Rasgão Reservoir, located close to the Metropolitan region of São Paulo, Brazil, has been analyzed previously, and its sediment was found to be highly toxic, with high levels of metals and polycyclic aromatic hydrocarbons and a complete absence of benthic life. Polychlorinated biphenyls also were present, as was mutagenic activity, detected with the Salmonella/microsome assay. Because of the extremely complex mixture of contaminants in these sediments, a toxicity identification evaluation was performed on the pore water and elutriate using Ceriodaphnia dubia and Vibrio fischeri. Toxicity characterization, identification, and confirmation procedures were performed in one representative sample of the reservoir, and the results indicated that ammonia was the main cause of the toxicity detected with C. dubia in both sediment pore water and elutriate. Chemical analysis corroborated this observation by revealing un-ionized ammonia concentrations as high as 5.14 mg/L in pore water and 2.06 mg/L in elutriate. These high ammonia levels masked possible toxicity caused by other classes of compounds. The toxicity detected with V. fischeri decreased with the time of sample storage and was related to the organic fraction of the pore water and the elutriate, in which compounds such as benzothiazole and nonylphenol were detected.

Bioavailability and toxicity of metals in sediment were assessed in three water bodies in the State of São Paulo, SE Brazil, in one of the first investigations of the topic in the country. Weakly bound metals in bulk sediment samples taken from a highly-polluted reservoir, Billings, São Paulo, exhibited enrichment factors from 2.4 to 30 fold, compared to two other water bodies in the study, Barra Bonita Reservoir and Diogo Lake. High mortality rates occurred when Hyalella azteca and Ceriodaphnia dubia were exposed to Billings reservoir sediment. After forty-eight days of sediment oxidation, laboratory microcosms showed increases in the weakly-bound fraction of metals in sediments from Billings Reservoir (all metals), Diogo Lake (all metals except Zinc) and Barra Bonita Reservoir (Cadmium and Lead only). Chironomus xanthus was not sensitive to toxicity in sediment from any of the three environments. Ceriodaphnia dubia was sensitive to chronic toxicity in bulk field sediments from all three environments, and showed an increase in mortality rate in tests with oxidized sediment. Billings sediment, with or without aeration, caused 100 percent mortality, while in contact with Barra Bonita field sediment, a mortality of 10 percent of the adult organisms occurred, increasing up to 80 percent in post-aeration sediments. In Diogo Lake sediment, 30 percent mortality was observed in field or pre-aeration sediment rising to up to 60 percent in post aeration sediments. The two factors caused by oxidation of reduced substances in sediments, a fall in pH in the water and mobilization of bound metal, are invoked to explain this response.