Sub-slab Depressurization Systems Research Articles

Impact of drainage pipe vapor intrusion pathways on sub-slab depressurization system mitigation performance

Sub-slab depressurization (SSD) systems are the most commonly employed technology for mitigating vapor intrusion (VI) impact to indoor air. However, as cases involving preferential VI pathways (e.g. sewers, land drains) are being reported, the efficacy of SSD systems to reduce VI impacts from such pathways is being questioned. In this study, field test and numerical modeling were performed to evaluate the effectiveness of an SSD system for the case of VI involving a drainage pipe preferential pathway. Field tests investigated the SSD performance at the study house under conditions with and without the impact of a drainage pipe pathway. A numerical model was developed to explore the influence of the preferential pathway on SSD mitigation under different scenarios. Both field and simulation results indicated that the sub-slab drainage system inhibits the performance of the SSD, consequently requiring a greater level of flow and depressurization for the system to protect the building. In addition, the SSD caused higher sub-slab vapor concentrations than under natural condition in both field testing and simulation results, which raised concerns for potential VI risk under standard SSD operation. Moreover, simulation results suggested the location of preferential pathways can affect the performance of an SSD system.

Water drainage and gas collection with geocomposites - Hydraulic software

Geosynthetic materials and, more specifically, drainage geocomposites are now widely used for water drainage and gas collection in applications as varied as final landfill covers, leachate collection in landfill cells, sub-slab depressurization systems under buildings, groundwater drainage under embankments, etc. The design methods used are based on the in-plane flow capacity of the geocomposites, which is determined by laboratory tests performed on 250-300 mm long product specimens. Fluid is injected into the thickness of the product and the drainage capacity is interpolated for an actual length of several meters. This paper presents the development of hydraulic design software for multi-linear drainage geocomposites, based on laboratory characterizations of the geocomposite and then validated with full-scale tests. The software gives a 3D model of the hydraulic curves in the geocomposite depending on the application for which the geocomposite is used, and the fluid to be drained (water, landfill gas, methane, air, etc.).

Emissions of Trichloroethylene from Bedroom Furniture Set as a Source of Indoor Air Contamination

AbstractShallow trichloroethene (TCE) groundwater and soil contamination associated with a Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) Superfund Site in Michigan resulted in a vapor intrusion (VI) investigation of overlying condominium units. Units with data suggesting a complete VI pathway received subslab depressurization systems (SSDs). Performance monitoring following the installation of an SSD at one unit indicated that the indoor air TCE concentrations remained elevated, despite pressure field extension tests that showed the system should effectively reduce VI from soil gas. Therefore, a cost‐efficient and incremental investigation was launched to identify other potential source(s) of TCE using a field‐portable gas chromatograph/mass spectrometer (GS/MS). The combination of room‐by‐room air sampling, potential VI entry point sampling, and emission tests of potential sources were used, which resulted in successfully identifying a bedroom furniture set as an indoor source of TCE for the unit. Although many common household products are recognized as indoor sources of TCE, emissions from finished furniture products have not been widely discussed in the VI literature. The findings of this study indicate that off gassing from furniture can lead to TCE concentrations in indoor air that exceed regulatory guidelines.

Cost Comparison of Soil Vapor Extraction and Subslab Depressurization for Vapor Intrusion Mitigation.

Soil vapor extraction (SVE) can be applied for remediation, and also as an alternative to sub-slab depressurization (SSD) for vapor intrusion (VI) mitigation. This study compares capital, operation, and treatment costs of SVE and SSD systems using data collected during a multi-year demonstration project conducted at eight buildings in an urban setting. The capital cost of the SVE system is substantially less than the estimated total capital cost of individual SSD systems. The SVE operating costs are higher, especially in the early operating years when it is being operated for mass removal and treatment. As a result, the cumulative SVE system cost rises above that of the SSD systems in the sixth year of operation. A significant portion of the operations and maintenance cost advantage of the SSD systems comes from the assumption that off-gas treatment is not required. Alternative cases show SVE costs are likely to be lower in scenarios where numerous small buildings requiring independent SSD systems overlie the SVE zone of influence. Conversely, SSD systems are less costly for cases with few small buildings overlying the SVE zone of influence. An additional benefit of SVE is continued mass removal. In a situation where an existing SVE can be repurposed for VI protection from residual volatile organic carbon (VOC) mass, the SVE cumulative costs over 30years can remain lower than the cost of installing and operating SSD systems in multiple buildings.

Method for assessing radon Re-Entrainment risks from above ground level discharges from sub-slab depressurization systems

Above ground level (ABL) discharges are commonly employed in northern climates to release radon-containing soil gases from sub-slab depressurization radon mitigation systems. While there are many practical reasons to employ this in cold climates (e.g., to avoid ice build-up), it does inherently elevate the outdoor radon concentrations, and raises the possibility of radon re-entrainment back into living spaces of the emitting house, and its neighbors. A method has been developed for assessing outdoor radon dispersion and re-entrainment risks. It links computational fluid dynamics calculations for evaluating outdoor concentrations, and regional multi-annual meteorological records that give the occurrence probability for different wind speed, wind direction, and atmospheric stability conditions. When combined with assumptions about indoor/outdoor concentration ratios, annual average exposure concentrations to radon can be estimated. Concentrations along surfaces of the emitting house are often quite high close to the exhaust vent, and air exchange into the house from nearby open windows, air intakes, etc., and can result in high indoor concentrations. Re-entrainment into the neighboring house is often less serious, though certain circumstances can lead to radon being trapped in the space between the two buildings, which leads to higher exposure.

Dispersion Simulations of Radon Discharges between Neighboring Buildings and Their Sensitivity to Meteorology, Discharge Rate, and Building Geometry.

In northern climates, it is common to install the discharges of radon sub-slab depressurization systems near ground level. However, this also elevates the ground level outdoor radon concentrations and raises the possibility of radon re-entrainment into homes. The study aims to assess outdoor radon concentrations near above-ground-level discharges along the surfaces of an emitting building and its close neighbor and identify parameters that most influence the dilution. This study employs a series of computational fluid dynamics calculations to assess concentrations along the exhaust-facing and non-exhaust-facing surfaces of the buildings. Different meteorological, venting, and building geometry parameters are explored. Boundary conditions for the CFD calculations are based on field measurements of the ground-level wind speeds and seasonal air temperatures and atmospheric stabilities. Outdoor concentrations can be as high as 7% of the discharge gas, although these become smaller at greater distances from the vent. The direction of the prevailing wind is a particularly important parameter, as it influences the formation of circulating building cavities and building wakes where radon could accumulate. The wind speed, atmospheric stability, and season (plume buoyancy) also have important influences on the outdoor radon concentrations, as do the velocity of the vent system and the size of the buildings. The study has assessed the dilution of the radon-laden exhaust gas and determined the outdoor concentrations that can be expected under a variety of conditions. These results can be used to inform regulators about the potential for radon re-entrainment into homes.

Research on Best Solution for Improving Indoor Air Quality and Reducing Energy Consumption in a High-Risk Radon Dwelling from Romania.

The purpose of this article is the assessment of energy efficiency and indoor air quality for a single-family house located in Cluj-Napoca County, Romania. The studied house is meant to be an energy-efficient building with thermal insulation, low U-value windows, and a high efficiency boiler. Increasing the energy efficiency of the house leads to lower indoor air quality, due to lack of natural ventilation. As the experimental campaign regarding indoor air quality revealed, there is a need to find a balance between energy consumption and the quality of the indoor air. To achieve superior indoor air quality, the proposed mitigation systems (decentralized mechanical ventilation with heat recovery combined with a minimally invasive active sub-slab depressurization) have been installed to reduce the high radon level in the dwelling, achieving an energy reduction loss of up to 86%, compared to the traditional natural ventilation of the house. The sub-slab depressurization system was installed in the room with the highest radon level, while the local ventilation system with heat recovery has been installed in the exterior walls of the house. The results have shown significant improvement in the level of radon decreasing the average concentration from 425 to 70 Bq/m3, respectively the carbon dioxide average of the measurements being around 760 ppm. The thermal comfort improves significantly also, by stabilizing the indoor temperature at 21 °C, without any important fluctuations. The installation of this system has led to higher indoor air quality, with low energy costs and significant energy savings compared to conventional ventilation (by opening windows).

