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Nuclear and isotopic techniques for investigating marine

A survey of selected applications that scientists are using to identify, trace, and investigate of the

by L.D. Mee I he oceans are a vast mixing pot for the ele- upon relatively simple physical measurements of and ments and compounds found on the Earth's salinity and temperature and the measurement of J.W. Readman crust, whether naturally formed or as a result of currents by current meters, drifting buoys, or activities. The "pot" is stirred about once calculations based on density gradients and every 1.6 thousand years — only about four geostrophy. These methods have provided good times since human history was recorded but at estimates of the horizontal of water the speed of a blender on the geological time masses but are inaccurate on the vertical scale. scale. Its constituents are therefore characterized A quantitative knowledge of vertical mixing is by major components which do not appear to vital if we are to understand the 's capacity change their relative concentration with time, for assimilating contaminants (introduced from and by a much smaller fraction (less than 0.1 % terrestrial sources or as a result of dumping), and of the dissolved salts) in disequilibrium as a as a receptor for "greenhouse gases" (thus result of their participation in basic life proces- providing the ultimate control mechanism for ses, their introduction by as con- global warming). taminants, or their geochemical reactivity. Much of our current knowledge of large- Marine scientists are often faced with huge scale oceanic mixing processes comes from practical challenges in order to measure in- measurements of natural series radionuclides — dividual trace components within this mixture such as cosmogenic carbon-14 which was used and to obtain data on their dynamics. The infor- to "date" the deep waters of the oceans — or by mation is essential for understanding marine fer- observing radio-geochemical disequilibria be- tility and for investigating the transport, fate, and tween primordial uranium and thorium series effects of marine contaminants. Nuclear and decay products. The introduction of "man- isotopic techniques are currently providing a made" radionuclides into the marine environ- unique source of information for identifying ment has provided a particularly valuable tool some non-nuclear contaminants and tracing their for studying pollution transport, since they were pathways in the environment and, potentially, introduced over a short period of time (as in the for investigating their biological effects. This case of atmospheric weapons testing, which was article surveys some major achievements in this effectively suspended after 1963) or as a point field and looks at the scope for future develop- source in the case of nuclear power or reprocess- ment. ing facilities. The literature on this topic is voluminous and a single example will be used to illustrate the impact of this work. Oceanographic tracers Perhaps the most noble of all the anthropogenic radiochemical tracers of oceanic is a solution of largely uniform mixing was tritium, which was produced in large composition which varies in concentration quantities during nuclear weapons tests and in- (salinity) from place to place. Most of our troduced to the oceans's surface, particularly in knowledge of ocean circulation is thus based northern latitudes, as tritiated water (HTO). "Was" is used since tritium, with an 11.3 year half-life, has now decayed away to levels which Mr Mee is section head of the lAEA-MEL's Marine Environ- can only be detected by sophisticated mass mental Studies Laboratory and Mr Readman is head of the spectrometric measurements of its daughter, Laboratory's Organic Chemistry Unit. helium-3. The IAEA, particularly the Isotope

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Hydrology Section, made good use of the period alter the geochemical equilibrium of the study when tritium was relatively easy to measure. system. Apart from original work in the field, the Agency This approach has been employed at IAEA- brought together specialists from all over the MEL and many other institutions worldwide world and edited a number of key publications. since the 1960s for investigating the partition of The work generated valuable data which show trace metals between and seawater that in the 25 year period since tritium was first and the biological uptake of metals. As a result introduced into the ocean in large amounts, the of this work, a compilation of / Studies of ocean plume of tritiated water can be seen to have water partition coefficients and biological con- pollution often include flowed over the sill of the Denmark straight and centration factors was published in 1985 for a the use of nuclear and isotopic techniques. penetrated 5 kilometers into the abyss. (See wide range of elements, both radioactive and (Credit: Kurt Adams, graph, next page.) This was far faster than the non-radioactive. The importance of studying UNEP-SELECT) rate predicted by physical oceanographers and demonstrates that water soluble airborne con- taminants produced by modern industries (e.g., PCBs, DDT, CFCs) could already have penetrated the deep ocean. Now that tritium is no longer convenient as an ocean-scale tracer, the search is continuing for other longer-lived radionuclides which reflect mixing processes. Suitable candidates are krypton-81 (half life 2.1 x 105 years), which is an inert but not very soluble fission product that may reflect air-sea exchange; neptunium-237, mainly introduced as a product of nuclear fuel or reprocessing plants but with a very long half-life (2.1 x 106 years); and iodine-129, a fairly conservative fission product with a half life of about 107 years. The problem with using these potential tracers is their low concentration and weak radioactivity. This makes conventional counting virtually impossible. For example, to measure neptunium-237 in contaminated seawater, the samples are typically 10 tons in size and, after concentration and chemical separation, samples are "counted" for two weeks! The IAEA's Marine Environment Laboratory (IAEA-MEL) in Monaco is currently investigating new "non- nuclear" techniques for measuring some of these tracers using inductively coupled plasma mass spectrometry (ICP-MS). This search may even- tually provide oceanographers with new tools to track the pathway and of con- taminants, whether nuclear or non-nuclear, and to model the ocean's capacity for assimilating them.

