Wind-induced contaminant transport in near-surface soils with application to radon entry into buildings

Abstract

Indoor air exposures to gaseous contaminants originating in soil can cause large human health risks. To predict and control these exposures, the mechanisms that affect vapor transport in near-surface soils need to be understood. In particular, radon exposure is a concern since average indoor radon concentrations lead to much higher risks than are generally accepted for exposure to other environmental contaminants. This dissertation examines an important component of the indoor radon problem: the impacts of wind on soil-gas and radon transport and entry into buildings. The research includes experimental and modeling studies of wind's interactions with a building's superstructure and the resulting soil-gas and radon flows 1 in the surrounding soil. In addition to exploring the effects of steady winds, we develop a novel modeling technique to examine the impacts of fluctuating winds on soil-gas and radon transport.The original impetus for this investigation arose from the design and development of a passive radon mitigation system at the Lawrence Berkeley National Laboratory. That effort was initiated in response to the relatively high costs of subslab depressurization (SSD), a common active mitigation system. Chapter 2 quantifies the regional and national energy requirements, operating expenses, and C02 emissions associated with SSD system use. We estimate that operating SSD systems in U.S. houses where it is both appropriate and possible (about 2.6 million houses) would annually consume 1 .7x104 TJ of end-use energy, cost $230 million (at current energy prices), and generate 2 . 0~1 0~ kg of C02. These figures are central estimates;Chapter 2 presents the associated uncertainties.The relatively high costs of SSD system operation prompted our research group to install and monitor a full-scale passive radon mitigation system in a house in Spokane, Washington. This study revealed that wind could strongly affect the mitigation system's performance. We therefore began to systematically investigate the impacts of wind on soil-gas radon transport and entry into buildings. Chapter 3 describes the results of a wind-tunnel experiment designed to measure the groundsurface pressure field established around a house in the presence of wind. We report wind-induced ground-surface pressure fields for several wind-incidence angles, two 2 atmospheric boundary layer structures, and two house geometries. Relatively small changes in the ground-surface pressure field resulted from perturbations in the atmospheric boundary layer structure (i .e., the surface roughness and displacement height) and the house geometry. The experimental results were compared to numerical predictions using a k-E turbulence model. In most cases, this turbulence model is inappropriate for geometries with separated flow, as in the case of air flow over a building. However, we found the numerical model predictions of the groundsurface pressure field sufficiently accurate to predict wind-induced soil-gas contaminant transport.The wind-induced ground-surface pressu...

The effects of chamber pressurization on soil‐surface CO2 flux and the implications for NEE measurements under elevated CO2

“…STEAC can simulate both Darcy and non‐Darcy flows of soil‐gas, as appropriate. For the cases considered here, the pressures imposed on the system result in flow rates within the linear regime, so Darcy’s law suffices to relate the pressure and velocity fields ( Riley 1996):…”

“…where C is the soil‐gas CO 2 concentration (μmol m –3 ), S is the CO 2 generation rate into the soil‐gas (μmol m –3 s –1 ), D is the molecular diffusivity of CO 2 in air reduced for air‐filled porosity and tortuosity (m 2 s –1 ), and ε is the air‐filled porosity of the soil (–). Riley (1996) describes the discretization of (1), (2), and (3) in more detail. The boundary conditions for the CO 2 concentration field simulations also impose a zero normal gradient at the two sides and the bottom of the soil block; the CO 2 concentration at the soil surface is held constant at 360 ppmv.…”

“…The soil‐gas pressure and velocity calculations of the model have been tested previously, as have the concentration calculations for the case of soil‐gas radon transport ( Riley 1996; chapter 5). STEAC modifies the concentration calculations in START simply by removing the radioactive decay necessary to simulate radon transport.…”