Radon exhalation and migration mechanisms across stratigraphic units: a case study from Inner Mongolia

Radon (²²²Rn) is a useful tracer for concealed fractures and subsurface heterogeneity. Using LTNA pore characterization and sealed-chamber exhalation tests on drill cores from a coal-mining area in Inner Mongolia, we observe a pronounced vertical zonation in pore volume (low in shallow clay-rich strata, higher in the middle transition zone, and reduced at depth) with stepwise shifts across lithologic boundaries. Radon exhalation exhibits an increase-then-decrease trend with depth, rising from 1.1 to 4.5 Bq·kg^-1·h^-1 to a peak of ∼7.6 Bq·kg^-1·h^-1 and then declining to ∼3.1-5.1 Bq·kg^-1·h^-1 in deeper mudstone. Exhalation correlates strongly with total pore volume (R² ≈ 0.95) and mesopore volume (2-50 nm; R² ≈ 0.98), suggesting that mesopores primarily reflect matrix-accessible pore space and diffusion supply at the core scale. Moisture-conditioning tests reveal an inverted-U response, confirming that water content modulates radon release by pore occupancy and gas-phase diffusion. These results support the use of radon anomalies to delineate high-permeability pathways and potential concealed fractures in coal-bearing strata.