Relative mobility of radioactive trace elements across the sediment-water interface in the MERL model ecosystems of Narragansett Bay

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Abstract
The "mobilities" of radioactive trace elements across the water sediment boundary of a coastal marine ecosystem ewere investigtated. The studies carried out included chemical speciation experiments of the solution and solid phases, as well as verification experiments in controlled model ecosystems ("MERL" tanks). The latter included backdiffusion experiments under oxic and anoxic conditions and experiments with artificially increased sediment resuspension rates. These studies have produced seven general conclusions: (1) The backdiffusion of Cs, Mn, Co, and Zn radiotracers across the sediment-water interface into oxic waters and of Mn and Co radiotracers into anoxic waters was predicted from laboratory experiments. (2) The removal from the water and the partial immobilization in the sediments of Cs, Zn and Cd tracers, during anoxic conditions, agreed with results from selective leaching experiments of surface sediments with dithionite-citrate solution, a mildly reducing agent which can reprecipitate liberated metals as sulfides. While most nuclides were leached by this solution to the same extent as by hydroxylamine, another reducing agent, Zn, Cd and Cs tracers were not, possibly due to the formation of sulfidic and other phases by the former solution. (3) Radioisotopes of particle-reactive elements (Sn, Fe, Hg and Cr) were shown by sequential extraction and ultrafiltration experiments to be involved in the dynamic cycle of colloid formation and aggregation in the water column and sediments. (4) In order to extend the information on nuclide behavior gained from the radiotracer methodology to stable tracer elements, (which are often introduced into coastal water in ionic form) stable metals were added to one tank. Radiotracer behavior in the water column (removal rates and extent of uptake by suspended particles) was quite similar to that of their stable metal counterparts at ambient concentrations (Mn, Cr, Fe, Cd and Zn), added simultaneously to one tank, and to the metal behavior in other tanks operating under similar conditions. (5) The experiments with increased resuspension rates without concomitant increased bioturbation rates had, as expected, only small effects of removal rates of the radiotracers. (6) Sediment profiles of the tracers revealed both seasonal and element-specific differences in mobility near the sediment interface. Tracer profiles allowed the calculation of bioturbation (tracer microspheres) and pore water diffusion (22Na) rates, as well as an investigation of the spacial and temporal dynamics of trace element cycling near the sediment-water interface. (7) Se and Cr nuclides which were added in different oxidation states to different tanks, showed that the higher oxidation state forms (Se-VI, Cr-VI) are removed more slowly from the water column than the lower oxidation state forms (Se-IV, Cr-III). Furthermore, speciation experiments have shown that the increase in the colloidal fraction of Se may be used to calculate the characteristic times of Se-reduction to elemental or organically-bound forms.