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AN INTRODUCTION TO RADIOACTIVITY IN THE MARINE ENVIRONMENT WAHYU RETNO PRIHATININGSIH IAEA/RCA Regional Training Course on Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems Mumbai, INDIA 27 May – 6 June 2008 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 AN INTRODUCTION TO RADIOACTIVITY IN THE MARINE ENVIRONMENT 1. INTRODUCTION Awareness of exposure to natural radioactivity and its technological enhancement in marine systems is growing. However, common misconceptions are the association of radioactive discharges only with the nuclear industry and that radioactivity is somehow unnatural. Natural Radioactivity in the Ocean - Gamma radiation Gamma Rays Cosmic Rays 0.01 pm Infra-Red Rays Ultra-Violet Rays X Rays 1 nm Visible Light Rays 10 nm 580 nm TV Micro Waves 1 m Electricity Radio Waves 10 cm 50 m 5000 km Energy Wavelength The Electromagnetic Spectrum Gamma rays : X rays : UV rays : Visible light : Infra-Red rays : Hertzian waves : Radiation from atomic nuclei Radiation from electrons & protons hitting a target Radiation from arcs & electric discharges in gases Radiation from stars, hot objects & electric discharges Radiation from warm objects Radiation from alternating electric currents, radar, etc Figure 1. The electromagnetic spectrum. Environmental radioactivity is a natural occurrence, it was the combination of nuclear reactions and radioactivity that created all matter and variations in the stability of nuclides determined which elements are abundant and which are rare in the universe. Of the more than 5000 atoms (nuclides) known, about 95% are radioactive - they are the norm rather than the exception. Virtually all materials and environments on our planet are both radioactive and naturally exposed to ionising radiation. The energy from nuclear decay powers major geological changes on the Earth, such as internal convection cycles, earthquakes, volcanic activity and continental drift. Natural radioactivity triggers and catalyses key stages in the evolution of life and fusion reactors, such as our sun, provide the essential energy to our planet in the form of light and heat which regulates climate. 2 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 Radioactivity in the Marine Environment (370 pCi/L) Nuclide Activity (pCi/L) Origin 40-potassium 320 primordial 3-hydrogen 0.6 - 3.0 cosmogenic 87-rubidium 2.9 primordial 234-uranium 1.3 primordial 238-uranium 1.2 primordial 14-carbon 0.2 cosmogenic 3-hydrogen 31 - 74 fallout 137-caesium 0.03 - 0.8 fallout 90-strontium 0.02 - 0.5 fallout 239&240-plutonium 0.0003 - 0.0012 fallout 14-carbon 0.01 - 0.04 fallout Table 1. Major components of radioactivity in the marine environment 2. TECHNOLOGICAL ENVIRONMENT ENHANCEMENTS OF RADIOACTIVITY (TENORM) IN THE MARINE Commonly used industrial and domestic energy sources like coal, oil, gas, wood, and peat contain radioactive elements which are often concentrated as a result of combustion processes then released into the environment, often as coastal discharges. It should be borne in mind that the lower activity limit for radioactive materials has traditionally been set in the range 100 to 400 microsieverts per year for solids, depending on context and usage. The fact that radioactivity levels in many everyday materials lie above these limits does not of course imply that they represent a significant hazard. The exposure route and duration must be taken into account in any such assessment. The key point here is that the naturally occurring radionuclides - primarily 40-potassium and the uranium and thorium decay series - occur at significantly enhanced concentrations in both natural and man-made materials and that they “could” be regarded as radioactive substances and “could” under certain circumstances generate a significant radiation exposure. Major estuarine or coastal contamination is often directly traceable to an industrial, powergenerating, or mining source, although urban run-off is also often responsible for increases in radionuclides, heavy metals and organics above natural baseline values. 