Abstract

The effects of five compounds on the endogenous methane production of sediment samples of the river Rhine were examined. The concentrations of a toxicant that inhibited the methane production for 10% and 50% are called EC10 and EC50. Benzene, 1,2- dichloroethane, pentachlorophenol and chloroform had EC10 values of >10000, 860, 140 and 5.5 mg/kg dry sediment, respectively. These values are well above the actual field concentrations, so the methane production is not inhibited. In one experiment, added zinc was toxic at concentrations far below the present background concentration in the sediment (800 mg Zn/kg) with an EC10 of 48 mg Zn.kg. In a second experiment with another sediment, however, the EC10 value was 1780 mg/kg. The difference in sensitivity is probably due to a different availability of zinc in the two experiments. Factors that govern the speciation of zinc in the sediment (e.g. the concentration of sulfide) are thus important when considering possible effects of zinc on microbial activity. Insight in these mechanisms, which also apply for other heavy metals, is needed as it was shown that zinc can be toxic at very low concentrations.