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.