Mine closure at Monteponi (Italy): effect of the cessation of dewatering on the quality of shallow groundwater

At the Monteponi Pb-Zn mine located in south-western Sardinia intensive dewatering has been carried out over about 100 a. A marked increase in the salinity and Hg concentration of deep groundwater occurred as the water table level was lowered from +15 to -160 m a.s.l. over this period. Closure of the mine implied the cessation of the expensive pumping system, and prompted the assessment of the contamination risk for the shallow groundwater system supplying Iglesias town. This study shows that deep water was mixed into the shallow aquifer as the water table level rose. An increase of salinity, mainly due to Na and Cl-, has been observed in shallow groundwater. The input of Cl- facilitates the dissolution of Hg. Moreover, the progressive mine flooding is causing an increase of other dissolved metals, mainly due to the weathering of primary sulphides and secondary minerals present in the ore; and remobilization of metals in the mine waste left in the galleries. A stratification process will allow the saline water at depth to settle, and this is expected to occur in a relatively short time (few years). The leaching of metals represents a greater concern for the quality of shallow groundwater, and the time necessary to clean up will be much longer (probably several decades) than that expected for the stratification of the water body. (C) 2001 Elsevier Science Ltd. All rights reserved.