandhi et al., 2002].

Estimation of Spatially Variable Aquifer Hydraulic Properties Using Kalman Filtering

Spatial variability of aquifer properties and hydraulic heads are modeled as random fields, and their estimates are achieved by Kalman filtering. The state-space equation is developed from a quasi-analytical solution of a first-order approximation of the governing stochastic flow equation. In the state-space system, the effects of aquifer heterogeneity and uncertainty in recharge are lumped linearly into a random noise vector that is correlated in time. To account for the correlated system noise, we reformulate the problem in terms of an augmented–state-space system and use the resulting Kalman filtering recursions to estimate transmissivities, storativities, and hydraulic heads, simultaneously. The filter is applied to a numerical experiment in which the statistical parameters of log aquifer properties are assumed to be known. The results indicate that under mild head gradients, head measurements alone are of limited value if the objective is to estimate the spatial distribution of aquifer transmissivity...

Coupled and partially coupled Eulerian‐Lagrangian Model of freshwater‐seawater mixing

The problem of density‐dependent transport of salt in unconfined coastal aquifers is solved numerically by means of an implicit Eulerian‐Lagrangian finite element formulation. Such a formulation leads to symmetric positive definite finite element matrices which are ideally suited for efficient solution by preconditioned conjugate gradient methods. Additional known advantages of the formulation are unconditional stability, reduced numerical dispersion and suitability for parallel computation. The method has been used to study the effect of dewatering on seawater intrusion within a vertical cross section through an aquifer in southern Italy, related to the construction of a thermoelectric power plant. To investigate the extent to which the dependence of fluid density on salt concentration affects the numerical solution, the flow and advection‐dispersion equations were solved in coupled (iterative), partially coupled (noniterative) and completely decoupled modes. Partial coupling was found to yield results very close to those obtained by full coupling but at great savings in computer time; the less rigorous decoupled approach led to results substantially different from those obtained through coupling and partial coupling. Effects of aquifer heterogeneity and the construction of a cutoff wall on seawater intrusion are discussed.