Drainage at abandoned mine sites: natural attenuation of contaminants in different seasons

Since 1996, hydrogeochemical surveys have been carried out in the abandoned Montevecchio Pb-Zn mining district (Sardinia, Italy), where mine drainages discharge directly into the local streams, to investigate variations in the aqueous occurrence of contaminants. Natural attenuation in dissolved contaminants occurs downstream of the abandoned mines. Iron is removed first (about 2 km downstream); lead, aluminum, and copper follow (about 4 km downstream). Other metals, such as zinc, cadmium, nickel, and rare earth elements, decrease further downstream. At high flow, natural attenuation is mainly due to dilution by rainwater. At low flow, natural attenuation processes are dominated by solid phase precipitation, efflorescent salt formation due to evaporation, and dilution by uncontaminated surface and/or groundwater inflow. Overall, concentrations of sulphate, Zn, Cd, Pb, Cu, Ni, and Co were less in 2010 than in 1996. The concentrations of dissolved sulphate and metals are lowest when the water is less acidic. However, despite ongoing natural attenuation, very high concentrations of toxic metals continue to be observed 15 years after the underground workings flooded. The amount of contaminants reaching the MarceddAnot sign lagoon increases as runoff increases following heavy rains