Source-dependent of dissolved organic matter interfacial transport and component fractionation retention in NAPL-contaminated aquifers.

Contaminated aquifers can alter the geochemical behavior and fate of dissolved organic matter (DOM) in groundwater systems. To date, research has almost entirely focused on the contaminants themselves, while the mechanisms by which DOM sources regulate their interfacial retention and transport within contaminated source areas remain unclear. This study uses field data from a coking site to confirm that the non-aqueous phase liquid (NAPL) zone is a hotspot for DOM accumulation in contaminated subsurface environments. Through integrated column and microfluidic experiments, we found that DOM is selectively and persistently retained at the aromatic NAPL (dioctyl phthalate, DOP) interface through directional hydrogen bonding and π-π stacking, while its binding to aliphatic NAPL (n-hexadecane, Hex) is primarily controlled by nonspecific van der Waals forces. When the residual NAPL saturation reaches approximately 0.3, the DOM transport mechanism shifts from being migration- to retention-dominated. Consistent with experimental observations, quantum chemical calculations show that DOM binds more strongly (-33.09 kcal/mol) to aromatic NAPL (DOP) with greater directionality, and is primarily driven by polar interactions (contributing 71.61% of the energy). While binding to aliphatic NAPL (Hex) is mainly achieved through non-specific van der Waals forces (-26.4 kcal/mol). Furthermore, the accumulation of DOM reduces interfacial tension, which may lead to the migration of residual NAPL, thereby exacerbating the long-term risk of groundwater contamination. This work establishes a multi-scale framework linking molecular interactions with macroscopic transport. The findings provide a new perspective for advancing pollution plume prediction and carbon accounting from the viewpoint of environmental structural chemistry.