Radon anomalies environmental radiation transfer and radiological risk assessment in a fault controlled Lake system

This study investigates the coupling between radon (222Rn) anomalies, seismic activity, hydrogeochemical transport, and radiological risk in Lake Hazar, an active fault controlled lake located along the East Anatolian Fault Zone (EAFZ), Türkiye. Soil gas and dissolved 222Rn concentrations were periodically monitored on a monthly basis from September 2024 to August 2025 using a DURRIDGE RAD7 detector coupled with RAD H2O accessories and the results were evaluated with regional seismicity within a 50 km radius. Natural radionuclide activities (40K, 238U, and 232Th) were determined using 2″ × 2″ NaI(Tl) well-type gamma spectrometry detector. 222Rn anomalies were assessed using μ + σ, μ + 2σ thresholds, the Standardized Radon Index (SRI), and the Dobrovolsky preparation radius model. Soil 222Rn ranged from 87.6 to 541.8 Bq/m3, with the highest value (SRI = 2.81) recorded in September 2024, approximately one month before the Mw 6.0 earthquake in October 2024. Although this temporal coincidence is consistent with previous observations reported for tectonically active regions, the present dataset does not allow a definitive causal interpretation. Dissolved 222Rn varied from 34.1 to 69.8 Bq/L and showed delayed but detectable responses during periods of Mw ≥ 5 seismic activity. Radiological indices calculated for fish tissues remained far below international safety thresholds: Hin and Hex were < 1, Raeq values were substantially lower than the recommended limit of 370 Bq/kg, and absorbed dose rates were below the global average value of 59 nGy/h reported by UNSCEAR. In addition, annual effective dose values from fish muscle consumption remained well below the public exposure limit of 1 mSv/year recommended by ICRP, and ELCR values were within the low risk range commonly used in environmental radiological assessments. These findings indicate that active fault controlled lake systems may provide integrated environmental settings where tectonic stress, 222Rn migration, aquatic radionuclide transfer, and radiological exposure processes interact. Novelty statement: This study presents an integrated environmental investigation of soil gas and dissolved water 222Rn variability, radionuclide transfer in aquatic biota, and radiological risk assessment within the tectonically active Hazar Lake system located on the East Anatolian Fault Zone (EAFZ), Türkiye. The novelty of the present study lies in the following aspects: • Simultaneous evaluation of soil and dissolved water 222Rn anomalies together with seismic activity in a fault controlled lake ecosystem. • Application of the Standardized Radon Index (SRI) approach for normalized temporal interpretation of radon anomaly behavior in both soil gas and aqueous environments. • Integration of tectonic hydrogeochemistry with environmental radioactivity and aquatic radionuclide transfer assessment. • Combined investigation of radon anomaly behavior and radionuclide accumulation in fish tissues within the same tectonically active environmental system. • Inclusion of radiological risk indices (Hin, Hex, Raeq, Iγ, AGED) together with internationally accepted reference values for environmental and public health interpretation. • Development of a multidisciplinary environmental monitoring framework linking lithospheric, hydrospheric, and biospheric processes in active fault controlled lake environments. To the best of our knowledge, this is among the limited studies combining 222Rn anomaly monitoring, aquatic radionuclide transfer, and radiological risk assessment within an integrated tectonic lake ecosystem framework.