An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark 2. Deduction of anaerobic (methanogenic, sulfate-, and Fe(III)-reducing) redox conditions

Redox conditions may be environmental factors which affect the fate of the xenobiotic organic compounds. Therefore the redox conditions were character ized in an anaerobic, leachate-contaminated aquifer 15-60 m downgradient fr om the Grindsted Landfill, Denmark, where an field injection experiment was carried out. Furthermore, the stability of the redox conditions spatially and over time were investigated, and different approaches to deduce the red ox conditions were evaluated. The redox conditions were evaluated in a set of 20 sediment and groundwater samples taken from locations adjacent to the sediment samples. Samples were investigated with respect to groundwater ch emistry, including hydrogen and volatile fatty acids (VFAs) and sediment ge ochemistry, and bioassays were performed. The groundwater chemistry, includ ing redox sensitive species for a large number of samples, varied over time during the experimental period of 924 days owing to variations in the leac hate from the landfill. However, no indication of change in the redox envir onment resulting from the field injection experiment or natural variation w as observed in the individual sampling points. The methane, Fe(II), hydroge n, and VFA groundwater chemistry parameters strongly indicated a Fe(III)-re ducing environment. This was further supported by the bioassays, although m ethane production and sulfate-reduction were also observed in a few samples close to the landfill. On the basis of the calculated carbon conversion, F e(III) was the dominant electron acceptor in the region of the aquifer, whi ch was investigated. Because of the complexity of a landfill leachate plume , several redox processes may occur simultaneously, and an array of methods must be applied for redox characterization in such multicomponent systems.