Sea water intrusion and mercury pollution of some coastal aquifers in the province of Grosseto (Southern Tuscany — Italy)

Abstract

Since about the mid-1990s the well waters of five areas located along the coast of the province of Grosseto, southern Tuscany, have started to present Hg concentrations above the limit for drinkable water (1 μg/l). In this part of Tuscany region, mercury occurs both as ore deposits and as clastic material within shallow alluvial sediments. A monitoring study of the geochemical characteristics of the delivered fluids, aimed at identifying the source of this unexpected presence of mercury in well waters, was carried out mainly during the summer of 1998. Seventy-one water samples were collected and analysed during this period. The studied groundwaters have salinity that varies between 0.7 and 34 g/l and they range mainly from Ca 2+–Mg 2+/HCO 3 − to Na +/Cl − in type. Clear mixing between diluted groundwater and sea water characterizes all the locations, as indicated also by the δ 18O data. Close relationships exist between Cl − and Hg in the different areas. Higher Cl − contents always correspond to higher mercury concentrations. The interaction between mixtures with different proportions of fresh and sea waters and mercury mineral phases, which are likely to be present within the studied aquifers, was modelled by means of the Phreeqc code. The results indicate that an increase in Cl −, forming stable complexes with Hg such as HgCl 3 −, HgCl 2 −, HgCl 4 2−, and HgBrCl −, leads to increased dissolution of mercury solid phases. This effect, however, depends greatly on the pH and redox state of the solution. The authors conclude that sea water intrusion in the studied areas is the main factor responsible for the dissolution of mercury minerals occurring naturally within the aquifers and for the consequent increased concentration of mercury in the well waters.