Contaminant Transport Model Research Articles

A theoretical model for the hydraulic conductivity of montmorillonite in relation to the concentration and valence of electrolyte cations in solution

This study is among the first to report on a theoretical relationship linking the hydraulic conductivity of soil and aquifer with environmental solution conditions. A mathematical model is developed for predicting the hydraulic conductivity of montmorillonite from the concentration and valence of electrolyte cations in solution. The model relies on the diffuse double layer and DLVO theories, the aggregation kinetics, and the Hazen equation, with a few plausible assumptions, for establishing three individual relationships, with each corresponding to one of three steps in the cation-to-particle-to-grain-to-hydraulic conductivity pathway. It is based on a mechanistic understanding that the concentration and valence of electrolyte cations controls the size of montmorillonite primary particles by influencing the electrical double layer, subsequently regulates the grain-size distribution of montmorillonite via inter-particle collision, and finally determines the hydraulic conductivity of montmorillonite. The model is validated by direct measurements of the surface charge density, the grain-size distribution and the hydraulic conductivity of montmorillonite in NaCl and CaCl2 solutions of various concentrations. The theoretical model of predicting hydraulic conductivity developed here is novel for implementing more accurate modeling of contaminant transport in the environment with advantage of cost-savings.

Study on the effect of municipal solid landfills on groundwater by combining the models of variable leakage rate, leachate concentration, and contaminant solute transport

The landfill with low economic cost and technical barrier has become a popular option for municipal solid waste treatment, but it is likely to seriously pollute groundwater by solute leaching. In this study, the pollutants concentration model, leakage rate model, and the solute transport model were coupled to investigate the effect of municipal solid waste landfill on groundwater quality. Major results obtained are, (1) the leakage rate of leachate differs significantly among the landfilling stage, covering stage and completely covered stage as the leachate depth varies with the infiltration rainfall. The contact condition between HDPE and CCL was found to be a key factor in determining the leakage rate of leachate. Ensuring good connection between HPDE and CCL is thus critical in protecting groundwater from being polluted by landfill. (2) The NH3–N as a proxy for organic pollutants was generated via the degradation process, and Cl− as a proxy for inorganic pollutants is a leachable fraction of mobilized substances. The concentration of Cl− is higher than that of NH3–N before 7600 days, then the concentration of NH3–N becomes roughly stable while that of Cl− continues to decrease. (3) The load of NH3–N as a proxy for organic pollutants declines linearly downwards before the completely covering stage. The load of Cl− as a proxy for inorganic pollutants increased during the first five years, and then declined. (4) In the case study, the path of maximum pollutants concentration is perpendicular to the groundwater contour, and the maximum pollutants concentration has two peaks, occurring on 7106 days and 11,554 days, respectively. The change laws of maximum pollutants concentration are similar for different connections between HPDE and CCL.

Simultaneous identification of contaminant sources and hydraulic conductivity field by combining geostatistics method with self-organizing maps algorithm

In the contaminant remediation of groundwater, the release history of contaminant sources and hydraulic conductivity field are two key parameters that need to know, but their actual values are difficult to obtain and can only be inversely identified by limited measured data. However, the process of solving the inverse problem needs to repeatedly call the forward model of contaminant transport, which is very time-consuming, especially for the high-dimensional inverse problems. In this study, based on the training data generated from a prior range of parameters (the release strength of contaminant sources and hydraulic conductivity at pilot points), the self-organizing maps (SOM) algorithm was employed to construct the surrogate model for the numerical model of contaminant transport in a simplified hypothetical aquifer, then the surrogate model was used to retrieve jointly the contaminant strength of sources and the hydraulic conductivity at pilot points, and the Kriging method of geostatistics was further used to process the estimated K-values at pilot points to obtain the hydraulic conductivity field. Also, to investigate the ability of the SOM-based surrogate model for retrieving both contaminant source strengths and hydraulic conductivity, we gradually expanded the prior range and increased the number of inversion terms in each prior range. Moreover, the robustness of the SOM-based surrogate model for inversion was illustrated by proposing the scarcity of data and different degrees of measurement error in the limited actual observation data. When the actual observation data is reduced by 2/3, the Root Mean Square Error (RMSE) of retrieving source strengths and hydraulic conductivity at pilot points are 1.07 and 0.09, respectively. The results indicated the SOM-based surrogate model shows remarkable inversion precision and robustness, and an accurate estimation of the actual hydraulic conductivity field could be obtained by the Kriging method based on that.

