Tracing the groundwater influx into the Yamuna River using radon (²²²Rn)

Understanding the surface water-groundwater exchange is essential for the sustainable water resource management, contaminant transport assessment, and overall health of the river ecosystem. Radon (²²²Rn) is a widely used natural tracer for identifying the groundwater discharge to rivers because of its conservative behavior, short half-life (3.82 days) and activity concentration relatively much higher in groundwater than surface water. Despite the hydrological importance of the Yamuna River, the groundwater exchange along its middle and lower reaches remains poorly understood. This study presents the first systematic ²²²Rn based assessment of groundwater influx into the Yamuna River along a ∼250 km reach from Panipat (upstream) to Agra (downstream). Radon activity was measured at 9 locations within the river and nearby groundwater wells, while Electrical Conductivity (EC) was recorded as a complementary hydrochemical tracer. Groundwater influent zones were identified based on the elevated riverine ²²²Rn activity. ²²²Rn activity concentration of the river ranged from 0.069 to 6.295 Bq/L (mean: 1.010 Bq/L), while, the groundwater showed consistently higher values (mean: 4.268 Bq/L). Two groundwater influx zones were identified. The primary zone occurred at Mohana, Mathura where river ²²²Rn reached 6.295 Bq/L and adjacent groundwater wells recorded 7.207-7.549 Bq/L. A secondary influx zone was detected at Wazirabad, Delhi, with river ²²²Rn of 1.387 Bq/L. The remaining river sampling stations exhibited near background activities, indicating limited groundwater contribution. The results demonstrate that the groundwater discharge to the Yamuna River is spatially discrete rather than diffuse, and they are concentrated at two locations associated with the presence of geological structure such as faults and lineaments. The identification of these preferential groundwater influx zones has direct implications for contaminant load estimation, sustainable groundwater abstraction policy, and river flow modelling in the Yamuna basin.