A full-scale experimental study of sub-slab pressure fields induced by underground perforated pipes as a soil depressurisation technique in radon mitigation

Sub-slab depressurisation systems have proven to effectively mitigate radon entry. A poor understanding of the fluid physics underlying the technique has been shown to lower the success rate substantially. This article describes a study of pressure fields in a sub-slab gravel bed induced by a soil depressurisation system consisting of perforated pipes run under the slab at a depth of 75 cm. The advantage of the approach is that pipes can be laid from outside the building to be protected. The study was conducted on a large-scale experimental facility where the variations in morphology and scope of pressure fields with different pipe combinations could be monitored and characterised. The findings showed that pressure was uniform across the entire area in the gravel bed, whereas the sensors buried in natural soil showed pressure to depend on distance from the source. Pressure transfer to the sub-slab plane was also observed to vary depending on the active pipe. Air-flow resistance studies in the layers of soil lying between the pipes and the gravel delivered different results for each pipe. That finding would appear to be related to the presence of preferential pathways in some parts of the soil. Total pressure when several pipes were activated was observed to be practically the same as the sum of the pressures transferred by each when working separately. The correlation between extraction fan power and pressure generated was also analysed. These and other factors are discussed and analysed from a perspective of the understanding of such highly effective techniques.

Field Study of Soil Vapor Extraction for Reducing Off-Site Vapor Intrusion.

Soil vapor extraction (SVE) is effective for removing volatile organic compound (VOC) mass from the vadose zone and reducing the potential for vapor intrusion (VI) into overlying and surrounding buildings. However, the relationship between residual mass in the subsurface and VI is complex. Through a series of alternating extraction (SVE on) and rebound (SVE off) periods, this field study explored the relationship and aspects of SVE applicable to VI mitigation in a commercial/light-industrial setting. The primary objective was to determine if SVE could provide VI mitigation over a wide area encompassing multiple buildings, city streets, and subsurface utilities and eliminate the need for individual subslab depressurization systems. We determined that SVE effectively mitigates offsite VI by intercepting or diluting contaminant vapors that would otherwise enter buildings through foundation slabs. Data indicate a measurable (5 Pa) influence of SVE on subslab/indoor pressure differential may occur but is not essential for effective VI mitigation. Indoor air quality improvements were evident in buildings 100 to 200 feet away from SVE including those without a measurable reversal of differential pressure across the slab or substantial reductions in subslab VOC concentration. These cases also demonstrated mitigation effects across a four-lane avenue with subsurface utilities. These findings suggest that SVE affects distant VI entry points with little observable impact on differential pressures and without relying on subslab VOC concentration reductions.

Vapor intrusion mitigation: Road map to successful sub‐slab depressurization at sites with shallow water tables

A common practice to prevent intrusion of contaminated vapor from the subsurface to overlying buildings is to install a sub‐slab depressurization system (SSDS) that creates lower pressure beneath the lowest level floor slabs, thereby preventing contaminated vapor from entering buildings. However, this standard approach cannot be implemented in the presence of a high groundwater table that is close to or in contact with lowest level building floor slabs. Therefore, for sites with a high groundwater table that require vapor intrusion mitigation, a unique “double slab” design has been developed. This alternate double slab design has been implemented and proven effective at new construction sites in New York City with sensitive receptors. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1758–1761, 2018

Radon mitigation in cold climates at Kitigan Zibi Anishinabeg.

Available radon mitigation results were gathered for 85 houses mainly by installing sub-slab depressurization systems (SSDS) with two types of discharge and fan locations: Above ground level discharge with the fan located in the basement (AGL) or above roof line discharge with the fan located in the attic (ARL). A comparative analysis was made of mitigation efficiency and of exhaust icing. Results show that both SSDS scenarios reduced radon levels similarly. The results of SSDS with AGL show that a sealed radon fan having proper fittings and sealed piping was able to reduce the radon to acceptable levels, and that these installations were less subject to obstructive icing of the exhaust in cold climates.

