Biotic and abiotic reductive dechlorination of chloroethenes in aquitards

Abstract

Chlorinated solvents occur as dense nonaqueous phase liquid (DNAPL) or as solutes when dissolved in water. They are present inmany pollution sites in urban and industrial areas. They are toxic, carcinogenic, and highly recalcitrant in aquifers and aquitards. Inthe latter case, they migrate by molecular diffusion into the matrix. When aquitards are fractured, chlorinated solvents also penetrateas a free phase through the fractures. The main objective of this study was to analyze the biogeochemical processes occurring insidethe matrix surrounding fractures and in the joint-points zones. The broader implications of this objective derive from the fact that,incomplete natural degradation of contaminants in aquitards generates accumulation of daughter products. This causes steepconcentration gradients and back-diffusion fluxes between aquitards and high hydraulic conductivity layers. This offers opportunitiesto develop remediation strategies based, for example, on the coupling of biotic and reactive abiotic processes. The main resultsshowed: 1) Degradation occurred especially in the matrix adjacent to the orthogonal network of fractures and textural heterogeneities,where texture contrasts favored microbial development because these zones constituted ecotones. 2) A dechlorinating bacterium notbelonging to the Dehalococcoides genus, namely Propionibacterium acnes, survived under the high concentrations of dissolvedperchloroethene (PCE) in contact with the PCE-DNAPL and was able to degrade it to trichloroethene (TCE). Dehalococcoides genus was able to conduct PCE reductive dechlorination at least up to cis-1,2-dichloroethene (cDCE), which shows again the potential of themedium to degrade chloroethenes in aquitards. 3) Degradation of PCE in the matrix resulted from the coupling of reactive abiotic andbiotic processes¿in the first case, promoted by Fe sorbed to iron oxides, and in the latter case, related to dechlorinatingmicroorganisms. The dechlorination resulting from these coupling processes is slow and limited by the need for an adequate supply ofelectron donors.