Rebound at Pb-Zn Mines Hosted in Carbonate Aquifers: Influence on the Chemistry of Ground Water

Closure of Pb-Zn mines in the Iglesiente district (SW Sardinia, Italy) caused the cessation of pumping in 1997 at Monteponi, and in 1998 at San Giovanni. Consequent flooding of underground workings occurred in the district and also involved Campo Pisano. In June 1998, as the water table rose from 160 to 20 m below sea level, the deep saline water mixed with the shallow ground water at Monteponi and nearby mines. In the same period, an increase in dissolved metals (especially Zn, Cd, and Pb) was observed under near-neutral pH conditions. Following peak concentrations, a marked decrease of Zn, Cd, and Hg occurred. Dissolved Pb showed fluctuating concentrations over the monitoring period (1996-2005). In January 2000, when the water table rose to 20 m above sea level, the salinity of ground water decreased significantly at all of the mines. Stratification caused the more saline water at depth to settle three years after rebound started. Depth profiles carried out in 2005 at Monteponi, San Giovanni, and Campo Pisano showed an increase in conductivity and dissolved metals in ground water at deeper levels, especially at depths below sea level.After eight years of rebound, a marine component was still present at depth in ground water at San Giovanni (about 2%), and to a lesser extent at Monteponi (about 0.4%).