PROCESSES AFFECTING THE FATE OF MERCURY IN THE KRKA RIVER ESTUARY

The concentrations of reactive and total mercury (in the u.v.-irradiated samples) were measured in the Krka River Estuary during the period from April 1988 to February 1991. Salinity, temperature, pH(NBS), calcium and magnesium were also measured. Only calcium and magnesium ions exhibited a linear conservative dilution property. Temperature measurements indicated a subsurface warming just below the freshwater-seawater interface (FSI) in spring and temperature inversion in winter. pH(NBS) profiles showed different patterns with variations up to one pH unit. The concentrations of reactive and total mercury, exhibiting different patterns, present a "momentum situation" and suggest composite scavenging processes. The fate of mercury in the Krka River Estuary is highlighted and qualitatively described as follows: -from increased surface mercury concentrations after a heavy rain, an atmospheric mercury input has been suggested -mercury species in the freshwater layer are chiefly connected with organic material -the adsorption of mercury at the FSI can occur on at least some of the mineral surfaces found at the FSI and identified in this work. (The identified phases are: calcite, quartz, calcium silicate hydrate and takanelite) -transport of mercury to sediments occurs, during calm weather, by sedimentation of particulates with the adsorbed mercury, and during a storm, by a wind-induced vertical gyre within the marine layer. The seawater at the bottom is replaced with the seawater from the FSI which accumulates organic material. Consequently, mercury is bound more to the organic material than to the inorganic complexes. The difference in total and reactive mercury concentrations observed in the whole vertical profile after the Bura wind, shows that mercury is bound to organic material on that occasion. This difference could be used as a useful parameter for observation of the vertical mixing in the seawater layer in the Krka River Estuary during a storm.