Carbon and chlorine isotopologue fractionation of chlorinated hydrocarbons during diffusion in water and low permeability sediments

Abstract

To identify reactive processes in diffusion dominated water-saturated systems using compound-specific isotope analysis (CSIA), the effect of the diffusive transport process on isotope ratios needs to be known. This study aims to quantify the magnitude of carbon and chlorine isotopologue fractionation of two chlorinated hydrocarbons (trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA)) during diffusion in the aqueous phase and to relate for the first time laboratory with field results. Diffusion coefficient ratios in the aqueous phase were experimentally quantified with a modified Stokes diffusion cell. The experiment revealed a significant shift of carbon and chlorine isotopologue ratios of TCE and 1,2-DCA during diffusion. For both TCE and 1,2-DCA, the magnitude of the shift of chlorine isotopologue ratios was larger , which is consistent with the larger mass difference between stable chlorine compared to carbon isotopes. Determined diffusion coefficients for carbon and chlorine isotopologues of TCE and 1,2-DCA follow an inverse power law form (D / m Àb ) with b < 0.5 revealing that the magnitude of isotopologue fractionation of TCE and 1,2-DCA is lower than in the previously postulated kinetic theory (D / m À0:5 ). To relate laboratory with field results, a water-saturated clay core from a VOC contaminated site was retrieved and subsampled as a function of depth to assess possible shifts in isotopologue ratios during downward diffusion of VOCs into the low permeable clay. Observed small shifts of TCE carbon and chlorine isotopologue ratio profiles were consistent with laboratory determined diffusion coefficient ratios, demonstrated by a 1D-diffusion model. Further 1D-simulations for shorter diffusion periods (5-10 years) than observed in the retrieved clay core (45 years), revealed a larger effect on TCE chlorine and carbon isotopologue ratio profiles. Thus, the diffusive transport process in water-saturated low permeability sediments only impairs the identification of reactive processes using compound-specific isotope analysis (CSIA) during short diffusion periods and for reactive processes with small enrichment factors.

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

“…23 However, the magnitude of the shifts of carbon isotope ratios is much larger (up to 23.9‰) mainly in clay cores 13-14, 14-10, and 15-13 ( Figures 3B−D) than what is expected due to diffusion only (∼2‰). 23 Earlier CSIA aquifer measurements performed by Vargas 32 showed that five and six years after contaminant injection isotope signatures had not changed significantly upgradient of multitlevel row 15 within the zone where clay cores were retrieved. This suggests minor degradation in the aquifer upgradient of multilevel row 15 within the first six years after the contaminants were released.…”

“…Besides reactive processes, it was assumed that the diffusive transport process is also associated with an isotope effect. Isotope fractionation factors due to diffusion were defined according to Wanner et al 23 The calibration of the model revealed that degradation is nonuniformly distributed in the aquitard (stronger close to the aquifer−aquitard interface than with increasing depth), which will be discussed in more detail in the "Results and Discussion" section. To quantify the quality of the fit between measured and modeled concentration and isotope data, the root mean squared error was used (RMSE; see eq 5 in the SI).…”

Carbon and Chlorine Isotope Fractionation Patterns Associated with Different Engineered Chloroform Transformation Reactions

“…Such information can be highly valuable in field situations. Non-destructive abiotic natural processes, such as sorption, volatilization or diffusion strongly affect concentrations of a contaminant, but generally do not cause significant isotopic fractionation [51][52][53][54][55][56][57] . Temporal or spatial shifts in isotope ratios, in contrast, are highly indicative of degradation and can, therefore, better monitor the success of remediation strategies at contaminated sites than mass balances alone 58 .…”

Normal and Inverse Diffusive Isotope Fractionation of Deuterated Toluene and Benzene in Aqueous Systems

“…[7][8][9][10] Despite the increased recognition of the quantitative importance and of the macroscopic impact of small scale diffusive processes on large scale transport of organic contaminants, only a few experimental and modeling studies have attempted to quantify diffusive isotope fractionation for organic compounds. [11][12][13][14][15][16] The lack of data and mechanistic understanding of organic chemicals' diffusive isotope fractionation becomes apparent when compared with the advances in the related field of inorganic isotope geochemistry, in which numerous studies have been carried out to investigate diffusive isotope effects of major cations, anions and dissolved gases in both aqueous solutions [17][18][19][20][21][22][23][24][25][26][27][28][29] and non-aqueous systems. [30][31][32][33] In this work we focus on diffusive transport of perdeuterated and non-deuterated benzene and toluene.…”