Experimental and Numerical Investigation of Soil Vapor Extraction

In recent years soil vapor extraction (SVE) has been used extensively to remove volatile organic compounds (VOCs) from the vadose zone. In order to investigate processes limiting the removal of VOCs during the later stages of SVE operations, multicomponent soil-venting experiments were performed at different water contents in a sand tank (80 × 66 × 5 cm) in the absence of a liquid organic phase. Four chlorinated VOCs were used as the model compounds. A homogeneous packing of quartz sand was used as the model soil. Gas phase concentrations were measured at several locations with different water saturations during each experiment. The compounds did not adsorb onto the sand. Volatilization was the key process for VOC removal. Gas concentrations decreased more slowly at locations with high water saturation and for compounds having a small Henry's law constant. Gas concentrations observed for experiments conducted at low water content were found to be a function of dimensionless time. Tailing in gas concentration distributions at large dimensionless times was attributed solely to diffusion in interparticle water and suggested that local nonequilibrium conditions prevailed. This hypothesis was supported by numerical simulations based on the local equilibrium assumption (LEA) and a first-order kinetics approach. For the experiments conducted at low water saturations the LEA was valid only for small dimensionless times. Tailing in the gas concentration distributions could then be described quite well by means of a first-order kinetic approach using calibrated mass transfer coefficients.

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