Multi-technological integration in a smelting site: Visualizing pollution characteristics and migration pattern

Hydrogeochemical analysis and tracing of heavy metal contamination in groundwater using self-organizing mapping

In recent years, heavy metal pollution in mines has garnered significant attention. However, systematic research on the hydrogeochemical characteristics and heavy metal migration patterns in groundwater at abandoned mine sites remains limited. Therefore we analyzed the hydrogeochemical characteristics and heavy metal migration patterns of groundwater at an abandoned iron ore mine in Jiangxi Province, China. A total of 64 groundwater samples were collected during the rainy season and 14 during the dry season. Self-organizing mapping (SOM), K-means clustering, multivariate statistical analysis, and the Piper diagram method were employed for analysis. The results revealed six distinct hydrogeochemical types of groundwater, with the hydrogeochemical evolution primarily being influenced by mining legacy waste and subsequent anthropogenic activities. Groundwater in the study area is predominantly of the HCO₃-Ca type, while areas near the tailing pond exhibit high total dissolved solids (TDS), low pH, and minor SO₄-Ca components. The exceedance rates of Mn and Fe were 60.0% and 53.8%, respectively. Positive matrix factorization (PMF) identified mining activities, farms, agricultural practices, and indirect mining impacts as the primary sources of heavy metal pollution. In contrast, elements such as As, Pb, Cd, Ni, and Be exhibited lower exceedance rates, possibly due to the specific adsorption behavior of Mn and Fe. These findings provide a scientific basis for environmental remediation and pollution control in abandoned mining areas.

Advanced three-dimensional prediction model based on stable machine learning for soil pollution: A case study from a contaminated site in Southern China

With over five million contaminated sites worldwide, accurately characterizing the three-dimensional (3D) distribution of soil contamination is critical for effective risk assessment and site remediation. However, current 3D interpolation methodologies often fail to simultaneously account for spatial correlation and spatial heterogeneity, both of which are critical for capturing the complex spatial structure of subsurface contamination. This study developed a refined 3D interpolation model that integrates site characteristics, spatial position, spatial correlation, and spatial heterogeneity to simulate site contamination and quantify prediction uncertainty. The proposed machine learning (ML) model achieved high predictive performance, with coefficient of determination (R2) values above 0.73 for four heavy metals (HMs). To enhance model generalizability, a stability analysis framework was developed alongside a novel model selection strategy based on random dataset partitioning and random ordering of input covariates, and 1000 random simulations could provide a reliable basis for model screening. This study introduces a new, precise 3D spatial interpolation method. Owing to the easy accessibility of its covariates, it offers high versatility, making a significant contribution to site assessment and remediation efforts.

Migration modeling of metal(loid)s in soil-groundwater systems from an abandoned mine: Based on multimethod integration

Metal(loid)s contamination of mine has been a global environmental challenge. Traditional investigations of metal(loid) distribution patterns and migration behavior in soil-groundwater systems are constrained by the high costs of drilling and sampling limitations, leading to significant uncertainties in contamination assessment. This study presents an integrated approach combining three-dimensional (3D) visualization with Random Forest (RF) modeling and GIS mapping to investigate metal(loid) contamination characteristics and migration behavior in a mining area's soil-groundwater system. We developed an RF model with 1000 decision trees to expand limited drilling data for comprehensive spatial coverage. Model performance was validated using R2 and Root Mean Square Error (RMSE) metrics. The validated predictions were integrated into 3D visualization models and analyzed in conjunction with GIS mapping to characterize spatial patterns. Through analysis of temporal groundwater sampling data across wet, dry, and transitional hydrological periods, combined with RF modeling, we visualized metal(loid) distribution patterns and characterized their migration behavior in the soil-groundwater system. This integrated methodology provides a novel framework for investigating metal(loid) distribution and migration in mine soil-groundwater systems, effectively bridging traditional exploration techniques with advanced numerical simulation.

Colloid mobilization and transport in response to freeze-thaw cycles: Insights into the heavy metal(loid)s migration at a smelting site

Smelting sites often exhibit significant heavy metal(loid)s (HMs) contamination in the soil and groundwater, which are inevitably subjected to environmental disturbances. However, there is limited information available regarding the migration behaviors of HMs in a disturbed scenario. Thus, this work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. Ultrafiltration results of soil effluents revealed that 61.5 %, 47.6 %, 68.0 %, and 59.2 % of Zn, Cd, Pb, and As were present in colloidal phase, and co-transported during treatments. Nanoparticle tracking analysis (NTA) further confirmed that freeze-thaw cycles were conducive to the generation of colloidal particles and showed the heteroagglomeration among different particles. Pore-network model (PNM) was used to quantify the soil macropore characteristics (macropore diameter, macropore number, coordination number, and Euler value) after treatments. It is evident that freeze-thaw cycles induced the formation of larger macropores while simultaneously enhancing macropore connectivity, thereby establishing an optimal pathway for colloid migration. These findings underscored the importance of environmental disturbances as a trigger for the release and migration of HMs in the smelting site, offering valuable insights for controlling HMs pollution. ENVIRONMENTAL IMPLICATION: The contaminated site has been subjected to prolonged environmental disturbances, causing the exacerbation of pollutants leaching and frequent occurrences of unstable pollution situations. This work explored the migration of HMs-bearing colloids in response to freeze-thaw treatments by laboratory simulation and pore-scale study. The distinct effects of freeze-thaw treatment on colloidal particle number concentration and macropore characteristics may explain the generation and migration of colloid-associated HMs driven by environmental disturbances. This work revealed the underlying mechanisms driving the redistribution of HMs under freeze-thaw cycles, offering valuable insights for risk assessment of soil and groundwater associated with HMs migration.

