THE FATE OF TRACE-METALS IN NARRAGANSETT BAY, RHODE-ISLAND - RADIOTRACER EXPERIMENTS IN MICROCOSMS
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abstract
Experiments designed to determine removal rate constants and removal mechanisms of various radioactive trace metals (51Cr, 54Mn, 58Co, 59Fe, 65Zn, 75Se, 115mCd, 134Cs, 210Po, 210Pb, 226Ra, 228Th) in controlled ecosystems simulating Narragansett Bay conditions in spring and early summer seasons are described. Overall removal could adequately be described with a model assuming first order removal to several reservoirs. Initial removal rates in two identical tanks replicated well during the early summer experiment. The removal behavior of Mn, Cr and Se changed after the first 1-2 weeks of the same experiment. Although adsorption to walls and other tank parts was significant, that fraction seemed to be effectively removed from further interaction and would be accounted for in the model. The major mechanisms for metal uptake that would be characteristic of the natural system were found to be scavenging by particles that settled from the water column (mostly resuspended sediment) and direct adsorption within the bioturbated layer of the sediment. By comparing the relative importance of these sinks and considering the similarities in respective half removal times, the 12 elements studied were grouped according to general behaviour: (1) hydrolysable elements including Fe, Th, Po, Cr (-III), and Se (as SeO2-3), with half removal times ranging from 4 to 40 days; (2) particle-reactive elements Mn and Co with half removal times ranging from 2 to 140 days; (3) Zn and Cd which were removed more slowly, with half removal times ranging from 50 to 400 days and about equally by settling particles and by adsorption within the bioturbated layer of the sediment; (4) Cs and Ra, which were removed most slowly, with half removal times ranging from 600 to 800 days, mostly by adsorption within the bioturbated layer of the sediments. Generally slower removal was observed during the early summer relative to the spring and was ascribed to a combination of trace metal association with low molecular weight organic compounds and increasing rates of return from the sediments. Bioturbation rates in the top 3 cm of the sediments in the spring experiments were similar to other experimental values for Narragansett Bay and other coastal regions: a mixing coefficient of 2-510-7 cm2 s-1 was calculated. 1980 Academic Press Inc. (London) Ltd.
