Degradation of chloroethenes in the transition zone between aquifers and aquitards

In the transition zone between aquifers and basal aquitards, the perchloroethenepools are more recalcitrant than those elsewhere in the aquifer. The aim of this study is to demonstrate that the transition zone is an ecotone where the biodegradation of chloroethenes from aged-pools of perchloroethene is favored. A field site was selected where an aged pool exists at the bottom of a transition zone. Two boreholes were drilled to obtain sediment and groundwater samples to perform chemical, isotopic and molecular analyses. The main results were: i) the transition zone is an ecotone with a high microbial richness; ii) partial reductive dechlorination coexists with denitrification, Fe and Mn reduction, and sulfatereduction; iii) the high concentrations of perchloroethene in this zone resulted in a decrease in microbial richness; iv) however, the reductive dechlorination in this area is not inhibited. These findings suggest that biostimulation and/or bioaugmentation could be applied to promote complete reductive dechlorination and to enhance the dissolution of more DNAPL.