Contaminant Transport In Aquifers Research Articles

Stochastic Assessment of Groundwater Contamination Risks From Onshore Gas Development Using Computationally Efficient Analytical and Numerical Transport Models

Regulators require the gas industry to assess the risks of unintentional release of chemicals to the environment and implement measures to mitigate it. Industry standard models for contaminant transport in aquifers do not explicitly model processes in the unsaturated zone and groundwater models often require long run times to complete simulation of complex processes. We propose a stochastic numerical-analytical hybrid model to overcome these two shortcomings and demonstrate its application to assess the risks associated with onshore gas drilling in the Otway Basin, South Australia. The novel approach couples HYDRUS-1D to an analytical solution to model contaminant transport in the aquifer. Groundwater velocities and chemical trajectories were derived from a particle tracking analysis. The most influential parameters controlling solute delivery to the aquifer were the soil chemical degradation constant and the hydraulic conductivity of a throttle soil horizon. Only 18% of the flow paths intercepted environmental receptors within a 1-km radius from the source, 87% of which had concentrations of <1% of the source. The proposed methodology assesses the risk to environmental assets and informs regulators to implement measures that mitigate risk down to an acceptable level.

PorousMedia4Foam: Multi-scale open-source platform for hydro-geochemical simulations with OpenFOAM®

porousMedia4Foam is a package for solving flow and transport in porous media using OpenFOAM® - a popular open-source numerical toolbox. We introduce and highlight the features of a new generation open-source hydro-geochemical module implemented within porousMedia4Foam, which relies on micro-continuum concept and which makes it possible to investigate hydro-geochemical processes occurring at multiple scales i.e. at the pore-scale, reservoir-scale and at the hybrid-scale. Geochemistry is handled by a third party package (e.g. PHREEQC) that is coupled to the flow and transport solver of OpenFOAM®. We conducted benchmarks across different scales to validate the accuracy of our simulator. We further looked at the evolution of mineral dissolution/precipitation in a fractured porous system. Application fields of this new package include the investigation of hydro-bio-geochemical processes in the critical zone, the modelling of contaminant transport in aquifers, as well as and the assessment of confinement performance for geological barriers.

New model for estimating geometric tortuosity of variably saturated porous media using 3D synchrotron microcomputed tomography imaging

AbstractTortuosity has a significant impact on flow and transport characteristics of porous media and plays a major role in many applications such as enhanced oil recovery, contaminant transport in aquifers, and fuel cells. Most analytical and theoretical models for determining tortuosity have been developed for ideal systems with assumptions that might not be representative of natural porous media. In this paper, geometric tortuosity was directly determined from three‐dimensional (3D) tomography images of natural unconsolidated sand packs with a wide range of porosity, saturation, grain size distribution, and morphology. One hundred and thirty natural unconsolidated sand packs were imaged using 3D monochromatic and pink‐beam synchrotron microcomputed tomography imaging. Geometric tortuosity was directly determined from the 3D images using the centroids of the connected paths in the flow direction of the media, and multivariate nonlinear regression analysis was adopted to develop a simple practical model to predict tortuosity of variably saturated natural unconsolidated porous media. Wetting phase saturation was found to provide a good estimate of relative tortuosity with an value of .93, even with a porosity variation between 0.3 and 0.5 of the porous media systems. The proposed regression model was compared to theoretical and analytical models available in the literature and was found to provide better estimates of geometric tortuosity with an value of .9 and a RMSE value of 0.117.

Groundwater Contamination by Hazardous Wastes

In the global race of development, a huge amount of wastes having different characteristics and potential for environmental pollution are generated from various activities such as manufacturing, mining, processing, treatments, agriculture, etc. Considering the pollution threats to all three components of the environment (air, water and soil), it is highly essential to assess the levels of pollution associated with these wastes so that suitable remedial measures can be adopted. When untreated hazardous wastes are disposed on the ground or in the landfills, there is always a possibility of groundwater contamination due to the transport of leachate generated from the disposed wastes. In order to predict the groundwater pollution levels at different locations of the aquifers and at different times, several models suited to the characteristics of the concerned hazardous wastes and aquifers have been widely reported in literature. This paper presents a comprehensive literature review on groundwater contamination by hazardous wastes and its different aspects, including the types of hazardous contaminants, aquifers, contaminant transport mechanisms, contaminant transport modeling, software available for modeling contaminant transport in aquifers, groundwater sustainability and a case study on groundwater quality prediction.