2.1. Coal Powered Electricity Production The best documented but largely unappreciated input of enhanced radioactivity to the marine environment is from fossil fuels (coal, oil & natural gas). Their radioactivity results from uranium, thorium, radon and polonium isotopes which are enhanced during fuel extraction and combustion. The generation of one gigawatt-electric (GWe) of coal fired electricity results in the environmental release in fly ash and gases of about 109-1010 becquerels (Bq) per year 3 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 each of 210Pb, 210Po, 226Ra, 228Ra, 232Th and 238U. These releases are comparable to release from nuclear power stations under normal operation. Globally, they add to the environment 5000 tonnes of uranium, 8000 tonnes of thorium plus daughter products which include 600 terabequerals (1012Bq) of alpha emitters. The natural radioactivity in the fly ash waste product is concentrated by volatilisation and sorption processes relative to the original coal burned. One immediate consequence of this enrichment of radioactivity in fly ash is that its common use in the production of construction materials results in correspondingly higher gamma-dose and radon-inhalation rates in buildings and environments built from bricks and building blocks manufactured in this way. The fly ash released in coal-burning is in the form of vitrified fine particles (1-100m). Despite stack filtration and other trapping methods, soils in the environment of coal-fired power plants are often found to be enriched in such air-borne radionuclides. Ash collected in stack filters, and coal residues, are often disposed of locally in dumps or wetlands and these again show enhanced levels of radioactivity. 2.2. Oil and Gas Industries Oil and gas burning is characterised by similarly interesting interactions with the natural decay series nuclides, particularly with the radium and radon isotopes. For example, 222Rn diffuses into natural gas and oil deposits within the Earth which, along with subsequently produced daughter products, are released in the power plants or homes where the fuels are burned. It had been estimated that the critical group near a gas-fired power plant can receive an effective dose equivalent of up to 20 microsieverts per year through ingestion of 222Rn through seafoods and leafy vegetables. A similar dose can result from inhalation in homes burning natural gas. The generation of one gigawatt-electric (Gwe) of coal fired electricity results in the release in fly ash and gases of about 109-1010 becquerels (Bq) per year each of 210Pb, 210Po, 226Ra, 228Ra, 232Th and 238U. Globally, they add to the environment 5000 tonnes of uranium, 8000 tonnes of thorium plus daughter products which include 600 terabequerals (1012Bq) of alpha emitters Baxter, M.S. (1993) Perhaps one of the better documented environmental exposure pathways in this area concerns the radionuclide concentration process which occurs within brines brought to the surface during extraction of oil and gas. Typically brines are over-saturated with sulfate and carbonate salts of alkaline earth metals and these deposit to form “scale” on the inside of pipes, pumps and associated holding tanks which eventually reduce the flow and routinely necessitate 4 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 dismantling and maintenance. Because the natural cations in these scales are calcium, strontium and barium the are extremely efficient in scavenging and concentrating their close relatives the radium isotopes (226Ra & 228Ra). Concentrations of these isotopes exceeding 106 Bq/kg have been observed and workers who clean and decontaminate such equipment must follow strict safety procedures comparable to those in high-radioactivity laboratories. It is not uncommon for effluents from these kinds of industrial sites to be discharged directly to the marine environment and enhancement of 210Pb, 210Po, 232Th and 238U in coastal sediments nearby discharge pipes have been observed. It must not be concluded that the radioactivity released by the fossil fuel power industry is either extensive or harmful. However, in some cases, the releases can be significant on local scales and are considerably less understood then those of similar magnitudes by the competing nuclear industry. 2.3. Other Industrial sources As well as the association with nuclear and fossil-fuel power-generation, radioactive enhancements are also, but not so publicly identified as, associated (Table 2) with industries producing pharmaceuticals, titanium oxides, rare earth element processing, mineral waters, paints, ceramics, dyes & colourings, clothing, lime burning and construction industries. Industrial Discharges of Radioactive Isotopes in the Ocean Nuclear operations - power generation, weapons, spent uranium munitions Power generation - coal, gas, oil, peat Mining operations Oil production Automobile emissions Domestic heating - coal, gas, oil, peat Pharmaceutical production Hospitals - sterilisation, diagnostic scans Titanium oxide - paints, catalysts Semiconductors - rare earth elements Mineral waters Clothing - dyes & colorings Construction - bricks, concrete, lime, sand Detergents - (uranyl) phosphates Fertilisers - superphosphate Table 2. Industrial sources of radioactivity in the oceans. Environmental enhancement of natural decay series nuclides have been regularly observed in the vicinity of phosphate processing plants, reflecting the natural chemical association of uranium and radium with the phosphate ion and