R. M. Trier

The residence time of thorium in surface sea water and its implications regarding the rate of reactive pollutants

Abstract The activity ratio of228Th/228Ra in the open surface ocean averages 0.21. This suggests that thorium is removed from surface water in about 0.7 yr. As plant matter is cycled within the surface sea on a similar time scale, the suggestion is made that highly reactive compounds are inadvertently removed by plants in their quest for the critical nutrients nitrate and phosphate. Combined with the coefficient for horizontal eddy diffusivity obtained from the distribution of228Ra in surface sea water this result provides a basis for the prediction of the distribution of “reactive” pollutants released to the surface sea from coastal areas.

Pu in coastal marine environments

Abstract Analysis of water samples from the New York Bight area and Narragansett Bay reveals that a small fraction of the total Pu (probably Pu (III + IV) species) is continuously removed to the sediments at a rate similar to that of the particle-reactive isotope 228 Th. A more “soluble” Pu species appears to be released at times from the sediments to the water column in these nearshore regions. Sediments in shallow areas of the New York Bight south of Rhode Island and Narragansett Bay have high Pu inventories and relatively deep penetration of this element, although the net sediment accumulation rate is generally low ( 2 yr). The high Pu inventories can be explained if both sediment resuspension and sediment mixing are assumed to be the major controlling factors for the effective transfer of Pu from the water column to the sediments. By simultaneous modelling of the depth distribution of three tracers which operate on vastly different time scales: 234 Th (half-life 24 days), 210 Pb (half-life 22 years) and 239,240 Pu (introduced into the environment during the past 30 years), bioturbation rates ranging from 4 to 32 cm 2 /yr in the surface mixed layer (5–10 cm thick) and from 0.3 to 2.5 cm 2 /yr in the layer below (up to 40 cm thick) and net sediment accumulation rates of approximately zero to 0.14 g/cm 2 yr were calculated for these areas.

Radium-228 profile at the second Geosecs intercalibration station, 1970, in the North Atlantic

Abstract 228Ra concentrations were measured in 12 subsurface water samples collected during August–September 1970 in the North Atlantic at the Second Geosecs Intercalibration Station.228Ra concentrations ranged from 1.5 to 2.1 dpm/100 kg in the water layer between 100 and 600 m depth, above the main thermocline, but decreased with increasing depth to less than 0.5 dpm/100 kg at 1.5 km, below the main thermocline. The two deepest samples, collected below 4 km depth, indicated that the228Ra concentration increased again to at least 1.2 dpm/100 kg as the interface with the sea floor sediments was approached. Above the 4 km level, the228Ra profile was roughly similar to the tritium profile measured by Roether and Munnich.

Radionuclides in Mono Lake, California

Several radioisotopes of the naturally occurring uranium and thorium decay series, in addition to fallout plutonium, have unusually high concentrations in the water column of Mono Lake, a natural alkaline, saline lake. Complexing by carbonate ions appears to be responsible for the enhanced solubility of actinide elements with oxidation states of IV to VI. In contrast, fallout strontium-90 has been largely removed from the water, probably as a result of coprecipitation with calcium carbonate. The daughter/parent activity ratios of thorium, radium, and uranium isotopes suggest that thorium is removed from the water column to the sediments on time scales substantially longer than a month and that the desorption of thorium from the sediments to the water column requires less than a few years.

Uranium and radium mobility in groundwaters and brines within the delaware basin, Southeastern New Mexico, U.S.A.

Concentrations of naturally occurring isotopes of U and Ra were measured in fresh waters, groundwaters and in sodium chloride brines within and near the Delaware Basin of southeastern New Mexico. We sought to determine the effect of high chloride concentrations in a wide range of redox conditions on the mobility of U and Ra in natural waters. Two important features of radionuclide mobility are evident from our results: (1) There is a slight tendency for U and Ra concentrations to correlate with the chloride content of the water samples. Thermodynamic speciation calculations suggest that this may result from complexation of these elements by Cl −1 ions. (2) Much more dramatic than the correlation with Cl − concentration is the effect of the redox state of the waters on U and Ra concentrations. Chemically reducing groundwaters contained much lower U concentrations and much higher Ra concentrations than were measured in oxic and su☐ic samples. The low U values are consistent with the expected reduction of U (VI) to insoluble U (IV). We suggest that the increased Ra concentrations result from mobilization of Mn which eliminates the strongest adsorption sites for Ra thereby greatly enhancing its concentration in the anoxic waters.

