Origin of Dissolved Organic Matter in Deep Groundwater of Marine Deposits and Its Implication for Metal Binding

Dissolved organic matter (DOM) plays important roles in the fate of contaminants and nutrients in nature. Nevertheless, our understanding of DOM in deep underground water is limited. This study tackled this issue by intensive groundwater sampling at various depths of an underground research laboratory. The origin and the binding properties of different fluorescent DOM components against Eu3+ were studied by fluorescence emission-excitation matrices (EEMs) and parallel factor analysis (PARAFAC). Four components with distinctive fluorescent properties were obtained by PARAFAC: one marine humic-like component, two terrestrial humic-like components, and one protein-like component. It was revealed that Eu3+ strongly bonded to the terrestrial humic-like components but less so to the marine humic-like component. Partial least-squares (PLS) regression further revealed the origin and distributions of the components. It was suggested that microbial decomposition of the DOM components, input of the marine humic-like component from sedimentary rocks, and mixing of fossil meteoric and sea waters determined their spatial distributions and affinities to Eu3+. These results indicated the dynamic nature of DOM in deep groundwater of marine deposits and provided important insight into evaluating their impact on the migration of radionuclides and other contaminants in deep underground environments.