External radiation exposure of turtles and soil biota in Ramsar high natural background radiation area: experimental dosimetry supported by Monte Carlo modeling

Abstract High natural background radiation areas (HNBRAs) can impose elevated external exposure on non-human biota, yet organism-relevant dose characterization remains limited for many extreme environments. Here, we quantify external radiation exposure in Ramsar’s Talesh Mahalleh (Iran) using controlled laboratory measurements supported by Monte Carlo (MC) modeling to interpret finite-geometry effects and soil-to-organism dose transfer. Depth-resolved dosimetry was performed in homogenized Ramsar soil (2–10 cm, 2 cm spacing) over 2 months using three thermoluminescent dosemeters (TLD-100, GR-200, and TLD-400). Cumulative doses in this biologically relevant layer ranged from 4.18 to 5.47 mGy, with no pronounced depth-dependent gradient. TLD-100 showed the most stable response across layers (mean 5.37 ± 0.08 mGy), while GR-200 and TLD-400 exhibited higher variability. In parallel, external turtle dosimetry using TLD-100 and GR-200 revealed a consistent ventral–dorsal asymmetry: plastron doses (6.75–6.83 mGy for TLD-100; 6.16–6.22 mGy for GR-200) exceeded carapace doses (2.15–2.79 mGy for TLD-100; 2.01–2.79 mGy for GR-200), yielding a measured plastron-to-carapace ratio of 3.18 (TLD-100). MC simulations reproduced this asymmetry (ratio 3.44) and indicated that boundary effects reduce wall-adjacent doses, whereas central positions remain within ~8%–12% of extended-soil conditions, supporting the robustness of relative dose ratios. Overall, the combined experimental and modeling results demonstrate that external exposure at the soil–air interface is governed primarily by geometric proximity to contaminated soil, and that soil-to-organism dose ratios provide a transferable basis for ecological dosimetry in HNBRAs.