Pulsed gas injection: A minimum effort approach for enhanced natural attenuation of chlorobenzene in contaminated groundwater Gerd Ulrich Balcke a, * , Heidrun Paschke b , Carsten Vogt c , Mario Schirmer d a Department of Hydrogeology, UFZ Helmholtz Centre for Environmental Research, Theodor-Lieser-Strasse 4, D-06120 Halle/Saale, Germany b Department of Groundwater Remediation, UFZ Helmholtz Centre for Environmental Research, Permoserstrasse 15, D-04318 Leipzig, Germany c Department of Isotope Biogeochemistry, UFZ Helmholtz Centre for Environmental Research, Permoserstrasse 15, D-04318 Leipzig, Germany d EAWAG, the Swiss Federal Institute of Aquatic Science and Technology, Department of Water Resources and Drinking Water, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland Minimum rate gas sparging resulted in sustained biodegradation of chlorobenzene in a polluted groundwater aquifer. article info Article history: Received 15 July 2008 Received in revised form 23 February 2009 Accepted 24 February 2009 Keywords: Groundwater Oxygen-deficient Remediation Oxygenation Chlorobenzene abstract Chlorobenzene-contaminated groundwater was used to assess pulsed gas sparging as a minimum effort aeration strategy to enhance intrinsic natural attenuation. In contrast to existing biosparging operations, oxygen was supplied at minimum rate by reducing the gas injection frequency to 0.33 day 1 . Field tests in a model aquifer were conducted in a 12 m long reactor, filled with indigenous aquifer material and continuously recharged with polluted groundwater over 3 years. The closed arrangement allowed yield balances, cost accounting as well as the investigation of spatial distributions of parameters which are sensitive to the biodegradation process. Depending on the injection frequency and on the gas chosen for injection (pure oxygen or air) oxygen-deficient conditions prevailed in the aquifer. Despite the limiting availability of dissolved oxygen in the groundwater, chlorobenzene degradation under oxygen-deficient conditions proved to be more effective than under conditions with dissolved oxygen being available in high concentrations. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction In Germany, the Bitterfeld region has gained a reputation for large-scale environmental pollution in soil and groundwater, reflecting a legacy of extensive releases of chlorinated organic compounds (COCs) into the environment. Consequently, aquifers are still heavily polluted with various cocktails of chlorinated hydrocarbons with concentrations up to 500 mM(Heidrich et al., 2004; Wycisk et al., 2005). The total volume of COC-contaminated groundwater is estimated to account for 200 Mm 3 spread over a densely populated 25 km 2 area (Weiß et al., 2001). Moreover, organic carbon-rich aquifer material, which has been serving as an absorptive sink for pollutants over many years, may become a secondary source for groundwater contamination by desorption of accumulated COCs (Dermietzel and Christoph, 2001; Dermietzel and Vieth, 2002). Chlorobenzene (CB) represents one of the main groundwater pollutants in the area (Heidrich et al., 2004; Wycisk et al., 2005). The compound is known to persist in anoxic environments for many years, but can be rapidly degraded by indigenous microor- ganisms if sufficient oxygen is available (Dermietzel and Vieth, 2002). Although recent studies have indicated that CB is slowly degraded under anoxic conditions (Kaschl et al., 2005; Nijenhuis et al., 2007), the rates are insufficient to address the large-scale contamination problems. Thus, natural attenuation of CB cannot play a significant role due to the strictly anoxic conditions within the aquifer. Several indigenous microorganisms capable of degrading CB have been identified at the site and the relevant metabolic path- ways have already been thoroughly investigated (Alfreider et al., 2003; Balcke et al., 2004; Nestler et al., 2007; Kiesel et al., 2008; Vogt et al., 2004b). However, aerobic conditions in a polluted aquifer, which is cut off from atmospheric gas exchange, cannot be ensured unless oxygen is permanently provided. Consequently, state-of-the-art groundwater aeration technologies such as hydrogen peroxide infiltration (Vogt et al., 2004a), in situ air sparging with soil vapor extraction (Yang et al., 2005; Klinchuch et al., 2007), or ex situ treatment of pumped groundwater in bioreactors (Langwaldt and Puhakka, 2000) are associated with high costs during a long-term operation. * Corresponding author. Metanomics GmbH, Tegeler Weg 33, D-10589 Berlin, Germany. Tel.: þ49 30 34807 148; fax: þ49 30 34807 300. E-mail address: gerd.balcke@metanomics.de (G.U. Balcke). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2009.02.030 Environmental Pollution 157 (2009) 2011–2018