A two-dimensional analytical model for contaminant transport in a finite domain subjected to multiple arbitrary time-dependent point injection sources

Predicting the fate and transport of contaminants in a subsurface environment is essential to the evaluation and remediation of soil and groundwater pollution. Point source models have been widely used to describe the accidental leakage of contaminants and the intended injection of chemicals, but previous researches are limited to a single source in an infinite/semi-finite domain with oversimplified injection types. Thus, this study establishes a two-dimensional analytical model for contaminant transport involving advection, dispersion, sorption and degradation in a finite domain with multiple arbitrary time-dependent point sources. Four representative time-dependent sources are applied to study the transport behavior of contaminants in a more realistic environment. Results reveal that assuming multiple sources as an equivalent-intensity single point source would significantly undervalue the polluted range by 17.7% in the early stage but overvalue the maximum downstream concentration, when the sources are spaced at an interval larger than 1 m. An infinite effluent boundary assumption would cause a noticeable deviation in the concentration near a real finite border, especially in a dispersion-dominated migration system. Moreover, a larger decay coefficient for decay injection will linearly reduce the area of contaminated region, while a larger injection rate for finite pulse injection will logarithmically expand the contaminated region. Thus, close attention should be paid to the time-dependent characteristics of point sources in practice to facilitate soil and groundwater remediation, and the present model is a promising tool to address this issue.

Simulating contaminant transport in unsaturated and saturated groundwater zones

The demand for clean and adequate water is rising rapidly with increasing population. This growing demand for water necessitates the measurement of the quantity and quality of water. Simulation modeling has become increasingly popular in the last two decades largely because of their predictive ability. This paper reviews the approaches for simulation modeling in groundwater resources management, focusing on models that have been used to simulate contaminant transport through the aquifer system. Recent research papers that have integrated the models for unsaturated and saturated zones have also been studied and described. Integrated models require assessment of the complex interactions between the groundwater zones and the movement of water and solute through them. Due to this, integrated models provide a more accurate modeling approach than models that have been independently developed for saturated and unsaturated zones. Application of such models is encouraged at the regional level to arrive at the best groundwater management decisions. PRACTITIONER POINTS: In the past few decades, modeling of contaminant transport in groundwater systems has seen tremendous applications. A number of models exist that independently simulate flow and solute transport in unsaturated and saturated zones. Recently, focus has been given on developing advanced coupled modeling approaches that require less inputs and run times.

Cover Image

The cover image is based on the Research Article A two-dimensional analytical model for organic contaminant transport in cutoff wall and aquifer system by Huaxiang Yan et al., https://doi.org/10.1002/nag.3179.

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.

Analytical solutions for contaminant fate and transport in parallel plate fracture-rock matrix systems with poiseuille flow

Modeling contaminant transport in fractured-rock matrix systems often approximates the effect of the parabolic flow field in the fractures (i.e., Poiseuille flow) on transport by adding a dispersion term to the uniform flow field. In this study, an analytical solution is derived to model contaminant transport in a parallel-plate fractured-rock matrix that explicitly simulates Poiseuille flow in the fractures, eliminating the need for the dispersion approximation. In addition to simulating Poiseuille flow in the fracture, the contaminant transport model developed here includes: (1) two-dimensional contaminant diffusion in the fractures and matrix, (2) first-order decay in the aqueous phase, and (3) rate-limited sorption onto matrix solids. It should be noted, however, that this model, much like the commonly employed Taylor dispersion approximation, neglects macro dispersion, thereby limiting the model’s applicability to systems having wide fracture apertures with extremely high flow velocities (Pe > 104). Model equations are analytically solved in the Laplace domain and numerically inverted. In addition, analytical expressions for the zeroth, first, and second spatial moments of the concentration profiles along the fractures are derived for both the new Poiseuille flow model as well as a model that approximates the effect of Poiseuille flow on transport by using a dispersion term. The first and second moment expressions are used to quantify how well the dispersion term approximates the effect of Poiseuille flow. Simulations confirm that the dispersion approximation will be adequate for natural fractures at long times. However, if a modeler is concerned with short-time transport behavior or transport behavior in systems with relatively wide-aperture fractures and high groundwater velocities where macro dispersion can be ignored, such as may be found at engineered geothermal systems and carbon capture and storage sites, there may be significant differences between model simulations that explicitly incorporate Poiseuille flow and those that approximate Poiseuille flow with a dispersion term. The model presented here allows the modeler to analytically quantify these differences, which, depending on the modeling objective, may cause the dispersion approximation to be inadequate. Simulations were also run to examine the effect of adsorption rate on remediation of fractured-rock matrix systems. It was shown that moderate adsorption rate constants could lead to very long remediation times, if remediation success is quantified by achieving low concentrations within the fracture.

Effect of Feed Water pH on the Partitioning of Alkali Metal Salts from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes.

The partitioning of solutes into the polyamide active layers of reverse osmosis (RO) membranes is a key membrane property determining solute permeation. Quantification of partition coefficients and their dependence on feedwater pH would contribute to the development of predictive transport models of contaminant transport through RO membranes; however, neither solute partitioning nor the effect of feed solution pH on partitioning has been thoroughly characterized in the literature. Accordingly, we characterized the partitioning of all chloride salts of alkali metals (CsCl, RbCl, KCl, NaCl, and LiCl) from the aqueous phase into the polyamide active layers of five polyamide RO membranes, including one prepared in-house and four commercial membranes. We evaluated the effect of pH on the partitioning of alkali metal salts and whether the effect of pH on salt partitioning and rejection is consistent with Donnan theory predictions. Results showed that for all membranes, the partition coefficients of all salts were less than one and did not differ substantially among RO membranes. Results also indicated that for all membranes tested, Donnan theory provided an appropriate theoretical framework to estimate the effect of pH on salt partitioning (evaluated for all chloride salts of alkali metals) and salt rejection (evaluated for NaCl). Thus, we conclude that changes in salt rejection resulting from feed solution pH are primarily driven by changes in salt partitioning with comparatively small changes in salt diffusion coefficients.

An Integrated Modeling System for the Evaluation of Water Resources in Coastal Agricultural Watersheds: Application in Almyros Basin, Thessaly, Greece

This study presents an integrated modeling system for the evaluation of the quantity and quality of water resources of coastal agricultural watersheds. The modeling system consists of coupled and interrelated models, including (i) a surface hydrology model (UTHBAL), (ii) a groundwater hydrology model (MODFLOW), (iii) a crop growth/nitrate leaching model (REPIC, an R-ArcGIS-based EPIC model), (iv) a groundwater contaminant transport model (MT3DMS), and (v) a groundwater seawater intrusion model (SEAWAT). The efficacy of the modeling system to simulate the quantity and quality of water resources has been applied to the Almyros basin in Thessaly, Greece. It is a coastal agricultural basin with irrigated and intensified agriculture facing serious groundwater problems, such as groundwater depletion, nitrate pollution, and seawater intrusion. Irrigation demands were estimated for the main crops cultivated in the area, based on precipitation and temperature from regional weather stations. The models have been calibrated and validated against time-series of observed crop yields, groundwater table observations, and observed concentrations of nitrates and chlorides. The results indicate that the modeling system simulates the water resources quantity and quality with increased accuracy. The proposed modeling system could be used as a tool for the simulation of water resources management and climate change scenarios.

