Molecular Response of Dissolved Organic Matter in Aquifer to Landfill Leachate Leakage

This study investigated changes in groundwater dissolved organic matter (DOM) composition resulting from leachate leakage and examined the causes of these changes by simulating the contamination process through soil column and adsorption experiments. The results showed that contaminated groundwater had higher humification degree, chemical diversity, and unsaturation of DOM compared to uncontaminated groundwater. Five fractions were identified in landfill leachate, with fulvic-like acid and building blocks of humic substance fractions (0.45–2.5 kDa) being poorly retained by soil upon leakage. In contrast, high-molecular humic-like acid and biopolymer fractions (>2.5 kDa) as well as low-molecular organic fractions (<0.45 kDa) were readily adsorbed in soil, leading to the appearance and intensification of characteristic fluorophores at excitation/emission wavelengths of 250, 310, 365 nm/460 nm in contaminated groundwater. The characteristic fluorophore representative molecules comprised highly unsaturated structures with low- and high-oxygen content (79.6%) with high oxidizability, aromaticity, and unsaturation. To the best of our knowledge, this was the first report of the medium-molecular-weight organics in leachate preferentially entering groundwater rather than the low-molecular weight organics. This study facilitates to deepen the understanding of groundwater contamination by leachate leakage and provides theoretical support for early warning of leachate leakage contamination.