Investigating contaminants

The introduction of a radiolabelled tracer into a controlled experimental system permits the marine scientist to investigate the interaction of individual contaminants with other environ- mental components. Such experiments have to be set up carefully so that the introduced radioisotope has the same chemical speciation as the component being studied and does not itself

IAEA BULLETIN, 2/1993 FEATURES

such interactions is not an academic matter. The development of work on the physical chemistry adsorption of metals onto particles suspended in of seawater/sediment interactions in national in- seawater and their subsequent deposition on the stitutions. This has led to a much better under- floor of the sea, is the principle mechanism of standing of the thermodynamics and kinetics of self-purification of the ocean. It maintains the adsorption processes and permits the develop- concentration of many toxic metals in the sea ment of more accurate models to describe and below levels which would otherwise damage the predict the dispersion of contaminant metals (in- integrity of the marine or cause cluding radionuclides) under real environmental detrimental consequences to the human popula- conditions. tion consuming or shellfish. A quantitative • Development of tracer techniques for or- knowledge of the sea's capacity to assimilate ganic compounds. The use of labelled organic contaminants allows scientifically based levels compounds in experiments designed along the to be set for discharging effluents to the marine lines described above has been widely explored environment from coastal sources. in the marine environment only in recent years. Current investigations using labelled com- In addition to providing an opportunity for pounds follow three general approaches, each studying the interaction of these compounds with considerable future scope for development: with biotic and abiotic components of the en- • Continuation of laboratory experiments vironment, well-designed experiments can in- on inorganic contaminants using controlled vestigate the degradation of the compounds microcosms (sediment/water, water/animals, themselves. (See related article in this edition on water/sediment/animals). Much work remains applications of these techniques for to be done to understand the mechanisms and studies.) The future key to wider success for this dynamics of metal accumulation by marine approach will be in the realm of preparatory plants and animals, particularly their ability to organic chemistry of labelled analogues of the remobilize and bioaccumulate metals many hundreds of organic contaminants posing transported to the sea floor by adsorption onto a potential threat to the marine and coastal en- sinking particles or as a result of waste disposal vironments. (dumping). IAEA-MEL has pioneered many of • Tracer release experiments in the marine these studies resulting in over 100 scientific pub- environment. For monitoring dispersion of toxic lications. The Agency's research contract materials from deep underwater effluent outfalls programme has also supported the further (pipelines from which waste is discharged into

Tritium in the Western Atlantic STA 74 68 67 64 60 58 56 54 49 48 40 39 37 34333231 30 29 27 35 11 14 15 16 17 Ocean in 1972

-1.0

50

50'S 40'S 30°S 20°S 10°S EQ 10'N 20°N 30°N 40°N 50°N 60=N 70°N Latitude Notes: The graph shows data from a tntium section of the Western Atlantic in 1972 from 70 degrees North to 50 degrees South versus depth in kilometers. Vertical exaggeration is 2000:1. Horizontal scale is proportional to cruise track. Source: "Oceanic distribution and transport of tritium", Ostlund and Fine, Behaviour of Tntium in the Environment. IAEA-SM-2322/67 (1979).

IAEA BULLETIN, 2/1993 FEATURES the ocean), it is necessary to conduct field sur- Bio-geochemical research veys with as many measurements as possible and pollution studies over a short period of time. Unfortunately, the chemical measurement of most contaminants is Stable isotopes of the same element have laborious and time-consuming and it is con- inherently small physical differences in atomic/ venient to add an easily measured tracer to the molecular properties. Frequently this to effluent itself. preferential selection of one isotope relative to In the past, widespread use has been made of another during natural environmental processes. highly fluorescent tracers, such as Rhodamine, These fractionations (especially between but these are carcinogenic and break down rela- isotopes of the elements hydrogen, carbon, nitro- tively slowly. An alternative is to release short- gen, , and sulphur) can provide a power- lived gamma-emitting radionuclides and follow ful tool to investigate physical, chemical, and their distribution by underwater gamma spectro- biological processes in nature. This provides the metry deployed from a small vessel or even a essential bio-geochemical information for un- helicopter. This approach is now in wide use by derstanding the environmental behaviour and the French Commission of Atomic Energy cycling of . (CEA) for studying the efficiency of Combined measurements of natural samples outfalls in the Mediterranean. The CEA system for their carbon-13/carbon-12 and -15/ employs -198 (half-life 2.7 days) as a tracer. nitrogen-14 abundances have proven useful to The anionic gold in solution converts irre- identify sources of marine organic matter, nutri- versibly to metallic gold in contact with solid tional sources for marine food webs, and trophic material and thus labels the solid effluent par- relationships among marine organisms. Research ticles irrespective of their size. The technique at IAEA-MEL has elucidated uptake mechanisms enables the particles to be traced in the water and inter-relationships between planktonic com- column and, within a one week period, at their munities which form the basis of the marine food point of deposition on the sea floor. (See figure.) web. Other research involving the Laboratory Similar systems are currently being investigated has provided insight into the allochthonous and by the IAEA using sand grains labelled by autochthonous inputs which feed into marine iridium-192 (half-life 74 days). and the trophic relationships therein.

