Although soil vapor extraction (SVE) is well established as an effective method of removing volatile organic compounds from soil, the spatial extent of extraction influence is often poorly understood. Typically, the limit of a detectable vacuum response in vadose monitoring wells is used to define radius of influence without regard for the pneumatic conductivity, vacuum measurement limitations, or barometric pressure effects. Vapor flow conditions are often assumed to be confined, but even beneath surface pavement or clay layers, vertical leakance of air is usually a dominant influence.
Transient-response pneumatic testing and analysis with a leaky aquifer solution can quantify vertical leakance of air as well as the horizontal pneumatic conductivity of the primary extraction zone. An additional benefit of automated transient-response testing is that the extent of SVE influence detected is usually substantially larger, because much smaller pressure changes can be detected and attributed to SVE than is possible with manual static measurements. Simple vapor flow models incorporating leakance based on the results of pneumatic testing can be developed as tools for design and evaluation of SVE systems.
Soil gas tracer (SGT) investigations utilizing sulfur hexafluoride (SF6) can be used to determine vapor travel times for assessing variability of pneumatic conductivity at an SVE site. The SGT technique can also be used evaluate extent of SVE influence, to investigate relative influence of extraction wells, and evaluate flow within and between different horizons. In a case study, the tracer data indicated the presence of preferential flow through macropores and provided the opportunity to calibrate and verify models of subsurface vapor flow.