Remediation of chlorinated ethenes in fractured sandstone by natural and enhanced biotic and abiotic processes: A crushed rock microcosm study

Biostimulation was identified as a potential technology to treat a fractured sandstone aquifer contaminated with trichloroethene (TCE) and cis-1,2-dichloroethene (cis-DCE). Most of the mass of TCE and cis-DCE resides within the rock matrix and strategies to restore ground water to pre-existing conditions are severely limited by back diffusion. A microcosm study using crushed rock and groundwater from the site was performed to assess biostimulation and natural attenuation. Lactate, hydrogen release compound (R) (HRC), and emulsified vegetable oil (EVO) significantly increased the rate of TCE reduction to cis-DCE. Lactate also stimulated dechlorination of cis-DCE to vinyl chloride (VC) and ethene, suggesting the presence of indigenous Dehalococcoides. Illumina sequencing and qPCR analyses suggest that reductive dechlorination of TCE to cis-DCE is mediated by Geobacter spp. while Dehalococcoides spp. perform reduction of cis-DCE to VC and ethene. The rate of VC reduction to ethene was much slower than the reduction of TCE to cis-DCE and cis-DCE to VC, indicating the indigenous Dehalococcoides perform the final step co-metabolically. This was confirmed in enrichment cultures fed with only VC. Consequently, biostimulation may create an elevated risk due to transient accumulation of VC. Abiotic transformation of TCE and cis-DCE was observed based on accumulation of C-14-labeled products from C-14-TCE and C-14-cis-DCE, as well as enrichment in delta C-13-cis-DCE in the absence of reductive dechlorination. Based on accumulation rates for C-14-products in unamended microcosms, pseudo-first-order rates for abiotic transformation were 0.038 yr(-1) for TCE and 0.044 yr(-1) for cis-DCE. These rates within the rock matrix may be sufficient to support natural attenuation in this diffusion controlled system. (C) 2018 Elsevier B.V. All rights reserved.