Groundwater Remediation Strategies Research Articles

Biogenic Palladium Improved Perchlorate Reduction during Nitrate Co-Reduction by Diverting Electron Flow in a Hydrogenotrophic Biofilm.

Microbial reduction of perchlorate (ClO4-) is emerging as a cost-effective strategy for groundwater remediation. However, the effectiveness of perchlorate reduction can be suppressed by the common co-contamination of nitrate (NO3-). We propose a means to overcome the limitation of ClO4- reduction: depositing palladium nanoparticles (Pd0NPs) within the matrix of a hydrogenotrophic biofilm. Two H2-based membrane biofilm reactors (MBfRs) were operated in parallel in long-term continuous and batch modes: one system had only a biofilm (bio-MBfR), while the other incorporated biogenic Pd0NPs in the biofilm matrix (bioPd-MBfR). For long-term co-reduction, bioPd-MBfR had a distinct advantage of oxyanion reduction fluxes, and it particularly alleviated the competitive advantage of NO3- reduction over ClO4- reduction. Batch tests also demonstrated that bioPd-MBfR gave more rapid reduction rates for ClO4- and ClO3- compared to those of bio-MBfR. Both biofilm communities were dominated by bacteria known to be perchlorate and nitrate reducers. Functional-gene abundances reflecting the intracellular electron flow from H2 to NADH to the reductases were supplanted by extracellular electron flow with the addition of Pd0NPs.

Groundwater heavy metal(loid)s risk prediction based on topsoil contamination and aquifer vulnerability at a zinc smelting site

Groundwater pollution is a recurrent problem in abandoned non-ferrous metal smelting sites, and its severity is influenced by topsoil contamination, hydrogeological characteristics, and hydrogeochemical conditions. In such unique areas, traditional methods for evaluating groundwater pollution risk are biased, as the long production history of these sites have led to highly polluted and heterogeneous soil and groundwater. Herein, based on a typical lead-zinc smelting site, As, Pb, Zn, Cd, Mn, and Ni were found to be the predominant heavy metal (loid)s in groundwater, with respective exceedance rates of 44.4%, 50.0%, 72.2%, 88.9%, 88.9%, and 61.1%. Combined with the groundwater pollution characteristics, the representative hydrogeochemical factors were screened out to optimize the following aquifer vulnerability evaluation using the AHP-DRASTICH method. A comprehensive evaluation model (DI-NCPI) for groundwater pollution risk was established by combining the DRASTICH index (DI) obtained after optimization and the Nemerow comprehensive contamination index (NCPI) of topsoil. The fit between DI-NCPI and groundwater heavy metal (loid) pollution index reached 0.956, which laterally confirms that the model has some reference value. In terms of distribution, the high-risk and very high-risk zones were mainly concentrated in the zinc smelting system, located in the southeastern and central-western parts of the site. These areas have relatively high levels of topsoil contamination and aquifer vulnerability and require focused attention in site remediation. This research highlights the importance of combining topsoil contamination and aquifer vulnerability to evaluate groundwater pollution risk in smelting areas. It provides a more targeted reference for groundwater remediation strategies in abandoned smelting sites, as well as severely polluted industrial areas.

Analysis of the quality, source identification and apportionment of the groundwater in a typical arid and semi-arid region

