Selenium distribution and mobility in Mediterranean karst soils from a coal mining region in Istria, Croatia

The coal from the Rasa mine located on the Istrian peninsula contains 10.5 % organic sulfur and 0.3 % pyritic sulfur and was used in the near by coal-firing power plant (Plomin) which is located in a typical karst landscape and hydrogeology. Mining activity in this region has lasted for centuries, while the power plant in Plomin is in operation since 1970. Arable soils mixed with coal dust from the have selenium concentrations ranging from 2.1 to 4.6mg kg-1. The ash from the power plant contains from 25 to 65 mg kg-1 Se. The content of Se in topsoil in the vicinity of the power plant, ranges from < 0.1 (regional background) to 18.5 mg kg-1 (mean 1.9 mg kg-1). The soils are composed of quartz, plagioclase, K-feldspar, kaolinite, micaceous clay minerals, vermiculite, hydroxy-interlayered vermiculite (HIV), chlorite, illite/chlorite mixed-layer clay mineral, hematite and goethite. Aqueous extracts of the soil samples containing coal dust have up to 0.1 mgL-1 of Se while ash samples contained up 1.9 mgL-1 water extractable Se (5% of total Se). Also 8% of Mo was found to be water extractable. Soil profiles overlain by coal ash were all found to contain high concentration of water extractable Se and Mo. The water that drains the abandoned mines and areas covered with ash deposits is enriched in Se (5  gL-1), as well as in Mo, Mn, Ni, Cr, Sb, U, Br, Fe and I. Groundwater from unpolluted areas has Se background concentrations of 0.4  gL-1 . Several centuries mining of a sulphur rich coal and its firing in a local power plant, as well as in homes of the local population has caused elevated concentrations of Se and Mo on both on the immediate surroundings as well as the wider region.