Site-specific design for dual phase recovery and stabilization of pooled DNAPL

Volume reduction and lowering of capillary pressure within a large DNAPL po ol are utilized as objectives in the design of a large-scale dual phase rec overy system at a chemical manufacturing facility in the United States. By reducing DNAPL pool height through mass removal, capillary pressure is lowe red, resulting in a reduced potential for future vertical and horizontal mo bilization of the chlorinated solvent DNAPL pool. The DNAPL pool extends ov er an approximately 200 m by 275 m area in low permeability fill deposits o verlying a clay aquitard. A three-dimensional multiphase flow model was emp loyed to arrive at a final design incorporating nine horizontal drains (tot al length 664 m) and a pulsed pumping system. The numerical model was calib rated to the results of a M-day field pilot-test involving the removal of a pproximately 25,000 L of DNAPL from a single, 55 m long horizontal drain. N umerical simulation revealed that gravity drainage, as opposed to hydraulic gradients in the water phase, is the dominant recovery mechanism at this s ite. This stems from the relatively high density and the viscosity of the D NAPL, and the relatively low permeability of the formation deposits. The us e of pulsed pumping is shown to reduce the volume of contaminated ground wa ter recovered from the 9-drain system, without significant reduction of the total volume of DNAPL recovered.