Divergent evolution of trace elements in groundwater-dependent lakes: Insights from multi-isotopic tracers, DOM molecular characteristics, and microbial evidence.

Groundwater-dependent lakes are highly vulnerable to trace element (TE) accumulation, yet the mechanisms controlling TE occurrence in coupled lake-groundwater systems remain poorly understood. Here, we investigated the occurrence, transport, and controlling mechanisms of TEs in a representative groundwater-dependent lake system. Concentrations are reported as mean ± standard deviation (SD; LW, n = 6; GW, n = 44). Lake water (LW) and groundwater (GW) exhibited distinct TE occurrence patterns, with significantly higher concentrations of V (89.50 ± 69.00 μg L⁻¹), Mo (32.40 ± 24.30 μg L⁻¹), and U (25.60 ± 20.60 μg L⁻¹) in LW, whereas Sc (3.78 ± 0.93 μg L⁻¹), Zn (11.95 ± 38.54 μg L⁻¹), and Ba (193.04 ± 94.58 μg L⁻¹) were significantly higher in GW (FDR-adjusted p < 0.05). Hydrochemical and isotopic evidence (δ¹¹B and ⁸⁷Sr/⁸⁶Sr) suggested that water-rock interactions and mineral dissolution contributed to lithogenic TE enrichment in GW, whereas alkaline and saline conditions (TDS = 7798 mg L⁻¹; pH = 9.4) were associated with elevated TE concentrations in LW. Furthermore, lignin-rich dissolved organic matter (DOM) and fungal communities were closely associated with lithogenic TE patterns in GW, whereas the more compositionally diverse DOM pool and bacterial communities showed distinct associations with TEs in LW, suggesting contrasting biogeochemical linkages between the two water bodies. The radon-222 (222Rn) mass balance model estimated a mean lacustrine groundwater discharge rate of 22.2 mm d⁻¹ , demonstrating substantial groundwater inputs to the lakes and highlighting groundwater as an important hydrological pathway for the transport of dissolved TEs. These findings provide a mechanistic understanding of TE evolution in groundwater-dependent lakes and improve predictions of TE accumulation and ecological risks under arid conditions.