Using MODFLOW and RT3D to simulate diffusion and reaction without discretizing low permeability zones

Integrated assessment for groundwater quality and flood vulnerability in coal mining regions

Coal mining activities greatly damage water resources, explicitly concerning water quality. The adverse effects of coal mining and potential routes for contaminants to migrate, either through surface water or infiltration, into the groundwater table. Dealing with pollution from coal mining operations is a significant surface water contamination concern. Consequently, surface water resources get contaminated, harming nearby agricultural areas, drinking water sources, and aquatic habitats. Moreover, the percolation process connected with coal mining could alter groundwater quality. Subsurface water sources can get contaminated by toxins generated during mining activities that infiltrate the soil and reach the groundwater table. The aims of this study are the creation of models and the provision of proposals for corrective measures. Twenty-five scenarios were simulated using MODFLOW; according to the percolation percentage and contamination, 35% of the study area, i.e., the middle of the research area, was the most affected. About 38.08% of the area around the mining zones surrounding Margherita is prone to floods. Agricultural areas, known for applying chemical fertilizers, are particularly vulnerable, generating a risk of pollution to surrounding water bodies during flooding. The outputs of this research contribute to identifying and assessing flood-vulnerable regions, enabling focused measures for flood risk reduction, and strengthening water resource management.

Simulating arsenic discharge flux at a relic smelting site in Guangxi Zhuang Autonomous Region, China

Anthropogenic groundwater arsenic (As) pollution is common in many aquifers in Southwest China. It is concerned that long-term random disposal of As smelting slag could induce the transport of high-As groundwater into previously uncontaminated aquifers. Here, we used HELP-MODFLOW-MT3DMS model simulations to integrate the percolation, groundwater flow, and solute transport processes at an aquifer at site scale, constrained by weather, hydrogeology, and monitoring data. Our simulations provide a new method framework of the simulated percolation by HELP model and have induced As spatiotemporal distribution in the aquifer. According to the HELP model simulation results, percolation volume accounts for 24% of rainfall over 18 years. This work determined that the As discharge trend was fitted by double-constants kinetics based on the leaching experiment. And this work calculates total mass distribution of As in the aquifer over 18 years. We have found that the sustained As pollution relies on the rainfall that acts as the primary contributor of elevated As concentrations. Model simulation results suggest that 51.70% of the total As mass (1.96 × 104 kg) was fixed in low permeability solid media. The total As mass discharged into groundwater reached 9.3 × 103 kg, accounting for 24.68%. The accumulative outflow mass of arsenic was 8.0 × 103 kg, accounting for 21.62%.

Control mechanism of trichloroethylene back diffusion by microstructure in a low permeability zone

The trailing effect caused by the back diffusion (BD) of contaminants in low-permeability zones (LPZs), which prolongs remediation time and increases remediation costs, has caused widespread concern. In this study, the BD of trichloroethylene (TCE) from the LPZ to the high-permeability zone (HPZ) was determined using flow cell experiments. The anomalous variance in the BD flux of the TCE-spanning 2-4 times the deviation under identical experimental conditions, attracted our attention. To determine the cause of this aberrant behavior, a micro computed tomography (micro-CT) characterization of the flow cell was conducted, which revealed significant microstructural disparities in the LPZ. The study found that the pore connectivity of LPZs determines the efficiency of BD and that LPZs with different porosities have different sensitivities to connectivity. The pore shape complexity indicates the possibility of BD retardation, and remediation is more difficult for these types of LPZs. Changing the structure of LPZs to improve their remediation efficiency may be a new research topic. Notably, correcting the model parameters through microstructural characterization significantly refined the prediction accuracy.

Mechanistic insights into contaminant transport dynamics in the saturated porous system in the presence of low permeability region using numerical simulations and temporal moment analysis

The influence of low permeability porous media (LPPM) on contaminant transport dynamics in saturated porous systems was investigated using numerical simulations and temporal moments of contaminant concentrations. Two-dimensional flow and contaminant transport simulations were conducted, considering various parameters such as longitudinal dispersivity (ranging from 15 to 60 m), the ratio of transverse to longitudinal dispersivity (ranging from 0.05 to 0.2), retardation factor (ranging from 1 to 4), and hydraulic gradient (ranging from 0.005 to 0.02) for both homogeneous and heterogeneous porous systems. The findings revealed significant differences in the transport behavior of conservative and highly reactive contaminants between the porous systems without and with the LPPM region. The center of mass of contaminant plume and peak concentration zone were observed inside the LPPM region for the heterogeneous porous system, especially during the source off-loading period. Furthermore, asymmetric distributions of the zeroth temporal moment (ZTM), mean residence time (MRT), and variance of the breakthrough curve (BTC) were observed along the longitudinal distance within the LPPM region for heterogeneous porous system, highlighting the impact of heterogeneity on contaminant plume evolution dynamics. The moment analysis results provided insights into the influence of LPPM region on time-averaged contaminant transport dynamics in adjacent porous systems. These findings can help risk managers understand the complex fate and transport dynamics in heterogeneous porous systems. Future studies could explore the modelling of multispecies contaminants in heterogeneous saturated porous systems subjected to fluctuating water table.

Simulation of nitrate pollution and vulnerability of groundwater resources using MODFLOW and DRASTIC models

Groundwater assets are the foremost imperative assets of freshwater accessible to people especially in arid and semi-arid regions. For the investigation of temporal changes in groundwater nitrate pollution and the role of agriculture and other sources in the pollution of groundwater, the information on 42 drinking water wells with suitable distribution in the plain in Bouin-Daran Plain in the center of Iran was used. The results showed that the amount of hydraulic conductivity in the plain for different areas after calibration in steady state was calculated between 0.8 and 34 m/day. After calibrating the model in permanent conditions, the model was calibrated in non-permanent conditions for 2 years. The results showed that in a wide area of the region, the nitrate ion concentration has values of more than 25 mg/L. This shows that the average concentration of this ion in the region is generally high. The highest level of pollution in the aquifer of the plain is related to the southern and southeastern parts of the plain. Due to the agricultural activities with the use of large amounts of fertilizers in this plain, there is a potential for pollution in all of the places, and it requires codified and executive planning for agricultural operations as well as the use of groundwater sources. The DRASTIC vulnerability estimation method is only useful for estimating the areas that have a high potential for contamination and according to the validation tests, it has also provided a suitable estimate.

Contaminant Back Diffusion from Low-Conductivity Matrices: Case Studies of Remedial Strategies

Recalcitrant groundwater contamination is a common problem at hazardous waste sites worldwide. Groundwater contamination persists despite decades of remediation efforts at many sites because contaminants sorbed or dissolved within low-conductivity zones can back diffuse into high-conductivity zones, and therefore act as a continuing source of contamination to flowing groundwater. A review of the available literature on remediation of plume persistence due to back diffusion was conducted, and four sites were selected as case studies. Remediation at the sites included pump and treat, enhanced bioremediation, and thermal treatment. Our review highlights that a relatively small number of sites have been studied in sufficient detail to fully evaluate remediation of back diffusion; however, three general conclusions can be made based on the review. First, it is difficult to assess the significance of back diffusion without sufficient data to distinguish between multiple factors contributing to contaminant rebound and plume persistence. Second, high-resolution vertical samples are decidedly valuable for back diffusion assessment but are generally lacking in post-treatment assessments. Third, complete contaminant mass removal from back diffusion sources may not always be possible. Partial contaminant mass removal may nonetheless have potential benefits, similar to partial mass removal from primary DNAPL source zones.