Hydrodynamic dispersion in heterogeneous anisotropic porous media: A simple model for anomalous diffusion emergence

Anomalous transport processes in porous media have become a central physical issue due to the increasing amount of processes and applications where it appears, ranging from contaminant transport in aquifers and oil recovery to anomalous diffusion in biological membranes. In all these cases, the existence of strong heterogeneities and anisotropies make proper physical and mathematical modelling a complicated problem, leading to simplified models where these spatial features and their implications for transport and diffusion are treated separately. In this work we propose a simple model for transport in porous media that includes anisotropy and strong heterogeneities. We show that depending on how strong the medium heterogeneities are, different regimes of superdiffusive transport emerge, going from superdiffusion due to long-range correlations to Lévy walks and ballistic diffusion.

Dynamics of a Highly Viscous Circular Blob in Homogeneous Porous Media

Viscous fingering is ubiquitous in miscible displacements in porous media, in particular, oil recovery, contaminant transport in aquifers, chromatography separation, and geological CO2 sequestration. The viscosity contrasts between heavy oil and water is several orders of magnitude larger than typical viscosity contrasts considered in the majority of the literature. We use the finite element method (FEM)-based COMSOL Multiphysics simulator to simulate miscible displacements in homogeneous porous media with very large viscosity contrasts. Our numerical model is suitable for a wide range of viscosity contrasts covering chromatographic separation as well as heavy oil recovery. We have successfully captured some interesting and previously unexplored dynamics of miscible blobs with very large viscosity contrasts in homogeneous porous media. We study the effect of viscosity contrast on the spreading and the degree of mixing of the blob. Spreading (variance of transversely averaged concentration) follows the power law t 3 . 34 for the blobs with viscosity ∼ O ( 10 2 ) and higher, while degree of mixing is found to vary non-monotonically with log-mobility ratio. Moreover, in the limit of very large viscosity contrast, the circular blob behaves like an erodible solid body and the degree of mixing approaches the viscosity-matched case.

Cu and Zn adsorption to a terrestrial sediment: Influence of solid-to-solution ratio

Laboratory batch adsorption experiments are commonly used to quantify contaminant adsorption to natural sediments. The distribution coefficients (KD) determined via these experiments are often incorporated into reactive transport models to predict contaminant movement in groundwater. The solid-to-solution ratio (SSR) in most laboratory experiments is much lower in comparison to that in aquifers, therefore it is questionable if distribution coefficients thus obtained can accurately quantify contaminant adsorption in the natural environment. SSR may also influence the leaching of multivalent cations and organic matters from natural sediments, which in turn could alter contaminant adsorption. The objective of this study is to determine how SSR influences heavy metal adsorption to a heterogeneous natural sediment. Cu and Zn adsorption was examined at SSRs of 250 and 25 g/L in the pH range of 3–8 using both batch experiments and surface complexation modelling. Results indicated that Ca, Mg, and DOC leaching depended on SSR, with higher SSR generally resulting in greater leaching. Conversely, Al and Fe leaching was less dependent on SSR. Cu distribution coefficients in the low pH range (3–6) and Zn distribution coefficients across the pH range (3–8) were not very sensitive to the SSR, despite higher leached concentrations of competing cations at a higher SSR. In contrast, Cu distribution coefficients at high pH (6–8) were more SSR-dependent, likely due to formation of non-adsorbing aqueous Cu-DOC complexes. This study demonstrates that cautions must be taken when distribution coefficients measured by laboratory batch experiments are used to predict contaminant transport in aquifers.

