THE RAETRAD MODEL OF RADON GENERATION AND TRANSPORT FROM SOILS INTO SLAB-ON-GRADE HOUSES

Remediation planning and Rn-222-related construction zoning require kn owledge of how close and strong Ra-226 sources can be in different fou ndation soils under different groundwater conditions without excessive ly elevating indoor Rn-222 levels. A two-dimensional numerical-analyti cal model was developed to simulate (a) Rn-222 emanation, decay, and m ovement by diffusion and advection in soils around houses and in their understructures; and (b) Rn-222 accumulation in a single-zone house. The model represents foundation soils and a house in elliptical-cylind rical geometry. Rn-222 may diffuse through its floor slab or may enter via idealized cracks and openings. The model was validated with analy tical calculations of two-dimensional air pressure fields and with one -dimensional calculations of Rn-222 generation with diffusion and diff usion combined with advection. Agreement generally was within <1% when finite-difference approximations were minimized. Benchmark comparison s with indoor Rn-222 measurements in two test-cell structures under pa ssive and depressurized conditions averaged within 11% of measured val ues, well within measurement uncertainty. The corresponding average bi as was only 3%. Larger variations were observed when applying the mode l to 50 houses. In this application, a negative bias of nearly 50% was observed due to data gaps and to poorly-characterized door slabs and crack distributions.