Soil Gas Movement in Unsaturated Systems

An understanding of gas transport in unsaturated media is important for evaluation of soil aeration or movement of O 2 from the atmosphere to the soil. Soil aeration is critical for plant root growth because roots generally cannot get enough O 2 from leaves. Evaluation of gas movement is also important for estimating transport of volatile and semivolatile organic compounds from contaminated sites through the unsaturated zone to the groundwater. The use of soil venting, or soil vapor extraction, as a technique for remediating contaminated sites has resulted in increased interest in gas transport in the unsaturated zone (Rathfelder et al., 1995). Migration of gases from landfills, such as methane formed by decomposition of organic material, is important in many areas (Moore et al., 1982; Thibodeaux et al., 1982). Soil gas composition has also been used as a tool for mineral and petroleum exploration and for mapping organic contaminant plumes. An understanding of gas transport is important for evaluating movement of volatile radionuclides, such as 3 H, 14 C and Rd from radioactive waste disposal facilities. The adverse health effects of radon and its decay products have led to evaluation of transport in soils and into buildings (Nazaroff, 1992). A thorough understanding of gas transport is required to evaluate these issues. Gas in the unsaturated zone is generally moist air, but it has higher CO 2 concentrations than atmospheric air because of plant root respiration and microbial degradation of organic compounds. Oxygen concentrations are generally inversely related to CO 2 concentrations because processes producing CO 2 generally deplete O 2 levels. Contaminated sites may have gas compositions that differ markedly from atmospheric air, depending on the type of contaminants. This chapter will focus primarily on processes of gas transport in unsaturated media. The following issues will be evaluated: