Soil-gas Entry Research Articles

High-frequency fluctuations of indoor pressure: A potential driving force for vapor intrusion in urban areas

In this study, we examine the impact of a building's indoor pressure fluctuations in drawing subsurface volatile contaminants into the building, and how the presence of an impervious pavement surrounding the building influences this. Even in the absence of communication between the subsurface soil gas and ambient air fluctuations of building indoor pressure can cause upward advection of contaminated soil gas from the subfoundation zone into a building. For cases with the paved ground surface, the simulated volumetric soil gas entry rates are lower than steady-state cases with constant −5 indoor-outdoor pressure difference, by at least half an order of magnitude. When the indoor pressure fluctuation rate exceeds about 5 Pa/h (which corresponds a sinusoidal fluctuation with a period of 2 h), the predicted indoor air concentration of paved scenarios will be higher than the conventional case. When both the building foundation and surrounding pavement block diffusional escape of the volatile soil gas contaminants to the atmosphere, high subfoundation soil gas contaminant concentrations can exist, and contaminant entry into the building through foundation breaches is enhanced beyond what would be expected from diffusion as the building undergoes normal pressure cycling. Upward advection into the building may be induced even when the indoor pressure appears, based on limited measurements, to be higher than that in the subslab, particularly when the indoor pressure in the building quickly fluctuates. This represents a limitation on VI mitigation approaches that rely on indoor pressurization, if those approaches cannot at the same time control significant fluctuation of indoor pressure.

An examination of the building pressure cycling technique as a tool in vapor intrusion investigations with analytical simulations

The building pressure cycling (BPC) technique has been developed and applied by vapor intrusion (VI) site investigators to obtain estimates of reasonable maximum exposures and to identify possible background sources of contaminant vapors. This method assumes that by application of consistent indoor depressurization one can increase the average contaminated soil gas entry rate into a building of interest. In this study, a one-dimensional analytical model was developed to examine this assumption and explore the mechanism of BPC application. We have established that contaminant entry rate can typically reach a new pseudo-steady state on a time scale of one day following the imposition of enhanced indoor depressurization. Considering the traditional source-soil-building pathway, the results indicate that BPC can increase building loading rate in the first 3−5 hours, to an extent linearly related to the strength of depressurization, and after half a day, the normalized rate would reach a pseudo-steady state of about twice the value before application of depressurization. More significant and substainble increases in building loading rate indicate alternative pathways such as land drain or sewer pipeline. These findings are fully consistent with available field observations, and could help investigators optimize the performance of the BPC operation.

A Petroleum Vapor Intrusion Model Involving Upward Advective Soil Gas Flow Due to Methane Generation.

At petroleum vapor intrusion (PVI) sites at which there is significant methane generation, upward advective soil gas transport may be observed. To evaluate the health and explosion risks that may exist under such scenarios, a one-dimensional analytical model describing these processes is introduced in this study. This new model accounts for both advective and diffusive transport in soil gas and couples this with a piecewise first-order aerobic biodegradation model, limited by oxygen availability. The predicted results from the new model are shown to be in good agreement with the simulation results obtained from a three-dimensional numerical model. These results suggest that this analytical model is suitable for describing cases involving open ground surface beyond the foundation edge, serving as the primary oxygen source. This new analytical model indicates that the major contribution of upward advection to indoor air concentration could be limited to the increase of soil gas entry rate, since the oxygen in soil might already be depleted owing to the associated high methane source vapor concentration.

Quantifying the Influence of Stack and Wind Effects on Vapor Intrusion

ABSTRACTThis article develops mathematical relationships for quantifying how the stack effect, wind effect, and effective leakage area influence the rates of subslab soil gas entry and outdoor air infiltration into residential buildings. The equations developed in the article are based on combining existing theory for air infiltration into buildings with existing vapor intrusion modeling methods. Use of the equations for estimating the subslab soil gas attenuation factor for assessing inhalation exposure via vapor intrusion into residential buildings is illustrated with example simulations using 1 year of hourly temperature and wind speed data from a Northeastern U.S. city and with several distributions of the effective leakage area from a U.S. residential air leakage database. The simulation results make clear that the soil gas entry rate and the building's ventilation rate are positively correlated, and this correlation mutes the influence of stack and wind effects on the subslab attenuation factor. The examples also suggest that the subslab attenuation factor for most residential buildings is likely to be less than 0.003 most of the time.

Radon entry into houses: the importance of scale-dependent permeability.

Soil permeability to air can increase substantially with measurement length scale. We tested the hypothesis that the scale effect could resolve large model underpredictions of radon and soil-gas entry into two experimental basement structures located in natural sandy-loam soil at a field site in Ben Lomond, CA. Previously, the model input for permeability at the site had been assessed based on 0.5-m scale measurements. After determining the soil-structure interaction scale (system scale) to be approximately 3 m, the model input was changed to reflect 3-m scale permeability measurements. This adjustment reduced unacceptably large model underpredictions, of a factor of 3 to 5, to a range near that of acceptable experimental error, 20 to 40%. The permeability scale effect may explain large and persistent model underestimates of radon entry into real houses. The results argue strongly for determining permeability at a length scale consistent with that of the system under study.

Soil-gas entry into an experimental basement driven by atmospheric pressure fluctuations—Measurements, spectral analysis, and model comparison

To study the effects of atmospheric pressure fluctuations on the entry of radon and soil-gas contaminants into houses, we have simultaneously measured the changes in atmospheric pressure and the gas flow rate between the interior of an experimental basement structure and the underlying soil. Atmospheric pressure fluctuations draw soil gas into the experimental basement without the indoor-outdoor pressure differences commonly associated with advective entry of soil-gas contaminants. The soil-gas flow rate induced by a change in atmospheric pressure depends on both the characteristic response time of the soil and the time-rate-of-change of the atmospheric pressure fluctuation. Spectral analysis indicates that relatively low-frequency fluctuations in atmospheric pressure are the most important for driving soil-gas into and out the of the experimental structure; more than 60% of the total power of the soil-gas flow spectrum occurs at frequencies less than 100 d −1. A transient finite-element model based on Darcy's law correctly predicts both the dynamics and the magnitude of the observed gas flow. Atmospheric pressure fluctuations may increase the long-term radon entry rate into the experimental structure by as much as 0.2 Bq s −1, which is more than twice the measured diffusive entry rate into the structure and comparable to the radon entry rate driven by a − 0.4 Pa, steady indoor-outdoor pressure difference.

Soil-gas entry into houses driven by atmospheric pressure fluctuations—The influence of soil properties

Atmospheric pressure fluctuations can draw soil gas into houses without the indoor-outdoor pressure differences commonly associated with the advective entry of radon and other soil-gas contaminants. To study this phenomenon, we employ a transient finite-element model based on Darcy's law to simulate the soil-gas flow around a prototypical basement caused by changes in atmospheric pressure. The characteristic response time and the capacitance of the soil are used to characterize how changes in permeability, air-filled porosity, and water-table depth affect this soil-gas flow. The shorter the characteristic response time and the larger the capacitance of the soil, the larger the soil-gas flow rate into a basement caused by a given fluctuation in atmospheric pressure. Such a soil must have a high permeability and a large air-filled porosity. The addition of a high permeability subslab gravel layer increases the soil-gas flow rate into the basement by a factor of ∼ 3. Relative to entry driven by steady indoor-outdoor pressure differences, contaminant entry induced by atmospheric pressure fluctuations will likely be most important in houses situated in a soil of low permeability (< 10 −12 m 2) and large air-filled porosity.

Indoor infiltration of volatile organic contaminants: Measured soil gas entry rates and other research results for Canadian houses

Indoor infiltration of volatile organic contaminants: Measured soil gas entry rates and other research results for Canadian houses

The influence of a subslab gravel layer and open area on soil-gas and radon entry into two experimental basements.

