Environmental Information

PBS Engineering and Environmental, Inc., the environmental consultant working with MPC Builders, is regularly monitoring both ambient air and soil gas at locations across the Millpond Crossing development. They use a combination of sensitive handheld instruments that are able to detect and report gas concentrations down to the parts-per-billion range.  

Soil gas is gas is trapped in subsurface pore spaces between the grains of sand, clay, silt, and wood. The soil gas being monitored is gaseous vapors that are generated from the biological decomposition of organic material in the soil. Under the right conditions, soil gas can migrate up from the subsurface to outdoor or indoor ambient air. 

The consultant is regularly monitoring a network of permanent soil gas monitoring points located throughout the developed and undeveloped portions of the neighborhood. These can be identified by their concrete monuments and steel lids that are flush with the ground surface. Shallow wells (or cylinders) are located inside each monument, designed to collect vapors trapped in the soil’s pore spaces. Monitoring provides the consultant information regarding gas concentrations and the relative pressure of soil gases. This provides information to understand if vapors have the potential to migrate laterally or vertically from the soil pore spaces to other areas, such as outdoor air, or potentially into crawl spaces, garages, or home’s indoor air.

Soil gas has the potential to reach ambient outdoor air by migrating through the pore spaces to the atmosphere. Measurable concentrations above background atmospheric concentrations have not been identified in outdoor air. The consultant monitors outdoor ambient air primarily to determine if background sources of gases are present prior to completing assessment of crawlspaces or other areas. 

The consultant is routinely assessing the potential for vapors to reach crawlspaces under the homes by directly monitoring ambient air inside the crawlspaces. This is accomplished by drawing a vacuum on dedicated sampling ports, accessible from the home’s exterior, that direct air from the crawl space’s central air column directly to the consultant’s instruments.

Much like a fluid, and under the right conditions, soil gas has the potential to migrate in the subsurface to other spaces, such as utility trenches, utility conduits, or vaults. These features can collect gases and prevent them from escaping to ambient air. The consultant has monitored accessible utility features located across the developed portion of the property, and routinely monitors a representative network of in-ground utility features to check for the potential accumulation of biogases. These features include underground communications vaults, water meter vaults, and sewer cleanouts.  

DEQ has established site-specific cleanup values for the Millpond Crossing Development. The cleanup values apply site-wide to the site boundaries. Cleanup values have been established for two compounds: methane and hydrogen sulfide. The cleanup values address each compound’s primary risk. For methane, the primary risk is flammability, or explosivity. For hydrogen sulfide, the primary risk is from toxicity (that is, impacts to human health from prolonged or acute exposure to the gas).

MPC Builder’s environmental consultant routinely monitors for both compounds within subsurface soil pores (soil gas), in outdoor ambient air (background), and in the ambient air of utility confined spaces (such as in-ground utility vaults) and crawl spaces below existing homes. 

Routine monitoring has found soil gas concentrations (in subsurface yard spaces) of methane and hydrogen sulfide that exceed the soil gas cleanup values in the yard spaces. These areas will continue to be monitored to assess if there are any health risks that should be addressed. But in crawl spaces, utility confined spaces, and ambient air, there have not been any concentrations of concern, suggesting that the gas is confined to subsurface soil pores where the gas is being generated. MPC Builder’s environmental consultant is completing routine monitoring and additional targeted studies to confirm this hypothesis.

Methane 

The site-specific cleanup value for methane in soil gas, and the air in utility confined spaces, crawl spaces, and outdoor spaces is 1.25 percent by volume (pbv). This value is 25 percent of the lower explosivity limit (LEL). This value also corresponds to 12,500 parts per million (ppm) methane.

Concentrations of methane equal to or greater than 5 pbv and less than 15 pbv in confined spaces (that is, without any circulation) fall within range where explosivity is a concern. Concentrations less than 5 pbv are below the LEL, while concentrations greater than 15 pbv are above the upper explosivity limit (UEL). 

Methane exceeding the UEL, if diluted with oxygen, can become explosive. But to create a flammable or explosive condition, you need (1) a fuel source (here, methane), (2) oxygen, and (3) sufficient heat. Without all three, a combustible condition is not possible. At Millpond Crossing, monitoring of crawl spaces, utility confined spaces, and outdoor ambient air have to date not identified the presence of methane, indicating that the gas is confined to the soil pores where it is being generated. For those site soils, routine monitoring has shown there is little to no oxygen present in the soil gas. And without oxygen, the presence of methane, even above the site-specific action level, is not explosive. 

Hydrogen Sulfide

DEQ has set the site-specific cleanup value for hydrogen sulfide in soil gas at 0.0467 parts per million (ppm). For utility confined spaces, it is 0.07 ppm. And for crawl spaces, indoor air, and outdoor ambient air, it is 0.0014 ppm.

For comparison purposes, the recommended exposure limit (REL) set by the National Institute of Safety and Health (NIOSH) for occupational worker exposure to hydrogen sulfide is 10 ppm for 10-minutes. And the NIOSH immediately dangerous to life or health (IDLH) value for hydrogen sulfide is 100 ppm.