ASSESSING METALS RELEASE FROM SEDIMENTS IN LAKES AND RESERVOIRS

Although sediments may represent indirect secondary sources of contaminants in water bodies, no criteria are known that determine such correlation. In the present study a method is proposed and scenarios are formulated to estimate risks of polluting water due to leaching of metals that are previously accumulated in sediments. The leaching of iron, manganese, cadmium, cupper, chrome, nickel, silver, and zinc was assessed experimentally under different redox conditions. Sediments were obtained from a reservoir in a tropical area and suspended in water with step-wise control of redox potential (Eh) between -450 and +300 mV. Leaching of metals was evaluated and extrapolated to field conditions by considering that water-sediment interactions occur in a 5-cm sediment layer of density 340 kg/m³. Three different scenarios for water depth were studied and resulting metals concentrations were compared to Mexican ecological criteria for quality of water as drinking water supply. Metals leaching from sediments were also investigated by application of the hydrogeochemical model PHREEQC. The results indicated that cadmium, cupper, chrome, nickel, silver, and zinc adsorb to iron and manganese oxides under oxidized conditions (positive Eh) while reduced conditions (or negative Eh) favor the dissolution of iron and manganese oxides, increasing the leaching of adsorbed metals. By further decreasing Eh below -200 mV, metals concentrations decreased due to coprecipitation with or adsorption on iron sulfide. The combination of experimental evaluation and hydrogeochemical modeling allows assessing different scenarios of pollution of water pollution in contact with sediments. These results show the importance to understand the reductive dissolution processes of contaminants to mitigate health effects and to make proper decisions on needs for water treatment.