Numerical investigations of optimal well spacing and the effect of screen length and surface sealing on gas flow toward an extraction well

Abstract

Interest in the use of vapor extraction to remove volatile and semi-volatile organic compounds (VOC's) from unsaturated soils has been growing recently. To quantify the lateral extent of the zone of effective influence of a vapor extraction well, a new term, distance of effective sweep ( R e) is introduced. This distance can be used to characterize the flow pattern, and it provides a guideline for determining the number and location of extraction wells needed at a contaminated site. In the present work, R e represents the distance of effective sweep from a line of wells (line sink). Analysis was performed to examine the role of the screen length, l, the extension of soil-surface sealing, r, and the vadose zone thickness, H, on R e, in a case of an array of wells where the flow is two-dimensional in the vertical plane. A regression equation was found to describe the relationships between R e and the ratio of l/ H. Additional analysis shows that vapor extraction treatment without soil-surface sealing may be most effective when the contaminant plume lateral dimension is smaller than one-half of the vadose zone height. Simulations with different r reveal that R e increases linearly with r. Simulations of transient scenarios indicated that the concept of distance of effective sweep is limited to permeable soils because the velocities developed in clay soils at the “distance of effective sweep” may be too low to be considered effective, and the time needed for the development of steady-state flow is very significant when compared to the duration of the vapor extraction operation. Additional analysis was performed on the distance of influence, R i, which is the conventional parameter used to characterize the flow. The analysis suggests that R i has linear relationship with l/ H, and that for the transient phase, R i( t), may depend on the permeability, K, in a logarithmic manner of the form: R i( t) = a( t) + b( t) ln K