Plutonium, radiocesium and radiocobalt in sediments of the Hudson River estuary

Abstract Anthropogenic radionuclides have reached the Hudson estuary as global fallout from nuclear weapons testing and through local releases from commercial nuclear reactors. Significant activities of 238 Pu and 239,240 Pu (fallout-derived), 134 Cs and 60 Co (reactor-released), and 137 Cs (derived from both sources), have accumulated in the sediments throughout the estuary, with the primary zone of accumulation near the downstream end of the system in New York harbor. The estuary appears to have trapped nearly all of the 239,240 Pu delivered as fallout, and consequently, ocean dumping of dredged harbor sediment is currently the primary means for the net transport of these nuclides to coastal waters. In contrast, only 10–30% of the 137 Cs, 134 Cs and 60 Co delivered to the estuary have been retained on the fine particles which accumulate at a rapid rate in the harbor. The primary factors which have governed the distribution of anthropogenic radionuclides in Hudson sediments are: (1) spread of fine particles labeled with both fallout and reactor nuclides throughout the axis of the estuary, (2) differences in timing of the peak fallout years (1962–1964) and years of maximum reactor releases (1971–1972), (3) large variations in sediment accumulation rates, ranging from a few millimeters per year or less to many tens of centimeters per year, (4) appreciable desorption of 137 Cs and 134 Cs from particles at higher salinities, and (5) possible enhanced desorption of 60 Co at higher salinities (relative to 134 Cs and 137 Cs) which may be associated with the release of reduced manganese from the harbor sediments.

228Ra and 228Th in coastal waters

Typically, the waters above the continental shelf contain higher concentrations of 228 Ra but lower concentrations of 228 Th than do the surface waters of the open ocean. The vertical profiles of 228 Ra concentration in the shelf waters of the Middle Atlantic Bight suggest that the shelf sediments are the chief source of 228 Ra, but that the sediments of bays and estuaries are a significant secondary source. In the mid-shelf and outer shelf waters of the Bight, the 228 Ra concentration increases and the 228 Th/ 228 Ra activity ratio decreases from the region bordering Nova Scotia to the region uust north of Cape Hatteras. The mean residence time of thorium in the surface waters of the Bight, calculated from the 228 Th/ 228 Ra activity ratios, ranges from 4 months in the slope waters and outer shelf waters and 1·5 months in the inner shelf waters of the region bordering Nova Scotia to 1·5 months in the slope waters, 20 days in the outer shelf waters and 10 days in the inner shelf waters of the region just north of Cape Hatteras. The vertical profiles of 228 Th concentration in the surface mixed layer and thermocline of the open ocean qualitatively resemble the profiles of concentrations of nutrients, such as phosphate. The process most likely to produce the observed rapid transport of thorium out of the surface waters probably involves uptake of thorium by phytoplankton, ingestion of the phytoplankton by zooplankton, and inclusion of the thorium in rapidly settling fecal pellets.

Reactor-released radionuclides and fine-grained sediment transport and accumulation patterns in Barnegat Bay, New Jersey and adjacent shelf waters*

Anthropogenic radionuclides have been introduced into the Barnegat Bay environment via global fallout from nuclear weapons testing and by low-level discharges from the Oyster Creek Nuclear Generating Station. A significant portion of these radionuclides have been adsorbed to fine-grained particles and consequently provide excellent tracers for fine-sediment transport and accumulation patterns in the Bay and adjacent shelf environment. Fine-grained particles tagged with reactor nuclides are presently accumulating along the landward shore of Barnegat Bay at rates between I and 7 cm year−1. There is some indication that reactor-tagged sediments are also being transported into the mouths of small tributaries which discharge into the Bay. Reactor-tagged particles that escape from the Bay, through Barnegat Inlet, onto the shelf, are accumulating during the summer with fine-grained material from other sources, in depressions along the shelf bottom. These near-shore mud deposits are apparently resuspended and dispersed by major storms probably on a seasonal basis. By comparing radionuclide ratios in the reactor releases with the ratios observed in the fine-grained sediments in the vicinity of the reactor, it appears that (I) 35% to 50% of the total 137Cs activity in the top 30 cm of the sediments in Oyster Creek is attributable to reactor releases, (2) if transuranic releases have occurred from the Oyster Creek Reactor, the increments of 239,240Pu and 238Pu are not distinguishable from the fallout plutonium burden in the sediments and (3) radiocobalt is removed from the liquid effluent and stored in the sediments at least 20× more effectively than radiocesium. A budget of 60Co, based on inputs from the reactor, storage in the Bay sediments, and outputs through Barnegat Inlet on particles and in the dissolved phase, indicates that 40% of the 60Co released by the Oyster Creek reactor resides in the sediments of Barnegat Bay. The accumulation and dispersal patterns of reactive activation products such as 60Co released during routine nuclear reactor operations provide important clues to the probable transport pathways of a wide variety of pollutants including transuranic nuclides if significant releases of such materials were to occur.

An estuarine fine-particle budget determined from radionuclide tracers

The sedimentary distributions of radiocesium and plutonium have been used to determine patterns of fine-particle accumulation, estimate net sediment fluxes from different sources, and develop a fine-particle budget for the Hudson-Raritan estuary. It is proposed that the rates and patterns of fineparticle accumulation reflect a sediment surface in dynamic equilibrium with the wave and current regimes. Rates of accumulation in most estuarine areas appear to equal the rate to sea-level rise or land subsidence. Localized areas, which have not yet attained or are presently out of equilibrium, serve as fine particle traps.