Hazardous concentrations of pollutants for micro-organisms in river sediment

The toxic effects of 5 toxicants on 5 anaerobic microbial processes in fresh water sediments were measured. Four 14-C labelled substrates were mineralized to 14-CO2 at concentrations from 1-4 mug/l in fresh sediment microcosms. The half-lives of the substrates acetate, benzoate and 4-monochlorophenol were 12-30, 24, and 84 min. respectively. The chloroform mineralization was much slower with a half-life of 4.5 - 12 days. The natural methane production in the sediment was also used to measure the toxic effects. A dangerous Concentration 5% (DC5) can be calculated from the toxicity data using methods previously adapted for single animal species toxicity tests. Above this toxicant concentration more than 5% of the possible mineralization reactions can be partly inhibited. The HC5 concentrations for benzene, pentachlorophenol, 1,2-dichloroethane, chloroform and zinc are 1, 0.39, 0.0004, 0.0001 and 270 mg/kg sediment dry weight respectively. The chlorinated alkanes are much more toxic for anaerobic micro-organisms than for aerobic fish.