Key Takeaways & Executive Findings
- •• • Groundwater at the landfill site exhibited severe contamination (PI > 3), with maximum exceedance multiples of 36 for total bacterial count, 9.5 for ammonia nitrogen, and 8 for total coliforms, indicating a direct leachate impact and necessitating urgent remediation. • • Soil concentrations of Cu, Pb, Cd, Ni, Hg, and As were all below GB 36600—2018 Class II screening values, with Nemerow pollution index (PI < 0.7) and potential ecological risk index (RI < 150) confirming overall soil safety, yet spatial heterogeneity (coefficient of variation > 36%) suggests localized hotspots. • • Soil DOM was dominated by humic-like (22.9%–34.9%) and fulvic-like (22.4%–27.5%) substances, with fluorescence intensity decaying exponentially with depth, indicating vertical transport and transformation of organic matter. • • Cadmium exhibited high mobility risk: active forms (exchangeable + Fe/Mn oxide-bound) accounted for 72.6%–86.7%, whereas Cu, Pb, As, Hg, and Ni were predominantly in residual fractions (52.33%–90.32%), highlighting Cd as a priority control target.
Abstract
The Holocene alluvial-diluvial stratum of the Quaternary is characterized by high soil hydraulic conductivity and intense surface water-groundwater interaction, which leads to rapid and extensive migration of contaminants from landfills. To investigate the contaminant characteristics of a municipal solid waste (MSW) landfill in such strata, a case study was conducted at a landfill in southwestern China. Methods including the Nemerow pollution index and the potential ecological risk index were employed to systematically analyze the contamination of groundwater and soil, as well as the spatial distribution of organic matter and heavy metals. The results showed that the groundwater was severely contaminated (PI > 3). The maximum exceedance multiples for total bacterial count, ammonia nitrogen (NH4+-N), and total coliforms relative to the standard limits were 36, 9.5, and 8, respectively. The composition of the contaminants in groundwater was highly consistent with the characteristics of landfill leachate. For the soil, the concentrations of six heavy metals (Cu, Pb, Cd, Ni, Hg, and As) were all below the Class II screening values of the standard GB 36600—2018. Both the Nemerow pollution index (PI < 0.7) and the potential ecological risk index (RI < 150) indicated that the soil environment was safe. Regarding soil dissolved organic matter (DOM), humic-like substances (22.9% to 34.9%) and fulvic-like substances (22.4% to 27.5%) were the dominant components, and their fluorescence intensities exhibited an exponential decay trend with increasing soil depth. The speciation of Cu, Pb, As, Hg, and Ni was dominated by the residual fraction (52.33% to 90.32%). However, over 70% of Cd existed in active forms (exchangeable + Fe/Mn oxide-bound), suggesting a high migration risk. The horizontal distribution of heavy metals showed regional specificity, with high-value areas mainly concentrated in the screening waste and soil stacking areas. Vertically, Cu and Cd exhibited surface enrichment, while As, Hg, Pb, and Ni were enriched in the groundwater fluctuation zone. These findings indicate that groundwater in alluvial-diluvial strata is highly susceptible to leachate contamination, while soil heavy metal contamination is not significant, with low levels in the aquifer but a tendency to accumulate at the water-soil interface. It is recommended that during landfill remediation, attention be paid to anti-seepage measures in waste excavation and stacking areas, as well as the interception and remediation of the groundwater fluctuation zone, to prevent secondary contamination of soil and groundwater.
1. Introduction
The Quaternary alluvial-diluvial strata, characterized by high hydraulic conductivity and strong surface water-groundwater interaction, pose a significant challenge for landfill siting and management. In such hydrogeological settings, contaminants from municipal solid waste (MSW) landfills can migrate rapidly and over extensive areas, threatening groundwater quality and ecological safety. Existing commercial remediation approaches often fail to account for the dynamic water-soil interactions that govern contaminant fate, leading to ineffective containment and prolonged environmental liability.
This study addresses this bottleneck by systematically characterizing the spatial distribution of typical contaminants—including heavy metals and dissolved organic matter—in both groundwater and soil at a landfill in southwestern China. By integrating Nemerow pollution index, potential ecological risk index, and speciation analysis, the research delineates contamination hotspots and vertical enrichment patterns, particularly within the groundwater fluctuation zone. The findings provide critical data for designing targeted remediation strategies that consider the unique hydrogeological conditions of alluvial-diluvial strata, thereby improving the efficacy and cost-efficiency of landfill aftercare.
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LI Weiqiang, ZHAO Ziliang, ZHU Hao, XU Yunsong, HAN Zhiyong (2026). Spatial Distribution Characteristics of Typical Contaminants in Municipal Solid Waste Landfills under Water-Soil Interactions in Alluvial-Diluvial Strata. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607025
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Frequently Asked Questions
What are the primary contamination pathways and key pollutants in groundwater at landfills in alluvial-diluvial strata?
The primary pathway is leachate infiltration through highly permeable soils, leading to severe groundwater contamination. Key pollutants include ammonia nitrogen (NH4+-N), total coliforms, and total bacterial count, with maximum exceedance multiples of 9.5, 8, and 36, respectively. The contaminant composition closely matches leachate characteristics, indicating direct leachate impact.
How does the vertical distribution of heavy metals vary, and what are the implications for remediation?
Cu and Cd show surface enrichment, while As, Hg, Pb, and Ni are enriched in the groundwater fluctuation zone. This differential behavior suggests that remediation efforts must target both the surface soil and the fluctuation zone to effectively reduce contaminant mobility and bioavailability.
What is the environmental risk posed by cadmium (Cd) in the soil, and why is it a priority?
Cd exists predominantly in active forms (exchangeable + Fe/Mn oxide-bound), accounting for 72.6%–86.7% of its total concentration. This high proportion of mobile and bioavailable Cd poses a significant risk of migration to groundwater and uptake by biota, necessitating focused remediation measures.
How does the composition of dissolved organic matter (DOM) change with depth, and what does it indicate?
DOM is dominated by humic-like (22.9%–34.9%) and fulvic-like (22.4%–27.5%) substances, with fluorescence intensity decreasing exponentially with depth. This indicates active vertical transport and transformation of organic matter, which can influence heavy metal mobility through complexation and redox reactions.
What are the limitations of this study, and how should future research address them?
The vertical analysis of heavy metal speciation and DOM was based on a single sampling point (S0), which may not capture the full spatial variability across the landfill. Future studies should incorporate multiple sampling points to validate the consistency of spatial distribution patterns and improve the robustness of risk assessments.
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