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On diffusive mass-transfer limitations in relation to remediation of polluted groundwater systems

Author: Griffioen, J. · Hetterschijt, R.A.A.
Type:article
Date:1998
Institution: Nederlands Instituut voor Toegepaste Geowetenschappen TNO
Source:Harder, W.Arendt, F., Consoil '98 ; 6th international FZK/TNO conference on contaminated soil, organized by Research Centre Karlsruhe (KZK, D), TNO, in cooperation with Scottish enterprise, May 17-21, 1998, EICC Edinburgh; vol. 1, 231-240
Identifier: 529136
Keywords: Environment · Diffusion · Immobile zones · In-situ groundwater remediation · Mass transport · Pumping

Abstract

Limited diffusive mass transfer is one cause for stagnant groundwater remediations. We characterise the diffusive mass transfer process as a three stage cyclic process of mass transfer into and out of stagnant or immobile zones. The first stage is the contamination stage, where net inward diffusive mass transfer happens and the last stage is the remediation stage where the opposite happens. An intermittent stage may happen when the immobile zone has reached equilibrium with the mobile zone. Using an analytical solution for diffusive mass transfer into and out of an immobile zone, we show that decades may be involved to reach this stage for natural systems. If this stage does not get reached, continuous inward diffusion happens during the remediation stage. Then, the remediation period required lasts much longer than the contamination period available. Numerical simulation for a simplified two-layer mobile/immobile system further shows that excessive pumping rates do not speed up remediation, since the rate-limiting process is not significantly affected by this. Intermittent pumping or continuous pumping at low rates is equally effective from remediation point of view. Other criteria are then decisive whether continuous pumping or intermittent pumping is most interesting.