INVESTIGATION OF MERCURY LEACHING FROM THE CONTAMINATED SOIL AT ROASTING SITES OF IDRIJA AREA

The Idrija mine in Slovenia was the second largest mercury deposit in the world. Although the production in the mine stopped in 1995, the soil in Idrija area contains from 10 to even 10, 000 mg Hg/kg at ancient roasting sites. Since Idrija is karst area, surface and rain waters can leach Hg from contaminated soil to deeper soil layers and then into groundwater. In order to estimate the potential leaching of Hg, the experiments have been conducted on contaminated soil sample containing 1347 mg Hg/kg. The leaching experiments have been performed in ultrapure water of different pH values in the range from 1.75 to 13.01, using standard batch method according to DIN 38414 S4. The concentration of total Hg in leachates obtained for pH range from 3.16 to 10.97 was very low. The highest concentrations of mercury have been obtained in leachates in acidic (pHo = 1.75) and in alkaline (pHo = 13.01) range, which is most likely caused by the destruction of the organo-mercury complexes and consequently the release of mercury. The complexation of Hg with organic matter, mostly humic and fulvic acids is considered to be the most dominant process controlling its mobility in soil. The increased concentration of leached Hg in alkaline conditions is most likely a result of the dissolving of humic acids. In acidic conditions it can be attributed to the dissolution of fulvic acids as well as organo- mineral complexes. The amount of leached Hg compared to its content in the soil is very small (from 0.0001% to 1.84%), even in highly acidic and the alkaline range, indicating that Hg is firmly bound to the soil matrix and is barely leached by water. Even the smallest detected concentration of leached Hg exceed the maximum permissible levels for drinking waters according to Croatian and Slovenian legislations, as well as to the Council Directive of European Community. This indicates the necessity of further monitoring the soil quality, as well as studying the Hg leachability at this contaminated sites of Idrija district.