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 o rder to investigate processes limiting the removal of VOCs during the later stages of SVE operations, multicomponent soil-venting experiment s were performed at different water contents in a sand tank (80 x 66 x 5 cm) in the absence of a liquid organic phase. Four chlorinated VOCs were used as the model compounds. A homogeneous packing of quartz san d was used as the model soil. Gas phase concentrations were measured a t several locations with different water saturations during each exper iment. The compounds did not adsorb onto the sand. Volatilization was the key process for VOC removal. Gas concentrations decreased more slo wly at locations with high water saturation and for compounds having a small Henry's law constant. Gas concentrations observed for experimen ts conducted at low water content were found to be a function of dimen sionless time. Tailing in gas concentration distributions at large dim ensionless times was attributed solely to diffusion in interparticle w ater and suggested that local nonequilibrium conditions prevailed; Thi s 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 d istributions could then be described quite well by means of a first-or der kinetic approach using calibrated mass transfer coefficients.