Groundwater flow and contaminant transport models – a short review

Groundwater flow and contaminant transport models – a short review

Optimal Design of Large-scale Bayesian Linear Inverse Problems Under Reducible Model Uncertainty: Good to Know What You Don't Know

We consider optimal design of infinite-dimensional Bayesian linear inverse problems governed by partial differential equations that contain secondary reducible model uncertainties, in addition to the uncertainty in the inversion parameters. By reducible uncertainties we refer to parametric uncertainties that can be reduced through parameter inference. We seek experimental designs that minimize the posterior uncertainty in the primary parameters while accounting for the uncertainty in secondary parameters. We accomplish this by deriving a marginalized A-optimality criterion and developing an efficient computational approach for its optimization. We illustrate our approach for estimating an uncertain time-dependent source in a contaminant transport model with an uncertain initial state as secondary uncertainty. Our results indicate that accounting for additional model uncertainty in the experimental design process is crucial.

Spatiotemporal Modelling of Groundwater Flow and Nitrate Contamination in An Agriculture-Dominated Watershed

In this study, both groundwater flow and nitrate transport were simulated in the Upper White River Watershed (Indiana, US) dominated by agricultural production. MODFLOW and MT3DMS were used for groundwater flow and contaminant transport modelling of the watershed under transient conditions. The input files for MODFLOW and MT3DMS were obtained by the GMS groundwater simulator. Model simulations were performed for 25 years between 1995 ~ 2019 with one-year intervals. Groundwater observation data (hydraulic heads and nitrate concentrations) for 1995 ~ 2013 and 2014 ~ 2019 were used for the model calibration and validation, respecttively. A heuristic optimization model based on the modified Clonal Selection Algorithm (a class of Artificial Immune Systems) was improved for calibration of the groundwater contaminant transport model. MODFLOW and MT3DMS were linked with the algorithm and run in MATLAB. Based on land use/cover, recharge and recharge nitrate concentrations were calibrated while other groundwater parameters (storage, porosity, longitudinal dispersivity, and denitrification rate coefficients) were calibrated for 7 aquifers. Furthermore, the models were run for different scenarios representing possible future conditions. The results demonstrated that the models performed well in terms of the fate and transport of nitrate in the Upper White River Watershed.

Simplified power law relationship in the estimation of hydraulic conductivity of unsaturated sands using electrical conductivity

The unsaturated zone is a complex multiphase system, and modelling and prediction of flow and contaminant transport in this zone remain a challenge. In order to understand the mechanisms of fluid flow in unsaturated sands, an accurate and efficient approach to estimate unsaturated hydraulic conductivity (K) is essential. In this study, a power law relationship was derived from a combination of Archie’s law and van Genuchten’s model to relate bulk (apparent) electrical conductivity (ECa) with unsaturated K. The laboratory sandbox experiments were conducted first to delineate the soil water characteristic curves (SWCCs). Time domain reflectometry was used to simultaneously measure volumetric water content (θ) and ECa. Then, the experimental relationships of the effective saturation (S) and ECa and simulated S–K were combined to establish the relationship between ECa and unsaturated K. The developed power law relationships described the relative EC (ECr) and relative K (Kr) very well by just using one parameter, exponent β. When fluid EC was low, the β values for the drainage and wetting processes ranged within 2.09–2.74 and 2.50–3.79 respectively. The variations of β values of homogeneous material were smaller that of heterogeneous material and the effect of hysteresis on the ECr–Kr relationship was observed. When pore space was filled with the high-EC solution, it easily mimicked the S–Kr relationship and resulted in a smaller β value. The β value acted as a lumped factor accounting for pore tortuosity, pore connectivity, shape of pore space, and fluid EC. The power law relationship of ECr–Kr developed in this study could lead to a direct estimation of the spatial and temporal variations of unsaturated K, once the measurements of SWCC are available from estimation of saturated K and combination of time-lapse ECa measurements. Accurate and efficient estimation of unsaturated K could improve the prediction of flow in the unsaturated zone and allow a comprehensive understanding of unsaturated zone processes.