On-site monitoring and numerical simulation on groundwater flow and pollution plume evolution in a hexavalent-chromium contaminated site

Accurately ascertaining spatiotemporal distribution of pollution plume is critical for evaluating the effectiveness of remediation technologies and environmental risks associated with contaminated sites. This study concentrated on a typical Cr(VI) contaminated smelter being currently remediated using pump-and-treat (PAT) technology. Long-term on-site monitoring data revealed that two highly polluted regions with Cr(VI) concentrations of 162.9 mg/L and 234.5 mg/L existed within the contaminated site, corresponding to previous chromium slag yard and sewage treatment plant, respectively. The PAT technology showed significant removal performance in these highly polluted areas (>160 mg/L) after six months of pumping, ultimately achieving complete removal of the pollutants in these high-pollution areas. Numerical simulation results showed that although the current remediation scheme significantly reduced the Cr(VI) pollution degree, it did not effectively prevent the incursion of the pollution plume into the downstream residential area after 20 years. Additionally, an improved measure involving supplementary pumping wells was proposed, and its remediation effects were quantitatively evaluated. Results indicated that the environmental pollution risk of groundwater downstream could be effectively mitigated by adding pumping wells, resulting in a reduction of the pollution area by 20 % in the case of adding an internal well and 41 % with the addition of external wells after 20 years. The findings obtained in this study will provide an important reference and theoretical guidance for the reliability analysis and design improvement of the PAT remediation project.

Mapping and monitoring dense non-aqueous phase liquid source zone by fused surface and cross-borehole electrical resistivity tomography

Effective characterization of dense non-aqueous phase liquid (DNAPL) source zones is crucial for remediating polluted sites. DNAPL often reside as residuals or pools within high-permeability lenses and above impermeable layers due to soil heterogeneity, gravity, and capillary barriers. Given the high cost of drilling, electrical resistivity tomography (ERT) techniques-including surface ERT and cross-borehole ERT, are commonly used for DNAPL source zone mapping and monitoring. However, the low spatial resolution of ERT increases uncertainty in source zone investigations. This study proposes a method for improving DNAPL mapping and monitoring by fusing surface and cross-borehole ERT data. Sandbox experiments were conducted to simulate a heterogeneous DNAPL source zone, employing both ERT methods for static mapping and dynamic monitoring. Reflective light imaging (RLM) was used to visualize DNAPL migration and provide saturation data, allowing for the quantification of ERT's effectiveness in characterizing DNAPL distribution. The results indicate that individual ERT methods face significant challenges in DNAPL source zone mapping due to background interference. Surface ERT alone tends to underestimate the extent of deeper DNAPL source zones. However, fusing surface and cross-borehole ERT results in a complementary enhancement of vertical spatial resolution, thereby improving the characterization of DNAPL source zones. The fusion of static and time-lapse ERT data substantially enhances DNAPL source zone mapping and monitoring capabilities. By calculating the ratio of the ERT-monitored area to the actual area using resistivity change contours (5 %, 10 %, 15 %), it was found that fusing surface and cross-borehole ERT data improved monitoring resolution by 50.48 % compared to surface ERT alone and by 22.95 % compared to cross-borehole ERT. Principal component analysis (PCA) was effective in fusing time-lapse data, while the weighted average method (WAM) outperformed PCA for static resistivity data fusion.

Pollution characteristics and quantitative source apportionment of heavy metals within a zinc smelting site by GIS-based PMF and APCS-MLR models

The abandoned smelters present a substantial pollution threat to the nearby soil and groundwater. In this study, 63 surface soil samples were collected from a zinc smelter to quantitatively describe the pollution characteristics, ecological risks, and source apportionment of heavy metal(loid)s (HMs). The results revealed that the average contents of Zn, Cd, Pb, As, and Hg were 0.4, 12.2, 3.3, 5.3, and 12.7 times higher than the risk screening values of the construction sites, respectively. Notably, the smelter was accumulated heavily with Cd and Hg, and the contribution of Cd (0.38) and Hg (0.53) to ecological risk was 91.58%. ZZ3 and ZZ7 were the most polluted workshops, accounting for 25.7% and 35.0% of the pollution load and ecological risk, respectively. The influence of soil parent materials on pollution was minor compared to various workshops within the smelter. Combined with PMF, APCS-MLR and GIS analysis, four sources of HMs were identified: P1(25.5%) and A3(18.4%) were atmospheric deposition from the electric defogging workshop and surface runoff from the smelter; P2(32.7%) and A2(20.9%) were surface runoff of As-Pb foul acid; P3(14.5%) and A4(49.8%) were atmospheric deposition from the leach slag drying workshop; P4(27.3%) and A1(10.8%) were the smelting process of zinc products. This paper described the distribution characteristics and specific sources of HMs in different process workshops, providing a new perspective for the precise remediation of the smelter by determining the priority control factors.

