title: Evolution of carbon isotope signatures during reactive transport of hydrocarbons in heterogeneous aquifers.
abstract: Compound-specific isotope analysis (CSIA) of organic pollutants has become a well-established tool for assessing the occurrence and extent of biodegradation processes in contaminated aquifers. However, the precision of CSIA is influenced by the degree to which assumptions underlying CSIA data interpretation hold under realistic field-scale conditions. For the first time this study demonstrates how aquifer analogs combined with reactive transport models offer an underexplored way to develop generic process understanding, evaluate monitoring and quantification strategies in highly heterogeneous subsurface settings. Data from high-resolution aquifer analogs were used in numerical experiments to track the propagation of a representative oxidizable organic compound (toluene) within a variety of realistic heterogeneous aquifers and to investigate its detailed fate. The simulations were used to analyze (1) the effects of physical aquifer heterogeneities on spatiotemporal patterns of contaminant concentrations and isotope signatures, (2) the performance of the commonly applied Rayleigh equation and (3) the applicability of an extension of the Rayleigh equation for complex hydrogeological conditions. The results indicate that if field-derived enrichment factors are applied without corrections for dilution, the conventional Rayleigh equation is inaccurate and estimates for biodegradation are typically overestimated and unreliable in heterogeneous aquifers. Underestimations can occur due to the partial source zone depletion. In contrast, if dilution can be accurately accounted for, field-derived enrichment factors comprise a suitable alternative to laboratory-derived and redox-specific enrichment factors. The study also examines to what extent variations in monitoring/sampling strategies influence the obtained results. Especially measurements from long-screened wells (>1 m) reveal to be inappropriate for the application of the Rayleigh equation in the investigated aquifer analogs, as low resolution data sampled from the simulated scenarios only enable a qualitative assessment of biodegradation. Measurements from both long- and short-screened wells employing the Rayleigh equation streamline approach are only partly viable for in situ biodegradation measurements in heterogeneous systems.
authors: University of Tübingen, Center for Applied Geoscience, Hölderlinstraße 12, 72074 Tübingen, Germany. Electronic address: dominik.hoeyng@uni-tuebingen.de.
authors: Dominik
authors: D
authors: Höyng
authors: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; University of Western Australia, School of Earth and Environment, Crawley, Perth, WA 6009, Australia; National Centre for Groundwater Research and Training, Flinders University, Adelaide, GPO Box 2100, SA 5001, Australia.
authors: Henning
authors: H
authors: Prommer
authors: Karlsruhe Institute of Technology (KIT), Institute for Applied Geosciences (AGW), Kaiserstraße 12, 76131 Karlsruhe, Germany.
authors: Philipp
authors: P
authors: Blum
authors: University of Tübingen, Center for Applied Geoscience, Hölderlinstraße 12, 72074 Tübingen, Germany.
authors: Peter
authors: P
authors: Grathwohl
authors: University of Tübingen, Center for Applied Geoscience, Hölderlinstraße 12, 72074 Tübingen, Germany; TIMGEO GmbH, Hölderlinstraße 29, 72074 Tübingen, Germany.
authors: Fernando Mazo
authors: FM
authors: D'Affonseca
mesh_terms: D001673:Biodegradation, Environmental; D002247:Carbon Isotopes / Q000032:analysis*; D005591:Chemical Fractionation / Q000379:methods*; D003198:Computer Simulation; D060587:Groundwater / Q000737:chemistry*; D006838:Hydrocarbons / Q000737:chemistry*
chemical_list: D002247:Carbon Isotopes; D006838:Hydrocarbons
keywords: Aquifer analogs; Biodegradation; Isotope fractionation; Rayleigh equation; Reactive transport modeling