Marine Dumping Runoff transfer and • Man transport by POLLUTANT INPUT • Marine Birds & Mammals biological AIR processes WATER Sorption Soluble excretion Upwailing S \ x ^ \1 1—7 v — \ OPLANKTON) \ RemineralizattonP*" / \ J Sorption \ ! ! V-— -^ \ J, ^ Biodetritus\ Migration *• —L . .. .. n V %> X I AdsorptionjReminer a x Vertical ~^-T 1T/ T — - Phyto detritus s | IZOOPIANKTONJ \ i i \ I Sorption \s 1 ! \~_^^\ i__^^ Biodetritus \ i 1 r" ------~f V — ' i ^Adsorption | NBCTON j i Biodetritus sedimentation \ / Sorption N X Bioturbation i 'feces, molts, eggs, carcasses) x^^^/S. I Biodetritus i ! V* N "" " "hl-'Jir / Epifauna WATER SEDIMENT V — — Microbial - »\ degradation NyWauiw^

Vl Dumping Input -

Source- "Biological Transfer and Transport Processes", S W Fowler, Pollutant Transfer and Transport in the Sea, Vol 2, (G Kullenberg, ed ) CRC Press Inc (1982)

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Principle of field experiments to study the efficiency of sewage pipelines to the ocean Chemical and biological analysis laboratories

Current meter Radioactivity probes Sample bottles Q Baskets of sample mi Sampling device

Source: "The Use of Artificial Radioactive Tracers for Measuring Dilution of Solid Pollutant Particles Released in the Sea". Caillot, Transport of Pollutants by Sedimentation (L.D. Mee. ed.) UNEP. Athens (1990)

A potentially important area of pollution re- Under these circumstances, it is possible to search which is likely to develop in future years take a sediment core and to estimate the relates to the fact that nitrogen is sedimentation rate by using suitable reduced in the heavy isotope. This offers a useful geochronological techniques. -210 (derived method to differentiate fertilizer inputs in studies from the natural uranium-238 decay series with on (or the unusual increase in the a half-life of 22.4 years) has proven to be the fertilization of the sea). Eutrophication has been most widely used nuclide in geochronological recognized as a most serious source of perturba- studies to investigate pollution histories. The tion of the marine environment caused by human lead's activity can be analyzed throughout the activities. Since it is impossible to chemically sediment core so that sections represent 22.4 distinguish between "natural" and "man-made" year time intervals. Layers can then be analyzed , the isotopic technique offers a unique for relevant pollutants whose flux and deposition possibility of assessing the significance of land- over time can be determined. based sources of nutrients and eventually con- Confirmation of dating can be achieved by trolling them. monitoring artificial nuclides associated with weapons testing (usually caesium-137 has been selected). These elements/isotopes peak at Dating of sediment cores around 1963. Although bioturbation within sedi- ments can mix materials within cores, the ap- It is only during the last century that man has proach described has been successfully used to significantly polluted his environment. This assess input histories of trace metals, , coincides with industrialization, advances in polychlorinated biphenyls. saturated hydrocar- technology, and the use of fossil fuels. With bons (oil), and polycyclic aromatic hydrocar- many pollutants — such as and bons (PAH). hydrophobic organics -- in aquatic environ- An example of the historical record of lead ments, these elements/compounds are bound to and PAH pollution from fossil fuel combustion paniculate material. Deposited sediments, there- has been obtained from a dated sediment core fore, represent a repository and once buried within from a United Kingdom . (See graph, the sediments these pollutants are frequently next page.) It demonstrates the dramatic increase trapped and stabilized (especially under anoxic in release of these contaminants during the last conditions). decades.