A detailed and thorough study of the quality and its distribution, source identification and apportionment of groundwater, as the major or even sole selection of clean water in Songyuan City (a typical arid and semi-arid district in China), is needed for local sustainable development. The groundwater quality was evaluated using a vague set (VS)-technique for order of preference by similarity to ideal scheme (TOPSIS) model to determine its contamination status. Additionally, to explore the source and quantitative analysis of the driving factors of the groundwater chemistry in Songyuan City, a hydrochemical graphical method and a Principle Component Analysis (PCA) and Positive Matrix Factorization (PMF) model were employed. Regarding the quality of the groundwater in Songyuan, 74.5% of the samples were categorized into Grade I–III (Excellent, Good, and Moderate) water, indicating that the majority of the groundwater can be used directly. Moreover, the confined water tended to be cleaner and more appropriate for use in comparison with the phreatic water in Songyuan City. The PCA and PMF model and hydrochemical graphs demonstrated water–rock interactions and the evaporation process acted as the main factor, which was followed by anthropogenic activities as they contributed 50.1% and 20.4% of the total, respectively. Besides, the fluoride and iron contamination mainly caused by geogenic processes cannot be ignored as they were identified as the two driving factors that controlled the water quality, accounting for 20.2% and 9.3%, respectively. The mutual influence of the natural geological processes and human interventions contribute to the groundwater deterioration in Low Plain. Furthermore, the inappropriate well-forming techniques (without water-stop processes) in Qianan County was the main reason for the low-quality of deep Neogene confined water. This research provides useful information and directions for the local government to carry out corresponding and effective groundwater remediation strategies.

Enhancing groundwater remediation efficiency through advanced membrane and nano-enabled processes: A comparative study

Groundwater contamination is a global environmental concern, and existing remediation technologies face challenges in terms of effectiveness, cost, and environmental impact. Advanced membrane and nano-enabled processes have shown promise in improving the efficiency of groundwater remediation. This study compares the performance of various advanced membrane and nano-enabled processes, such as reverse osmosis, nanofiltration, ultrafiltration, and electrocoagulation, for removing common groundwater contaminants, including heavy metals, nitrates, and organic compounds. Laboratory-scale experiments using synthetic groundwater samples spiked with contaminants are conducted. Key parameters, including removal efficiency, energy consumption, and membrane fouling, are measured, and the environmental impact of each process is evaluated by analyzing the wastewater generated. The findings offer valuable insights into the potential of advanced membrane and nano-enabled processes for groundwater remediation, which can aid policymakers, researchers, and environmental practitioners in developing and implementing effective and sustainable groundwater remediation strategies. This study aims to evaluate the efficiency of different advanced membrane and nano-enabled processes for groundwater remediation and will provide valuable insights into their potential for enhancing groundwater remediation effectiveness.

Influence of porosity and temperature on light non-aqueous phase liquid diffusion in soils

Soil characteristics play an important role in distribution of light non-aqueous phase liquid (LNAPL) spilled from buried pipeline, and enhanced understanding of distribution is significant to the effective design of soil and groundwater remediation strategies. Therefore, distribution of diesel in soils with different porosity and temperature on the temporal evolution of the diesel migration following the saturation profiles of the two-phase flow in soils were investigated in this study. The diffusion ranges, areas and volumes in both the radial as well as in axial directions of leaked diesel in soils with different porosity and temperature increased with time. Soil porosities played an important role in the distributions when soil temperatures had no effect on distributions of diesel in soils. The distribution areas were 0.385 m2, 0.294 m2, 0.213 m2, and 0.170 m2 at 60 min when the soils porosities were 0.1, 0.2, 0.3, and 0.4, respectively. The distribution volumes were 0.177 m3, 0.125 m3, 0.082 m3, 0.060 m3 at 60 min when the soils porosities were 0.1, 0.2, 0.3, and 0.4, respectively. But the distribution areas were 0.213 m2 at 60 min when the soil temperatures were 286.15 K, 296.15 K, 306.15 K and 316.15 K, respectively. The distribution volumes were 0.082 m3 at 60 min when the soil temperatures were 286.15 K, 296.15 K, 306.15 K and 316.15 K, respectively. The calculation formulas of distribution areas and volumes of diesel in soils with different porosity and temperature for developing prevention and control strategies in the future were fitted. The seepage velocities of diesel changed sharply around the leakage port and decreased from about 4.9 m/s to 0 within a few millimeters in soils with different porosity. Additionally, the diffusion ranges of leaked diesel in soils with different porosity were different, indicating that soil porosity had a significant impact on seepage velocities and pressures. The seepage velocities fields and pressures fields of diesel in soils with different temperature were same at the leakage velocity of 4.9 m/s. And the study could provide some supports for determination of the safety zone and formulation of emergency response plans for LNAPL leakage accidents.

