Disentangling spatial and seasonal controls on heavy metal pollution in Mediterranean rivers through fluorescence DOM

Mediterranean river basins are subject to heavy metal pollution driven by legacy mining, diffuse anthropogenic pressures, and strong hydrological seasonality. Dissolved organic matter (DOM), particularly its fluorescent fraction (fDOM), plays a key role in controlling metal mobility and ecological risk, yet its basin-scale influence remains poorly constrained. This study investigates spatial and seasonal controls on trace metal pollution across the Ombrone River Basin (central Italy), a representative Mediterranean catchment impacted by historical mercury mining. Monthly sampling along four rivers during wet and dry seasons combined excitation–emission matrix fluorescence spectroscopy and PARAFAC modelling with inductively coupled plasma–mass spectrometry. By combining chemometric analyses, six fDOM components, including humic-like and protein-like fractions, were evaluated alongside fifteen trace metals. A heavy metal pollution index (HPI) revealed frequent exceedances of quality thresholds, largely driven by mercury, highlighting persistent legacy contamination. Multivariate analyses showed that humic-like fDOM strongly mediated the distribution of Fe, Cu, Ni, and Se, favouring metal persistence in the dissolved phase, whereas metalloids such as Sb exhibited negative associations with humic components, indicating transport pathways dominated by mineral phases and colloids. Protein-like fDOM was linked to more transient, seasonally driven metal signals in hydrologically responsive river reaches. Overall, integrating optical fDOM properties with chemometric approaches provides a transferable framework for disentangling geogenic and anthropogenic controls on metal pollution in Mediterranean river systems. Highlights : • Basin-scale monitoring revealed persistent mercury-driven metal contamination. • Humic-like fDOM mediated dissolved Fe, Cu, Ni and Se under variable hydrology. • Sb and As showed contrasting behaviour linked to mineral and colloidal transport. • Protein-like fDOM tracked transient, hydrologically driven metal signals. • Complementary chemometric approaches improved pollution tracking.