Geological and hydrogeological characterization of the landfill areas located around Bucharest city in the context of environmental management

The process of risk assessment and investigation of potentially contaminated soil and groundwater is highly dependent on a proper understanding of the geological and hydrogeological conditions of the investigated site. The current paper aims at identifying the general geological and hydrogeological conditions of the landfill areas around Bucharest city based on interpretations of the data available in research articles, books, public reports, and geological and hydrogeological maps. Several soil samples were collected, and laboratory analyses were conducted to validate the initial expectations in terms of physical properties of the underlying strata. Results were presented as cross-sections and piezometric maps suitable for developing tridimensional models, conceptual site models and contaminant fate and transport modeling with sufficient accuracy for environmental urban planning. The research provides a quick method which may be used when assessing the general geological and hydrogeological conditions of various sites in environmental studies. The vulnerability of the Romanian legislation in data management and decision-making was also highlighted, therefore a series of recommendations to improve the data availability and quality were provided.

Improvement of Experimental Equipment Based on the One‐Step Outflow Test and Research on Rapid Determining SWCC

The soil‐water characteristic curve (SWCC) is an important hydraulic parameter for modeling water flow and contaminant transport in the unsaturated soil. However, direct measurement of the SWCC is still difficult. The commonly used measuring instrument in the laboratory is the pressure plate extractor. In this paper, the original pressure plate extractor has been improved based on the one‐step outflow test. Compared with the original pressure plate extractor, three devices, water storage, data acquisition system, and bubbles scouring device, are added to the improved pressure plate extractor. The improved equipment eliminates the problems of long test time consumption and inaccurate test data. A one‐step outflow test is performed utilizing the improved pressure plate extractor. The test data (the amount of spilled water and the time of pressurization) are substituted into the one‐dimensional moisture migration analysis software HYDRUS‐1D. The parameters (α, m, and n) are identified by the numerical inversion method in the inversion module of the HYDRUS‐1D, which are associated with the VG model to fit SWCC. The fitted SWCC is highly consistent with the measured SWCC, which is obtained from the conventional test of the original pressure plate extractor. Results confirm that the improved pressure plate extractor not only saves considerable time but also effectively measures the SWCC. The improved pressure plate extractor also involves a simple operation. The influence factors of controlling the geometry of SWCC are also discussed, the results of the discussion confirm that the geometry of SWCC is directly controlled by pore distribution, and the consolidation pressure is an indirect factor.

Evaluation of contamination of manganese in groundwater from overburden dumps of Lower Gondwana coal mines

Inappropriate management of coal mine overburden (OB) dumps creates environmental hazards. In many countries, high concentration of Mn was observed in groundwater near the coal mine OB dump sites. Comparatively new coal mines in Barjora coalfield were chosen in this study to assess the impact of OB dumps in groundwater. Acid Digestion (AD), Toxicity Characteristics Leaching Procedure (TCLP) and Batch Leaching Test (BLT) were performed to estimate Mn concentration in OB dumps and its leaching behaviour. Shale present in OB dump has potential to leach high concentration of Mn. The aquifer in this region is mainly unconfined type and water table is very shallow. In situ soil is characterized by 44.3% sand, 37.5% silt and 18.2% clay with a saturated hydraulic conductivity of 5.58 cm/day. Column study and HYDRUS 1D numerical modelling were used to simulate the migration of Mn in vadose zone. Subsurface soil does not possess appreciable attenuating potential to retard the transport of Mn in vadose zone. HYDRUS 1D model explains that 1.6 m thick soil profile of the study area may release Mn in groundwater after 11th year and it can attain the complete breakthrough at 17th year. A groundwater contaminant transport model based on available hydrogeological data has been developed using VISUAL MODFLOW 2009.1 software. The movement of Mn in aquifer has been determined using MT3DMS code. It indicates ≃ 8 km2 area around the two Open Cast Coal Mines (OCCMs) namely OCCM 1 and OCCM 2 may suffer groundwater contamination of Mn by 2050.

