Spatial variability and seasonal toxicity of dredged sediments from Guanabara Bay (Rio de Janeiro, Brazil): acute effects on earthworms

The toxicity of dredged sediments from Guanabara Bay (Rio de Janeiro, Brazil) was evaluated using acute bioassays with Eisenia andrei and metal determination. The sediments were collected in August 2014 (winter) and February 2015 (summer) and in five areas distributed along the Bay: Port of Rio de Janeiro, Port of Niterói, Meriti River mouth, Iguaçu River mouth, and the Environmental Protection Area (APA) of Guapimirim. The sediments were mixed with a ferralsol (a representative Brazilian tropical soil) in proportions varying between 0 (pure soil) and 30%. The acute bioassays with E. andrei followed a standard protocol (ISO 11268-2:2012). Total metal determination in the sediments was performed by ICP-OES. The medium lethal earthworm concentration (LC₅₀) was estimated through PriProbit analysis. The sediments from the APA of Guapimirim, which is a control area at the Guanabara Bay, were the only ones whose total metal concentrations were in agreement with the limits established by Brazilian law for land disposal of dredged sediments. However, the sediments collected in the APA of Guapimirim were the most toxic ones among the study areas due to very high contents of salts in these materials. Winter sediments were generally more toxic compared to the summer ones due to the increase of metal concentrations and salt precipitation to bottom sediments during the winter. The exceptions were (i) the sediments from APA of Guapimirim, where the toxicity in the summer (LC₅₀ = 3.99%) and winter (LC₅₀ = 4.60%) were relatively similar to each other, since the toxicity is linked to salt in excess; and (ii) the Iguaçu River mouth, where the presence of mangrove areas might be associated with the filtering of pollution sources (winter LC₅₀ = 12.67%; summer LC₅₀ = 11.58%). In the Port of Rio de Janeiro, LC₅₀ obtained in the winter (7.30%) was almost three times lower than that found in the summer (19.64%). The sediments from Meriti River mouth showed the highest total metal concentrations, were the most toxic sediments among the study areas (excluding the APA of Guapimirim), and its winter LC₅₀ (6.64%) was almost twice lower than that obtained in the summer (12.55%). By following the same tendency, summer LC₅₀ (17.52%) found for the sediment collected in the Port of Niterói was also higher than the value found in the winter (12.34%). Finally, the dredged sediments from Guanabara Bay were toxic to earthworms in mixtures with pure ferralsol and winter samples were generally more toxic than the summer ones, in agreement with the increase of metal and salt concentrations during the winter.