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

Since about the mid-1990s the well waters of five areas located along the c oast of the province of Grosseto, southern Tuscany, have started to present Hg concentrations above the limit for drinkable water (1 mug/l). In this p art of Tuscany region, mercury occurs both as ore deposits and as elastic m aterial within shallow alluvial sediments. A monitoring study of the geoche mical characteristics of the delivered fluids, aimed at identifying the sou rce 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 a nd they range mainly from Ca2+-Mg2+/HCO3-; to Na+/Cl- in type. Clear mixing between diluted groundwater and sea water characterizes all the locations, as indicated also by the delta O-18 data. Close relationships exist betwee n 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 se a waters and mercury mineral phases, which are likely to be present within the studied aquifers, was modelled by means of the Phreeqc code. The result s indicate that an increase in Cl-. forming stable complexes with Hg such a s HgCl3-, HgCl2-, HgCl42-, and HgBrCl-, leads to increased dissolution of m ercury solid phases. This effect, however, depends greatly on the pH and re dox state of the solution. The authors conclude that sea water intrusion in the studied areas is the m ain factor responsible for the dissolution of mercury minerals occurring na turally within the aquifers and For the consequent increased concentration of mercury in the well waters. (C) 2000 Elsevier Science B.V. All rights re served.