Investigating radon (222Rn) distribution in groundwater of a tropical river basin, India: Development of a sensitivity potential index framework

• 222 Rn in groundwater exhibits strong post-monsoon amplification linked to recharge-driven fracture activation. • Enhanced radon mobilization occurs in zones marked by deep weathering and fracture-controlled groundwater flow. • A novel Radon Sensitivity Potential Index (RSPI) developed to quantitatively integrate geological, structural, and seasonal drivers of 222 Rn. • RSPI provides a robust and transferable framework for assessing radon sensitivity in across diverse geological settings. The study examines the seasonal variability of radon ( 222 Rn) with reference to the geology and hydrogeology of the Ithikkara River Basin (IRB) in Kerala, India. Groundwater samples ( n = 35) were collected during the monsoon and post-monsoon seasons of 2023. The RAD7, RAD-H 2 O accessory (Durridge Co. USA) was used to analyze 222 Rn. No correlation was observed between radon and physico-chemical parameters. 222 Rn activities ranged between 296 and 21,100 Bq/m 3 (monsoon) and 720–69,200 Bq/m 3 (post-monsoon), with up to 14.3% of post-monsoon samples exceeding the USEPA limit. The findings demonstrate that 222 Rn distribution is strongly modulated by lithological heterogeneity, fracture networks, and seasonal controls. High radon activities were observed in midland zones underlain by migmatite and khondalite lithologies with deep weathering and proximity to lineament zones. Conversely, highland aquifers with steeper slopes exhibited limited radon mobilization due to lower recharge and faster flow paths. Importantly, a novel Radon Sensitivity Potential Index (RSPI) was developed to assess sensitivity of radon in groundwater to geological and seasonal influences. The RSPI provided a clear framework to distinguish fracture-dominated flow regimes from matrix-controlled systems, offering valuable insights into recharge processes and structural controls in hard rock aquifers. This study highlights the potential of integrating 222 Rn analysis with RSPI as an effective approach for identifying hydrogeologically active zones and enhancing groundwater resource management in structurally complex terrains.