accumulation in phosphogypsum waste products. The fertiliser and detergent industries are the main focuses for the release of enhanced natural radioactivity in phosphatic wastes. Discharges from the Rotterdam industrial region of the Netherlands amount to 1012 Bq/yr each of 210Po and 226Ra leading to enhancement of about 100Bq/kg of 210Po in edible parts of mussels and prawns living >100km from the industrial zone. 5 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 3. OCEAN GEOCHEMISTRY OF NATURAL RADIONUCLIDES Natural radioactive nuclides in the marine environment are acted upon by the same processes which affect stable elements, and the behaviour of a primary long-lived nuclide, such as 238U, can be viewed in the same terms as that of any other conservative trace metal (Table 3). With most radioactive nuclides, however, their half-lives, genetic relations to other nuclides and consequent specialised modes of production introduce additional aspects to their behaviour which are not paralleled among the stable elements. These difference can afford useful insights into geochemical processes and many radionuclides have important applications as indicators of the time-scales of various oceanic processes, such as the chronology, transport and mixing of water masses, sediment accumulation, geochemical and air-sea gas exchange processes - we will consider some of the applications later. Geochemical Behaviour Trace Metals Radionuclides Conservative gold, rubidium, tungsten uranium, radon, radium, iodine Nutrients arsenic, cadmium, copper, iron, silver polonium, beryllium, plutonium Scavenged aluminium, arsenic, cobalt, copper, iron thorium, bismuth, lead, iodine Redox-controlled arsenic, chromium, iron, manganese, selenium iodine, uranium, plutonium Table 3. Oceanic biogeochemical behaviour of some trace metals and radionuclides. It is convenient to discuss natural occurring radionuclides in three groups; 1. Primordial nuclides of elements which also have stable isotopes: the most important of which are 40-potassium and 87-rubidium; 2. Primordial parent nuclides of the three natural decay series: 238-uranium, 235uranium and 232-thorium and the families of shorter-lived daughter isotopes which are continuously produced through radioactive decay; and 3. Naturally occurring nuclides, other than the daughter products of primordial nuclides which have short half-lives relative to the rates of ocean processes which are continuously produced by natural nuclear processes. This group contains 7Be, 10 Be, 14C, 26Al and 32Si, but as these nuclides generally do not reach levels in the environment to be of concern, they will not be discussed further. 3.1. Cosmogenic Radioisotopes 6 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 The important nuclides in relation to ocean processes are produced by the interaction of cosmic radiation with either atmospheric constituents or extra-terrestrial material accreted by the Earth. Ocean cycling of cosmogenic isotopes ATMOSPHERE 3H, 7Be, 10Be, 14C, 26Al, 32Si, 32P, 33P, 36Cl wind transport air-sea exchange precipitation aerosol production land-sea flux horizontal advection 36Cl, 3H SURFACE OCEAN 36Cl upwelling 14C, 32Si, 32P, 33P 7Be, 10Be biological pump particle scavenging vertical advection CONTINENTAL SHELF DEEP OCEAN faecal pellet remineralisation horizontal advection 14C sedimentation 14C, 32Si 10Be, 26Al SEA FLOOR SEDIMENTS VENT & VOLCANO TRENCH Figure 2. Cosmogenic radionuclides in the ocean 3.2. Uranium, Thorium and Actino-Uranium Series Isotopes The uranium, thorium and actino-uranium series have as their parent nuclides 238U, 232Th and 235 U, respectively. Each parent gives rise to complex chain of decay products. In a closed system of sufficient age each daughter isotope in a given series would be in secular equilibrium (steady state) so that the radioactive decay rates of each would equal that of the primary parent and the total activity would decrease only with the half-life of the parent nuclide. Under these conditions each daughter product is produced at the same rate at which it decays. The parent nuclide regenerates the daughter at a rate which is governed by the half-life of the parent and secular equilibrium will be maintained if the system remains closed. On separation of a shorter-lived nuclide from its immediate parent, the unsupported daughter nuclide decays with its own half-life. A period of about six daughter half-lives gives practically complete restoration of secular equilibrium and this period also allows decay of an unsupported nuclide to only a few percent of its original concentration. Other situations may arise in which there are different relationships between parent and daughter half-lives but the case of secular equilibrium is most important in this context. In the marine environment the equilibria in the radioactive families are profoundly disrupted by geochemical processes. The decay of unsupported nuclides and regrowth of daughter products then provide many approaches to the measurement of kinetics (time-scales) of environmental processes. 