Analytical model for organic contaminant transport in a cut-off wall and aquifer dual-domain system considering barrier arrangements

Groundwater pollution management with source remediation and composite geomembrane cut-off wall: An analytical model and field investigation

This study proposes a two-dimensional analytical model for contaminant migration through the composite GMB cut-off wall (CGCW) and aquifer system considering the effects of the leakage and source remediation. A comprehensive sensitivity analysis based on the Sobol's method is conducted to investigate the key impacts of contaminant distribution and degradation rate in the source, cut-off wall retardation factor and Darcy velocity in CGCW. The results show that the performance of the CGCW significantly is controlled by the leakage. The peak contaminant concentration at the outlet of CGCW for large leakage (10-8 m/s) can be 77.1 times larger than that for small leakage (10-11 m/s), with a performance deterioration of 98.7 %. Additionally, the performance of the CGCW can be significantly enhanced by the source remediation technique. The CGCW will not be broken through when contaminant source degradation rate is larger than 2.9 × 10-8 /s. Increasing the cut-off wall retardation factors (> 14.7) and decreasing the standard deviation of contaminant source distribution (< 0.1) can also improve the CGCW performance. Moreover, the analytical model coupled with the genetic algorithm is used for the CGCW leakage detection and performance prediction at an abandoned pesticide site based on the monitoring data. Accurate leakage detections in the field (with errors of < 5 m) can be achieved through the analytical solution, which is validated by the non-destructive high-voltage technique. The analytical solution can be an easy-to-use tool for the design, installation and construction of the CGCW at polluted sites.

Analytical study for two-dimensional transport of organic contaminant in a polymer material-enhanced composite cutoff wall system

Polymer material (PM) is a novel vertical barrier material, demonstrated to be effective in impeding pollutants. However, the associated transport research is limited. This study aims to develop an analytical solution for two-dimensional transport of organic contaminant in the PM-enhanced composite cutoff wall (CCW) system, where the variable substitution and Fourier transform methods are used. This analytical solution, available in various simplifications, is effectively validated via several comparisons. Following this, the analyses show that an increase in the non-uniformity of pollution source concentration distribution shortens the PM-enhanced CCW's breakthrough time (tb), while exhibits a marginal effect on the total flux at its exit. The increment of aquifer horizontal thickness prolongs the tb to some extent, whereas an increase in its hydraulic conductivity slightly reduces the tb. Additionally, the PM layer location is found to have a little effect on the PM-enhanced CCW's barrier performance. Furthermore, the equivalent performance assessment reveals that the improvement gained from increasing the PM layer thickness far surpasses that from increasing the single-layered cutoff wall thickness, and this difference may exceed 10. For a PM layer with low hydraulic conductivity, it is more suitable for engineering scenarios with the higher hydraulic head difference. Totally, the proposed analytical solution offers a valuable tool for designing the PM-enhanced CCW.

Investigation of preferential flow and leakage location in landfill: A field tracer test and numerical analysis

A field tracer test was carried out in an uncontrolled valley-type landfill. Fluorobenzoic acid (FBA) was firstly used in the landfill to study the preferential flow. A two-dimensional advection-diffusion dual-porosity model coupled with indirect streamline and terrain conditions was developed to analyze the breakthrough curves. The leakage location method was proposed based on the volume proportion of fracture domain in the total domain wf distribution. The results show that FBA is an excellent tracer due to its lower dosage, high peak concentration and long residence time at monitoring wells. The tracer transport depth and length can reach up to 15 m and 86.3 m, respectively. Diffusion drive the tracer flow to upstream with high velocity. The anisotropy value is mainly influenced by the effect of compression rather than the waste age. The horizontal preferential flows dominate in the landfill. The preferential flow is observed to be more obvious with the increasement of the depth due to the increasement of the content of 2D particles. The leakage probability of different part in the landfill is determined by the proposed dual-porosity model and leakage location method. The proposed leakage detection method can be used for active landfills, especially those with thick layers of wastes. It can also provide scientific guidance for the design of subsequent vertical barrier for the landfills.

