Iodine speciation in the Nile River estuary

During high- and low-flood periods, surface and bottom water samples were c ollected along the Nile River estuary for the voltammetric determination of dissolved iodine species. Iodine occurs in the estuary as iodate, iodide a nd organic iodine. Total iodine increases with salinity, showing a source f eature in surface and bottom waters during high-flow indicating iodine inpu t. During low-flow, total iodine showed both addition and removal throughou t the course of mixing. Maximum input was associated with turbidity and chl orophyll maxima originating from the agricultural anthropogenic input from Lake Burullus. At the bottom, total iodine enrichment in water marches sulp hide production in the anoxic zone. The annual total dissolved iodine flux from the Nile to the S.E. Mediterranean is about 100 x 10(6) g yr(-1) durin g 1993. Iodate was nor measured below salinity 19, nor in the anoxic waters . It is derived mainly from marine waters that contribute between 38-65% an d 44-90% of total dissolved iodine during high- and low-flow periods, respe ctively. Iodate in the surface layer behaves almost conservatively. Iodate behavior during the low-flow period showed three end members. seawater, riv er, lake water (source) and/or anoxic bottom waters (sink). Iodide, of rive rine: marine and sedimentary origins, is almost without exception the domin ant iodine fraction in the surface and bottom waters of the Nile estuary. A t high- and low-flow periods, iodide showed an increase with salinity in th e upper estuary followed by a dramatical decrease seawards beyond salinitie s 17-23 and 16-28 at surface and bottom layers, respectively. During low-fl ow, iodine invades the estuary from seawater as iodate which is converted a t the bottom layer to iodide by sulphide ion present during anoxic conditio ns. Organic iodine is a river-derived species, appearing at the surface and bottom waters, coexisting with iodate and iodide in the river side and con tributing between 9-40% and 23-29% of total iodine. A flux of 6 +/- 2.3 mu M I- m(-2) day(-1) is estimated to be released from the bottom oxic sedimen ts, increasing to 13.4 mu M I- m(-2) day(-1) at the anoxic zone, and is suf ficient to raise the iodide concentration to near 500 nM in the bottom writ ers. (C) 1999 Elsevier Science B.V. All rights reserved.