Global perspectives on quartet radium isotopes in lakes

Lake-groundwater systems sustain critical ecosystems and socioeconomic activities, yet global radium (Ra) distribution patterns, radiation risks and quantified potential remain understudied, particularly regarding Ra's utility in tracing lake salinization timescales. This study characterizes the ionic composition and Ra dynamics of ∼50 global lake-groundwater systems using an integrated framework of comprehensive data retrieval and geochemical analysis. Saline lakes (∼46 %, SO₄·Cl-Na-type) dominate arid endorheic basins, contrasting with freshwater systems (similar proportion, Ca·Mg-HCO₃-type) in humid catchments. Hydraulic connectivity, rock weathering, and evaporite dissolution drive the solute evolution in lake-groundwater systems. Elevated Ra activities (∼29 % of samples, N = 798) exceed US drinking standards in hypersaline lakes and freshwater systems, yielding annual radiation doses of 1.76-1.81 mSv (18 ×World Health Organization-WHO limits; ²²⁶Ra >80 %). Freshwater Ra anomalies necessitate urgent monitoring due to direct exposure risks. We provided Ra transport models demonstrating system-specific controls between saline lakes and freshwater lakes, involving adsorption-desorption, decay-recoil, and weathering-co-precipitation. LGD (Lacustrine groundwater discharge) and saline lake water intrusion into local aquifers are equally important. Modified ²²⁶Ra-Ba-Cl mass balance model, incorporating solid-liquid distribution coefficients, quantifies precipitation amounts to determine salinization timescales in arid lakes, yielding 15.4-17.3 ky for Lake Urmia (Iran) and Daqaidam Lake (China), corroborated by independent chronometers. Disparate salinity accumulation rates highlight evolving resource potentials. This work establishes Ra isotopes as tracers for global radiation risk mapping and aquifer-lake dynamics, while providing a scalable framework to manage brine resources and assess hydrological resilience under anthropogenic forcing.