Downhole Flux Chamber Testing
The principles of surface flux chamber testing have been applied to subsurface testing using a small, cylindrical chamber designed to fit inside a hollow-stem auger.
The need for subsurface flux testing came about on an uncontrolled hazardous waste site investigation. The challenge was to collect "disturbed waste" flux data from a buried chemical solid waste material so that an air emissions control strategy could be developed supporting a Remedial Investigation/Feasibility study (RI/FS) without excavating and exposing the waste.
A small scale excavation to 20 feet below land surface to collect flux data would expose hundreds of square feet of materials off-gassing potentially toxic gases. In addition, pilot-scale excavations are costly and require permits, health and safety protection for workers, and fence line air monitoring. In order to collect subsurface flux data without excavation, CE Schmidt, Ph.D developed the "downhole" flux chamber.
The downhole chamber is a "plug-flow" chamber. The flux measurement is performed by drilling to the depth of interest using a hollow-stem auger, removing the drill bit or plug, lowering the downhole chamber down the hollow-stem auger to the point of advance, and conducting a flux test at the freshly exposed surface at the advance of the auger. In-situ flux data are collected including monitoring the "emissions profile" from the soil/waste using an appropriate real-time analyzer. The compound identification, combined with the measured flux and the emissions profile, provides valuable site characterization data.
The downhole flux emission profile is useful for determining the source of compounds found in soil gas and, as such, can be used to isolate the various sources of soil gas and potential exposure. Data from the downhole chamber have been used to address a variety of project objectives other that waste site RI/FS objectives, including: differentiating sources of contamination on-site (i.e, surface contamination from soil column, from groundwater contamination); identifying off-site sources of contamination such as a migrating groundwater plume; identifying volumes of subsurface soil waste that qualifies for removal as part of a site remedial effort; determining potential exposure to below grade structures not yet constructed from deeper sources such as contaminated groundwater; and determining if soils on-site have been adequately treated by soil vapor extraction.
The downhole flux chamber technology is recommended by the US EPA for subsurface flux measurement applications like the surface flux chamber technology is recommended for surface flux measurement application.