Pollutant advective spreading in beach sand exposed to high-energy tides

This paper presents field measurements in which dye solute was injected into coastal sand to investigate contaminant advection in intertidal beach sand. The measurements show the pathways of a contaminated plume in the unsaturated zone during both the flood and ebb tides. A prescribed amount of dye tracer solution was directly injected through the topsoil, with average porosity 0.3521 ± 0.01, at predetermined locations of the River Mersey's outer estuarial beach during ebb-tide. The injected dye was monitored, sampled and photographed over several tidal cycles. The distinctive features of the plume (full two dimensional cross-sections), sediments and water-table depth were sampled in-situ, close to the injection point (differing from previous contaminant monitoring tests in aquifers). The advective movement is attributed to tidal impact which is different from contaminant transport in aquifers. The experimental results show that plumes have significantly large spatial variability, diverging upwards and converging downwards, with a conical geometric shape which is different from the usual spherical/elliptical shape reported in literature. The mean vertical motion of the plume reaches three times the top-width within ten tidal cycles, exceeding the narrow bottom-width by a factor of order 2. The observed transport features of the plume within the beach sand have significant relevance to saltwater intrusion, surface water and groundwater quality. The field observations are unique and can serve as a valuable benchmark database for relevant numerical studies.

Accurate phase-shift velocimetry in rock

Spatially resolved Pulsed Field Gradient (PFG) velocimetry techniques can provide precious information concerning flow through opaque systems, including rocks. This velocimetry data is used to enhance flow models in a wide range of systems, from oil behaviour in reservoir rocks to contaminant transport in aquifers. Phase-shift velocimetry is the fastest way to produce velocity maps but critical issues have been reported when studying flow through rocks and porous media, leading to inaccurate results. Combining PFG measurements for flow through Bentheimer sandstone with simulations, we demonstrate that asymmetries in the molecular displacement distributions within each voxel are the main source of phase-shift velocimetry errors. We show that when flow-related average molecular displacements are negligible compared to self-diffusion ones, symmetric displacement distributions can be obtained while phase measurement noise is minimised. We elaborate a complete method for the production of accurate phase-shift velocimetry maps in rocks and low porosity media and demonstrate its validity for a range of flow rates. This development of accurate phase-shift velocimetry now enables more rapid and accurate velocity analysis, potentially helping to inform both industrial applications and theoretical models.

Leachate transport through advection-diffusion and linear sorption in saturated land

A long-term large-scale variably-saturated subsurface numerical solution of flow and transport are conducted and presented as a simple and fast computational method. Therefore, it is appropriate to develop such a simplified model that focus on the main processes operating at the pertinent time and space scales, based on the error introduced by the simpler model which is acceptable and also was compared to the uncertainties associated with the spatial and temporal variation of boundary conditions. To assess leachate transport as contamination potential it requires calculation of drainage water volume and contamination concentration. A practical, quantitative pollution transfer assessment tool can be developed to enable technical service providers and other practitioners to estimate potential pollution transfer and design best management practices for land waste management pollution systems in order to estimate and control pollution transport to soil and groundwater. The objective of this paper is to describe governing numerical equation and solution algorithm of pollution transport mechanisms and also essential factors which are included in developing relatively simple and practical tools to quantify pollution loss, advection, diffusion and sorption in pollution transport into the groundwater at landfill sites. The paper presents the development of a two-dimensional numerical model that can be used for quantifying groundwater inputs and associated contaminant discharge from a landfill into the affected aquifer. The presented computing results reveal that the proposed model can be used for the simulation of contaminant transport in aquifers in any scale. This numerical solution is established on finite difference, finite-volume solution advection-diffusion linear sorption with first order decay equation. To show the capability of proposed model, the results of a case study are presented in the paper as simulating leachate transport at a 2000 ton/day landfill facility assesses leachate migration away from the landfill in order to control associated environmental impacts, particularly on groundwater wells gradient of the site. Leachate discharge from landfills is the main route for release of the organic and inorganic contaminants through subsurface, commonly encountered in the refuse. Leachate quantity and potential percolation into the subsurface are estimated by the proposed model. A comprehensive sensitivity analysis to leachate transport control parameters was also conducted. Sensitivity analysis suggest that changes in source strength, aquifer hydraulic conductivity, and dispersivity have the most significant impact on model output indicating that these parameters should be carefully selected when similar modeling studies are performed. The sensitivity of the model to variations in input parameters results in two opposing patterns of contaminant concentration. While higher groundwater velocities increase the speed of plume spread it also increases the dilution ratio and hence decrease the concentration.

Review: Groundwater flow and transport modeling of karst aquifers, with particular reference to the North Coast Limestone aquifer system of Puerto Rico

Karst systems have a high degree of heterogeneity and anisotropy, which makes them behave very differently from other aquifers. Slow seepage through the rock matrix and fast flow through conduits and fractures result in a high variation in spring response to precipitation events. Contaminant storage occurs in the rock matrix and epikarst, but contaminant transport occurs mostly along preferential pathways that are typically inaccessible locations, which makes modeling of karst systems challenging. Computer models for understanding and predicting hydraulics and contaminant transport in aquifers make assumptions about the distribution and hydraulic properties of geologic features that may not always apply to karst aquifers. This paper reviews the basic concepts, mathematical descriptions, and modeling approaches for karst systems. The North Coast Limestone aquifer system of Puerto Rico (USA) is introduced as a case study to illustrate and discuss the application of groundwater models in karst aquifer systems to evaluate aquifer contamination.

Effective reaction parameters for mixing controlled reactions in heterogeneous media

Sound understanding of mixing‐controlled reactions in heterogeneous media is needed for the realistic modeling of contaminant transport in aquifers and is a precondition for the evaluation of natural attenuation processes, the design of nuclear waste disposal, and the engineered remediation of contaminated sites. In this work, we study the bimolecular dissolution‐precipitation equilibrium reaction, adapted after De Simoni et al. (2005). Because of advective and dispersive transport of the reacting species, the system is globally in nonequilibrium because the effective reaction rate is limited by the finite rate of transport and thus is affected by the heterogeneity of the formation. We study the macroscopic formulation of such a reactive transport system in terms of mixing‐controlled reaction parameters which integrate the impact of spatial heterogeneity. The apparent chemical saturation is found to be a function of the concentration variance and is generally greater than its local‐scale equivalent. This explains why water samples taken from pumping wells are normally nonequilibrium with respect to minerals existing in the aquifer, even when local equilibrium is to be expected. The reaction rate is given by the product of a reaction factor, associated with the local equilibrium constant and concentration variance, and a mixing factor, which is the product of the microdispersion coefficient and the square gradient of the mean and perturbation concentration fields. The mixing factor dominates the description of the reaction rate in the upscaled macroscopic models. The reaction rate predicted by macroscopic models is controlled by two competing effects: The large heterogeneity‐induced macrodispersion coefficient leads to an increase of reaction rate, while a more smoothed concentration gradient may lead to a decrease of the reaction rate. Macroscopic models may only give a good approximation at large time and away from the plume center of mass because of the balanced variance budget but significantly underestimate reaction rate near the plume center because of the smoothed concentration gradient field. In fact, in an application example, reaction rate is maximum around the plume center, where a homogeneous equivalent medium would predict zero rates.

Positive Solution of Two‐Dimensional Solute Transport in Heterogeneous Aquifers

The transport of contaminants in aquifers is usually represented by a convection-dispersion equation. There are several well-known problems of oscillation and artificial dispersion that affect the numerical solution of this equation. For example, several studies have shown that standard treatment of the cross-dispersion terms always leads to a negative concentration. It is also well known that the numerical solution of the convective term is affected by spurious oscillations or substantial numerical dispersion. These difficulties are especially significant for solute transport in nonuniform flow in heterogeneous aquifers. For the case of coupled reactive-transport models, even small negative concentration values can become amplified through nonlinear reaction source/sink terms and thus result in physically erroneous and unstable results. This paper includes a brief discussion about how nonpositive concentrations arise from numerical solution of the convection and cross-dispersion terms. We demonstrate the effectiveness of directional splitting with one-dimensional flux limiters for the convection term. Also, a new numerical scheme for the dispersion term that preserves positivity is presented. The results of the proposed convection scheme and the solution given by the new method to compute dispersion are compared with standard numerical methods as used in MT3DMS.

The effect of non-Fickian diffusion into surrounding rocks on contaminant transport in a fractured porous aquifer

Solute transport in a fractured porous confined aquifer is modelled by using an equation with a fractional-in-time derivative of orderγ, which may vary from 0 to 1. Accounting for non-Fickian diffusion into the surrounding rock mass, which is modelled by a fractional spatial derivative of orderα, leads to the introduction of an additional fractional-in-time derivative of orderα/(1+α) in the equation for solute transport. Closed-form solutions for solute concentrations in the aquifer and surrounding rocks are obtained for an arbitrary time-dependent source of contamination located at the inlet of the aquifer. Based on these solutions, different regimes of contaminant transport in aquifers with various physical properties are modelled and analysed.