Experiments and Simulations on Plume Spreading by Engineered Injection and Extraction in Refractive Index Matched Porous Media

AbstractEngineered injection and extraction (EIE) is an in situ groundwater remediation strategy that imposes an engineered spatially and temporally varying velocity field to spread an amendment into the contaminant plume to promote mixing and contaminant degradation through reaction. Here we present a coordinated suite of laboratory experiments and numerical simulations that apply EIE using two pumping sequences: A folding sequence that manipulates plume geometry by stretching and folding, and an oscillating sequence that imposes velocity perpendicular to the plume interface in accordance with previously‐reported active spreading theory. Laboratory experiments investigated spreading and mixing of a non‐reactive tracer using refractive index matched porous media and laser‐induced fluorescence, with remarkable reproducibility between replicates. Numerical simulations matched the experimental results and simulated reactive transport by post‐processing sequentially injected non‐reactive tracer plumes representing the contaminant and the amendment assuming rapid reaction (i.e., high Damköhler number). This study provides the first experimental verification of EIE using Darcy‐scale imaging, demonstrates a novel experimental method to mimic reactive transport at the high‐Damköhler number limit, and confirms active spreading theory.

Inexact multi-attribute decision analysis of groundwater remediation strategies with inputs of interval data

This study introduced an inexact interval-based multi-attribute decision analysis (IMADA) method with AHP for identifying the compromised groundwater remediation strategies in different periods. This IMADA method has the following advantages: (1) provides a systematic and quantitative analysis procedure for comparing potential pumping alternatives under four remediation duration; (2) reflects the priorities of the project based on the points of view of different decision makers; (3) compromises among possibly tangible and intangible attributes according to a final ranking of pumping alternatives. The method is applied to a contaminated aquifer located in southeastern China, where a period of remediation action should be taken. Ten influential attributes were considered and 50 alternative remediation strategies were generated. Results from the case study indicate that action 48 is the optical remediation strategy under 5-year period, action 5 in 10-year, action 33 during 15-year period and action 26 in 20-year.

A Computational Framework for Design and Optimization of Risk-Based Soil and Groundwater Remediation Strategies

Soil and groundwater systems have natural attenuation potential to degrade or detoxify contaminants due to biogeochemical processes. However, such potential is rarely incorporated into active remediation strategies, leading to over-remediation at many remediation sites. Here, we propose a framework for designing and searching optimal remediation strategies that fully consider the combined effects of active remediation strategies and natural attenuation potentials. The framework integrates machine-learning and process-based models for expediting the optimization process with its applicability demonstrated at a field site contaminated with arsenic (As). The process-based model was employed in the framework to simulate the evolution of As concentrations by integrating geochemical and biogeochemical processes in soil and groundwater systems under various scenarios of remedial activities. The simulation results of As concentration evolution, remedial activities, and associated remediation costs were used to train a machine learning model, random forest regression, with a goal to establish a relationship between the remediation inputs, outcomes, and associated cost. The relationship was then used to search for optimal (low cost) remedial strategies that meet remediation constraints. The strategy was successfully applied at the field site, and the framework provides an effective way to search for optimal remediation strategies at other remediation sites.

