Marine Radioactivity

MARINE RADIOACTIVITY

RADIOACTIVITY IN THE ENVIRONMENT A companion series to the Journal of Environmental Radioactivity Series Editor M.S. Baxter Ampfield House Clachan Seil Argyll, Scotland, UK Volume 1: Plutonium in the Environment (A. Kudo, Editor) Volume 2: Interactions of Microorganisms with Radionuclides (F.R. Livens and M. Keith-Roach, Editors) Volume 3: Radioactive Fallout after Nuclear Explosions and Accidents (Yu.A. Izrael, Author) Volume 4: Modelling Radioactivity in the Environment (E.M. Scott, Editor) Volume 5: Sedimentary Processes: Quantification Using Radionuclides (J. Carroll and I. Lerche, Authors) Volume 6: Marine Radioactivity (H.D. Livingston, Editor)

MARINE RADIOACTIVITY

Editor

Hugh D. Livingston International Atomic Energy Agency, Marine Environment Laboratory, Principality of Monaco

2004 AMSTERDAM – BOSTON – HEIDELBERG – LONDON – NEW YORK – PARIS SAN DIEGO – SAN FRANCISCO – SINGAPORE – SYDNEY – TOKYO

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Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vii

1. Natural radionuclides applied to coastal zone processes by J. K. Cochran & P. Masqué . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2. Linking legacies of the Cold War to arrival of anthropogenic radionuclides in the oceans through the 20th century by T. F. Hamilton . . . . . . . . . . . . .

23

3. Transuranium nuclides in the world’s oceans by L. L. Vintró, P. I. Mitchell, K. J. Smith, P. J. Kershaw & H. D. Livingston . . . . . . . . . . . . . . . . . .

79

4. Overview of point sources of anthropogenic radionuclides in the oceans by G. Linsley, K.-L. Sjöblom & T. Cabianca . . . . . . . . . . . . . . . . . . .

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5. Reactive radionuclides as tracers of oceanic particle flux by M. P. Bacon . . .

139

6. Radionuclides in the biosphere by S. W. Fowler & N. S. Fisher . . . . . . . .

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7. Radiological assessment of ocean radioactivity by G. J. Hunt . . . . . . . . .

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8. Developments in analytical technologies for marine radionuclide studies by P. P. Povinec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Index of Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Foreword Hugh D. Livingston

This series of books on Radioactivity in the Environment is ambitious in that it sets out to review and document the steady growth in knowledge in this young field. While it may date back a century or so in respect of the discovery of radioactivity, it really has been over only about fifty years that the application of the basic knowledge in nuclear physics and chemistry has been applied to understand the impact of radioactivity on the environment and, conversely, what can be learned about the terrestrial and marine environments using the unique new toolset of radionuclides. The oceans, covering 70% of the surface of the planet and containing the bulk of its water, form a major compartment of the global environment generally and one which holds a corresponding large share of radioactivity on our planet. In recent years, we have become increasingly aware of the importance of the oceans to life on the planet in diverse ways such as their influence on climate, their potential to provide food for an ever-expanding population and the vital role of the coastal ocean for mineral resources, transportation and a locale used by a very large fraction of the Earth’s population. This book on Marine Radioactivity sets out to cover most of the aspects of marine radioactivity which have been the focus of scientific study in recent decades. The authors and their reviews divide into topic areas which have defined the field over its history. They cover the suite of natural radioisotopes which have been present in the oceans since their formation and quantitatively dominate the inventory of radioactivity in the oceans. Also addressed are the suite of artificial radionuclides introduced to the oceans as a consequence of the use of the atom for development of nuclear energy, nuclear weapons and various applications of nuclear science. The major source of these continues to derive from the global fallout of atmospheric tests of nuclear weapons in the 1950s and 1960s but also includes both planned and accidental releases of radioactivity from both civilian and military nuclear technology. The other division of the major study direction depends on whether the objective is to use the radionuclides as powerful tools to study oceanic processes, to describe and understand the ocean distribution of the various natural or artificial radionuclides or to assess the different radionuclides’ impact on and pathways to man or marine organisms. The subject of natural radionuclides in the oceans has been often covered in general in books and reviews. This book features two very current and broad reviews of specific topics vii

