Quantification of Degradation of Chlorinated Hydrocarbons in Saturated Low Permeability Sediments Using Compound-Specific Isotope Analysis

Abstract

This field and modeling study aims to reveal if degradation of chlorinated hydrocarbons in low permeability sediments can be quantified using compound-specific isotope analysis (CSIA). For that purpose, the well-characterized Borden research site was selected, where an aquifer−aquitard system was artificially contaminated by a three component chlorinated solvent mixture (tetrachloroethene (PCE) 45 vol %, trichloroethene (TCE) 45 vol %, and chloroform (TCM) 10 vol %). Nearly 15 years after the contaminant release, several highresolution concentration and CSIA profiles were determined for the chlorinated hydrocarbons that had diffused into the clayey aquitard. The CSIA profiles showed large shifts of carbon isotope ratios with depth (up to 24‰) suggesting that degradation occurs in the aquitard despite the small pore sizes. Simulated scenarios without or with uniform degradation failed to reproduce the isotope data, while a scenario with decreasing degradation with depth fit the data well. This suggests that nutrients had diffused into the aquitard favoring stronger degradation close to the aquifer−aquitard interface than with increasing depth. Moreover, the different simulation scenarios showed that CSIA profiles are more sensitive to different degradation conditions compared to concentration profiles highlighting the power of CSIA to constrain degradation activities in aquitards.

Multi-method assessment of the intrinsic biodegradation potential of an aquifer contaminated with chlorinated ethenes at an industrial area in Barcelona (Spain)

“…Likewise, co-contamination and off-site sources of the contaminants make difficult site degradation assessments. To overcome this bottleneck, a more reliable method that is increasingly used to monitor in situ chlorinated ethenes transformation is the compound specific isotope analysis (CSIA) (Hunkeler et al, 1999;Nijenhuis et al, 2007;Palau et al, 2014;Wanner et al, 2016).…”

Strategies for Managing Risk due to Back Diffusion

“…The long‐term significance of abiotic or biotic degradation in aquitards or other unconsolidated LPZ media relative to back diffusion has also been demonstrated experimentally (Wanner et al 2016; Schaefer et al 2018) and in modeling simulations (Chambon et al 2010; Wanner et al. 2018).…”

Three Decades of Solvent Bioremediation: The Evolution from Innovation to Conventional Practice

“…Within these slower advection zones, microbial species still proliferate and have access to electron donor. Similarly, substrate transport via diffusion also occurs across the immediate interface between slow advection and true storage zones where microbial populations can still contribute meaningful treatment benefit (Wanner et al ). While these transport processes take some time, sustained treatment across these interfaces is fostered by routine carbon delivery, substrates with extended longevity, and the recycling of cellular biomass to generate additional hydrogen via endogenous cell decay.…”