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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Occurrence and Rates of Terminal Electron-Accepting Processes and Recharge Processes in Petroleum Hydrocarbon-Contaminated Subsurface

^a Finnish Environment Institute, P.O. Box 140, FI-00251 Helsinki, Finland
^b Geological Survey of Finland, P.O. Box 96, FI-02151 ESPOO, Finland

^* Corresponding author (jani.salminen{at}ymparisto.fi)

Received for publication February 20, 2006. The occurrence and rates of terminal electron acceptor processes, and recharge processes in the unsaturated zone of a boreal site contaminated with petroleum hydrocarbons in the range C₁₀ to C₄₀ were examined. Soil microcosms were used to determine the rates of denitrification, iron (Fe) reduction, sulfate (SO₄) reduction, and methanogenesis in two vertical soil profiles contaminated with oil, and in a noncontaminated reference sample. Furthermore, the abundances of the 16S rRNA genes belonging to Geobacteracaea in the samples were determined by real-time quantitative polymerase chain reaction (PCR). Analyses of ground water chemistry and soil gas composition were also performed together with continuous in situ monitoring of soil water and ground water chemistry. Several lines of evidence were obtained to demonstrate that both Fe reduction and methanogenesis played significant roles in the vertical profiles: Fe reduction rates up to 3.7 nmol h^–1 g^–1 were recorded and they correlated with the abundances of the Geobacteracaea 16S rRNA genes (range: 2.3 x 10⁵ to 4.9 x 10⁷ copies g^–1). In the ground water, ferrous iron (Fe²⁺) concentration up to 55 mg L^–1 was measured. Methane production rates up to 2.5 nmol h^–1 g^–1 were obtained together with methane content up to 15% (vol/vol) in the soil gas. The continuous monitoring of soil water and ground water chemistry, microcosm experiments, and soil gas monitoring together demonstrated that the high microbial activity in the unsaturated zone resulted in rapid removal of oxygen from the infiltrating recharge thus leaving the anaerobic microbial processes dominant below 1.5 m depth both in the unsaturated and the saturated zones of the subsurface.

Abbreviations: bgs, below ground surface • BTEX, benzene, toluene, ethylbenzene, and xylenes • DSMZ, Deutsche Sammlung von Mikroorganismen und Zellkulturen • FID, flame ionization detector • GC, gas chromatograph • HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid • PCR, polymerase chain reaction • rRNA, ribosomal ribonucleic acid