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Hugh D. Livingston

in this field. In Chapter 1 (Cochran & Masque), the uses of natural radionuclides to address practical problems in the vital coastal ocean are reviewed. In Chapter 5 (Bacon), the use of reactive natural radionuclides in the study of particle transport processes is covered. Three chapters focus specifically on aspects of the nature of oceanic contamination with artificial radionuclides. Chapter 2 (Hamilton) provides a broad account of the history and evolution in the world oceans of the artificial radionuclides introduced both by accident and on purpose, from both military and civilian activities since the beginning of the nuclear age. More specifically, Chapter 3 (Leon Vintro et al.) reviews the parallel history of the long lasting transuranic radionuclides in the oceanic domain. Finally, in Chapter 4, Linsley et al. cover, in a very comprehensive fashion, the nature and distribution of all known locales where manmade radionuclides are located – the so-called point sources. The chapter by Fowler and Fisher (Chapter 6) focuses on the interactions of both artificial and natural radionuclides with marine biota – a broad subject of wide interest and application in ocean science and very relevant to understanding the pathways back to man for effects evaluations. Such evaluations are fully described and reviewed in Chapter 7 (Hunt) where a complete assessment of the radiological impact of natural and artificial radionuclides is made and set in the context of the international standards and guidelines adopted to assess and control the effects of radioactivity on man and biota. Finally, Chapter 8 (Povinec) documents the current ‘state-of-the-art’ critical analytical methodologies which are in use in both ocean sampling and analyses in connection with marine studies of radioactivity. While the book thus deals with most salient aspects of marine radioactivity, coverage of tracer applications of radionuclides to characterize ocean current flows, advective and diffusive processes in the water column etc. is not the subject of a planned dedicated chapter since unfortunately at a late date the invited author was unable for personal reasons to deliver the manuscript. However, there are references throughout the book to such applications and so hopefully the reader will assimilate much on this topic if in indirect manner. The authors are to be congratulated in their efforts to bring together a scholarly and topical selection of papers in this key area of environmental studies of radioactivity. I wish to thank them for their efforts knowing that they will be much valued by students in the field who will be carrying their efforts forward in the 21st century.

MARINE RADIOACTIVITY Hugh D. Livingston (Editor) © 2004 Elsevier Ltd. All rights reserved

1

Chapter 1

Natural radionuclides applied to coastal zone processes J. Kirk Cochran, Pere Masqué1 Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794-5000, USA

1. Introduction The coastal zone represents the portion of the world ocean that is most directly affected by human activities. Development of the coastal zone, exploitation of its natural resources, and its use as a receptacle for societal wastes are resulting in problems ranging from contamination of sediments and living marine resources to eutrophication and the development of harmful or nuisance algal blooms. Radionuclides provide tracers for many of the processes related to coastal zone problems. Indeed, increasingly in the last 25 years, natural radionuclides have been used to quantify the rates of coastal ocean processes and many of these results are directly applicable to providing important information that may be used by managers tackling problems in the coastal zone. This chapter had its inception in a request from the International Atomic Energy Agency to the senior author to provide an overview of some of the principal applications of natural radionuclides to studying processes in the coastal zone, with an emphasis on the management problems that these radionuclides are well placed to address. These applications are indeed numerous: virtually all are the focus of current research and application, and any one of them could serve as the basis for a review article. In preparing this chapter we have of necessity, therefore, been selective in the examples we have chosen to include. It is fortuitous that the naturally occurring radionuclides, including those of the U and Th decay series and those produced by the interactions of cosmic rays with atmospheric gases (i.e. cosmogenic), comprise some of the most useful tracers for coastal zone processes. The choice of an appropriate radionuclide tracer is dependent on matching (i) its distinctive geochemical behavior with the process being characterized, as well as (ii) its half-life to the rate of the process. 1 Present address: Institut de Ciència i Tecnologia Ambientals – Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain.

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J. Kirk Cochran, Pere Masqué

Indeed many coastal zone processes operate on time scales that are rapid relative to similar processes in the open or deep ocean, and thus require radionuclides with short half-life for proper characterization. The suite of natural radionuclides used in studying coastal zone processes includes (halflives in parentheses): •

•

•

•

•

•

234 Th

(24 d) – This 238 U series radionuclide, produced in solution from decay of dissolved 238 U, is rapidly scavenged onto particles and removed to bottom sediments. It is used to determine rates of scavenging, sediment mixing by organisms or physical processes (such as storms) or rates of sediment accumulation in cases of rapidly depositing sediments. 32 P (14.3 d) and 33 P (25.3 d) – These cosmogenic isotopes of phosphorus are beta emitters produced by the spallation of atmospheric argon by cosmic rays. They quickly become associated with aerosols and are input to the oceans, where they equilibrate with dissolved inorganic phosphorus. Only a small fraction of 32 P and 33 P is produced in situ in the surface ocean, and thus the principal source of both isotopes is wet deposition. 7 Be (53 d) – This cosmogenic radionuclide is produced by the interaction of cosmic rays with atmospheric gases (mainly N and O). It is delivered to the Earth’s surface by wet and dry deposition and, where suspended particle concentrations are high, can be scavenged onto particles and deposited in bottom sediments. As with 234 Th, 7 Be can be used to determine mixing rates, or in certain instances, sediment accumulation rates. 210 Pb (22.3 y) – Lead-210 is produced from decay of 226 Ra via 222 Rn in the 238 U decay series. In the coastal ocean, the dominant supply of 210 Pb is typically via the atmosphere where it is produced from the decay of 222 Rn that has emanated from terrestrial rocks and soils. Owing to its long half-life, 210 Pb profiles in coastal sediments reflect both mixing and accumulation processes. 226 Ra (1622 y), 228 Ra (5.7 y), 223 Ra (11.4 d), 224 Ra (3.7 d), 222 Rn (3.8 d) – The element radium includes isotopes in the 238 U, 235 U and 232 Th decay series and 222 Rn is the immediate daughter of 226 Ra. All are the result of alpha decays and are produced dominantly in mineral structures. Recoil associated with the production of the nascent Ra or Rn atoms mobilizes these nuclides to sediment pore waters or to groundwater. Subsequently Ra and Rn can be released from sediments to overlying water. Recent interest in the distributions of Ra and Rn in coastal waters has focused on their use as tracers of the rate of groundwater inflow. 14 C (5730 y) – Although radiocarbon specific activities in the environment have been affected by the burning of fossil fuels as well as anthropogenic production associated with atmospheric nuclear testing, this radionuclide is also produced naturally as a cosmogenic radionuclide. Radiocarbon can be used, especially at depth in sediment cores, to determine long-term rates of accumulation. Such determinations provide an assessment of the accuracy of accumulation rate estimates produced using the shorter-lived radionuclides listed above. Recent advances in measurement using accelerator mass spectrometry (AMS) provide errors of only ±50 years on the radiocarbon age for ∼2000 year old carbon, permitting sediment accumulation rates to be determined over time intervals of just a few hundred years.

