A Multitracer Test Proving the Reliability of Rayleigh Equation-Based Approach for Assessing Biodegradation in a BTEX Contaminated Aquifer

Abstract

Compound-specific stable isotope analysis (CSIA) is one of the most important methods for assessing biodegradation activities in contaminated aquifers. Although the concept is straightforward, the proof that the method cannot be only used for a qualitative analysis but also to quantify biodegradation in the subsurface was missing. We therefore performed a multitracer test in the field with ring-deuterated (d5) and completely (d8) deuterium-labeled toluene isotopologues (400 g) as reactive tracers as well as bromide as a conservative tracer. The compounds were injected into the anoxic zone of a BTEX plume located down-gradient of the contaminant source. Over a period of 4.5 months the tracer concentrations were analyzed at two control planes located 24 and 35 m downgradient of the injection well. Deuterium-labeled benzylsuccinate was found in the aquifer, indicating the anaerobic biodegradation of deuterated toluene via the benzylsuccinate synthase pathway. Three independent methods were applied to quantify biodegradation of deuterated toluene. First, fractionation of toluene-d8 and toluene-d5 using the Rayleigh equation and an appropriate laboratory-derived isotope fractionation factor was used for the calculation of the microbial decomposition of deuterated toluene isotopologues (CSIA-method). Second, the biodegradation was quantified by the changes of the concentrations of deuterated toluene relative to bromide. Both methods gave similar results, implying that the CSIA-method is a reliable tool to quantify biodegradation in contaminated aquifers. The results of both methods yielded a biodegradation of deuterated toluene isotopologues of approximately 23-29% for the first and 44-51% for the second control plane. Third, the mineralization of deuterated toluene isotopologues was verified by determination of the enrichment of deuterium in the groundwater. This method indicated that parts of deuterium were assimilated into the biomass of toluene degrading microorganisms.

Utilization of monocyclic aromatic hydrocarbons individually and in mixture by bacteria isolated from petroleum-contaminated soil

“…In a hydrocarbon-contaminated site, it is necessary to understand what the microbial population is present and associated activities (i.e., biosurfactant production) as well as what factors influence their metabolic activities to take advantage of their remedial capacity . Perturbations to microbial populations could explain different rates of BTEX biodegradation often observed in heterogeneous environments (Fischer et al 2006). From an applied point of view, the results indicate that, in remediating contaminated environments the site-specific microbiological activities should be taken into account.…”

“…While physical/chemical routes may remove some BTEX groundwater contamination, only microbial degradation leads to a long-term removal of BTEX associated with a significant mass removal (Fischer et al 2006).…”

Multidimensional analytical models for isotope ratios in groundwater pollutant plumes of organic contaminants undergoing different biodegradation kinetics

“…This is a typical value when estimating this factor from the K OC of the compound and a typical organic matter content of aquifer sediments [33]. However, field tracer tests with aromatic compounds have indicated that retardation factors may be generally lower and lie close to 1 [16,36]. If so, sorption would not create any significant isotope fractionation.…”

Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers

“…3 Biodegraded mass of deuterated toluene calculated for the first and second control plane of a multitracer test by methods based on CSIA (dark grey), bromide (grey), and deuterium enrichment in groundwater (white). Reprinted with permission from Fischer et al (2006). Copyright © 2006, American Chemical Society verifying that in situ benzene biodegradation indeed mainly takes place under sulfate-reducing conditions at the site.…”

“…The isotope fractionation associated with sorption in the Zeitz aquifer was investigated by Fischer et al (2006) in their field tracer experiment to check potential impacts on biodegradation assessment with CSIA. Although the model suggested by Kopinke et al (2005a) predicted that sorption would significantly alter the ratio of toluene-d 8 to toluene-d 5 within the migrating front of the tracer breakthrough curve (Thullner et al 2012), this effect was not confirmed in the field.…”