Optimizing pump-and-treat method by considering important remediation objectives

The efficiency of groundwater remediation by pump-and-treat (PAT) method is affected by several components. The most important of these components include the pumping wells location, pumping rate, and remediation period. In this research, hybrid optimization-simulation models were developed to find the appropriate groundwater remediation strategy by PAT method. The GA-FEM and NSGA-II-FEM models were used to solve four optimization problems for a hypothetical and real aquifer. These optimization problems were investigated from one objective problem to a four-objectives problem. In the multi-objective problems, in each step, one objective function is added to the previous set of objective functions. In the one-objective case, the objective function was defined as minimizing the contaminant concentration by pumping at a constant rate, while in the two-objectives problem, minimizing the drawdown of groundwater head by pumping at a constant rate was added. In the three-objectives problem, the pumping rate was variable and the mean pumping rate from all the wells is minimized. Finally, minimizing the remediation period is added in the four-objective case. The results indicated that locating the pumping wells in the path of the contaminant flow and close to the source improves the efficiency of the PAT system. The wells with higher pumping rates would be in the path of contamination flow and the wells with lower pumping rates should be located in nodes near the Dirichlet boundary. It is concluded that the remediation period in the hypothetical and real aquifer cannot be less than almost 3000 and 760 days, respectively. Finally, it can be said, the most important component in choosing the proper PAT strategy is the proper location of pumping wells.

Enhanced attenuation (EA) to manage PFAS plumes in groundwater

AbstractRemediation of per‐ and polyfluoroalkyl substances (PFAS) in groundwater is particularly challenging because of their unique chemical and fate and transport properties. Many conventional in‐situ remediation technologies, commonly applied to address other groundwater contaminants, have proven ineffective for treatment of PFAS. Given their stability, destruction of PFAS in‐situ has remained elusive as an in‐situ treatment option. Consequently, new approaches to manage PFAS groundwater plumes are of great interest to environmental practitioners. We propose that enhancing PFAS retention can play an important role in reducing PFAS mass flux and providing long‐term protection of downgradient groundwater receptors. Enhanced retention of PFAS fits directly into the enhanced attenuation (EA) framework, an established groundwater remediation strategy that was developed in the first decade of the 2000s for other groundwater contaminants. In this paper, we propose eight EA approaches for PFAS in groundwater, including technologies that are currently being implemented at PFAS sites (e.g., injection of particulate carbon amendments), applications of conventional remediation technologies to PFAS sites (e.g., capping to retain PFAS in the vadose zone), and novel, innovative approaches (e.g., intentional food grade LNAPL emplacement to retain PFAS) for enhanced PFAS retention. These EA approaches leverage the properties of PFAS to (i) facilitate sorption to conventional and novel sorbents, (ii) concentrate PFAS at air/water interface via gas sparging, and/or (iii) encourage retention via tidal pumping and PFAS salting out processes. For each of the proposed EA approaches, we describe the methodology or concept and discuss the key processes, potential applications, anticipated increases in PFAS retention compared to natural systems, potential challenges, alternate designs, and current likelihood of large‐scale adoption.

Enhanced bioremediation of RDX and Co-Contaminants perchlorate and nitrate using an anaerobic dehalogenating consortium in a fractured rock aquifer

The potential neurotoxic and carcinogenic effects of the explosives compound RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) on human health requires groundwater remediation strategies to meet low cleanup goals. Bioremediation of RDX is feasible through biostimulation of native microbes with an organic carbon donor but may be less efficient, or not occur at all, in the presence of the common co-contaminants perchlorate and nitrate. Laboratory tests compared biostimulation with bioaugmentation to achieve anaerobic degradation of RDX, perchlorate, and nitrate; a field pilot test was then conducted in a fractured rock aquifer with the selected bioaugmentation approach. Insignificant reduction of RDX, perchlorate, or nitrate was observed by the native microbes in microcosms, with or without biostimulation by addition of lactate. Tests of the RDX-degrading ability of the microbial consortium WBC-2, originally developed for dehalogenation of chlorinated volatile organic compounds, showed first-order biodegradation rate constants ranging from 0.57 to 0.90 per day (half-lives 1.2 to 0.80 days). WBC-2 sustained degradation without daughter product accumulation when repeatedly amended with RDX and lactate for a year. In microcosms with groundwater containing perchlorate and nitrate, RDX degradation began without delay when bioaugmented with 10% WBC-2. Slower RDX degradation occurred with 3% or 5% WBC-2 amendment, indicating a direct relation with cell density. Transient RDX daughter compounds included methylene dinitramine, MNX, and DNX. With WBC-2 amendment, nitrate concentrations immediately decreased to near or below detection, and perchlorate degradation occurred with half-lives of 25–34 days. Single-well injection tests with WBC-2 and lactate showed that the onset of RDX degradation coincided with the onset of sulfide production, which was affected by the initial perchlorate concentration. Biodegradation rates in the pilot injection tests agreed well with those measured in the microcosms. These results support bioaugmentation with an anaerobic culture as a remedial strategy for sites contaminated with RDX, nitrate, and perchlorate.

