Effects of periodic atmospheric pressure variation on radon entry into buildings

Abstract

Using a mathematical model, we have investigated the temporal variations of radon entry into a house basement in the presence of time‐dependent periodic variations of barometric pressure as well as a persistent small steady depressurization within the basement. The tool for our investigation is an integral finite difference numerical code which can solve for both diffusive and advective flux of radon in the soil gas which is treated as a slightly compressible fluid. Two different boundary conditions at the house basement are considered: (1) a dirt floor basement so that diffusion is equally or more important than advective transport, and (2) an “impermeable” cement basement except for a 1‐cm‐wide crack near the perimeter of the basement floor; in which case, advective transport of radon flux dominates. Two frequencies of barometric pressure fluctuation with representative values of amplitudes, based on a Fourier decomposition of barometric pressure data, were chosen in this study: one with a short period of 0.5 hour with pressure amplitude of 50 Pa, the other a diurnal variation with a period of 24 hours with the typical pressure amplitude of 250 Pa. For a homogeneous soil medium with soil permeability to air between 10−13 and 10−10 m2, we predict that the barometric fluctuations increase the radon entry into the basement by up to 120% of the steady radon inflow into the basement owing to a steady depressurization of 5 Pa. If soil permeability heterogeneity is present, such as the presence of a thin layer of higher permeability aggregate immediately below the basement floor, radon flux due to atmospheric pumping is further increased. Effects of pressure pumping on radon entry are also compared to diffusion‐only transport when the steady depressurization is absent. It is found that contribution to radon entry is significant for the basement crack configuration. In particular, for pressure pumping at 0.5‐hour period and for a homogeneous medium of permeability of 10−10 m2, the radon entry is a factor of 10 larger than that predicted by the diffusion‐only transport. This may help to explain indoor radon concentrations during times of low steady state driving force. Extending beyond radon transport, the results of this case study establish the importance of transient advective transport resulting from atmospheric pressure variation. These results may have relevance in the estimations of the transfer of trace gases such as methane and nitrous oxide across the soil‐atmosphere interface and their impact on global climate changes.

Radon entry into buildings: Effects of atmospheric pressure fluctuations and building structural factors

“…Advective radon entry driven by atmospheric pressure fluctuations represents a possible explanation for these observed but unexplained elevated indoor radon concentrations. A recent theoretical study by Tsang and Narasimhan (1992) indicates that cyclical changes in atmospheric pressure can drive advective radon entry into buildings without sustained indoor-outdoor pressure differences.…”

“…The theoretical study by Tsang and Narasimhan (1992) is the most persuasive evidence for radon entry driven by atmospheric pressure fluctuations. Their results suggest that sinusoidal changes in atmospheric pressure may drive significant radon entry into buildings for certain combinations of soil types, basement types and atmospheric pressure signals.…”

“…Recent theoretical studies have shown that atmospheric pressure fluctuations can draw soil gas into houses Tsang and Narasimhan, 1992). We are interested in the different mechanisms which induce soil-gas flow because the advective entry of radon-bearing soil gas into houses is believed to be the dominant transport mechanism of radon into most homes with elevated indoor concentrations (e.g.…”

“…The phrase "indoor-outdoor pressure difference" refers to the pressure difference between the ambient atmosphere at the soil surface and the indoor air at an opening between the basement and the soil. Recent theoretical Tsang and Narasimhan, 1992) studies indicate that changes in atmospheric pressure can draw soil gas into a building in the absence of these persistent indoor-outdoor pressure differences.…”

“…Unfortunately, such studies provide little detailed insight into the effect of atmospheric pressure fluctuations on radon entry because indoor radon concentrations depend on both the entry rate and the building ventilation rate. Using a numerical model, Tsang and Narasimhan (1992) In Chapters 3 and 4, we present a detailed examination of the gas flow between a building and the underlying soil in response to changes in atmospheric pressure. The goal of these chapters is to characterize the relationship between this flow, soil properties, and typical atmospheric pressure fluctuations.…”