Estuaries represent a crucial link between land and sea where river fluxes of trace
elements are subjected to a variety of physical and biogeochemical processes. The boundary
conditions are extremely variable in the estuarine mixing zone affecting trace element
speciation and their partitioning among different phases.
The Isonzo/Soča River represents the main source of Hg in the Gulf of Trieste due to past
mining activity at the Idrija mercury mine (Slovenia). The aim of this work is to evaluate
trace element occurrence, size partitioning and mixing behaviour at the mouth of the
Isonzo/Soča River. For this purpose, large volume water samples were collected from
surface, mixing and bottom layers under various seasonal conditions. Size fractionation was
performed using vacuum filtration and cross-flow ultrafiltration to isolate the suspended
particulate matter (SPM) and the colloidal retentate solution, respectively, from the truly
dissolved. Trace elements appear to be mainly associated with the SPM and dilution effects
between riverine and marine particles are responsible for the decrease in their concentration
along the water column. As expected, Hg was notably present during winter in the surface
freshwater, in particular under conditions of elevated river discharge. On the contrary, Mn
was found to be higher at the bottom, particularly during summer, as a consequence of
oxidation processes followed by the precipitation of Mn oxy-hydroxides promoted by high
water temperature. The strong physico-chemical gradients, especially in terms of salinity,
are often responsible for the non-conservative behaviour of several trace elements. Iron, Mn
and Cu displayed a non-conservative behaviour during mixing and simultaneously showed
the highest content of colloidal material. On the contrary, conservative behaviour was found
for As and Cs which are often present under ionic dissolved forms in natural water systems