Analytical solutions to the transient, unsaturated transport of water and contaminants through horizontal porous media

We present a range of analytical solutions to the combined transient water and solute transport for horizontal flow. We adopt the concept of a scale and time dependent dispersivity used for contaminant transport in aquifers and apply it to transient, unsaturated horizontal flow to develop similarity solutions for both constant solute concentration and solute flux boundary conditions. Through the use of a specific form of the water profile as used by Brutsaert [Water Resour Res 1968:4;785], the solute profiles can be reduced to a simple quadrature. We also derive a solution for the instantaneous injection of water and solute into a horizontal media for an arbitrary dispersivity. It is found that the solute concentration remains constant in both space and time as the water redistributes, suggesting that the solute does not disperse relative to the water.

Biosorption of phenol and chlorophenols by acclimated residential biomass under bioremediation conditions in a sandy aquifer

Phenol and chlorophenols are common environmental contaminants. The fate and transport of these chemicals must be sufficiently understood to predict detrimental environmental impacts and to develop technically and economically appropriate remedial action to minimise environmental degradation. In order to gain a better understanding of the many mechanisms influencing the fate of phenol and chlorophenols in a sandy aquifer, we conducted biosorption experiments with biomass collected from a simulated aquifer polluted by consecutive accidental spills of phenol, 2-monochlorophenol, 2,4,6-trichlorophenol and pentachlorophenol under continuous bioremediation conditions following a closed-loop configuration during 180 days. A comparative study of the biosorption capacity of phenol and chlorophenols characterised by different physicochemical properties, at different pHs in the range of 6.0±0.1 to 9.0±0.1 showed the following: (i) the biosorption of phenol and chlorophenols on resident biomass was rapid (equilibrium reached in less than 2 h); (ii) the experimental data followed the Freundlich isotherm; (iii) changes in pH from 6.0±0.1 to 9.0±0.1 resulted in a decrease in the equilibrium biosorption capacity ( q eq); (iv) both Freundlich parameters ( K F, n) should be used together as predictive parameters in mathematical models to simulate the fate of phenol and chlorophenols in the aquifer; (v) q eq of phenol and chlorophenols investigated in this study were satisfactorily correlated to their hydrophobicity ( K ow) with a correlation factor 0.98. In addition, available data from other reported studies fell in the same correlation curve. The results of the present study should be introduced in mathematical models developed to predict the effect of biomass fate and transport of contaminants in aquifers during bioremediation conditions.

CONNEC3D: a computer program for connectivity analysis of 3D random set models

Geostatistical simulation is used in risk analysis studies to incorporate the spatial uncertainty of experimental variables that are significantly under-sampled. For example, the values of hydraulic conductivity or porosity are critical in petroleum reservoir production modelling and prediction, in assessing underground sites as waste repositories, and in modelling the transport of contaminants in aquifers. In all these examples connectivity of the permeable phase or permeable lithofacies is a critical issue. Given an indicator map on a regular two- or three-dimensional grid, which can be obtained from continuous-valued or from categorical variables, CONNEC3D performs a connectivity analysis of the phase of interest (coded 0 or 1 by an indicator function). 3D maps of multiple indicators, categories or continuous variables can also be analysed for connectivity by suitable coding of the input map. Connectivity analysis involves the estimation of the connectivity function τ( h) for different spatial directions and a number of connectivity statistics. Included in the latter are the number of connected components (ncc), average size of a connected component (cc), mean length of a cc in the X, Y and Z directions, size of the largest cc, maximum length of a cc along X, Y and Z and the numbers of percolating components along X, Y and Z. In addition, the program provides as output a file in which each cc is identified by an integer number ranging from 1 to ncc. The implementation of the program is demonstrated on a random set model generated by the sequential indicator algorithm. This provides a means of estimating the computational time required for different grid sizes and is also used to demonstrate computationally that when the semi-variogram of the indicator function is anisotropic the connectivity function is also anisotropic. There are options within the program for 6-connectivity analysis, 18-connectivity analysis and 26-connectivity analysis. The software is provided in two formats, as a stand-alone program that can perform connectivity analysis of an indicator map and as a subroutine that can be repeatedly called in order to calculate averages of connectivity analyses of a large number of realizations of indicator maps, or to identify critical realizations generated by conditional simulations of continuous variables or categorical variables.