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Isotopic techniques to "fingerprint" oils Distribution of lead Date (derived from Lead-210 analyses) and polycyclic genesis induces a wide range of aromatic isotopic signals which, in general, differ sig- 1980-- hydrocarbons in a nificantly from the isotopic compositions of the sediment core in natural modern marine environment. Stable the United carbon isotope ratios have been valuable in the Kingdom exploration for petroleum and in the under- standing of formation processes. Although present in oils at lesser quantities than carbon, sulphur and nitrogen isotopic abun- dances have also been shown to reflect source and geological histories of formation. Thus they also afford oil-field specific ratios. The charac- teristic isotopic ratios of these elements can be exploited to "fingerprint" oils spilled into the environment in order to determine the source or sources. Studies have demonstrated that the isotopic compositions are comparatively resis- tant to change during weathering of the oil in the marine environment. Stable isotope ratios were recently used by -1900 IAEA-MEL in conjunction with non-nuclear 4321 0 10203040506070 fingerprinting techniques (including fluores- PAH (pg/g) Lead (iig/g) cence spectrophotometry. capillary gas chroma- tography-flame ionization detection, and capil- Source: Adapted from "A Record of PAH Pollution Obtained from Accreting Sediments of the Tamar Estuary, UK: Evidence lary gas chromatography-mass spectrometry) to for Non-equilibrium Behaviour of PAH", Readman et al, The investigate the extent of contamination of the Science of the Total Environment, Vol. 66 (1987). Gulf marine environment following the massive spillage of petroleum during the Gulf War. (See related article in this edition.) In addition to the stable isotopes, carbon-14 ways arises is, "Are the levels sufficient to cause measurements can potentially provide informa- detrimental biological effects?" tion on oil contamination. Petroleum-derived or- In recent years much effort has been focused ganic matter contributes a zero carbon-14 signal on the evaluation of relationships between con- to the specific activity of modern marine organic taminant exposure and observed biological ef- carbon (which is 261 mBq/g) or to the carbon fects in marine organisms. Measures of sub- activity of current terrestrial matter (271 mBq/g), acute exposure have included investigations of which thus is still around 20% higher than the cytological changes, induction of enzymes, and purely natural baseline specific activity because quantification of chromosomal aberrations. of the recent influence of nuclear weapons test- Recently attention has been directed towards ing. The coastal marine organic reservoir is rela- "genotoxic" compounds. These refer to pol- tively depleted in carbon-14 through a combina- lutants which act by damaging the DNA, thereby tion of factors. However, the zero carbon-14 providing the opportunity for induction of can- signal from petroleum contamination (because cer. Such compounds include PAH and poly- of complete decay during the oil's geological chlorinated biphenyls (PCBs), both of which are formation) should provide a useful quantitative classes of ubiquitous environmental pollutants. indication of the contribution of petroleum carb- In chemical carcinogenesis, chemical on to the total organic matter of the marine or- modification of the DNA is believed to be the ganic pool. critical initial step. This frequently involves binding of the organic pollutant (or its "active" metabolite) to the DNA. Ultra sensitive techni- ques are required to detect low levels of car- Labelling techniques in cinogen-DNA binding. -32 post- "molecular biology" labelling methods developed for base composi- tion and sequencing analyses of nucleic acids While the presence of many contaminants in have proven particularly successful to inves- the environment can be quantified using chemi- tigate carcinogen-DNA adducts. The technique cal analytical techniques, the question that al- has proven effective to determine adducts arising

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from low-level environmental exposure. Scien- IAEA's reference materials used by hundreds of tists have demonstrated that in some marine or- analysts worldwide. In this manner, the benefits ganisms the adducts persist for months, thus af- of nuclear techniques are transferred to the many fording information on the exposure history of institutions which do not have the technical or organisms to chemical carcinogens. financial resources to apply them on a routine basis.

Neutron activation analysis An ongoing challenge Neutron activation analysis (NAA) is one of the most sensitive and accurate techniques for The few examples selected for this brief trace metal measurement in solid matrices. It is review serve to illustrate the major role that thus readily applicable for the study of many nuclear techniques have already played in trace metals in marine matrices. Activation of marine pollution studies (as with major components of the sea salt itself limits its in general) and the important role which they utility in seawater analysis; however, this is easi- should play in the future. ly facilitated by chemical separation of the The IAEA, and particularly its Marine En- analyte trace metals (by co-precipitation) prior to vironment Laboratory in Monaco, have worked analysis. for over 30 years in the development and fur- Unfortunately, many institutes do not have therance of applied nuclear techniques for study- ready access to the rather costly facilities re- ing the marine environment. This work will quired for instrumental NAA. Nevertheless, its never be complete. The challenges facing human power as a reference technique is well employed society are continuously increasing, requiring by the intercomparison exercises of IAEA's the continual development of new and better Collecting marine Analytical Quality Control Service. The par- approaches to assessing, predicting, and mini- organisms off the of Monaco. (Credit: ticipation of more than 20 institutions applying mizing the impact of human development on the IAEA-MEL) NAA helps to assure an accurate calibration of . L~l

8 IAEA BULLETIN, 2/1993