Model Complexity Needed for Quantitative Analysis of High Resolution Isotope and Concentration Data from a Toluene-Pulse Experiment

Abstract

Separating microbial- and physical-induced effects on the isotope signals of contaminants has been identified as a challenge in interpreting compound-specific isotope data. In contrast to simple analytical tools, such as the Rayleigh equation, reactive-transport models can account for complex interactions of different fractionating processes. The question arises how complex such models must be to reproduce the data while the model parameters remain identifiable. In this study, we reanalyze the high-resolution data set of toluene concentration and toluene-specific δ(13)C from the toluene-pulse experiment performed by Qiu et al. (this issue). We apply five reactive-transport models, differing in their degree of complexity. We uniquely quantify degradation and sorption properties of the system for each model, estimate the contributions of biodegradation-induced, sorption-induced, and transverse-dispersion-induced isotope fractionation to the overall isotope signal, and investigate the error introduced in the interpretation of the data when individual processes are neglected. Our results show that highly resolved data of both concentration and isotope ratios are needed for unique process identification facilitating reliable model calibration. Combined analysis of these highly resolved data demands reactive transport models accounting for nonlinear degradation kinetics and isotope fractionation by both reactive and physical processes such as sorption and transverse dispersion.

Pesticide fate on catchment scale: conceptual modelling of stream CSIA data

“…isotope fractionation; Elsner, 2010;Schmidt and Jochmann, 2012). In contrast, non-destructive processes such as dispersion or sorption may lead to significant isotope fractionation effects only under specific conditions (Eckert et al, 2013;van Breukelen and Prommer, 2008;van Breukelen and Rolle, 2012). Therefore, CSIA allows for the detection and even quantification of contaminant degradation in polluted environmental systems.…”

Reply to Reviewer 1

“…Quantification and CSIA of S-metolachlor and acetochlor are described in detail elsewhere (Elsayed et al, 2014). Briefly, 1 L water samples were filtered, extracted by solidphase extraction (SPE), concentrated under nitrogen flux to one droplet, and resuspended in 500 µL dichloromethane (DCM).…”

“…Although CSIA may provide evidence of pesticide degradation (Fenner et al, 2013), CSIA data of pesticides remain restricted to the analysis of isotope fractionation under laboratory conditions (Hartenbach et al, 2008;Meyer and Elsner, 2013;Meyer et al, 2009;Penning et al, 2010;Reinnicke et al, 2011;Wu et al, 2014) and grab samples of groundwater and streamwater (Milosevic et al, 2013;Schreglmann et al, 2013). Degradation of chloroacetanilide herbicides and associated isotope fractionation have been recently studied in lab-scale wetlands (Elsayed et al, 2014), but CSIA of herbicides has not yet been applied on catchment scale to evaluate in situ degradation of pesticides. This study presents the first field CSIA data of pesticides in surface runoff and streamwater from an agricultural catchment.…”

Evolution of carbon isotope signatures during reactive transport of hydrocarbons in heterogeneous aquifers

“…Furthermore, Qiu et al (2013) and Eckert et al (2013) systematically investigated and discriminated the effects of biodegradation-induced, sorptioninduced, and transverse-dispersion-induced effects on the overall isotope shift in a meso-scale tank experiment and reactive modeling study. Their results show that for process identification highly resolved data for concentrations and isotope ratios are required as well as reactive transport models that account for nonlinear degradation kinetics and isotope fractionation by reactive and physical processes.…”