Potential release of zinc and cadmium in soils contaminated by heavy metals under flooding

Zinc and cadmium are two of the most common heavy metals found in contaminated sites, the main sources being anthropogenic activities such as mining and smelting operations. When a highly contaminated soil becomes flooded, a number of chemical reactions may occur and define if the metals will be retained or released. The aims of this study were to evaluate the potential release and discuss the mechanisms that regulate Zn and Cd solubility in two mining soils under flooding conditions, using chemical extractions, mesocosm experiments and geochemical modelling. Mine-affected soil samples were collected in the mining basin of Gorno. Following a previous characterization study, we collected two soil samples contaminated by Zn (65 - 112 g/kg) and Cd (160 - 360 mg/kg), representative of the mining area. The waterlogging of the soils resulted in two somehow different behaviors. In Soil 1, Mn levels in the soil solution increased gradually while in Soil 2 the Mn reduction started immediately after flooding and also Fe was slowly reduced and solubilized after the 5th week of the experiment. Zn contents in soil solution increased during the first week in both soils, decreasing almost linearly from the 2nd week and until the end of the experiment. Regarding Cd, in Soil 1 the soluble concentration decreased gradually over time while in Soil 2 a small peak of release appeared during the first days of the experiment followed by a sharp decrease after 28 days of experiment. The geochemical modelling predicted both metals precipitating in Soil 1 as Zn and Cd carbonate. Clays and Fe oxides seem not to have a great influence on the concentrations, as both metals are preferentially complexed by SOM. In Soil 2, metals precipitate as Zn and Cd carbonate during the first days while sulphides are calculated after 60 days of submersion. In this case, Fe oxides seem to adsorb Zn while not Cd and both metals are preferentially complexed by SOM. These results show that, during an occasional flooding, the metals could be promptly released both in the soil solution and in the aquifer. Thus, we performed a leaching experiment to evaluate the risk for the ground water due to intense rain episodes. Zn and Cd concentrations released in the soil solution were very high, 10 times the legislative limit in the waterbodies thus, in this case, a carbonatic soil seem not to protect the environment from the instantaneous release of heavy metals subsequent to a flooding.