<p>In oxic waters, Re<sup>VII </sup>is the stable oxidation state which undergo hydrolysis to the relatively unreactive perrhenate ion, Re<sup>VII</sup>O<sub>4</sub><sup>- [1]</sup>. The oceanic dissolved Re exhibits quite conservative behaviour with the concentration of about 40 pM <sup>[2]</sup>. Despite the frequent utilization of Re for the atmosphere and the ocean past redox state reconstructions, the geochemical behaviour of Re in the modern surface environments such as rivers, estuaries as well as in seawater is not well studied. Understudy is partially arising from the fact that Re has low seawater and riverine concentration of 4&#160;pM and 16.5 pM, respectively<sup></sup><sup>[1, 3]</sup>. In the Amazon and the Hudson estuaries, in crease of Re concentration at low and middle salinity regions is observed<sup> [4]</sup>. On the other hand, Re exhibits complete conservative behaviour in Indian river estuaries, i.e. Narmada, Tapi and the Mandovi estuaries in the Arabian Sea and the Hooghly estuary in the Bay of Bengal <sup>[5]</sup>. Deviation from conservative behaviour in Re can be explained as the interplay of variety of factors including the nature and composition of the particles, Eh-pH conditions, biological productivity and fate of the organic matter. <sup>[5]</sup>.</p><p>Here we present the Re concentration profiles in the freshwater part of the karstic Krka river (Croatia) and its 23 km long estuarine segment, covering a full salinity range (0.1 to 38.6). Analysis of Re was performed by its preconcentration and separation using an anion exchange resin (Dowex 1X8) followed with the ICP-MS quantification using isotope dilution (ID) method. The Krka River spring is characterised by the low Re concentration (~6 pM). A noticeable anthropogenic influence at the point of the wastewater discharge of the Knin town was observed (27 pM). This input probably caused a progressive downstream increase of Re concentration to 12 pM at the freshwater end-member in the winter period (with a high Krka River discharge) and 17 pM in the summer period (low Krka River discharge). In the estuarine segment, a near-conservative behaviour of Re was found, with the "oceanic" concentration of 38 pM in the seawater end-member.</p><p>&#160;</p><p>References:</p><p>[1] Hasse AA et al., (2019) Coordination chemistry reviews 394: 135-161.</p><p>[2] Anbar AD et al., (1992) Geochimica et Cosmochimica Acta<strong> </strong>56:4099-4103.</p><p>[3] Miller CA et al., (2011) Geochimica et Cosmochimica Acta 75:7146-7179.</p><p>[4] Colodner D et al., (1993) Earth and Planetary Science Letters 117:205-221.</p><p>[5] Rahaman W and Singh SK (2010) Marine Chemistry 118: 1-10.</p>