Air flow models for sub-slab depressurization systems design

The entry of soil gas pollutants (Radon, VOCs, …) into buildings can cause serious health risks to inhabitants. Various systems have been developed to limit this risk. Soil Depressurization System (SDS) is one of the most efficient mitigation systems to prevent buildings against these pollutants. Two operating modes of SDS are currently used: active and passive systems. Active systems use a fan which enables to extract air from the sub-slab. Passive systems use the stack effect and the wind to extract air from the sub-slab. Until now, no airflow model has been developed that leads to the effective design of these systems. In this paper, an analytical method has been used to develop airflow models to design these systems. The developed models take into account different type of substructures. These models are integrated in a multizone airflow and heat transfer building code. This integration permits to take into account climate conditions (stack effect, wind), building envelope characteristics and ventilation systems. Preliminary field verification results for the extracted flow by the SDS in an experimental building are presented and discussed. The results show that the airflow models are accurate to design SDS. A first application of the models is illustrated by the impact of climate conditions on the operation of the passive SDS. A second application is illustrated by the study of the impact of the ventilation strategies (natural ventilation, exhaust ventilation, supply ventilation and balanced ventilation systems) on the passive SDS operation.

Sub-slab depressurisation systems used in the Czech Republic and verification of their efficiency.

Design principles for sub-slab depressurisation systems recently used in the Czech Republic for remediating existing buildings are described. Results of an efficiency analysis performed on the basis of data from >60 single-family houses are presented. It was found out that the efficiency varies between 70 and 98 %, which is twice greater than when additionally applied radon-proof insulation is used. The efficiency is mainly influenced by the vertical profile of the soil permeability and by the air-tightness of floors resting on the soil. Higher efficiency was found for houses with greater air-tightness of the floors and with a sub-floor layer with permeability higher than the permeability of the subsoil.

Residential Radon Mitigations at Kitigan Zibi Anishinabeg

Radon mitigations in nine houses were conducted by installing sub-slab depressurization systems (SSDS) with two types of discharge and fan locations: Ground level discharge with the fan located in the basement or roof-discharge with the fan located in the attic. This paper presents a detailed comparative analysis of the radon reduction efficiency, condensation problems, and the cost-effectiveness of both SSDS installation scenarios in nine houses. The mitigations from both SSDS scenarios were successful in reducing radon. The results of rim-joist installations discharging above ground level with the fans located in the basement show that a sealed radon fan with proper fittings and sealed piping were able to reduce the radon to acceptable levels in a cost-effective manner.

Implementation of radon barriers, model development and calculation of radon concentration in indoor air

Norway has some of the highest concentrations of radon in indoor air in the world. Based on large-scale surveys by direct measurements of radon in indoor air it has been estimated that nearly 9% of the housing stock has an annual mean indoor radon concentration which is higher than the current action level of 200 Bq/m3. Preventive measures that focuses on saving energy and avoiding moisture problems in a cold climate, and by not introducing any specific measures to reduce the infiltration of radon and/or balanced ventilation of the indoor air, can lead to high indoor radon concentrations. It is of vital importance that the ground floor structure is as airtight as possible; both to reduce the infiltration of soil gas radon by using, for example, airtight and resistant membranes, and as a premise for other preventive measures to function, for example, sub-slab depressurization systems. Sufficient airtightness may be achieved by using a radon barrier towards the ground, for example, by avoiding perforations and ensuring sufficient airtightness in joints and feed-throughs. Various factors influencing the radon concentration in indoor air are discussed. Based on these factors a simplified but yet versatile and powerful model for calculating the radon concentration in indoor air is presented. Furthermore, the examples are depicted in selected 2 and 3D graphical plots for visualization. By incorporating various and realistic values in a spreadsheet version of the indoor radon concentration model, valuable information about the different parameters influencing the indoor radon level is gained. Hence, the presented model may be utilized as a tool for examining which preventive or remedial measures should be carried out in order to achieve an indoor level of radon below the reference level as set by the authorities. The radon transport into buildings might be dominated by diffusion, pressure driven flow or something in between depending on the actual values of the various parameters. The results of our work indicate that with realistic or typical values of the parameters, most of the transport of radon from the building ground to the indoor air is due to air leakage driven by pressure differences through the construction.

