ANAEROBIC MICROBIAL REDOX PROCESSES IN A LANDFILL LEACHATE CONTAMINATED AQUIFER (GRINDSTED, DENMARK)

The distribution of anaerobic microbial redox processes was investigat ed along a 305 m long transect of a shallow landfill-leachate polluted aquifer. By unamended bioassays containing sediment and groundwater, 37 samples were investigated with respect to methane production, sulfa te, iron, and manganese reduction, and denitrification. Methane produc tion was restricted to the most reduced part of the plume with rates o f 0.003-0.055 nmol CH4/g dry weight/day. Sulfate reduction was observe d at rates of maximum 1.8 nmol SO42-/g dry weight/day along with metha ne production in the plume, but sulfate reduction was also observed fu rther downgradient of the landfill. Iron reduction at rates of 5-19 nm ol Fe(II)/g dry weight/day was observed in only a few samples, but thi s may be related to a high detection limit for the iron reducing bioas say. Manganese reduction at rates of maximum 2.4 nmol Mn(Il)/g dry wei ght/day and denitrification at rates of 0.2-37 nmol N2O-N/g dry weight /day were observed in the less reduced part of the plume. All the redo x processes were microbial processes. In many cases, several redox pro cesses took place simultaneously, but in all samples one process domin ated accounting for more than 70% of the equivalent carbon conversion. The bioassays showed that the redox zones in the plume identified fro m the groundwater composition (e.g. as methanogenic and sulfate reduci ng) locally hosted also other redox processes (e.g. iron reduction). T his may have implications for the potential of the redox zone to degra de trace amounts of organic chemicals and suggests that unamended bioa ssays may be an important supplement to other approaches in characteri zing the redox processes in an anaerobic plume. (C) 1998 Elsevier Scie nce B.V. All rights reserved.