Development and application of an analytical model to aid design and implementation of in situ remediation technologies

Innovative in situ remediation technologies using injection/extraction well pairs which recirculate partially treated groundwater as a method of increasing overall treatment efficiency have recently been demonstrated, An important parameter in determining overall treatment efficiency is the fraction of flow that recycles between injection and extraction points. Numerical and semi-analytical methods are currently available to determine this parameter and an analytical solution has been presented for a single injection/extraction well pair. In this work, the analytical solution for fraction of recycled flow for a single injection/extraction well pair is extended to multiple co-linear well pairs. In addition, a semi-analytical method is presented which permits direct calculation of fraction recycle for a system of arbitrarily placed injection/extraction wells pumping at different rates. The solution is used to investigate the behavior of multiple well pairs, looking at how well placement and pumping rates impact capture zone width and fraction recirculation (and, therefore, overall treatment efficiency). Results of the two-well solution are compared with data obtained at a recent field-scale demonstration of in situ aerobic cometabolic bioremediation, where a pair of dual-screened injection/extraction wells was evaluated. A numerical model is then used to evaluate the impact of hydraulic conductivity anisotropy on the overall treatment efficiency of the dual-screened injection/extraction well pair. The method presented here provides a fast and accurate technique for determining the efficacy of injection/extraction systems, and represents a tool that can be useful when designing such in situ treatment systems. (C) 1999 Published by Elsevier Science B.V. All rights reserved.