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

Abstract

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 groundwater 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® (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 14C-labeled products from 14C-TCE and 14C-cis-DCE, as well as enrichment in δ13C-cis-DCE in the absence of reductive dechlorination. Based on accumulation rates for 14C-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.