Radioecology of Bombay harbour - a tidal estuary

Abstract

Low level liquid radioactive waste from nuclear facilities at Bhabha Atomic Research Centre, Bombay, are released into Bombay harbour after monitoring and dilution. The interactions of gamma-emitting fission product nuclides, especially cesium-137, with sedimentary particles and biota were studied during 1968-71. Cesium-137 is first scavenged by the sedimentary particles, followed by cerium-144 and ruthenium-106. Zirconium/niobium-95, though present in the effluent, was not sorbed. Cesium-137 was distributed throughout the harbour, whereas cerium and ruthenium deposition were limited to a few stations off Trombay coast. Maximum deposition of cesium-137 activity was around the discharge zone off Trombay; it decreased significantly with distance, the concentration being 100 times lower towards the mouth of the harbour in south. Over the period of surveillance there was about two-three times increase in the accumulation of radioactivity by bed material, though the total radioactivity in the effluent discharge had increased five times. Maximum deposition of activity was found in the upper 5 cm layer of the sediment column; decreasing thereafter either intermittently or exponentially with depth. The sorption sequence of cesium-137, cerium-144 and ruthemium-106 has been explained in terms of rate of sorption, mineral structure and the physicochemical parameters controlling the diffusion mechanism. The occurrence of artificial radioactivity in fish and shell-fish of economic importance in 1965 before regular discharge began was below detection limit. Analysis since then showed accumulation of cesium-137 only. The absolute concentration as such, however, varied from species to species. Other nuclides, cerium-144, ruthenium-106 and zirconium/niobium-95, could not be detected except in the ark-shell bivalve, Anadara granosa, which showed specific accumulation of cesium, cerium and ruthenium nuclides but not of zirconium/niobium-95, although this nuclide was present in the effluent. A. granosa was therefore used as an indicator to detect contamination due to cerium and ruthenium radionuclides. Lamellibranchs and crustaceans, in general, were found to be the most effective integrators of cesium-137. In these benthic species the concentration factors (Cfs) for the nuclide varied from 10² to 10⁵. Pelagic species, on the other hand, were poor integrators of cesium-137 (Cfs: 10–50). Monthly variation in concentration of cesium-137 in A. granosa, Placenta placenta, Scylla serrata and Periopthalmus sp. could be related to the variation in the amount of activity released. Maximum concentration of cesium-137 occurred in muscle tissues of different species. The radiation dose through the contaminated environment to the benthic communities was far below the limits required to produce any detectable radiation damage. The radiation dose to fishermen, both internally through the consumption of contaminated marine products and externally through fishing over the contaminated bed, was also well below the permissible dose limit.