Age and hydrogeological conditions of landfill site control groundwater pollution in landfill site

Landfilling has historically been the primary method for disposing municipal solid waste. As impermeable liners age, leachate poses a long-term and insidious threat to surrounding groundwater quality. Hence, accurately identifying its pollution characteristics and migration processes is crucial for groundwater protection. Accordingly, this study analyzed 129 typical landfills within the region, examining the chemical evolution characteristics of leachate and groundwater during the wet season, pollutant sources, and influencing mechanisms. The results indicated that the primary pollutants in the leachate and groundwater within the study area were high concentrations of Cl^-, NH₄⁺-N, heavy metals, and recalcitrant organic compounds, with their composition exhibiting systematic changes according to landfill age. Younger landfills (<5 years) exhibited predominant heavy metal (Fe, Mn) contamination, NH₄⁺-N was the primary pollutant in middle-aged landfills (5-10 years), and older landfills (>10 years) demonstrated overall reduced pollutant concentrations but increased proportions of refractory organic compounds. The migration and distribution of pollutants in groundwater were significantly influenced by the hydrogeological conditions. The rate of groundwater pollutants exceeding standards in mountainous landfill sites (26.8 %) was significantly lower than that in plain areas (85.8 %), Further, the rate of these exceedances at landfills exhibited a highly significant negative correlation with burial depth (r = -0.98, p < 0.01). Shallow groundwater (<10 m) served as the primary accumulation zone for pollutants. Areas with a vadose zone, comprising silt-clay layers, showed a significantly lower exceedance rate (approximately 38.4 %) compared to regions with gravel layers (approximately 81.6 %), Porewater aquifers exhibited the most widespread contamination, with over 70 % of the samples exceeding regulatory standards.