Standard Guide for Evaluating Potential Hazard in Buildings as a Result of Methane in the Vadose Zone

Importancia y uso:

5.1 Several different factors should be taken into consideration when evaluating methane hazard, rather than, for example, use of a single concentration-based screening level as a de-facto hazard assessment level. Key variables are identified and briefly discussed in this section. Legal background information is provided in Appendix X3. The Bibliography includes references where more detailed information can be found on the effect of various parameters on gas concentrations.

5.2 Application—This guide is intended for use by those undertaking an assessment of hazards to people and property as a result of subsurface methane suspected to be present based on due diligence or other site evaluations (see 6.1.1).

5.2.1 This guide addresses shallow methane, including its presence in the vadose zone; at residential, commercial, and industrial sites with existing construction; or where development is proposed.

5.3 This guide provides a consistent, streamlined process for deciding on action and the urgency of action for the identified hazard. Advantages include:

5.3.1 Decisions are based on reducing the actual risk of adverse impacts to people and property.

5.3.2 Assessment is based on collecting only the information that is necessary to evaluate hazard.

5.3.3 Available resources are focused on those sites and conditions that pose the greatest risk to people and property at any time.

5.3.4 Response actions are chosen based on the existence of a hazard and are designed to mitigate the hazard and reduce risk to an acceptable level.

5.3.5 The urgency of initial response to an identified hazard is commensurate with its potential adverse impact to people and property.

5.4 Limitations—This guide does not address potential hazards from other gases and vapors that may also be present in the subsurface such as hydrogen sulfide, carbon dioxide, and/or volatile organic compounds (VOCs) that may co-occur with methane. If the presence of hydrogen sulfide or other potentially toxic gases is suspected, the analytical plan should be modified accordingly.

5.4.1 The data produced using this guide should be representative of the soil gas concentrations in the geological materials in the immediate vicinity of the sample probe or well at the time of sample collection (that is, they represent point-in-time and point-in-space measurements). The degree to which these data are representative of any larger areas or different times depends on numerous site-specific factors. The smaller the data set being used for hazard evaluation, the more important it is to bias measurements towards worst-case conditions.

5.5 Variables and Site-Specific Factors that May Influence Data Evaluation: 

5.5.1 Gas Transport Mechanisms—Methane migration in soil gas results from pressure-driven flow, advection and diffusion. Advective transport (for example, biogas within a soil gas matrix) and pressure-driven flow (for example, pure or nearly pure biogas) has been associated with methane incidents (for example, fires or explosions), whereas no examples are known of methane incidents resulting from diffusive transport alone. Therefore, diffusion is not considered a key transport mechanism when evaluating methane hazard.

5.5.1.1 The potential for significant rates of soil gas transport can often be recognized by relatively high differential pressures (for example, >500 Pa [2 in. H₂O]), high concentrations of leaked or generated gas, and concurrent displacement of atmospheric gases (nitrogen, argon) from the porous soil matrix. Alternatively, gas flowrates can be measured directly (see Appendix X4).

5.5.2 Effect of Gas Transport Mechanisms: 

5.5.2.1 Near-Surface Advection Effects—Within buildings, across building foundations, and in the immediate subsurface vicinity of building foundations, advective flow may be driven by temperature differences, the on-off cycling of building ventilation systems, the interaction of wind and buildings, and/or changes in barometric pressure. These mechanisms can pump air back and forth between the soil and the interior of structures. The effects may be significant in evaluation of VOC or radon migration between buildings and the subsurface, but generally are relatively minor factors in evaluation of methane migration and hazard unless the source of methane is in very shallow soils.

5.5.2.2 Source Zone Flow Effects—Biogenic (microbial) gas generation (methanogenesis) results in a net increase in molar gas volume near the generation source. The resulting increased gas pressure causes gas flow away from the source zone. This gas flow typically originates near sources of buried organic matter. Pressure-driven flow can also result from pressurized subsurface gas sources including leaks from natural gas distribution systems, subsurface gas storage, or seeps from natural gas reservoirs. The evaluation of pressurized sources of gas themselves (for example, pipelines, reservoirs, or subsurface storage) is outside the scope of this guide (see 1.5.3 – 1.5.6).

5.5.2.3 Subsurface soil gas pressure change can also occur in other instances, such as with a rapidly rising or falling water table in a partially confined aquifer or barometric pumping of fractured bedrock or very coarse gravel. This effect may occur in conjunction with advection of either dilute or high-concentration soil gases and may be irregular or intermittent. Induced pressure driven flow in response to diurnal barometric pressure changes is both upward and downward and there is no net upward pressure gradient. The CSM should consider the potential for induced pressure-driven flow (which is sometimes referred to as repressurization).