3.2.1. Uranium 7 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 There is considerable agreement between the reported values for uranium concentrations in oceanic waters with an average of 3.3g/L, corresponding to an activity of 2.2pCi/L, although a degree of variability is evident. The real extent of these variations is diminishing with more reliable studies and analytical techniques and it is clear that uranium has a more uniform distribution in open ocean waters than do the majority of trace elements, probably attributable to reduction in its reactivity through formation of the stable [UO2(CO3)3]4- complex. Particulate uranium concentrations are about 0.01g/L and represents an insignificant fraction relative to the dissolved component. The conservative nature of uranium thus lends this isotope and its daughter products to applications in tracing in situ processes. Uranium-238 series isotopes Nuclide Half-life Origin - Mode of Supply 238U 4.47x109 y primordial - fluvial input 234Th 24.1 d in situ decay of 238U 234Pa 1.18 m in situ decay of 234Th 234U 2.48x105 y in situ decay of 234Pa & fluvial input 230Th 7.52x104 y in situ decay of 234U 226Ra 1.62x103 y diffusion from 230Th decay in sediments 222Rn 3.83 d in situ decay of 226Ra 218Po 3.05 m in situ decay of 222Rn 214Pb 26.8 m in situ decay of 218Po 214Bi 19.7 m in situ decay of 214Pb 214Po 10-4 s in situ decay of 214Bi 210Pb 22.3 y in situ decay of 214Po & atm deposition 210Bi 5.0 d in situ decay of 210Pb 210Po 138 d in situ decay of 210Bi 206Pb stable in situ decay of 210Po Table 4. 238-uranium series of radionuclides. A great deal of variation is however evident in the concentrations of uranium reported in river waters, making difficult the estimation of global averages and oceanic residence times. Global averages for river water range from 0.3 - 0.6m/L. A uranium flux to the oceans (based on a global riverine U:dissolved solids ratio and global flux of dissolved solids to the oceans) has been calculated at 1 x 1010g/yr. This value is plausible considering an average U concentration of 3.3g/L and a water discharge rate of 3.5 x 1016L/yr. Though speculative, given this flux, for a mean oceanic uranium concentration of 3.3g/L and an oceanic volume of 1.37 x 1021L, the total uranium content of the oceans is 4.5 x 1015g and, hence the residence time is approximately 2 x 105 years. Although 235U and 238U have a global activity ratio of one, the preferential mobilisation of 234U due to recoil reactions during chemical weathering gives rise 8 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 to a 234U/238U activity ratio greater than unity (1.2-1.3) in rivers and implies that the 234U/238U activity ratio in the oceans will be similar. The average 234U/238U activity ratio for seawater is 1.14 +/- 0.03. Natural concentrations of uranium in sediments range from 0.6 - 6.0g/g, the higher levels associated with clays minerals and lower concentrations found in carbonate deposits with elevations associated with hydrothermal activity, ferromanganous deposits and anoxic sediments. Uranium substitutes for calcium and/or silicon in fish bones and teeth, foraminifera and coral skeletons, molluscs and diatom frustules. Ocean cycling of 238U, 222Rn 232Th and 235U series isotopes 222Rn wind transport 222Rn 210Pb global air circulation 210Bi 210Po precipitation emanation ATMOSPHERE air-sea exchange U & Th series isotopes land-sea flux 230Th 226Ra 226Ra 222Rn 232Th 228Ra 228Ra 228Th horizontal advection CONTINENTAL SHELF boundary processes source 238U SURFACE OCEAN 234Th particle scavenging 210Pb 210Po particle scavenging food web ocean circulation 234U 230Th 235U 231Pa vertical advection 226Ra DEEP OCEAN 222Rn 210Pb hydrothermal activity diffusion from sediment sink sedimentation 230Th decay 226Ra SEA FLOOR SEDIMENTS 222Rn 210Pb VENT & VOLCANO TRENCH Figure 3. Uranium & thorium series radionuclides in the ocean 3.2.2. Thorium The primary characteristic of thorium in the marine environment is its high particle reactivity (the opposite of conservative uranium). Primordial 232Th enters the oceans primarily in detrital form, although there is some mobilisation during chemical weathering and soil formation. Unlike uranium which is dissolved during this process thorium is largely retained on particle surfaces. The naturally occurring thorium isotopes 234Th, 230Th and 228Th have in situ sources in seawater, resulting from the decay of U and Ra parents. Because the activities of thorium isotopes in the oceans are generally very low and their measurement is still subject to a degree