USE OF SIMULINK TO ADDRESS KEY FACTORS FOR RADON MITIGATION IN A FAIRBANKS HOME

Hilly areas around Fairbanks, Alaska, are known to have elevated soil radon concentrations. Due to geological conditions, cold winters, and the resulting stack effect, houses in these areas are prone to higher indoor radon concentrations. Key variables with respect to radon mitigation were addressed in this paper by using a dynamic model implemented in MATLAB Simulink. These variables included the ventilation rate; the foundation flow resistance, which can be affected by sealing the foundation during the construction of a house; and the differential pressure between the subslab and the house interior, which can be affected by using a subslab depressurization system. The model was used for the scenario of a varying differential pressure and then for the scenario of a varying ventilation rate at a Fairbanks home where real-time radon concentrations were measured. The correlation coefficients between the model-predicted and measured radon concentrations were 0.96 and 0.94, for both scenarios respectively, which verified the feasibility of the model for predicting indoor radon concentrations.

Numerical modelling as a tool for optimisation of sub-slab depressurisation systems design

The following paper is focused on possibilities of a numerical modelling of sub-slab depressurisation systems, which belong among the most effective radon protective measures. Three recently developed computer programs are briefly described in the paper—mainly from the point of view of governing equations and basic numerical analysis approach. The paper also presents results of sensitivity tests for these numerical models. In addition, a comparison of numerical simulation results with measured data is included as well.

A case study of sub-slab depressurization for a building located over VOC-contaminated ground

A sub-slab depressurization system for protecting indoor air quality in buildings located over VOC-contaminated ground is described. Included in the design is a means of assessing the performance of the protection measures during the lifetime of the building. A case study is presented of the sub-slab depressurization system implemented in two renovated historic buildings located at the site of an old gas works facility with extensive tar and PAH contamination in the soil. Measurements of under pressure under the slab and airflow rates in the sub-floor exhaust ventilation ducts from performance checks both immediately after the systems were installed and then 2 years later demonstrated that the protection measures in each building functioned as planned. VOC measurements in indoor air in the buildings during the two performance checks suggested that the air quality in the buildings was not adversely affected by the contamination in the soil.

96/06512 Heat recovery in ventilation systems with exhaust air humidification

Radon is the largest source of risk to human health caused by an indoor pollutant in the industrial countries. Subslab ventilation (SSV) is one of the most effective and common methods of reducing indoor radon concentrations in houses with a basement. In this paper, the impact of SSV on the air exchange rate is quantified, through numerical modeling of a prototype house with basement for a range of permeabilities of soil and subslab aggregate and various sizes of the cracks in the basement floor. We show that a SSV system can increase the air exchange rate by as much as a factor of 4.5. Then the energy and capital costs of a subslab depressurization (SSD) system are compared with those of direct ventilation of the basement which is required to lower the indoor radon concentration to an acceptable level for a Chicago climate. We show that (1) an exhaust ventilation cannot significantly reduce the indoor radon concentration and may even increase it, and (2) a balanced ventilation with heat recovery is only effective for low premitigation radon concentrations. However, both SSV and balanced ventilation systems are probably too expensive to be recommended in houses with low premitigation radon concentrations. A SSD system is the most cost-effective technique for reduction of high radon concentrations.

Regional and national estimates of the potential energy use, energy cost and CO 2 emissions associated with radon mitigation by sub-slab depressurization

Active sub-slab depressurization (SSD) systems are an effective means of reducing indoor radon concentrations in residential buildings. However, energy is required to operate the system fan and to heat or cool the resulting increased building ventilation. We present regional and national estimates of the energy requirements, operating expenses and CO 2 emissions associated with using SSD systems at saturation (i.e. in all US homes with radon concentrations above the EPA remediation guideline and either basement or slab-on-grade construction). The primary source of uncertainty in these estimates is the impact of the SSD system on house ventilation rate. Overall, individual SSD system operating expenses are highest in the Northeast and Midwest at about $99 y −1, and lowest in the South and West at about $66 y −1. The fan consumes, on average, about 40% of the end-use energy used to operate the SSD system and accounts for about 60% of the annual expense. At saturation, regional impacts are largest in the Midwest because this area has a large number of mitigable houses and a relatively high heating load. We estimate that operating SSD systems in US houses, where it is both appropriate and possible (about 2,6 million houses), will annually consume 1.7 × 10 4 (6.4 × 10 3 to 3.9 × 10 4) TJ of end-use energy, cost $230 (130 to 400) million (at current energy prices), and generate 2.0 × 10 9 (1.2 × 10 9 to 3.5 × 10 9) kg of CO 2. Passive or energy efficient radon mitigation systems currently being developed offer opportunities to substantially reduce these impacts.