Rn-222 gas-phase release from shallow Ra-226 sources under representative Taiwan climate conditions: A HYDRUS-1D study.

Naturally occurring Ra-226 is routinely detected in scrap metal entering Taiwan's steel recycling industry; of the 69 radiation anomaly cases reported during 2022-2023, 38 involved Ra-226, motivating the contamination case examined here. However, the potential for Ra-226 to migrate into the underlying vadose zone through rainfall infiltration during interim outdoor storage, and the consequent Rn-222 gas-phase release has not been systematically evaluated. This study employs numerical simulation with HYDRUS-1D to model Rn-222 transport through unsaturated soil columns under an idealized, hypothetical case, using site-realistic climate forcing derived from five-year (2021-2025) meteorological records at three stations spanning northern, central, and southern Taiwan. Twenty-six cases were constructed, encompassing three co-located soil-climate pairings (clay loam, loam, sandy loam), a source-depth series, sustained-rainfall and drought perturbations, a water-table series, and a soil-climate factorial that separates the soil and climate contributions. Because the site-specific Ra-226 inventory is unknown, fluxes are reported per unit source term (in cm-2 day-1 for an unspecified solute-mass unit), and comparisons across cases are interpreted on a relative basis. Results show that soil texture is the dominant control: sandy loam produces roughly twice the annual mean surface flux of clay loam with minimal seasonal variation, while clay loam exhibits nearly sixfold seasonal modulation. Adding 10 cm of overburden reduces flux by approximately 37% in clay loam but only marginally in sandy loam. A pronounced climate asymmetry emerges: drought increases summer flux substantially, whereas the sustained-rainfall-induced suppression, though substantial in fine-textured soil, is smaller than the drought enhancement. Across the fifteen baseline and perturbation cases, surface flux spans less than half an order of magnitude, governed primarily by the inherent soil-climate combination at the source location rather than engineering measures or extreme weather.