of uncertainty due to both sampling and analytical difficulties, and the fact that seawater concentrations will vary according to the concentration and flux rate of suspended particles, reported concentrations vary over several orders of magnitude. Secular equilibrium with respective parent nuclides (234Th/238U, 230Th/234U, 228 Th/228Ra) is dramatically disrupted by geochemical scavenging and particle flux rates are determined by analysis of the activity ratios. Surface waters show extremely variable activity ratios of 234Th to 238U with very low ratios in coastal waters and values, at times, approaching unity in the open ocean. Particulate residence times reported for 234Th have been observed to vary from less than one day to more than 20 days in various marine environments, with a general trend of increasing residence times with distance offshore. The general increase in 9 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 thorium removal rates (and consequent increased secular disequilibrium) in the near-shore environment maybe attributed to increased suspended particle concentrations due to biological productivity, as well as more efficient oxidation-reduction cycling of iron and manganese. Concentrations of 232Th in river waters have been reported in the range of 0.1 - 0.01 g/L and an oceanic residence time of 60 years has been estimated (Burton, 1975), although such a low value has little real meaning and indicates very rapid removal from seawater. Concentrations for 230Th in seawater represent less than 0.01% of the equilibrium with respect to the parent (234U) with in situ formation being the main source to the oceans, with less than 4% arriving via rivers. 230Th and is influenced by particle scavenging in a similar way to 234Th, but with its longer half-life the influence of interactions with particulate matter is different. The amounts of 228Th in ocean waters are substantially in excess of those predicted by secular equilibrium with 232Th, and as only 20-40% of this excess can be explained by riverine inputs, the departure is considered to be supported from in situ production by 228Ra released from sea floor sediments. Although thorium is largely removed from the water column by scavenging onto particles and co-precipitation with (ferromanganous) mineral deposits, significant concentrations have also been found associated with biological compartments such as in fish bones and teeth, foraminifera and coral skeletons, molluscs and diatom frustules. 3.2.3. Radium Most of what we know about the behaviour of radium in seawater is based on studies of the longest-lived isotopes 226Ra and 228Ra, although some studies have been recently concerned with the shorter lived 224Ra. In most ocean regions there is a marked vertical gradient in concentration of 226Ra with higher values in deep waters and an average concentration in the order of 10-13g/L amounting to about 10% of the equilibrium value with respect to the uranium parent, but is 2-3 orders of magnitude greater than could be supported by the immediate parent 230Th in ocean water. Large variations are evident in values reported for river waters (0.1 - 14 x 10-14g/L), but this range confirms that river sources are inadequate to sustain the observed oceanic concentrations (Cochran ,1982). This implied release from interstitial waters of sediments which accords with the mobility of this nuclide within the sediment column. Sourced from the sea floor, with a half-life of 1620 years this nuclide has applications as a tracer of deep water mass mixing. Similar to 226Ra, 228Ra is supplied to the oceans primarily by release from bottom sediments where it is produced by decay of 232Th. However, the shorter half-life of 228Ra (5.7 years) leads to a greatly modified subsequent distribution. This isotope also has application to mixing studies of water masses, but is restricted to shorter time-scales typical of eddy diffusivity. Radium isotopes are generally considered conservative geochemically, however as with uranium isotopes, limited associations with particulate matter, mineral deposits and biological matrices have been observed. 3.2.4. Radon The decay of 226Ra produces 222Rn which, because of its short half-life of 3.82 days relative to large-scale mixing processes, is generally in equilibrium with Ra in the oceans. Exceptions to this pattern occur near the air-sea and sediment-water interfaces. In surface waters, 222Rn is 10 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 deficient with respect to its parent 226Ra (the 222Rn/226Ra activity ratio is less than 1 because it is lost to the atmosphere. The pattern near the sediment-water interface is one of excess 222Rn which is caused by diffusion out of bottom sediments into the overlying water column. The mobilisation of radon to sediment pore water occurs in a similar manner to radium isotopes, however, because it is chemically less reactive than 226Ra, the activity of 222Rn in near-bottom waters is generally greater. The patterns of 222Rn/226Ra disequilibrium in these two regions provide tracers for the study of gas exchange of the surface ocean with the atmosphere, and near-bottom water movements. 