Measurements of steady-state soil-gas and 222Rn entry rates into two room-sized experimental basement structures were made for a range of structure depressurizations (0-40 Pa) and open floor areas (0-165 x 10(-4) m2). The structures are identical except that in one the floor slab lies directly on native soil whereas in the other the slab lies on a high-permeability gravel layer. The subslab gravel layer greatly enhances the soil-gas and radon entry rate into the structure. The radon entry rate into the structure with the subslab gravel layer is four times greater than the entry rate into the structure without the gravel layer with an open floor area of 165 x 10(-4) m2; however the ratio increases to 30 for an open floor area of 5.0 x 10(-4) m2. The relationship between open area and soil-gas entry rate is complex. It depends on both the amount and distribution of the open area as well as the permeability of the soil near the opening. The entry rate into the experimental structures is largely determined by the presence or absence of a subslab gravel layer. Therefore open area is a poor indicator of radon and soil-gas entry into the structures. The extension of the soil-gas pressure field created by structure depressurization is a good measure of the radon entry. The measured normalized radon entry rate into both structures has the same linear relationship with the average subslab pressure coupling is an estimate of the extension of the soil-gas pressure field. A three-dimensional finite-difference model correctly predicts the effect of a subslab gravel layer and different open area configurations on radon and soil-gas entry rate; however, the model underpredicts the absolute entry rate into each structure by a factor of 1.5.

Concrete Blocks' Adverse Effects on Indoor Air and Recommended Solutions

Air infiltration through highly permeable concrete blocks can allow entry of various serious indoor air pollutants including radon. An easy approach to avoiding these pollutants is to select a less-air-permeable concrete block. Tests show that air permeability of concrete blocks can vary by a factor greater than 50 (0.63--35 standard L/min/m{sup 2} at 3 Pa). The surface texture of the blocks correlates well with air permeability; test results of smoother, closed-surface-texture blocks were usually less air-permeable. During construction, air infiltration can be minimized by capping walls and carefully sealing around openings for utilities or other penetrations. Structures with indoor air-quality problems due to soil-gas entry can be mitigated more effectively with less coating material if the blocks have a closed surface texture. All coatings evaluated--cementaceous block filler (which has the lowest applied cost and is more than 99.5% effective), surface bonding cement, water-based epoxy, polysulfide vinyl acrylic, and latex (three coats)--were highly effective (more than 98%) in reducing air permeability when adequately applied. Coating selection should be influenced by expected service life, considering surface condition and cost.

Cost-Effective Strategies to Control Radon in Residences

Many control strategies can be used to address health and comfort concerns posed by high levels of indoor air contaminants. Sufficient experience exists to develop models that estimate the effectiveness and cost of radon (Rn) mitigation strategies in single-family houses. In this paper, cost curves are developed that use one or several control strategies to achieve a range of concentration reductions. The results indicate preferred “cost-optimal” strategies at various mitigation levels. Rn control strategies considered include sealing, block-wall ventilation, subslab ventilation, drain-tile ventilation, ventilation with and without heat recovery, and water treatment. For a case-study residence typical of U.S. single-family homes, cost-effective control is provided by sealing soil-gas entry routes, block-wall ventilation, and heat-recovery ventilation. Other strategies can provide similar Rn reductions, but at higher costs. The sensitivity of results to climate and energy-price assumptions is explored. Identifying strategies that are cost-effective will help to increase their use and reduce risks.

Soil Gas and Radon Entry into a Simple Test Structure: Comparison of Experimental and Modelling Results

A radon test structure has been established at a field site at Riso National Laboratory. Measurements have been made of soil gas entry rates, pressure couplings and radon depletion. The experimental results have been compared with results obtained from measured soil parameters and a two-dimensional steady-state numerical model of Darcy flow and combined diffusive and advective transport of radon. For most probe locations, the calculated values of the pressure couplings and the radon depletion agree well with the measured values, thus verifying important elements of the Darcy flow approximation, and the ability of the model to treat combined diffusive and advective transport of radon. However, the model gives an underestimation of the soil gas entry rate. Even if it is assumed that the soil has a permeability equal to the highest of the measured values, the model underestimates the soil gas entry rate by a factor of two.