Characterization of shallow groundwater chemistry in the Yogyakarta basin, Central Java

Sufficient water supply can still be a big challenge in developing countries. A large number of households in Yogyakarta still relay on private dug wells to meet their daily demand. But water from shallow aquifers is prone to pollution from sewage systems, industry and agriculture. To assess the water quality in the basin of Yogyakarta, this study determines the chemical characteristics of the shallow groundwater in anthropogenically influenced and uninfluenced areas in and around the city of Yogyakarta. Therefore, a combination of on-site and laboratory analysis is used to distinguish areas of different water types. The observations give a major ion distribution for the whole research area and show significant anomalies in nitrate and phosphate concentrations especially in the city. Furthermore, high DOC values point out that the aquifer is widely polluted. There is a need for a better groundwater protection and remediation strategy to facilitate future use of those groundwaters.

1,2-DCA Natural Attenuation Evaluation in Groundwater: Insight by Dual Isotope 13C/37Cl and Molecular Analysis Approach

Natural attenuation (NA) processes represent a valuable option in groundwater remediation. At a heavily 1,2-dichloroethane (1,2-DCA) contaminated site, Compound-Specific Isotope Analysis (CSIA) in combination with Biological Molecular Tools (BMTs) were implemented as a rigorous characterization approach to evaluate the occurrence of Natural Attenuation in the proximity of the source area. By the use of microcosm experiments, the potential for natural and enhanced biodegradation under anaerobic conditions was documented, following the dichloroelimination pathway. Enrichment factors of −9.1‰ and −11.3‰ were obtained for 13C while Geobacter spp. and reductive dehalogenase genes (rdhs) were identified as main site-specific biomarkers. At pilot scale, enrichments of 13.5‰ and 6.3‰ for δ13C and δ37Cl, respectively, high levels of reductive dehalogenase (rdh group VI) along with the dominance of Geobacter spp. indicated the occurrence of significant dichloroelimination processes in groundwater under anaerobic conditions. By using the site-specific enrichment factors, degradation extents over approximately 70–80% were estimated, highlighting the relevant potential of NA in 1,2-DCA degradation in the vicinity of the source area at the site. The proposed fine-tuned protocol, including CSIA and BMTs, is proven to be effective as a groundwater remediation strategy, properly assessing and monitoring NA at site scale.

An adaptive dynamic surrogate model using a constrained trust region algorithm: application to DNAPL-contaminated-groundwater-remediation design

Simulation optimization is a robust tool in the design of groundwater remediation strategy. Simulation models for groundwater contaminated by dense non-aqueous phase liquids (DNAPLs) are usually computationally expensive, especially when used within a simulation optimization framework. Surrogate models have been proved to possess the potential to speed up complex models without sacrificing accuracy or detail. In this study, a constrained trust region (CTR)-based adaptive dynamic surrogate model was proposed and applied to a DNAPL-contaminated-groundwater-remediation-design scenario. First, the multiphase flow simulation model was constructed to simulate the groundwater remediation process. Then, preliminary input–output sample points were generated using the Latin hypercube sampling method and the developed simulation model. Next, the static Kriging surrogate model, as well as the nonlinear optimization model, were constructed. Finally, the surrogate model and optimization model were updated based on the CTR method and the optimal remediation strategy was obtained. The application of the CTR-based dynamic surrogate model reduced the mean relative error by 46% compared with the static surrogate model, and greatly improved the reliability of the optimal remediation strategy. The CTR-based dynamic surrogate model had the best fitting accuracy of multiphase flow simulation when compared with the expected improvement-based dynamic surrogate model and the optimal solution-based dynamic surrogate model. The corresponding optimization model also had the minimum remediation cost. This study demonstrates that the proposed CTR-based dynamic surrogate model is an effective tool to increase the accuracy of surrogate models and the reliability of their optimization results.