Analytical study of water infiltration and contaminant transport in barrier systems

An analytical model was developed to assess the service time of the barrier system consisting of a two-layer cover system and a cut-off wall. The recursive method is used to evaluate the influence of the variable head loss boundary condition caused by the water infiltration. The impact of the types of cover systems and cut-off walls on the barrier system performance is assessed. The results show that cover system types are more likely to influence the long-time performance of barrier systems. Contaminant concentrations with H1* = 0.5 m and H2* = 0.3 m when t = 40 and 100 years are 1.17 and 1.42 times larger than those with H1* = H2* = 0.5 m, respectively. The decrease in hydraulic conductivity of cut-off wall and the increase in the thicknesses and retardation factors of it can also significantly improve the performance of barrier systems. Among all of the parameters, the cut-off wall thickness poses the most significant influence on the contaminant cumulative concentrations, followed by the retardation factor of the cut-off wall, the thickness and hydraulic conductivity of the lower cover layer, the hydraulic conductivity of the cut-off wall, and the thickness and hydraulic conductivity of the upper cover layer. Additionally, the proposed solution is used for the barrier system design of a mine legacy site. The minimum design thicknesses of the cut-off walls for three different cover system types and service times are obtained.

Statistical and machine learning analysis for the application of microbially induced carbonate precipitation as a physical barrier to control seawater intrusion

Seawater intrusion in coastal aquifers is a significant problem that can be addressed through the construction of subsurface dams or physical cut-off barriers. An alternative method is the use of microbially induced carbonate precipitation (MICP) to reduce the hydraulic conductivity of the porous medium and create a physical barrier. However, the effectiveness of this method depends on various factors, and the scientific literature presents conflicting results, making it challenging to generalise the findings. To overcome this challenge, a statistical and machine learning (ML) approach is employed to infer the causes for the reduction in hydraulic conductivity and identify the optimum MICP parameters for preventing seawater intrusion. The study involves data curation, exploratory analysis, and the development of various models to fit the input data (k-Nearest Neighbours - kNN, Support Vector Regression - SVR, Random Forests - RF, Gradient Boosting - XgBoost, Linear model with interaction terms, Ensemble learning algorithms with weighted averages - EnL-WA and stacking - EnL-Stack). The models performed reasonably well in the region where permeability reduction is sensitive to carbonate increase capturing the permeability reduction profile with respect to cementation level while demonstrating that they can be used in initial assessments of the specific conditions (e.g., soil properties). The best performing algorithms were the EnL-Stack and RF followed by XgBoost and SVR. The MICP method is effective in reducing hydraulic conductivity provided that the various biochemical parameters are optimised. Critical biochemical parameters for successful MICP formulations are the bacterial optical density, the urease activity, calcium chloride concentration and flow rate as well as the interaction terms across the properties of the porous media and the biochemical parameters. The models were used to identify the optimum MICP formulation for various porous media properties and the maximum permeability reduction profiles across cementation levels have been derived.

Pollutant transport behavior through polymer cutoff wall: Laboratory test and analytical model investigation

Polymer cutoff wall has emerged as a new and promising technology for anti-seepage and anti-pollution in geotechnical engineering. With notable advantages such as rapid sealing, high efficiency, and environmental friendliness, this technology has garnered significant attention. This study presents a systematic investigation into the transport characteristics of pollutants in polymer materials, with a specific focus on the transport mechanisms through polymer cutoff wall. The research investigates various factors that influence the pollutant transport characteristics in polymer materials. The objective is to analyze the pollutant transport behavior and obtain the transport parameters. Moreover, the study develops and solves a one-dimensional transport model incorporating partition-diffusion-partition mechanisms, aiming to determines the long-term service performance of polymer wall. These findings contribute to a better understanding of pollutant transport through polymer walls, which is crucial for the future advancement and utilization of this technology.