Hydrologic Contaminant Transport Modeling: A Novel Analytical and Computational Approach Title

We have developed a method for modeling contaminant transport in aquifers with rectangular boundaries utilizing an analytical solution to the porous medium flow equation and a finite difference solution to the advection-dispersion equation. Any number of wells may be placed within the aquifer, as well as any number of non-interacting contaminants. Constant head boundaries that simulate rivers may be included by means of a source term. With realistic parameters we are able to successfully model and predict contamination transport in an on campus well site used for both undergraduate pedagogy and research.

Scale-Dependent Dispersion from a Pit

It is fairly well accepted that contaminant transport in aquifers can be modelled accurately only if dispersion coefficients are allowed to increase with distance downstream. However, this also increases the difficulty of obtaining exact solutions of the contaminant transport equation. Therefore, singular perturbation methods are used to obtain a relatively simple approximate solution for scale-dependent dispersion from a pit located in a region of uniform flow. Comparison of exact and approximate perturbation solutions for two simpler but related problems suggests that the perturbation approach used here is likely to give sufficient accuracy for many, if not most, applications.

Solute transport under non-linear sorption and decay

Contaminant transport in aquifers is usually represented by a solution to the advective–dispersive differential equation. When the contaminant is subject to non-linear degradation or decay, or it is characterized by a chemical constituent that follows a non-linear sorption isotherm, the resulting differential equation is non-linear. Using the method of decomposition, series solutions were obtained for the non-linear equation. The series were used to derive and test “simulant” solutions that arise using the concept of double decomposition. Simulant solutions are closed-form analytic expressions that approximate part of the series. These expressions are simple, stable, and flexible. They permit an accurate forecasting of contaminant propagation under non-linearity in laboratory or field investigations at early or prolonged times after the spill. In this article, the practical scenario of an instantaneous spill, and that of a constant concentration boundary condition, is studied for situations of non-linear decay, non-linear Freundlich isotherm, and non-linear Langmuir isotherm. The solutions are verified with limited well-known analytical solutions of the linear reactive and non-reactive equations with excellent agreement, and with limited finite difference solutions. Plumes undergoing non-linear decay experience a profile re-scaling with respect to that of linear decay, the degree of which is controlled by the magnitude of the non-linear parameter b. The direction of the scaling (scaling up or scaling down with respect to the linear decay plume) is controlled by the magnitude of C (whether greater or less than 1) in relation to the magnitude of b (whether greater or less than 1). When C>1, values of b<1 produce plumes that experience less decay (i.e., are scaled up) than that of the linear decay, whereas values of b>1 produce non-linear plumes that experience more decay (i.e., are scaled down) than that of the linear decay. The opposite effect is observed when concentrations are less than 1. In other words, when C<1, values of b<1 produce non-linear plumes that experience more decay (i.e., are scaled down) than that of the linear decay, whereas values of b>1, produce non-linear plumes that experience less decay (i.e., are scaled up) than that of the linear decay. A plume undergoing non-linear sorption according to a Freundlich isotherm retards the processes of advection and dispersion with respect to a plume with no sorption. Similar to the case of non-linear decay, whether this retardation is more or less pronounced than that of the linear sorption plume depends on whether the values of b and C are greater or less than 1. The solution presented here for the advective dispersive equation subject to a Freundlich sorption isotherm is restricted to concentration greater than 1. When C>1 and b<1, the decrease in mobility in the non-linear plume is not as pronounced as that of a plume modeled by a linear isotherm. Plume shape may be quite sensitive to the values of the non-linear parameters. Plumes with parameter values b<1 (and C>1) exhibit the well-known lack of symmetry with respect to their center of mass, sharp fronts, and the tailing effects observed at hazardous waste sites. As the magnitude of the non-linear parameter increases, the non-linear plume approaches the linear one. This partial non-linear “retardation” can now be observed quantitatively with the models presented herein. The models developed also simulate the case of b >1 (i.e., “unfavorable” sorption), which produce a plume even more retarded than the linear. The shape of a contaminant plume following a non-linear Langmuir isotherm is very sensitive to the magnitude of the non-linear parameter α. Approximate solutions for mild non-linearity are presented.