Assessing Human Health Risk to DNAPLs Exposure in Bayesian Uncertainty Analysis

The human health risk (HHR) assessment to dense non-aqueous phase liquids (DNAPLs) exposure has become an important part of groundwater environment management. Usually, DNAPL transport models are applied to simulate the concentration distribution of contaminant for HHR assessment. The present paper studied the influences of model uncertainties on the HHR assessment, and the metric of Incremental Lifetime Cancer Risk (ILCR) was used to quantify HHR. The impacts of permeability’s heterogeneity and the structure of DNAPL transport model (e.g., the constitutive model) on HHR assessment were evaluated based on a synthetical DNAPL transport model. The results demonstrate that, compared with the low heterogeneity, the high heterogeneity leads to lower average ILCR value at the control planes near the source zone, and higher average ILCR value at the control planes far away from the source zone. In addition, the HHR assessments would be inconsistent for the two constitutive models, i.e., Stone-Parker (S-P) and Coreyvan Genuchten (C-v) models. Compared with the HHR assessment depending on C-v model, the mean of ILCR’s probability distribution produced by S-P model is larger at the control planes near the source zone, and smaller at the control planes far away from the source zone. Moreover, based on a sandbox experiment, the impact of parameter uncertainty of DNAPL transport model on HHR assessment was evaluated by Markov chain Monte Carlo (MCMC) simulation. The results show that it is infeasible and risky to assess HHR by the specific parameters of contaminant transport model and ignoring parameter uncertainty. The HHR assessment by incorporating Bayesian uncertainty analysis could provide more flexible information. In addition, the sparse grid (SG) surrogate is an effective way to reduce computation burden caused by the larger number of model executions in the MCMC based HHR assessment.

A two‐dimensional analytical model for organic contaminant transport in cutoff wall and aquifer system

AbstractCutoff walls are widely used as in‐situ geoenvironmental barriers to isolate the contaminants caused by landfill leakage, chemical storage tank accidents, and mining activities. A two‐dimensional analytical solution is developed to investigate the behavior of organic chemical transport in cutoff wall and aquifer. The method of Laplace transforms and finite Fourier cosine transform is adopted to derive the solution for a finite domain. The proposed analytical solutions are verified by the numerical approach. The effects of contaminant source distribution, aquifer scale, direction of flow rate, degradation, adsorption, and thickness of cutoff wall were investigated. The investigation of non‐uniformly distribution of contamination sources indicates that the 1‐D models lead to a more conservative result compared to the 2‐D models. Biodegradation will be a dominant process and significantly enhance the performances of the barrier system when the half‐life of contaminant is less than 4 years. Furthermore, the effect of dimension on breakthrough time can be augmented with the increase of distribution coefficient.

Probabilistic Groundwater Flow, Particle Tracking and Uncertainty Analysis for Environmental Receptor Vulnerability Assessment of a Coal Seam Gas Project

The production of coalbed methane, or coal seam gas (CSG) in Australia increased 250-fold since the 1990s to around 1502 petajoules in 2019 and continues to expand. Groundwater flow in the aquifers intersected by gas wells could potentially facilitate a transport pathway for migration of contaminants or poorer quality water from deeper formations. While regulatory and mitigation mechanisms are put in place to minimize the risks, quantitative environmental impact assessments are also undertaken. When many gas wells are drilled in a wide area where many potential receptors are also spatially distributed, potential source-receptor combinations are too numerous to undertake detailed contamination risk assessment using contaminant transport modelling. However, valuable information can be gleaned from the analysis of groundwater flow directions and velocities to inform and prioritise contamination risk assessment and can precede computationally challenging stochastic contaminant transport modelling. A probabilistic particle tracking approach was developed as a computationally efficient screening analysis of contamination pathways for a planned CSG development near Narrabri in northern New South Wales, Australia. Particle tracking was run iteratively with a numerical groundwater flow model across a range of plausible parameter sets to generate an ensemble of estimated flow paths through the main Great Artesian Basin aquifer in the area. Spatial patterns of path lines and spatial relationships with potential receptors including neighbouring groundwater extraction wells and hydrologically connected ecological systems were analysed. Particle velocities ranged from 0.5 to 11 m/year and trajectories indicated dedicated contaminant transport modeling would be ideally focused at the local scale where wells are near potential receptors. The results of this type of analysis can inform the design of monitoring strategies and direct new data collection to reduce uncertainty and improve the effectiveness of adaptive management strategies and early detection of impacts.