Simulation Of The Steady-state Transport Of Radon From Soil Into Houses With Basements Under Constant Negative Pressure.

Normal conditions in a house can produce negative pressures as high as 20 Pa relative to the outside. This underpressure can induce a flow of soil gas into the house, through cracks or any other openings in the understructure of the building. Radon (Rn-222), which is produced in the soil and mixed in the soil gas, can then be transported into the house through a complex combination of molecular diffusion and forced convection. A theoretical model was developed to simulate this phenomenon, under some specific assumptions. The model simulates: (1) the generation and decay of radon within the soil; (2) its transport throughout the soil due to diffusion and convection induced by the pressure disturbance applied at a crack in the basement; (3) its entrance into the house through the crack; and (4) the resultant indoor radon concentration. The most important assumptions were: (1) a steady-state condition; (2) a house with a basement; (3) a geometrically well-defined crack at the wall-floor joint in the basement; and (4) a constant negative pressure applied at the crack in relation to the outside atmospheric pressure. Two three-dimensional finite-difference computer programs were written to solve the mathematical equations of the model. The first program was used to calculate: (1) the pressure distribution within the soil as a result of the applied disturbance pressure at the crack; and (2) the resultant velocity distribution of the soil gas throughout the soil matrix. The second program was used to: (1) solve the radon mass-transport equation, and to calculate the concentration distribution of radon in the soil gas within the whole soil; and (2) to calculate the entry rate of radon through the crack into the basement, and the final indoor radon concentration. A parametric sensitivity analysis performed on the model, revealed several features of the mechanisms involved in the transport of radon into the house. Among all the parameters analyzed, it was concluded that the most important are: (1) k --the soil permeability; (2) the pressure differential from inside to outside; and (3) the radium (Ra-226) concentration in the soil particles. For k $\leq$ 1.0 $\times$ 10$\sp{-12}$ (m $\sp2$), the entry rate of radon into the house was dominated by diffusion, and consequently the resultant indoor radon concentration varied very slowly with soil permeability. For k $\leq$ 1.0 $\times$ 10$\sp{-12}$ (m $\sp{-2}$), the convective transport of radon predominated over diffusion, and the indoor radon concentration was found to be strongly dependent (almost linearly) on the soil permeability. These effects were observed for an applied delta pressure of 5.0 (Pa). It is expected that a variation of the delta pressure would affect directly this turning point of k = 1.0 $\times$ 10$\sp{-12}$ (m $\sp2$). The indoor radon concentration was found to be: (1) directly, though not linearly, related to the pressure differential; and (2) directly, linearly, related to the concentration of Ra-226 in the soil particles.Ph.D.Environmental scienceHealth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128060/2/8720308.pd