The Raetrad Model of Radon Generation and Transport from Soils Into Slab-on-grade Houses

Abstract

Remediation planning and ²²²Rn-related construction zoning require knowledge of how close and strong ²²⁶Rn sources can be in different foundation soils under different groundwater conditions without excessively elevating indoor ²²²Rn levels. A two-dimensional numerical-analytical model was developed to simulate (a) zzzRn emanation, decay, and movement by diffusion and advection in soils around houses and in their understructures; and (b) ²²²Rn accumulation in a single-zone house. The model represents foundation soils and a house in elliptical-cylindrical geometry. ²²²Rn may diffuse through its floor slab or may enter via idealized cracks and openings. The model was validated with analytical calculations of two-dimensional air pressure fields and with one-dimensional calculations of ²²²Rn generation with diffusion and diffusion combined with advection. Agreement generally was within < 1% when finite-difference approximations were minimized. Benchmark comparisons with indoor ²²²Rn measurements in two test-cell structures under passive and depressurized conditions averaged within 11% of measured values, well within measurement uncertainty. The corresponding average bias was only 3%. Larger variations were observed when applying the model to 50 houses. In this application, a negative bias of nearly 50% was observed due to data gaps and to poorly-characterized floor slabs and crack distributions.