Transmissivity Identification by Combination of CVFEM and Genetic Algorithm: Application to the Coastal Aquifer

The solution of inverse problems in groundwater flow has been massively invested by several researchers around the world. This type of problem has been formulated by a constrained optimization problem and this constraint is none other than the direct problem (DP) itself. Thus, solving algorithms are developed that simultaneously solve the direct problem (Darcy's equation) and the associated optimization problem. Several papers have been published in the literature using optimization methods based on computation of the objective function gradients. This type of method suffers from the inability to provide a global optimum. Similarly, they also have the disadvantage of not being applicable to objective functions of discontinuous derivatives. This paper is proposed to avoid these disadvantages. Indeed, for the optimization phase, we use random search-based methods that do not use derivative computations, but based on a search step followed only by evaluation of the objective function as many times as necessary to the convergence towards the global optimum. Among the different algorithms of this type of methods, we adopted the genetic algorithm (GA). On the other hand, the numerical solution of the direct problem is accomplished by the CVFEM discretization method (Control Volume Finite Element Method) which ensures the mass conservation in a natural way by its mathematical formulation. The resulting computation code HySubF-CVFEM (Hydrodynamic of Subsurface Flow by Control Volume Finite Element Method) solves the Darcy equation in a heterogeneous porous medium. Thus, this paper describes the description of the integrated optimization algorithm called HySubF-CVFEM/GA that has been successfully implemented and validated successfully compared to a schematic flow case offering analytical solutions. The results of this comparison are qualified of excellent accuracy. To identify the transmissivity field of the realistic study area, the code HySubF-CVFEM/GA was applied to the coastal "Chaouia" groundwater located in Western of Morocco. This aquifer of high heterogeneity is essential for water resources for the Casablanca region. Results analysis of this study has shown that the developed code is capable of providing high accuracy transmissivity fields, thus representing the heterogeneity observed in situ. However, in comparison with gradient method optimization the HySubF-CVFEM/GA code converges too slowly to the optimal solution (large CPU-time consuming). Despite this disadvantage, and given the high accuracy of the obtained results, the HySubF-CVFEM/GA code can be recommended to solve in an efficient and effective manner the identification parameters problems in hydrogeology

Performance evaluation of equipment used for volumetric water content measurements

Various methods can be used to evaluate in situ volumetric water content; however, when continuous measurements are required, only dielectric methods can be used. For equipment based on these methods, in the mining environment and particularly in environments having high sulfide contents (or acid-generating minerals), the accuracy of measurements can be affected by the chemistry of the solid and liquid phases. To evaluate the real impact of water chemistry and mineralization on volumetric water content measurement, three instruments were tested on five materials with varying sulphur contents and two types of water mineralization. The results of these investigations show that: i) increasing sulfur content of mine tailings increases the response output of the tested probes; ii) the obtained volumetric water contents are systematically higher than the real volumetric water contents obtained gravimetrically; iii), time-domain reflectometry probes are able to provide measurements in materials with high sulphur contents; and iv) 5TM and GS3 probes are not suitable for use in reactive materials with higher sulphur contents

Dewatering of Mine Tailings Slurries Using Superabsorbent Polymers (SAPs) Reclaimed from Industrial Reject of Baby Diapers: A Preliminary Study

Traditional deposition of tailings slurry in a tailings storage facility (TSF) can create risks of dike failure. In order to minimize these risks and slurry spillage, the surface deposition technique of densified tailings (DT) through dewatering of the slurry has emerged. The DT technique has the potential to maximize water reuse, improve the shear strength of surface tailings, and reduce the ecological footprint of TSF. The consistency of DT covers a continuum ranging from thickened state, to paste state, to dry state. Despite its efficiency and economic feasibility, DT densification using thickeners sometimes proves unable to achieve the design target solids mass concentration (Cw%). Hence, the use of superabsorbent polymers (SAPs) seems to represent a promising alternative, owing to their higher water absorbent capacity. In this paper, superabsorbent polymers (SAPs) reclaimed from industrial reject of baby diapers (Na-polyacrylates) are explored as a promising alternative to mine tailings slurries dewatering. To this end, laboratory-scale dewatering tests have been performed using two grades of Na-polyacrylate SAPs (grade 1 SAP = SAG-A06P coarse-grained, and grade 2 = SAG-M01P-100 medium-grained) for the tailings slurries densification. A higher water absorbency (or swelling capacity) was observed using the coarser SAPs (SAG-A06P) compared to the finer SAPs (SAG-M01P-100). The preliminary results showed that a SAP volume dosage in the range 10–13 kg of SAP/m3 of slurry allowed achieving a final solids mass concentration (Cw%_final) ≥ 70%, despite the occurrence of gel-blocking phenomenon

Physicochemical quality of surface water: Background study prior of the Milky river sub-basin, Abitibi, Canada

