Soil gas consists of the various gases found within the pores between soil particles. These pores, when not filled with water, are occupied by air. The makeup of soil gas varies significantly with location and environmental conditions.
Natural Composition and Processes
The air within soil pores is a dynamic mixture, primarily composed of nitrogen (around 79.2%), oxygen (about 20.6%), and carbon dioxide (approximately 0.25%). Other naturally occurring gases found in soil include methane, nitric oxide, nitrous oxide, and ammonia. The concentrations of these gases are not static; they fluctuate due to ongoing chemical and biological processes within the soil.
One significant process influencing soil gas composition is soil respiration. This refers to the release of carbon dioxide from the soil surface, resulting from the metabolic activities of plant roots and soil microorganisms as they decompose organic matter. The rate of soil respiration is affected by factors such as soil organic matter content, temperature, moisture levels, and aeration.
Gas movement within the soil also occurs through diffusion, where molecules move from areas of higher concentration to lower concentration. Environmental factors like temperature, moisture content, and even the thawing of permafrost can further influence the variability and movement of soil gases. Wet, oxygen-poor soils, such as those found in peatlands or rice fields, are common sources of methane emissions.
Health Risks from Contaminants
Soil gas can also contain harmful contaminants, posing health risks if they migrate into indoor spaces. Two categories of these contaminants are radon and volatile organic compounds (VOCs). Radon is a naturally occurring radioactive gas formed from the decay of uranium found in rocks and soil. It is colorless, odorless, and tasteless, undetectable without specialized testing.
Breathing radon gas increases lung cancer risk, and this risk is significantly higher for individuals who also smoke. Radon can accumulate in buildings, especially in basements and confined areas, as it seeps up from the ground. The U.S. Environmental Protection Agency (EPA) estimates that radon is a leading cause of lung cancer among non-smokers, accounting for thousands of deaths annually in the United States.
Volatile organic compounds (VOCs) are human-made chemicals with a high vapor pressure that readily vaporize into the air. They originate from industrial processes, fuel spills, dry cleaning operations, and common household products like paints, cleaning supplies, and pesticides. Exposure to VOC vapors can cause a range of health effects, including irritation to the eyes, nose, and throat, headaches, nausea, and damage to organs like the liver, kidneys, or central nervous system. Some VOCs are also known or suspected carcinogens.
The migration of these harmful gases, like radon and VOCs, from the soil into buildings is known as vapor intrusion. This occurs when negative air pressure inside a building draws vapors through cracks in foundations, utility penetrations, or sumps. Once inside, these contaminants impact indoor air quality and create health risks for occupants.
Detection and Management
Detecting harmful soil gases involves specific methods. The most common technique uses a hollow metal rod inserted into the ground. For assessing soil gas beneath existing buildings, holes may be drilled into the basement floor, and a rod is then pushed into the underlying soil. The gas is drawn through this rod into a specialized sampling container or canister, which is then sent to a laboratory for analysis.
Monitoring soil gas evaluates potential risks to human health and the environment. Data from soil gas sampling can be compared to risk-based screening levels to determine if mitigation is needed. Soil gas sampling should not be conducted within 48 hours of significant rainfall (e.g., 0.5 inches or more) to avoid inaccurate results due to water-filled soil pores.
Strategies for managing or mitigating harmful soil gases, especially to prevent vapor intrusion into buildings, involve several approaches. Sealing foundation openings, such as cracks and gaps around pipes, can reduce vapor entry. Installing vapor barriers (sheets of durable plastic or geomembrane) beneath a building’s foundation is another effective method. Active systems, such as sub-slab depressurization (SSD) systems, are frequently used to mitigate vapor intrusion. These systems create negative pressure beneath the slab using a fan-driven vent system, drawing vapors from the soil and expelling them safely above the roofline.