Hydrochemical and isotopic effects associated with petroleum fuel biodegradation pathways in a chalk aquifer Michael J. Spence a , Simon H. Bottrell b, * , Steven F. Thornton a , Hans H. Richnow c , Keith H. Spence b a Groundwater Protection and Restoration Group, Department of Civil and Structural Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK c UFZ-Leipzig Halle GmbH, Permoser Strasse 15, 04318 Leipzig, Germany Received 2 July 2004; received in revised form 28 April 2005; accepted 5 June 2005 Abstract Hydrochemical data, compound specific carbon isotope analysis and isotopic enrichment trends in dissolved hydrocarbons and residual electron acceptors have been used to deduce BTEX and MTBE degradation pathways in a fractured chalk aquifer. BTEX compounds are mineralised sequentially within specific redox environments, with changes in electron acceptor utilisation being defined by the exhaustion of specific BTEX components. A zone of oxygen and nitrate exhaustion extends approximately 100 m downstream from the plume source, with residual sulphate, toluene, ethylbenzene and xylene. Within this zone complete removal of the TEX components occurs by bacterial sulphate reduction, with sulphur and oxygen isotopic enrichment of residual sulphate (e s = 14.4x to  16.0x). Towards the plume margins and at greater distance along the plume flow path nitrate concentrations increase with d 15 N values of up to +40x indicating extensive denitrification. Benzene and MTBE persist into the denitrification zone, with carbon isotope enrichment of benzene indicating biodegradation along the flow path. A Rayleigh kinetic isotope enrichment model for 13 C-enrichment of residual benzene gives an apparent e value of  0.66x. MTBE shows no significant isotopic enrichment (d 13 C=  29.3x to  30.7x) and is isotopically similar to a refinery sample (d 13 C=  30.1x). No significant isotopic variation in dissolved MTBE implies that either the magnitude of any biodegradation-induced isotopic fractionation is 0169-7722/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.jconhyd.2005.06.003 * Corresponding author. Tel.: +44 113 3435228; fax: +44 113 3435259. E-mail address: simon@earth.leeds.ac.uk (S.H. Bottrell). Journal of Contaminant Hydrology 79 (2005) 67 – 88 www.elsevier.com/locate/jconhyd