Magnitude of Diffusion- and Transverse Dispersion-Induced Isotope Fractionation of Organic Compounds in Aqueous Systems

Determining whether aqueous diffusion and dispersion lead to significant isotope fractionation is important for interpreting the isotope ratios of organic contaminants in groundwater. We performed diffusion experiments with modified Stokes diaphragm cells and transverse-dispersion experiments in quasi-two-dimensional flow-through sediment tank systems to explore isotope fractionation for benzene, toluene, ethylbenzene, 2,6-dichlorobenzamide, and metolachlor at natural isotopic abundance. We observed very small to negligible diffusion- and transverse-dispersion-induced isotope enrichment factors (epsilon < -0.4 %), with changes in carbon and nitrogen isotope values within +/- 0.5% and +/- 1%, respectively. Isotope effects of diffusion did not show a clear correlation with isotopologue mass with calculated power-law exponents beta close to zero (0.007 < beta < 0.1). In comparison to ions, noble gases, and labeled compounds, three aspects stand out. (i) If a mass dependence is derived from collision theory, then isotopologue masses of polyatomic molecules would be affected by isotopes of multiple elements resulting in very small expected effects. (ii) However, collisions do not necessarily lead to translational movement but can excite molecular vibrations or rotations minimizing the mass dependence. (iii) Solute-solvent interactions like H-bonds can further minimize the effect of collisions. Modeling scenarios showed that an inadequate model choice, or erroneous choice of beta, can greatly overestimate the isotope fractionation by diffusion and, consequently, transverse dispersion. In contrast, available data for chlorinated solvent and gasoline contaminants at natural isotopic abundance suggest that in field scenarios, a potential additional uncertainty from aqueous diffusion or dispersion would add to current instrumental uncertainties on carbon or nitrogen isotope values (+/- 1%) with an additional +/- 1% at most.

Automated summary

The study found that isotope fractionation caused by aqueous diffusion and dispersion for organic contaminants at natural isotopic abundance is very small, with unclear correlation to isotopologue mass changes. In field scenarios, the additional uncertainty from aqueous diffusion or dispersion may impact the instrumental uncertainties on carbon or nitrogen isotope values by up to +/- 1%.