Land disposal of dredged sediments from an urbanized tropical lagoon: toxicity to soil fauna

Urban tropical lagoons are commonly impacted by silting, domestic sewage and industrial wastes and the dredging of their sediments is often required to minimize environmental impacts. However, the ecological implications of land disposal of dredged sediments are still poorly investigated in the tropics. Aiming to contribute to filling this gap, an ecotoxicological evaluation was conducted with dredged sediments from Tijuca Lagoon (Rio de Janeiro, Brazil) using different lines of evidence, including soil and sediment characterization, metal determination, and acute and avoidance bioassays with Eisenia andrei. Two different dredged sediment samples, a sandy sediment and another muddy one, were obtained in two distinct and spatially representative sectors of the Tijuca Lagoon. The sediments were mixed with an artificial soil, Ferralsol and Spodosol to obtain doses between 0 (pure soil) and 12%. The sediment dose that caused mortality (LC₅₀) or avoidance responses (EC₅₀) to 50% of the organisms was estimated through PriProbit analysis. Metal concentrations and toxicity levels were higher in the muddy sediment (artificial soil LC₅₀ = 3.84%; Ferralsol LC₅₀ = 4.58%; Spodosol LC₅₀ = 2.85%) compared to the sandy one (artificial soil LC₅₀ = 10.94%; Ferralsol LC₅₀ = 14.36%; Spodosol LC₅₀ = 10.38%), since fine grains tend to adsorb more organic matter and contaminants. Mortality and avoidance responses were the highest in Spodosol due to its extremely sandy texture (98% of sand). Metal concentrations in surviving earthworms were generally low, except sodium whose bioaccumulation was high. Finally, the toxicity is probably linked to marine salts, and the earthworms seem to accumulate water in excess to maintain osmotic equilibrium, increasing their biomass.