THE ROLE OF PARTICLES AND COLLOIDS IN THE TRANSPORT OF RADIONUCLIDES IN COASTAL ENVIRONMENTS OF TEXAS

Many bays and estuaries of the USA are heavily impacted by man's activities. It is therefore important to characterize the self-cleansing capacity of these waters. To this effect, Th, Pb and Be nuclides in the water column can be used as tracers for the removal behavior of particle-reactive substances. Both sub- and supra-micron sized particles play a major role in the transport of particle-reactive nuclides and pollutants in estuarine and coastal waters. We report here the results of a study of the fate of radionuclides, carried out during different times of the year in six Texas bays and estuaries, and in Galveston's coastal waters, after a one-time pulse injection by wet fallout of Pb and Be nuclides. We investigated both removal residence times and the partitioning of Th, Pb and Be isotopes between filter-retained particles (0.5 m or greater), filter-passing dissolved (0.5 m or less) and, in a separate experiment, a colloidal phase. Recent studies indicate that a significant fraction of conventionally defined dissolved (0.4 m or less) Th isotopes is associated with colloidal material (less than 0.4 m). In this investigation, we extend this approach to two other particle-reactive nuclides, Be-7 and Pb-210. Our results show that the extent of association of Th, Be and Pb nuclides with colloidal material appears to follow the same sequence as that for the association of these nuclides with filter-retained particles. A major portion, i.e. 70% or greater, of Be-7 and Pb-210 injected into the surface coastal water by wet fallout, is removed from the water with an overall residence time of less than 1 day. Suspended particles and their associated nuclides settle and are turned over on time scales of hours by sediment resuspension. The rate of sediment resuspension and deposition appears to control the particle association and removal from the water of these nuclides. The extent of particle association of Pb-210 and Th-234 isotopes appears to be highest in the samples taken during the summer. This is attributed to the more energetic particle transport owing to the large amounts of freshwater inputs and stormy weather conditions during that time.