3.2.5. Lead Although a number of lead isotopes exist, the only one of significance to our ocean studies is 210 Pb which is in excess relative to its parent 222Rn (from 226Ra) and a half-life of 22.3 years. The most significant source of excess 210Pb to the surface oceans is via wet and dry deposition from the atmosphere, where it is produced from the decay of 222Rn. The emanation of radon to the atmosphere is primarily from continental rocks and soils, with only about 2% resulting from flux from the surface oceans. The supply of excess 210Pb delivered from continents by rivers, largely in particulate form, is considerably less than that derived from direct atmospheric input. The excess supply of 210Pb to the surface oceans varies with distance from the continental sources. Latitudinal and longitudinal variation in the flux is controlled by (1) air mass transit times which affects the total production of 210Pb as a function of distance away from continents, and (2) the mean aerosol residence time which governs the deposition rate of 210Pb. An additional source of 210Pb in the oceans is in situ decay of dissolved 226Ra, via 222Rn. This source is a function of the integrated 226Ra activity in the water column, and although at depths of 4-5km it may be comparable to the atmospheric flux (at that depth), is not important in the coastal and surface ocean at depths of less than 100m. The surface ocean distribution pattern observed for 210Pb results from inputs from the atmosphere, in situ 226Ra decay, upwelling, chemical and biologically mediated particle removal processes and radioactive decay. 3.2.6. Polonium Despite its relatively short half-life of 138 days, significant radioactive disequilibrium exists, between 210Po and its (once removed via 210Bi) parent 210Pb, in the oceans. Rapid biological removal of 210Po results in a general deficiency relative to 210Pb in surface waters and concomitant excesses in phytoplankton and zooplankton compartments (Cochran, 1982). Recent studies have reported elevated 210Po and trace element levels in seawater and zooplankton from oligotrophic marine systems, where concentrations were inversely and nonlinearly related to zooplankton biomass and particle flux. These studies postulated the enhancing effect of oligotrophy on the bioaccumulation in zooplankton of 210Po, the most significant radionuclide with respect to the collective radiological dose to the global population from their consumption of seafoods. 4. RADIONUCLIDES AS TRACERS OF OCEAN PROCESS Natural radioactive nuclides in their passage through the marine environment are acted upon by the same processes which affect stable elements, and the behaviour of a primary long-lived nuclide, such as 238U, can be viewed in the same terms as that of any other conservative trace 11 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 metal. With most radioactive nuclides, however, their half-lives, genetic relations to other nuclides and consequent specialised modes of production introduce additional aspects to their behaviour which are not paralleled among the stable elements. These difference can afford useful insights into geochemical processes and many radionuclides have important applications as indicators of the time-scales of various oceanic processes, such as the chronology, transport and mixing of water masses, sediment accumulation, geochemical and air-sea gas exchange processes. The parent-daughter association of primordial, chemically conservative and long-lived (half-life 4.5x109 years) uranium-238 and thorium-234, with a half-life of 24.1 days and high affinity to bind with seawater particles, provides an excellent tracer to study these processes with temporal ranges from days to months. When combined with phase associations measurements of other elements (heavy metals, rare earth elements and noble metals) observations of disequilibrium in 238U /234Th ratios provide an understanding of the time dependence of particle formation and elemental scavenging, or removal, processes. 