Spatial Distribution Characteristics of Typical Contaminants in Municipal Solid Waste Landfills under Water-Soil Interactions in Alluvial-Diluvial Strata
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.