Integrated framework to assess soil potentially toxic element contamination through 3D pollution analysis in a typical mining city

Soil potentially toxic elements (PTEs) pollution of contaminated sites has become a global environmental issue. However, given that previous studies mostly focused on pollution assessment in surface soils, the current status and environmental risks of potentially toxic elements in deeper soils remain unclear. The present study aims to cognize distribution characteristics and spatial autocorrelation, pollution levels, and risk assessment in a stereoscopic environment for soil PTEs through 3D visualization techniques. Pollution levels were assessed in an integrated manner by combining the geoaccumulation index (Igeo), the integrated influence index of soil quality (IICQs), and potential ecological hazard index. Results showed that soil environment at the site was seriously threatened by PTEs, and Cu and Cd were ubiquitous and the predominant pollutants in the study area. The stratigraphic models and pollution plume simulation revealed that pollutants show a decreasing trend with the deepening of the soil layer. The ranking of contamination soil volume is as follows: Cu > Cd > Zn > As > Pb > Cr > Ni. According to the IICQs evaluation, this region was subject to multiple PTE contamination, with more than 60% of the area becoming seriously and highly polluted. In addition, the ecological hazard model revealed the existence of substantial ecological hazards in the soils of the site. The integrated potential ecological risk index (RI) indicated that 45.7%, 10.13%, and 4.15% of the stereoscopic areas were in considerable, high, and very high risks, respectively. The findings could be used as a theoretical reference for applying multiple methods to integrate evaluation through 3D visualization analysis in the assessment and remediation of PTE-contaminated soils.

The formation of multi-metal(loid)s contaminated groundwater at smelting site: Critical role of natural colloids

The occurrence and migration of colloids at smelting sites are crucial for the formation of multi-metal(loid)s pollution in groundwater. In this study, the behavior of natural colloids (1 nm-0.45 µm) at an abandoned smelting site was investigated by analyzing groundwater samples filtered through progressively decreasing pore sizes. Smelting activities in this site had negatively impacted the groundwater quality, leading to elevated concentrations of zinc (Zn), lead (Pb), arsenic (As), and cadmium (Cd). The results showed that heavy metal(loid)-bearing colloids were ubiquitous in the groundwater with the larger colloidal fractions (∼75 -450 nm) containing higher abundances of pollutants. It was also observed that the predominant colloids consisted of Zn-Al layered double hydroxide (LDH), sphalerite, kaolinite, and hematite. By employing multiple analytical techniques, including leaching experiments, soil colloid characterization, and Pb stable isotope measurements, the origin of groundwater colloids was successfully traced to the topsoil colloids. Most notably, our findings highlighted the increased risk of heavy metal(loid)s migration from polluted soils into adjacent sites through the groundwater because of colloid-mediated transport of contaminants. This field-scale investigation provides valuable insights into the geochemical processes governing heavy metal(loid) behavior as well as offering pollution remediation strategies specifically tailored for contaminated groundwater.

Sb/As immobilization and soil function improvement under the combined remediation strategy of modified biochar and Sb-oxidizing bacteria at a smelting site

Antimony (Sb) and arsenic (As) lead to soil pollution and structural degradation at Sb smelting sites. However, most sites focus solely on Sb/As immobilization, neglecting the restoration of soil functionality. Here, we investigated the effectiveness of Fe/H2O2 modified biochar (Fe@H2O2-BC) and Sb-oxidizing bacteria (Bacillus sp. S3) in immobilizing Sb/As and enhancing soil functional resilience at an Sb smelting site. Over a twelve-month period, the leaching toxicity of As and Sb was reduced to 0.05 and 0.005 mg L-1 (GB3838-2002) respectively, with 1% (w/w) Fe@H2O2-BC and 2% (v/v) Bacillus sp. S3 solution. Compared to CK, the combination of Fe@H2O2-BC and Bacillus sp. S3 significantly reduced the bioavailable As/Sb by 98.00%/93.52%, whilst increasing residual As and reducible Sb fractions by 210.31% and 96.51%, respectively. The combined application generally improved soil aggregate structure, pore characteristics, and water-holding capacity. Fe@H2O2-BC served as a pH buffer and long-term reservoir of organic carbon, changing the availability of carbon substrates to bacteria. The inoculation of Bacillus sp. S3 facilitated the transformation of Sb(III)/As(III) to Sb(V)/As(V) and differentiated the composition and functional roles of bacterial communities in soils. The combination increased the abundance of soil saprotrophs by 164.20%, whilst improving the relative abundance of N- and S-cycling bacteria according to FUNGuild and FAPROTAX analysis. These results revealed that the integrated application was instrumental in As/Sb detoxification/immobilization and soil function restoration, which demonstrating a promising microbially-driven ecological restoration strategy at Sb smelting sites.