ORIGINAL PAPER Assessing trace metal contamination and organic matter in the brackish lakes as the major source of potable water Vlado Cuculic ´ . Neven Cukrov . Z ˇ eljko Kwokal . Slad¯anaStrmec ˇki . Marta Plavs ˇic ´ Received: 21 October 2016 / Accepted: 9 March 2017 Ó Springer Science+Business Media Dordrecht 2017 Abstract On small and medium karstic coastal islands in the Adriatic Sea, brackish lakes are often the only source of freshwater. Therefore, it is impor- tant to adequately evaluate the biogeochemical pro- cesses occurring in these complex water systems, as well as to determine the origin of contaminants present. In this study, the distribution and origin of trace metals (Tl, Hg, Cd, Pb, Cu, Zn, Ni, Co) and organic matter in the water column, sediment, and surrounding soil of the brackish lakes on Mljet Island, South Adriatic Sea, Croatia, were evaluated. Thallium and mercury concentrations in the lake water were up to two orders of magnitude higher compared to ranges found in the adjacent coastal sea water. Elevated thallium concentrations were of anthropogenic origin resulting from previous use of rodenticide, while elevated mercury content was naturally enhanced. Levels for the other metals were characteristic of uncontaminated water systems. Speciation modelling showed that dissolved trace metals such as Cu, Pb, and Zn were mostly associated with organic matter, while Tl, Co, and Ni were present predominantly as free ions and inorganic complexes. The presence of organic matter (OM) clearly influenced the speciation and distribution of some trace metals. OM was charac- terised by the determination of the complexing capacity for Cu ions (CuCC), surface active sub- stances, and catalytically active compounds. Reduced sulphur species (glutathione and other thiols) repre- senting significant Cu-binding ligands were deter- mined and discussed as well. Keywords Brackish water Á Karstic coastal island Á Trace metals Á Organic matter Á Speciation Introduction The people of the Mljet Island (Adriatic Sea, Croatia) have not yet been connected with a mainland potable water supply network. Local authorities desalinise brackish water from drill holes, which local community uses for personal and agricultural pur- poses. In several places, on the southern part of the island, surface water collects in lakes formed by sea- level rise during a postglacial period, approximately 10,000 years ago (Gus ˇic ´ et al. 1995; Wunsam et al. 1999). These lakes (‘‘blatine’’—muds in local term) are connected hydrologically with the coastal Adriatic Sea surface water but only through the porous karsticated carbonated rock. Therefore, lakes are brackish with salinity ranging between 1 and 25, Electronic supplementary material The online version of this article (doi:10.1007/s10653-017-9935-4) contains supple- mentary material, which is available to authorized users. V. Cuculic ´(&) Á N. Cukrov Á Z ˇ . Kwokal Á S. Strmec ˇki Á M. Plavs ˇic ´ Division for Marine and Environmental Research, Rud¯er Bos ˇkovic ´ Institute, Bijenic ˇka c. 54, 10000 Zagreb, Croatia e-mail: cuculic@irb.hr 123 Environ Geochem Health DOI 10.1007/s10653-017-9935-4