Natural radionuclides applied to coastal zone processes

3

2. Scavenging rates of particle-reactive contaminants in coastal waters The simplest use of natural radionuclides to determine scavenging rates assumes a balance between the production of the radionuclide from decay of a dissolved parent and the decay and scavenging of the radionuclide itself (Matsumoto, 1975): λP = λD + kd D,

(1)

where: λ is the decay constant for the daughter radionuclide, P and D are the activities of parent and daughter respectively in a water sample and kd is the scavenging rate constant (first-order) of the daughter radionuclide. If the parent and daughter activities are measured, all quantities in equation (1) except kd are known and kd can be calculated; the inverse of kd is τscav , the mean residence time of the daughter radionuclide with respect to scavenging. A variation of equation (1) includes separate measurements of the particulate and dissolved fractions and setting up analogous equations that lead to calculation of the residence time of D with respect to removal from solution onto particle surfaces and removal from the water column by sinking particles (e.g. Krishnaswami et al., 1976). The thorium isotopes are perhaps the quintessential particle-reactive radionuclides, and both 234 Th and 228 Th have been used to measure scavenging rates of Th in coastal waters (see Moore, 1992, for a review). Indeed the high suspended particle concentrations and shallow water columns of coastal waters and the consequent intense benthic–pelagic coupling that results produces short residence times with respect to scavenging. For example, early measurements by Aller & Cochran (1976) showed that 234 Th had a mean residence time of about 1 day in the waters of Long Island Sound, USA. An extreme example of rapid scavenging of Th is seen in the waters of the Venice Lagoon (Italy) (Cochran et al., 1995). The shallow water column (0.22 µm) material. In pond water effluents, the corresponding percentages were found to be somewhat lower (Pentreath et al., 1985). Differences in chemical speciation were also noted, with reduced plutonium (i.e. Pu(III, IV)) predominating in the filtrate of the effluent from the sea tanks and oxidised plutonium (i.e. Pu(V, VI)) predominating in the filtrate of the effluent from the cooling ponds. However, as the bulk of the effluent came from the sea tanks, only about 1% of the total would appear to have been in an oxidised form upon discharge. Nuclides of americium and curium were present only in the Am(III) and Cm(III) forms (i.e. chemically reduced), while for neptunium both reduced (Np(IV)) and oxidised (Np(V)) forms were present in approximately equal proportions in the total 237 Np discharge. These observations were supported by a more recent study carried out in 1991 on sea tank and SIXEP effluent streams (Leonard et al., 1995). In the case of sea tank effluent, almost all of the 239,240Pu and 241Am activity present was found to be associated with the iron floc formed upon the neutralisation of acid liquors containing ferrous sulphamate – a chemical used to control the valency of plutonium during fuel reprocessing. What little plutonium was in the solution phase was determined to be in the reduced (tetravalent) form. Further, nuclides such as 137 Cs, 90 Sr and 99 Tc were found to be almost entirely in the solution phase. In SIXEP effluent, on the other hand, all of the radionuclides considered were almost entirely in a dissolved form, presumably as a result of the absence of particulate material in this waste stream. Laboratory experiments to determine the colloidal size distribution of a suite of radionuclides in each of the effluent streams (SIXEP and seatank) were also carried out using ultrafiltration techniques (Leonard et al., 1995). Overall, the results suggest that colloidal forms of individual radionuclides, originating from the solution phase, are more likely to occur in the SIXEP rather than in the sea tank effluent, with significant fractions of the 239,240Pu(V), 239,240Pu(IV) and 241Am in the former being in colloidally-bound form, as evidenced by the level of retention upon ultrafiltration (