Soil Gas and Radon Entry into a Simple Test Structure: Comparison of Experimental and Modelling Results

Abstract A radon test structure has been established at a field site at Riso National Laboratory. Measurements have been made of soil gas entry rates, pressure couplings and radon depletion. The experimental results have been compared with results obtained from measured soil parameters and a two-dimensional steady-state numerical model of Darcy flow and combined diffusive and advective transport of radon. For most probe locations, the calculated values of the pressure couplings and the radon depletion agree well with the measured values, thus verifying important elements of the Darcy flow approximation, and the ability of the model to treat combined diffusive and advective transport of radon. However, the model gives an underestimation of the soil gas entry rate. Even if it is assumed that the soil has a permeability equal to the highest of the measured values, the model underestimates the soil gas entry rate by a factor of two.

Analysis of Radon Mitigation Techniques Used in Existing US Houses

Abstract This paper reviews the full range of techniques that have been installed in existing US houses for the purpose of reducing indoor radon concentrations resulting from soil gas entry. The review addresses the performance, installation and operating costs, applicability, mechanisms, advantages, and disadvantages of each radon mitigation technique. Active soil depressurisation (ASD) techniques are the measures most widely used by mitigation contractors. ASD techniques consistently and reliably provide the highest radon reductions, at costs often competitive with the less effective alternatives. The large majority of existing US houses having elevated radon can be effectively treated using ASD. Various other techniques can be used in houses where ASD is difficult or impractical to apply, or where lesser radon reductions are acceptable. However, these other techniques are always less effective, less reliable, or less well demonstrated than ADS.

Analysis of Radon Mitigation Techniques Used in Existing US Houses

This paper reviews the full range of techniques that have been installed in existing US houses for the purpose of reducing indoor radon concentrations resulting from soil gas entry. The review addresses the performance, installation and operating costs, applicability, mechanisms, advantages, and disadvantages of each radon mitigation technique. Active soil depressurisation (ASD) techniques are the measures most widely used by mitigation contractors. ASD techniques consistently and reliably provide the highest radon reductions, at costs often competitive with the less effective alternatives. The large majority of existing US houses having elevated radon can be effectively treated using ASD. Various other techniques can be used in houses where ASD is difficult or impractical to apply, or where lesser radon reductions are acceptable. However, these other techniques are always less effective, less reliable, or less well demonstrated than ADS.

Soil-gas entry into an experimental basement: model measurement comparisons and seasonal effects

Previous studies have reported a large and persistent discrepancy between field measurements and model predictions of pressure-driven entry of soil gas into houses, the phenomenon that causes high concentrations of radon indoors. The discrepancy is often attributed to poor understanding of inherently complex field sites. This paper compares measurements of soil-gas entry made at a full-scale test basement located in natural solid with predictions of a three-dimensional finite difference model. The results corroborate the earlier findings, with the model underpredicting the soil-gas entry rate by a factor of 7. The effect of seasonal changes in soil conditions on soil-gas entry is also examined. Despite large seasonal changes in near-surface soil moisture content and air permeability, there is no observable effect on soil-gas entry, apparently because critical soil conditions near the soil-gas entry location in the structure floor remain relatively constant. 24 refs., 5 figs., 1 tab.

Modeling Radon Entry into Houses with Basements: The Influence of Structural Factors

Where indoor concentrations are high, radon entry into houses with basements is usually due primarily to the convective transport of soil gas through openings in the subsurface part of the building shell. The factors determining the rate of entry may conveniently be divided into those associated with the undisturbed soil and those associated with the structure and its surroundings. This paper uses a numerical model to determine the influence of the latter factors on the soil gas and radon entry rates. The most important of these is the presence or absence of a gravel layer below the slab; the presence of the gravel can increase the radon entry rate through the perimeter gap betureen the foundation footer, slab, and wall (slab-footer gap) by as much as a factor of 5 over that for homogazeous soil. The permeability of the gravel becomes important when the soil permeability is unusually high, i.e., greater than 10−10 m2. Of lesser importance are the thickness of the gravel layer and the radium content of the gravel. The sizes and numbers of openings in the slab are relatively unimportant so long as the total opening area is vey small compared to the slab area. If cracks in the basement walls are major radon entry paths, as in concrete-block construction, the permeability of the soil restored to the region adjacent to the walls after completion of construction (backfill) is the determining factor in convective radon entry through these openings; if the soil is packed loosely, so that there is a gap between wall and soil, radon entry through a wall crack may be further increased by as much as a factor of 7.5. Radon entry rates through the slab-footer gap and through openings in the slab are only weakly influenced by the permeability of the backfill. The resistance of the perimeter gap to soil gas entry becomes significant when the gap width falls below 0.001 m, assuming a soil permeability of 10−11 m2.

