Long‐Term Impacts on Groundwater and Reductive Dechlorination Following Bioremediation in a Highly Characterized Trichloroethene DNAPL Source Area

Abstract

High‐resolution soil and groundwater monitoring was performed to assess the long‐term impacts of bioremediation using bioaugmentation with a dechlorinating microbial consortium (and sodium lactate as the electron donor) in a well‐characterized trichloroethene (TCE) dense nonaqueous phase liquid (DNAPL) source area. Monitoring was performed up to 3.7 years following active bioremediation using a high‐density monitoring network that included several discrete interval multi‐level sampling wells. Results showed that despite the absence of lactate, lactate fermentation transformation products, or hydrogen, biogeochemical conditions remained favorable for the reductive dechlorination of chlorinated ethenes. In locations where soil data showed that TCE DNAPL sources persisted, local contaminant rebound was observed in groundwater, whereas no rebound or continuous decreases in chlorinated ethenes were observed in locations where DNAPL sources were treated. While ethene levels measured 3.7 years after active treatment suggested relatively low (2 to 30%) dechlorination of the parent TCE and daughter products, carbon stable isotope analysis showed that the extent of complete dechlorination was much greater than indicated by ethene generation and that the estimated first‐order rate constant describing the complete dechlorination of TCE at 3.7 years following active bioremediation was approximately 3.6 y–1. Overall, results of this study suggest that biological processes may persist to treat TCE for years after cessation of active bioremediation, thereby serving as an important component of remedial treatment design and long‐term attenuation.

Assessment of 2,4-Dinitroanisole Transformation Using Compound-Specific Isotope Analysis after In Situ Chemical Reduction of Iron Oxides

“…Several approaches have been evaluated to enhance the availability of reactive Fe(II) including biostimulation, 7,8 bioaugmentation, 9,10 additions of external electron donors (i.e., in situ chemical reduction; ISCR) including dithionite, 11,12 polysulfides, 13 and (sulfidized) ZVI, 14,15 and combinations thereof. 16 To date, several of these strategies have been successfully applied to a number of environmental contaminants including heavy metals 12,17,18 and chlorinated solvents.…”

High‐Resolution Characterization of a Chlorinated Solvent Impacted Aquifer Using a Passive Profiler

“…After enhanced dissolution and bioremediation, the expected remaining zone of residual DNAPL at the time of this study was in the region where HRPP‐3 (Figure 2) was located (Schaefer et al. 2018). However, overall site data collected during this demonstration revealed primarily reductive dechlorination daughter products of TCE, cis ‐dichloroethene ( cis ‐DCE) and vinyl chloride (VC), suggesting either rapid bioconversion of dissolved TCE or the general absence of DNAPL in the test area.…”

“…In addition, relative permeability was estimated based on MIP/HPT data (MIP-EC measures EC, and the HPT measures hydraulic pressure which inversely correlates with hydraulic conductivity). The site-average velocity, calculated using hydraulic gradient, porosity, and hydraulic conductivity (K) measured with well pump tests, is estimated as 2 cm/ day as shown in Figure 6 (Schaefer et al 2018). Velocities measured from three PFMs (Annable et al 2005) installed in SPW 3-1 (5.2-5.9 m bgs), SPW 3-2 (5.9-6.7 m bgs), and PEW 02 (4.6-7.6 m bgs) were 4, 3, and 8 cm/day, respectively ( Figure 6).…”

“…Velocity measurements at the Alameda site include an estimated average site velocity based on well pump tests (Schaefer et al. 2018) and PFMs installed in monitoring wells. In addition, relative permeability was estimated based on MIP/HPT data (MIP‐EC measures EC, and the HPT measures hydraulic pressure which inversely correlates with hydraulic conductivity).…”

“…The site‐average velocity, calculated using hydraulic gradient, porosity, and hydraulic conductivity (K) measured with well pump tests, is estimated as 2 cm/day as shown in Figure 6 (Schaefer et al. 2018). Velocities measured from three PFMs (Annable et al.…”