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

Abstract

The "mobilities" of radioactive trace elements across the water sediment boundary of a\ncoastal marine ecosystem were investigated. The studies carried out included chemical speciation\nexperiments of the solution and solid phases, as well as verification experiments in controlled\nmodel ecosystems ("MERL" tanks). The latter included backdiffusion experiments under oxic\nand anoxic conditions and experiments with artificially increased sediment resuspension rates.\nThese studies have produced seven general conclusions: <BR/>\n( 1) The backdiff usion of Cs, Mn, Co, and Zn radiotracers across the sediment-water interface\ninto oxic waters and of Mn and Co radiotracers into anoxic waters was predicted from laboratory\nexperiments. (2) The removal from the water and the partial immobilization in the sediments of\nCs, Zn and Cd tracers, during anoxic conditions, agreed with results from selective leaching\nexperiments of surface sediments with dithionite-citrate solution, a mildly reducing agent which\ncan reprecipitate liberated metals as sulfides. While most nuclides were leached by this solution\nto the same extent as by hydroxylamine, another reducing agent, Zn, Cd and Cs tracers were\nnot, possibly due to the formation of sulfidic and other phases by the former solution. (3)\nRadioisotopes of particle-reactive elements (Sn, Fe, Hg and Cr) were shown by sequential\nextraction and ultrafiltration experiments to be involved in the dynamic cycle of colloid\nformation and aggregation in the water column and sediments. (4) In order to extend the\ninformation on nuclide behavior gained from the radiotracer methodology to stable trace\nelements, (which are often introduced into coastal water in ionic form) stable metals were added\nto one tank. Radiotracer behavior in the water column (removal rates and extent of uptake by\nsuspended particles) was quite similar to that of their stable metal counterparts at ambient\nconcentrations (Mn, Cr, Fe, Cd and Zn), added simultaneously to one tank, and to the metal\nbehavior in other tanks operating under similar conditions. (5) The experiments with increased\nresuspension rates without concomitant increased bioturbation rates had, as expected, only small\neffects on removal rates of the radiotracers. (6) Sediment profiles of the tracers revealed both\nseasonal and element-specific differences in mobility near the sediment interface. Tracer profiles\nallowed the calculation of bioturbation (tracer microspheres) and pore water diffusion (<SUP>22</SUP>Na)\nrates, as well as an investigation of the spacial and temporal dynamics of trace element cycling\nnear the sediment-water interface. (7) Se and .Cr nuclides which were added in different\noxidation states to different tanks, showed that the higher oxidation state forms (Se-VI, Cr-VI)\nare removed more slowly from the water column than the lower oxidation state forms (Se-IV,\nCr-III). Furthermore, speciation experiments have shown that the increase in the colloidal\nfraction of Se may be used to calculate the characteristic times of Se-reduction to elemental or\norganically-bound forms.