Geochemical dynamics and colloidal transfer of major and trace elements in the Hers-Mort − Girou watershed: distinction between natural and anthropogenic signatures evidenced by REE anomalies

Abstract

Small watersheds are highly sensitive to anthropization, which deeply alters elemental biogeochemical cycles. This study focuses on the Hers-Mort and Girou watersheds near Toulouse (SW France), originating in the rural Lauragais region. A sampling campaign in January 2025 combined laboratory ultrafiltration (0.22 μm, 0.025 μm, 3 kDa) with measurements of physicochemical parameters (conductivity, pH, alkalinity) and elemental concentrations (DOC, anions, major cations, and trace elements).

Results reveal a clear distinction between natural and anthropogenic signals. Linear relationships between carbonates, nitrates, and Ca–Mg highlight the dominant role of lithology, with carbonate dissolution naturally supplying major cations, further enhanced by nitrates. Ultrafiltration showed that most elements occur in the labile low-molecular-weight <3 kDa (LMW <3 kDa) fraction, indicating high potential bioavailability and little downstream variation in DOC, Fe, Mn, Cu, and As.

Conversely, Zn and Gd displayed strong co-variation in their colloidal patterns, with the <3 kDa fraction increasing downstream in parallel with urbanization, reaching up to 80 % in the most impacted areas. These elements thus represent robust tracers of anthropogenic influence. Rare Earth Element (REE) patterns in headwaters were controlled by molassic lithology, whereas downstream sections showed pronounced positive Gd anomalies (0.89–25) and systematic LREE depletion with HREE enrichment. Anthropogenic Gd accounted for 10–90 % of total Gd, evidencing clear artificial inputs and the progressive shift from natural to anthropogenic geochemical dynamics.