4.1. Residence times and flux of particles in surface ocean waters Particularly suited to studies of residence times and flux of particles chemical scavenging in marine systems is the observed disequilibrium in the 234Th/238U pair as 234Th, with a half-life of 24.1 days, provides a mechanism to study processes with temporal ranges of days to months. Interpretation of data from open ocean systems has progressed the development of 234 Th scavenging and export models through steady state to non-steady state concepts. Tracing the Residence Times & Flux of Marine Particles 238 U decay 234 Th half-life 4.47 x 109 years “conservative” half-life 24.1 days “particle reactive” removal to sea floor, via aggregation & sedimentation Figure 4. Tracing the residence times of marine particles. However, particles in coastal systems may form in situ, due to biological and chemical processes, or arrive in the water column from aeolian, riverine, or resuspension inputs. This variety in the composition and nature of particles is further complicated by short-term variations in the concentration and nature of other water column components such as dissolved organic matter, particle loading and planktonic blooms as well as localised oceanographic events. Tropical coastal systems have other unique factors such as low rates of 12 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 productivity, high precipitation, high insolation and massive dissolved organic and particulate loadings. These characteristics, typical of coastal systems, have the potential to greatly influence the efficiency of sorption processes and rates of particle removal. Many coastal water bodies are impacted by human activities, and despite recent advances in surface and solution chemistry, it is still not possible to deduce residence times for trace metals in seawater from model calculations. It is therefore important to assess the chemical residence times, or “self cleansing ability” of these waters according to the nature of discharges and in situ biogeochemical scavenging processes. These processes allow natural water bodies to cope with some level of disturbance, however, if the rate of chemical discharges exceeds the rate of removal, deleterious impacts may occur. Radiotracer techniques can quantify this balance. Tracing the Accumulation Rates of Marine Sediments half-life 3.83 days “gas phase” 222Rn half-life 1.62x103 y “terrestrial” 226Ra 210Pb decay seawater 210Pb half-life 22.3 years “atmospheric wet/dry deposition” seawater 230Th (very small) decay 226Ra decay 222Rn decay 210Pb Figure 5. Tracing the accumulation rates of marine particles. Similar to the thorium isotopes, lead has a high affinity for particle surfaces and is rapidly removed from solution by detrital matter and other sinking particles. Residence times for 210 Pb have been calculated to be in the order of 54 years for deep ocean waters, and 0.63 years for the surface ocean above 100m. Although 210Pb (in combination with 210Po) has been applied to studies of the fate of marine particulate matter, and biological productivity in the water column, the main application in coastal and marine studies has been as a geochronological tracer of sediment accumulation on the sea floor - this will be dealt with later. 5. REFERENCES AND READING LIST 13 IAEA/RCA Regional Training Course on E Application of Agreed Nuclear Techniques to Measurement of Nuclear Contaminants in Marine Systems, Mumbai, INDIA, 27 May to 6 June 2008 1. Aller, R.C. and Cochran, J.K. (1976) 234Th/238U disequilibria and diagenetic time scales, Earth and Planetary Science Letters, 29, 37-50. 2. Baskaran, M. and Santschi, P.H. (1993) The role of particles and colloids in the transport of radionuclides in coastal environments of Texas, Marine Chemistry, 43, 95-114. 3. Baxter, M.S. (1993) Environmental radioactivity: a perspective on industrial contributions, IAEA Bulletin, 2, 33-38. 4. Burton, J.D. (1975) Radioactive nuclides in the marine environment. In, Chemical Oceanography, 2nd Edition, Volume 3, Eds., J.P. Riley and G. Skirrow (Academic Press, London), p. 91-139. 5. Cochran, J.K. (1982) The ocean chemistry of the U- and Th-series nuclides. In, Uranium Series Disequilibrium: Application to Environmental Problems, Eds., M. Ivanovich and R.S. Harmon (Clarendon Press, Oxford), p. 385-429. 6. Chesselet, R. and Lalou, C. (1988) The use of natural radionuclides in oceanography. In, Radionuclides: A Tool for Oceanography. (Eds Gury,J.C., Guegueniat,P. & Pentreath,R.J.), Elsevier Applied Science, London (UK), p. 1-11. 7. Cochran, J.K. (1982) The ocean chemistry of the U- and Th-series nuclides. In, Uranium Series Disequilibrium: Application to Environmental Problems, Eds., M. Ivanovich and R.S. Harmon (Clarendon Press, Oxford), p. 385-429. 8. Jeffree, R.A. and Szymczak, R. (2000) Enhancing effect of marine oligotrophy on environmental concentrations of particle-reactive trace elements, Environmental Science & Technology, 34, 1966-1969. 9. Ku, T.K., Knauss, K.G. and Matheau, G.G. (1977) Uranium in the open ocean: concentration and isotopic composition, Deep Sea Research, 24, 1005-1017. 10. Szymczak, R. (1997) Radionuclides as tracers of dynamics biogeochemical processes in tropical marine systems, Radioprotection, 32(C2), 183-188. 14