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

Abstract

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.