Effects of variable wind speed and direction on radon transport from soil into buildings: model development and exploratory results
We describe a novel modeling technique, based on Duhamel’s theorem, to study the effects of time-varying winds on radon transport in soil near buildings. The technique, implemented in the model RapidSTART, reduces computational times for transient, three-dimensional, wind-induced soil-gas and radon transport by three to four orders of magnitude compared with conventional finite-difference models. To test model performance, we compared its predictions to analytical solutions of one-dimensional soil-column flow, finite-difference simulations of flow around a full-scale house, and measurements of transient soil–gas and radon entry into an experimental basement structure. These comparisons demonstrate that RapidSTART accurately simulates time-dependent radon transport through soil and its entry into buildings. As demonstrated in a previous study, steady winds can significantly affect radon entry. In this paper, we extend the findings of that study by applying RapidSTART to explore the impacts of fluctuating wind speed and direction on radon entry into a prototypical house. In soils with moderate to high permeability, wind fluctuations have a small to moderate effect on the soil-gas radon concentration field and entry rate into the building. Fluctuating wind direction dominates the impact on radon entry rates, while fluctuating wind speed has little effect. For example, in a soil with a permeability of 10-10 m2, diurnal oscillations in wind direction can increase the predicted radon entry rate by up to 30% compared to steady-state predictions.