Sustainability appraisal of desired contaminated groundwater remediation strategies: an information-entropy-based stochastic multi-criteria preference model

This study proposes an information-entropy-based stochastic multi-criteria preference model (SMCPM) to assess the sustainability of groundwater remediation strategies. The model was applied to a real-world site, which includes the following steps: (1) selecting fifty remediation alternatives with four criteria, including total pumping rate, remediation cost, average contaminant concentration and general cancer risk in light of uncertainty parameter (e.g., slope factor); (2) evaluating the health risks associated with the strategies under three confidence levels (i.e., 68.3%, 95.4% and 99.7%); (3) computing the weight by information entropy; and (4) determining the ranking of remedial alternatives by establishing a random evaluation matrix. Results from the case study indicate that: (1) the most desirable actions are action A39 (i.e., the pumping rates at six wells are 3.499, 2.722, 2.624, 1.458, 7.776 and 2.527 m3/h, respectively) for the 5-year; (2) action A25 (i.e., the six pumping rates at injecting and extracting wells are 2.819, 2.722, 0, 3.791, 0.972 and 1.166 m3/h, respectively) for the 10-year; (3) action A24 (i.e., the pumping rates of wells P1–P6 are 1.944, 1.555, 1.166, 3.110, 7.193 and 0.583 m3/h, respectively) for the 15-year; (4) action A37 (i.e., the optimal pumping rates of P1 to P6 become 2.138, 2.722, 3.694, 2.722, 3.694 and 7.582 m3/h, respectively) for the 20-year remediation, respectively. Compared to the traditional MCPM, the proposed SMCPM would be more acceptable and reasonable since various uncertainties be addressed.

Dissolution of poorly soluble uranyl phosphate phases in the Metaautunite Subgroup under uranyl peroxide cage cluster forming conditions

Abstract Uranyl phosphate minerals are widespread in uranium deposits and normally exhibit very low solubility in aqueous systems. Uranyl phosphates of the autunite group and metaautunite subgroup impact the mobility of uranium in the environment and have inspired groundwater remediation strategies that emphasize their low solubility. The importance of soluble uranium-bearing macro-anions, including nanoscale uranyl peroxide cage clusters, is largely unexplored relative to solubilization of normally low-solubility uranium minerals. Eight synthetic analogs of metaautunite subgroup minerals have been prepared and placed in various alkaline aqueous solutions containing hydrogen peroxide and tetraethylammonium hydroxide. Each uranyl phosphate studied has a topologically identical anionic sheet of uranyl square bipyramids and phosphate tetrahedra combined with various cations (Li+, Na+, K +, Rb+, Cs+, Mg2+, Ca2+, Ba2+) and water in the interlayer. Uranyl peroxides formed under many of the experimental conditions examined, including solid studtite [(UO2)(O2)(H2O)2](H2O)2 and soluble uranyl peroxide cage clusters containing as many as 28 uranyl ions. Uranyl phosphate solids in contact with solutions in which uranyl peroxide cage clusters formed dissolved extensively or completely. The greatest dissolution of uranyl phosphates occurred in systems that contained cations with larger hydrated radii, Li+ and Na+. The details of the uranium speciation in solution depended on the pH and counter cations provided from the interlayers of the uranyl phosphate solids.

