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 stud y the effects of time-varying winds on radon transport in soil near buildin gs. The technique, implemented in the model RapidSTART, reduces computation al times for transient, three-dimensional, wind-induced soil-gas and radon transport by three to four orders of magnitude compared with conventional f inite-difference models. To test model performance, we compared its predict ions to analytical solutions of one-dimensional soil-column flow, finite-di fference simulations of how 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-de pendent radon transport through soil and its entry into buildings. As demon strated in a previous study, steady winds can significantly affect radon en try. In this paper, we extend the findings of that study by applying RapidS TART to explore the impacts of fluctuating wind speed and direction on rado n entry into a prototypical house. In soils with moderate to high permeabil ity, wind fluctuations have a small to moderate effect on the soil-gas rado n concentration field and entry rate into the building. Fluctuating wind di rection dominates the impact on radon entry rates, while fluctuating wind s peed has little effect. For example, in a soil with a permeability of 10(-1 0) m(2), diurnal oscillations in wind direction can increase the predicted radon entry rate by up to 30% compared to steady-state predictions. (C) 199 9 Elsevier Science Ltd. All rights reserved.