Soil-Gas Contamination and Entry of Volatile Organic Compounds into a House Near a Landfill

Toxic volatile organic compounds (VOC) are commonly found in landfills, including those accepting only municipal waste. These VOC can migrate away from the site through the soil and result in contaminated off-site soil gas. This contaminated soil gas can enter houses built near landfills and is a potential source of human exposure to VOC. This study investigated soil-gas contamination and the mechanisms of entry of VOC into a house with a basement sited adjacent to a municipal landfill. The VOC were identified and quantified in the soil gas and in indoor and outdoor air. Pressure coupling between the basement and the surrounding soil was measured. Using soil-gas tracers, the pressure-driven advective entry of soil gas was quantified as a function of basement depressurization. From the measurements, estimates were made for the diffusive and advective entry rates of VOC into the house. A comparison of the chlorinated hydrocarbons found in soil gas at the site and in the landfill suggests that the landfill i...

Developing Soil Gas and 222Rn Entry Potentials for Substructure Surfaces and Assessing 222Rn Control Diagnostic Techniques

Research-based procedures for characterizing the causes of elevated indoor 222Rn levels and guiding the selection of an appropriate control technique were evaluated at seven New Jersey houses. Procedures such as thorough visual inspections, blower door air leakage tests, pressure field mapping, subsurface vacuum extension tests, sampling of 222Rn concentrations throughout the substructure, and measurements of the additional depressurization caused by various appliances all were found to furnish important information to the mitigation contractor or researcher. An analysis of data from these and other diagnostic techniques performed at the seven houses also indicated: (1) regions of very high permeability existed directly adjacent to the exterior of substructure walls and floors; (2) the additional substructure depressurization caused by operation of forced-air furnaces and attic exhaust fans could exceed 1 Pascal; (3) 222Rn concentrations below basement slabs and slabs-on-grade adjoining below grade basement walls were approximately seven times higher than those within block wall cavities; and (4) air leakage areas of crawlspace and basement ceilings were quite large, ranging up to 0.15 m2. The pressure field mapping tests identified the areas surrounding the substructure that were well coupled to the indoors. Using flow, pressure difference, and 222Rn concentration data, indices of soil gas entry potential and 222Rn entry potential were developed to indicate the areas of the substructure that may have high entry rates of soil gas and 222Rn, respectively. These indices could be helpful for quantifying the relative resistance to soil gas movement of substructure surfaces and surrounding soils and for determining the placement of 222Rn control systems.

Characterizing the Occurrence, Sources, and Variability of Radon in Pacific Northwest Homes

A compilation of data from earlier studies of 172 homes in the Pacific Northwest indicated that approximately 65 percent of the 46 homes tested in the Spokane River Valley/Rathdrum Prairie region of eastern Washington/northern Idaho had heating season indoor radon (222Rn) concentrations above the U. S. EPA guideline of 148 Bq m-3 (4 pCi L-1). A subset of 35 homes was selected for additional study. The primary source of indoor radon in the Spokane River Valley/Rathdrum Prairie was pressure-driven flow of soil gas containing moderate radon concentrations (geometric mean concentration of 16,000 Bq m-3) from the highly permeable soils (geometric mean permeability of 5 x 10(-11) m2) surrounding the house substructures. Estimated soil gas entry rates ranged from 0.4 to 39 m3h-1 and 1 percent to 21 percent of total building air infiltration. Radon from other sources, including domestic water supplies and building materials was negligible. In high radon homes, winter indoor levels averaged 13 times higher than summer concentrations, while in low radon homes winter levels averaged only 2.5 times higher. Short-term variations in indoor radon were observed to be dependent upon indoor-outdoor temperature differences, wind speed, and operation of forced-air furnace fans. Forced-air furnace operation, along with leaky return ducts and plenums, and openings between the substructure and upper floors enhanced mixing of radon-laden substructure air throughout the rest of the building.