Evaluation of groundwater remediation strategies at petroleum contaminated sites based on groundwater modelling approach

Based on the hydrogeological survey and groundwater environmental investigation at an abandoned oil refinement factory, a site-specific groundwater flow and solute transport model was constructed to support the development of groundwater remediation strategy. The contamination plume of Benzene is simulated and predicted in 30-year scale for designed scenarios. The results indicate that the groundwater quality takes long time to recover even though the contamination source above the water table is cleaned up. This is due to the large quantity of solved contaminants in local aquifer, and additionally the contribution from groundwater circulation is limited. The groundwater remediation actions, such as Pump & Treat (P&T) and In-Situ Chemical Oxidation (ISCO) could greatly improve the groundwater quality. However, under the site-specific circumstance, the ISCO technique is efficient but costly; the P&T takes longer time but more flexible and effective in protecting target drinking wells. Therefore, a combined remediation approach including both P&T and ISCO would be recommended. In summary, by the quantitative analysis regarding the feasibility, time scale, and effectiveness for the potential applicable remediation approaches, the groundwater modelling framework provided a insight on the system behavior and reliable support for the decision making in groundwater protection.

Identifying optimal groundwater remediation strategies through a simulation-based PROMETHEE-TOPSIS approach: An application to a naphthalene-contaminated site

The aim of this paper is to develop a simulation-aided PROMETHEE-TOPSIS approach for the selection of the most desirable groundwater remediation strategies. The combination methods enables a careful evaluation of the identified remediation alternatives in which their strong and weak points can be detected and a ranking is provided which facilitates the final selection for the decision-maker. The capabilities and effectiveness of the developed method are illustrated through a case study on the remedial alternative selection for a naphthalene contaminated site in Anhui, China. Four attributes (i.e., total pumping rate, total cost, mean residual contaminant concentration and maximum excess life time cancer risk) for fifty remedial alternatives in each duration are considered and analytic hierarchy process is used to determine the weight of attributes importance. The results demonstrates that the developed method could help decision makers obtain the useful ranking information strategies that covering a variety of decision-relevant remediation options, which is beneficial for public health and environmental protection.

A Genetic Programming-Based Model for Colloid Retention in Fractures.

Understanding the behavior of colloids in groundwater is critical as some are pathogenic while others may facilitate or inhibit the transport of dissolved contaminants. Colloid behavior in saturated fractured aquifers is governed by the physical and chemical properties of the groundwater-particle-fracture system. The interaction between these properties is nonlinear, and there is a need for a mathematical model describing the relationship between them to advance the mechanistic understanding of colloid transport in fractures and facilitate modeling in fractured environments. This paper coupled genetic programming and linear regression within a multigene genetic programming framework to develop a robust mathematical model describing the relationship between colloid retention in fractures and the physical and chemical parameters that describe the system. The data employed for model development and validation were collected from a series of 75 laboratory-scale colloid tracer experiments conducted under a range of conditions in three laboratory-induced discrete dolomite fractures and their epoxy replicas. The model sufficiently reproduced the observed data with coefficients of determination (R2 ) of 0.92 and 0.80 for model development and validation, respectively. A cross-validation demonstrated the model generality to 86% of the observed data. A variance-based global sensitivity analysis confirmed that attachment is the primary retention mechanism in the systems employed in this work. The model developed in this study provides a tool describing colloid retention in factures, which furthers the understanding of groundwater-particle-fracture system conditions contributing to the retention of colloids and can aid in the design of groundwater remediation strategies and development of groundwater management plans.

A Multi-objective Optimal Model for Groundwater Remediation under Health Risk Assessment in a Petroleum contaminated site

This study proposes a multi-objective model (i.e. MOHRA) for groundwater remediation in a petroleum-contaminated site under health risk assessment. The optimization system is designed based on the pump and treat technology considering both cost and human health risk, which can provide reliable groundwater remediation strategies.