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Previously, the authors have developed the concept of in-situ reduction of hexavalent chromium in contaminated groundwater, using various forms of active reagent delivery, based on site geohydrological conditions. While these approaches are highly successful and cost effective, in some cases, such active approaches may not be appropriate. Rather, it may be appropriate to limit the spread of chromium contamination by formation of permeable reactive barriers or reactive zones through the borehole placement of aqueous reductants. A passive approach, utilizing bore-hole placed reactive barriers and Monitored Natural Attenuation (MNA), offers real advantages for groundwater remediation, especially in situations involving groundwater in fractured or cavernous bedrock, where the water table is at great depth, or where on-going activities in the source area limit access for more active approaches. A geochemical reactive barrier was used at a South Australian timber preservation facility sited over a cavernous limestone. After remedial activities within the plume achieved significant reduction of the mobile chromium mass, the applicable regulatory agency authorized MNA for control of residual contamination. A phased approach has been utilized at a Central Valley, California timber preservation site, to develop data on the radial spread of reductant injection through diffusion, and the longevity of the effect of such injection, and a full-scale remedial approach designed, recognizing limitations on injection imposed by limited access. The paper discusses the requirements to demonstrate the effectiveness of MNA. Case histories of successful application of the integrated approach of passive reduction and MNA are presented, as a cost-effective and environmentally-protective means of accomplishing remediation of hexavalent chromium in groundwater.

Hexavalent Chromium Remediation by Bore-Hole Placed Reduction Barriers and Monitored Natural Attenuation