Mercury in groundwaters of the cross-border Isonzo River upper alluvial plain (Northeastern Italy-Western Slovenia)

The Idrija mine (western Slovenia) is recognized to be the main source of mercury (Hg) in the Isonzo River watershed as well as in the Gulf of Trieste [1]. The present work aims at evaluating Hg occurrence and speciation in groundwaters of the Isonzo River upper alluvial plain. A fairly good number of wells and piezometers were sampled both in static (through bailer sampler) and dynamic (by pumping) mode during four sampling campaigns between 2014 and 2015. Generally, it was observed that most of the samples collected in static mode showed higher concentrations of total Hg. Besides, distribution of dissolved and particulate Hg was not uniform. At low river stage, Hg prevailed in the dissolved form whereas, at high river stage, the dominant form of the metal was associated with suspended fine particles. On the basis of hydrochemistry and isotopic composition, the main sources of groundwaters were established. Surprisingly, Hg concentrations in aquifers associated to the Isonzo River freshwaters are comparable to those measured in the Slovenian area, where surface and groundwaters are mainly feeded by local rainfalls. In addition to Eh and pH values, it has been established that major ions composition, especially chloride, may affect formation of different soluble mercury species. Since Isonzo River Hg-contaminated alluvial sediments may act as a secondary source of Hg in groundwaters, a pilot drilling was achieved down to the water table and the related sediment profile was investigated for Hg content and speciation. It can be hypothesized that the raising/lowering of the water table, i.e. groundwater level, may be locally responsible for slight variability of Hg concentrations in the aquifer.