(1) Significant gas flow due to barometric pressure fluctuations may occur for nearby subsurface gas void volumes (nominal gas volumes of 4000 m³ or greater) in confined coarse sand or gravel connected to a building or enclosure

(2) Significant gas flow due to water table changes may occur for changes of 10 cm/day or greater in confined coarse sand or gravel connected to a building or enclosure.

5.5.3 Effect of Land Use—Combustible soil gas is a concern mostly for sites with confined habitable space because of the safety risk. Combustible soil gas can also be a concern at sites with other types of confined spaces, such as manholes or buried vaults where a source of ignition may be present. Proximity or entry to such spaces may require consideration of hazards associated with methane.

5.5.4 Pathways—Pathways into buildings from the soil can include cracks in slabs, unsealed space around utility conduit penetrations, the annular space inside of dry utilities (electrical, communications), elevator pits (particularly those with piston wells), basement sumps, sewer lines with dry water traps, and other avenues.

5.5.5 Effect of Hardscape and Softscape—Any capping of the ground surface can impede the natural venting of soil gas with concrete being generally less permeable than asphalt. Hardscape and well irrigated softscape both present barrier conditions. Existing hardscape/softscape conditions should be noted during soil gas investigations. Proposed hardscape/softscape conditions should be considered when formulating alternatives for action at sites where methane hazard is to be mitigated. The potential for future hardscape/softscape conditions also should be taken into account when evaluating the representativeness of methane and pressure data.

5.5.6 Effect of Soil Physical Properties—The diffusion of gas through soil is controlled by the air-filled porosity of the soil, whereas the advection and pressure-driven flow of gas through soil is controlled by the permeability of the soil. Two soils can have similar porosities but different permeabilities and vice-versa. The effective porosity of a soil may be different than the total porosity depending on whether the soil pores are connected or not. For methane transport, advective and pressure-driven flow is of much more concern than diffusive flow, so permeability is a more important variable than porosity. Large spaces such as fractures in fine-grained soils can impart a high permeability to materials that would otherwise have a low permeability. Soil moisture can reduce the air-filled porosity of soil and the gas permeability thereby reducing both diffusive and advective flow of soil gas.

5.5.7 Effect of Environmental Variables—A number of environmental variables can affect the readings taken in the field and can be important in interpreting the readings once taken. The effect of environmental variables tends to be greatest for very shallow measurements in the vadose zone and typically is of limited importance at depths of 1.5 m and greater.

5.5.8 Atmospheric Pressures and Barometric Lag—A falling barometer may leave soil gas under pressure as compared with building interiors enabling increased soil gas flux out of the soil and into structures. The interpretation of barometric lag data should take into account the type of soil. Barometric lag is most pronounced in tight (clayey) soils in which the flow of gases is retarded; barometric lag is least pronounced in granular (sandy) soils that provide the greatest permeability for the flow of gas. The potential for pressure-driven gas transport through soil is significant only for permeable soil pathways (that is, air-filled coarse sands and gravels).

5.5.9 Precipitation—Normal outdoor soil gas venting (that is, emissions at soil surface) is impeded when moisture fills the surface soil pore space. Infiltrating rainwater may displace soil gas and cause it to vent into structures. Increases in soil moisture following rain or other precipitation events can lead to enhanced rates of biogas generation, which may be evaluated through repeated measurements.

5.5.10 Effect of Sampling Procedures—Sampling probes (test wells) typically are designed to identify soil gas pressures and maximum soil gas concentrations at the point of monitoring. The sequence of steps (for example, purging, pressure and concentration readings, and so forth) can affect the results. For differential pressure measurements, gages capable of measuring 500 Pa (2 in. H₂O) may be used. Ideally, the gage or gages should be capable of measurements over a range of pressures (for example, 0 to 1,250 Pa (0 to 5 in. H₂O)) and have a resolution of at least 25 Pa (0.1 in. H₂O). See the Bibliography for references on equipment for concentration and differential pressure measurements. Initial readings of pressure should be taken before any gas readings, as purging can reduce any existing pressure differential and steady-state conditions may not be reestablished for some time afterwards. Soil gas pressures and soil gas concentrations should also be measured after purging. The recovery, or change of pressure with time, may also be of interest. Gas pressure readings taken in groundwater monitoring wells may not be representative of vadose zone pressures.

5.6 Applicability of Results—Instantaneous data from monitoring probes represent conditions at a point in space and time. Worst-case, short-term impacts are of interest in a methane evaluation because of the acute risk posed by methane. Single-sampling events in which data are collected from a number of points at different locations may be sufficient if there is a robust CSM (that is, accounting for worst-case conditions) and the site is well understood. If site results are inconsistent with the CSM, additional data may be needed to address uncertainties and increase the statistical reliability and confidence in the results.

Subcomité:

E50.02

Referida por:

E3300-21, E3382-24, E3361-22

Volúmen:

11.05

Número ICS:

13.080.05 (Examination of soil in general)

Palabras clave:

hazard; measurement; methane; soil gas; vadose zone;