In spite of its economic benefits, the mining industry can have many negative impacts on the environment, more specifically, on surface water quality. The evaluation of the impact of the mining activity must therefore be addressed adequately with a background study prior to mining operation areas. Thirteen active and past producing mines are located inside the sub-basin of the Milky. These mining sites may contribute to the water mineralization of the surface waters and sediments of the Milky river system. In the present study, nine metals including As, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were measured at thirty (30) surface water locations during two years in the catchment area of the Milky river. The data collected served to calculate the Metal pollution index (MPI) and the Metal index (MI) to evaluate the surface water quality. The MI values suggest that all the samples are contaminated, while only some of the samples are considered contaminated according to the MPI values. However, it is impossible to discriminate the origin of this contamination between the natural enriched geochemical background of this sub-basin and the anthropic activities. This discrepancy between the two pollution evaluation methods demonstrate that their interpretation needs to be adapted to the context of mining districts that have both a high natural geochemical background and are affected by past and present mining activities

Analyzing variation of the water table level with three-dimensional numerical simulations to assess reclamation techniques for an acidic tailings impoundment

RÉSUMÉ: Tailings with sulphide minerals exposed to oxygen and water can oxidize and produce acid mine drainage (AMD). This study evaluated the impact of natural conditions and of a thin cover deposition on the water table level, with respect to selecting the reclamation technique to control AMD on the elevated portion of an abandoned tailings impoundment. The reactive tailings were partly covered with alkaline tailings transported as a pulp from a nearby mine. A three-dimensional numerical variably saturated groundwater flow model of the tailings impoundment and its surrounding area was built. The simulation results reproduced well the field observations before, during, and after the deposition of a thin layer of tailings. The calibrated model was then used to forecast the evolution of the water table position for the next 100 years under various site conditions, including a period of 5 years with dry summers. The results show that the water table levels are on average several metres below the interface between the reactive tailings and the cover, thus failing to meet the target criterion to control the production of AMD determined from previous column tests. The results are analyzed and discussed in terms of the site configuration and atmospheric recharge. ABSTRACT: Les résidus miniers sulfureux exposés à l'oxygène et à l'eau peuvent engendrer la production de drainage minier acide (DMA). Cette étude évalue l'impact de la déposition d'une couverture sur l'établissement d'une nappe phréatique surélevée (NPS) comme méthode de contrôle du DMA sur un parc à résidus miniers abandonné. Les résidus réactifs ont été recouverts partiellement par des résidus alcalins sous forme de pulpe provenant d'une mine voisine. Un modèle numérique tridimensionnel d'écoulement souterrain à saturation variable du parc à résidus et ses environs a été construit. Les résultats de ces simulations reproduisent bien les niveaux phréatiques observés avant, durant et après la déposition des résidus. Le modèle calibré a ensuite été utilisé pour prédire l'évolution du niveau phréatique pour les 100 prochaines années et dans le cas de périodes estivales plus sèches. Les résultats ont montré que la nappe phréatique serait en moyenne plusieurs mètres sous l'interface entre les résidus réactifs et le recouvrement, soit nettement sous la cible permettant de contrôler la production de DMA selon des essais antérieurs. Les résultats sont analysés et discutés selon la configuration du site et la recharge atmosphérique

Evaluation of the Performances of a Monolayer Cover with an Elevated Water Table Used for the Reclamation of the Abandoned Aldermac Mine Site (Québec, Canada)

Until 2008, the Aldermac mine was considered one of the most problematic abandoned mines sites in Québec due to the high production of acid mine drainage caused by mine tailings spread over a large area. The site was reclaimed between 2008 and 2009 to reduce the ecological footprint and to minimize any further tailings oxidation (production of acid mine drainage). For this purpose, a monolayer cover with an elevated water table was used as an oxygen barrier on the north zone of the site. To assess the performance of the monolayer with an elevated water table cover, a network of observation wells was set up to monitor the water table level and collect groundwater samples for the analysis of physical and chemical parameters. Results of water table level measurements show that the cover successfully maintains the tailings within the water table at all times, thus preventing further tailings oxidation. Hydrogeochemical results, interpreted with principal component analysis, show that the quality of water is improving, thus indicating that reclamation is helping to reduce groundwater contamination at the site

A Simple Relationship to Estimate Parameters of the Optimum Compaction Point

Compacted clay covers have been the most commonly used materials for hydraulic barrier layers. During their construction, the control of some parameters such as compactive effort and molding water content is required. These last parameters affect the hydraulic conductivity, which is considered as one of the important key parameters for cover design. To reach the target in terms of hydraulic conductivity, the cover must be compacted to a pre-determined dry unit weight that usually corresponds to a certain percentage of the maximum dry unit weight (γdmax). During the prefeasibility stage of a project (before conducting the required tests), γdmax and the optimum water content (wOpt) can be estimated to obtain an early overview of the conditions that can be anticipated. In this regard, a new approach was proposed to estimate these parameters using the liquid limit (wL) and the plastic limit (wP). The proposed equations were developed using data from 56 compacted clay liners and validated using 44 others taken from published data. Results presented in this paper indicate how the proposed equations can successfully estimate parameters of the optimum compacted point