Assessment of the methane oxidation capacity of compacted soils intended for use as landﬁll cover materials Ingke Rachor ⇑ , Julia Gebert, Alexander Gröngröft, Eva-Maria Pfeiffer University of Hamburg, Institute of Soil Science, Allende-Platz 2, 20146 Hamburg, Germany article info Article history: Received 31 May 2010 Accepted 2 October 2010 Available online 9 November 2010 abstract The microbial oxidation of methane in engineered cover soils is considered a potent option for the mit- igation of emissions from old landﬁlls or sites containing wastes of low methane generation rates. A lab- oratory column study was conducted in order to derive design criteria that enable construction of an effective methane oxidising cover from the range of soils that are available to the landﬁll operator. There- fore, the methane oxidation capacity of different soils was assessed under simulated landﬁll conditions. Five sandy potential landﬁll top cover materials with varying contents of silt and clay were investigated with respect to methane oxidation and corresponding soil gas composition over a period of four months. The soils were compacted to 95% of their speciﬁc proctor density, resulting in bulk densities of 1.4– 1.7 g cm 3 , reﬂecting considerably unfavourable conditions for methane oxidation due to reduced air- ﬁlled porosity. The soil water content was adjusted to ﬁeld capacity, resulting in water contents ranging from 16.2 to 48.5 vol.%. The investigated inlet ﬂuxes ranged from 25 to about 100 g CH 4 m 2 d 1 , covering the methane load proposed to allow for complete oxidation in landﬁll covers under Western European climate conditions and hence being suggested as a criterion for release from aftercare. The vertical dis- tribution of gas concentrations, methane ﬂux balances as well as stable carbon isotope studies allowed for clear process identiﬁcations. Higher inlet ﬂuxes led to a reduction of the aerated zone, an increase in the absolute methane oxidation rate and a decline of the relative proportion of oxidized methane. For each material, a speciﬁc maximum oxidation rate was determined, which varied between 20 and 95 g CH 4 m 2 d 1 and which was positively correlated to the air-ﬁlled porosity of the soil. Methane oxi- dation efﬁciencies and gas proﬁle data imply a strong link between oxidation capacity and diffusive ingress of atmospheric air. For one material with elevated levels of ﬁne particles and high organic matter content, methane production impeded the quantiﬁcation of methane oxidation potentials. Regarding the design of landﬁll cover layers it was concluded that the magnitude of the expected methane load, the tex- ture and expected compaction of the cover material are key variables that need to be known. Based on these, a column study can serve as an appropriate testing system to determine the methane oxidation capacity of a soil intended as landﬁll cover material. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Methane production in landﬁlls and the resulting emissions to the atmosphere, representing the second largest anthropogenic methane source, have been frequently evaluated in the past (EEA, 2009; IPCC, 2007; US EPA, 2010). Closed landﬁlls are provided with a variety of cover systems, the efﬁcacy of which to prevent such emissions varies strongly. In Western Europe, the state-of-the- art, depending on the waste classiﬁcation, is a cap intended to pre- vent inﬁltration of rainwater on the one hand and gas emissions on the other hand. This sealing is usually combined with a gas extrac- tion system coupled to a gas engine (Council of the European Union, 1999). When the landﬁll gas production declines, the engine is commonly replaced by a ﬂare. However, an enormous number of landﬁlls are older than this legislation and thus they do not possess a sealing or an operating gas extraction system. Consequently, methane is emitted from almost all of these sites. Therefore, mech- anisms to mitigate landﬁll methane emissions are of great concern. The potential of microbial methane oxidation in landﬁll covers as a means to reduce methane emissions has been frequently veriﬁed in the last years (for reviews see Hilger and Humer, 2003; Huber-Humer et al., 2008; Scheutz et al., 2009). The potential of soils and other media for microbial methane oxidation is well known, and different factors, mainly moisture, temperature, and nutrient supply as well as the availability of the substrates meth- ane and oxygen, have been suggested to govern oxidation efﬁ- ciency (e.g. Whalen et al., 1990; Boeckx et al., 1996; Humer and Lechner, 1999; Gebert et al., 2003; Scheutz et al., 2004; Scheutz 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.10.006 ⇑ Corresponding author. Tel.: +49 40 42838 6595; fax: +49 40 42838 2024. E-mail address: i.rachor@ifb.uni-hamburg.de (I. Rachor). Waste Management 31 (2011) 833–842 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman