origins, concentrations and distributions of artificial radionuclides discharged by the rhône river...

J. Environ. Radioactivity 11 (1990) 105-139

Origins, Concentrations and Distributions of Artificial Radionuclides Discharged by the Rh6ne River to the

Mediterranean Sea

J. M. Martin & A. J. Thomas

Institut de BiogOochimie Marine (Unit6 associOe au CNRS n ° 386), Ecole Normale SupOrieure, 1 rue Maurice Arnoux, F-92120 Montrouge, France

(Received 5 August 1987; revised version received 15 April 1988; accepted 13 July 1989)

A B S T R A C T

This study represents a comprehensive assessment of artificial radionuclides discharged by the RhOne river to the Mediterranean Sea. Fourteen different artificial radionuclides, mainly originating from the Marcoule reprocessing plant, have been measured in water and in deposited and suspended sediments. The total activity supplied by the river averages about 70 GBq/day and is one order of magnitude less than the effluent discharges from La Hague reprocessing plant. However, because of a lower degree of dispersion and dilution in a relatively small water body _ compared to the situation at La Hague, the artificial radionuclide concentrations (gamma-emitters and plutonium isotopes) still remain high at the river mouth (1200 Bq/kg in suspended sediments and 0.05-0.8 Bq/l in waters). Overall particulate activities are the highest that have been measured so far in French estuarine systems.

These nuclides allow tracing of the RhOne river input to the Mediterra- nean Sea. However, because of the different 'speciations' of the stable and radioactive isotopes and of the variable nature of the input, caution must be exercised in interpreting the data. A preliminary assessment shows that most radionuclides settle within a few kilometres of the river mouth.

I N T R O D U C T I O N

The R h r n e river is the most important river entering the Medi ter ranean Sea since the damming of the Nile river. It represents the major source of

105 J. Environ. Radioactivity 0265-931X/90/$03-50 (~) 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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J. M. Martin, A. J. Thomas

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Fig. 1, Per cent of optical transmissivity (inversely related to water turbidity) in a north-south cross-section of the mixing zone in February 1985 extending southwards from the Rh6ne river mouth (top axis in km). A three layer stratification is observed including the brackish-water surface plume, intermediate marine waters of lower turbidity, and the bottom nepheloid layer where suspended sediment is transferred to the deep Mediterra-

nean Sea via a submarine canyon.

river water, dissolved salts, nutrients and sediments to the Gulf of Lyon and exerts a major effect upon the primary production of the whole area. This is shown as an offshore plume of chlorophyll and can be detected far from the coast by the ocean colour images transmitted from the CZCS satellite (A. Morel, pers. comm. 1988). However, the surface sediment plume is usually limited to the nearshore area, i.e. to less than 20 km from the river mouth.

The transfer of sediments through the canyon off the deep sea fan by nepheloid layers would represent an important contribution to deep sea sedimentation (Aloisi et al., 1982), bringing terrestrial material rapidly to the abyssal plain. A typical cross-section of the transition area between the Rh6ne river and the Mediterranean Sea (Fig. 1) shows the thin surface plume and the deep nepheloid layer further connected to the submarine canyon. In order to specify the actual significance of the Rh6ne river to the central Mediterranean Sea, it is necessary to trace the importance of the

Artificial radionuclides discharged by the Rhfne river 107

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Fig. 2. Location of nuclear plants along the Rh6ne river. Water and sediment samples (this study) were collected in the eastern branch of the delta (Grand Rh6ne) from Arles (river

reference) to the mouth at kilometre point (KP) 331, and up to 20 km seaward.

material discharged via these two major sedimentary units (surface plume and nepheloid layer). This paper is restricted to the study of the radionuclide distributions in the surface plume and in the bottom sediments deposited beneath the nepheloid layer.

To gain more insight into these processes, it has been decided to use as tracers the artificial radionuclides discharged to the Rh6ne river by various nuclear power plants and by the reprocessing plant at Marcoule located 120 km upstream from the river mouth (Fig. 2). However, the inventories and concentrations of the various radionuclides discharged to the river are still poorly known. The more recently published value of the total beta activity (excluding tritium) discharged by the Marcoule plant is 38 TBq in 1980, 5-12% and 70--90% of which is due to 137Cs and l°6Ru, respectively (Luykx & Fraser, 1983). More recent data up to 1985 are unknown but liquid discharges (gamma plus beta) from 1985 to 1987 do not show strong variations (48 TBq/year; CEA, 1985, 1986, 1987). Alpha discharges are 3

108 J. M. Martin, A. J. Thomas

orders of magnitude less. In 1980, the annual total alpha discharge was 89 GBq; no important changes are observed for 1985-1987 (73-113 GBq/ year). In all cases, the detailed elemental compositions of effluents remain unknown.

The global activity (total beta) discharged to the Rh6ne river in the liquid effluents from two power plants has been published (Bidard & Bardin, 1987), but elemental composition is not specified. Previous studies of particulate radioactivity in the lower Rh6ne river have considered only deposited sediments; gamma-emitters and 239+24°pu w e re measured upstream and downstream of the Marcoule plant (Foulquier & Pally, 1984; Foulquier et al. , 1987). Ranges of riverine concentrations of dissolved gamma-emitters downstream from Marcoule were also published by Foulquier et al. (1987) and by Lambrechts & Foulquier (1987); dissolved 137Cs was measured in Mediterranean coastal waters by Calmet et al. (1988).

We did not, however, find any published data for the Rh6ne plume and associated marine deposits. Therefore, the aim of this preliminary study was:

(1) To assess the origin of the artificial radionuclides encountered in Rh6ne river water and sediments.

(2) To estimate their yearly river discharges to the Mediterranean Sea in both dissolved and particulate forms.

(3) To make, so far as proved possible, a first assessment of their distributions in the mixing zone between river water and sea waters.

SAMPLING AND ANALYSIS

From 1982 to 1985 large volume (200 litres) water samples were collected from Arles (located on the major branch of the delta, 49 km upstream) to the mouth, with 30 litre Niskin bottles, and recent fine-grained sediments were carefully recovered by hand from the banks (Table 1). Water samples were similarly collected in the surface plume (Table 2) along two transects carried out in April and September 1984, from the mouth to 20 km south in the marine system (CI = 20 g/l). Bottom sediments of the pro-delta were sampled at the same stations using a Shipeck grab sampler, only the surface layers (about 0-2 cm) being subsampled and stored for analysis.

Suspended matter was immediately preconcentrated by continuous flow tangential filtration at 0.45 p,m followed by continuous centrifugation at 32 000 rpm. The suspended matter samples were freeze-dried for subsequent analysis.

The filtrates were immediately acidified with I-ICI to pH 1. A carrier

Artificial radionuclides discharged by the Rh6ne river 109

(25 mg/l Fe3+), 242pu and different stable spikes including COC12, CsCI, MnCI2 and SbCI3 were equilibrated with the samples for 10-24 hours to determine coprecipitation efficiencies. Plutonium and gamma-emitters were coprecipitated with Fe(OH)3 using NH4OH at pH 9. Hydroxides were recovered by filtration and oven-dried at 105°C. Ammonium molybdophosphate (AMP) was then added to the filtrate at pH 2 to fix dissolved Cs isotopes and was recovered after equilibration and oven- dried.

Laboratory coprecipitation and evaporation experiments using filtered (0.45/zm) Rh6ne and Mediterranean waters and radioactive tracers allowed us to determine average collection efficiencies for those elements which were not spiked in the field. A coprecipitation efficiency of 90% has been found for Ce, Zr, Ag, Am and of 30% for Ru. These values have been applied in calculating dissolved activities. There was good agreement between laboratory and field results for the spiked elements.

Hydroxides and deposited or suspended sediments were ground and homogenized for gamma-counting. Low-level gamma-ray spectroscopy was carried out on the sediments, hydroxides and AMP samples adjusted to a constant density, with a high purity Ge detector (32% efficiency and 1.80 keV resolution at 1.332 MeV) placed inside a 1 m 3 lead shield 15 cm thick. Efficiency calibrations were checked during I A E A intercalibration exercises and using international NBS standards. Hydroxides and sediments were later dissolved with HCI and a mixture of HF + HCI + HCIO4 respectively, for plutonium analysis according to current methods (Talvitie 1971, 1972; Wong, 1971; Ballestra et al., 1979). Briefly, the solution was radiochemically purified using the anionic resin Bio--Rad AG1 x 8, 50--100 mesh and the plutonium was finally electroplated and counted by alpha-spectrometry with 300 mm 2 Si-Au surface barrier detectors.

Some selected suspended matter samples were also submitted to sequential chemical extractions according to the procedure of Tessier et al. (1979), slightly modified (Nirel et al., 1986; Nirel, 1987). The advantages and drawbacks of this procedure, which allows distinction of 5 operationally defined chemical fractions in sediments, have been discussed elsewhere (Martin et al., 1987). Only a few results will be summarized briefly in this paper.

RESULTS

Radionuclide concentrations in dissolved and suspended matter and in bottom sediments are given in Tables 1 to 3.

I 10 J . M . Martin, A. J. Thomas

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The nature of the deposited and suspended sediments is rather variable from the river to the submarine delta. The large fluctuations of the AI/Si ratios indicate a mixture of fine-grained detrital alumino-silicates with quartz particles. The CaCO3 content may reach 50% in the coastal zone. An interesting feature is the overall increase of POC/AI ratio from about 0.5 in the river section to 3.0 or more at the extremity of the surface plume, showing a mixture of terrigenous particles with an increasing proportion of plankton debris in the marine suspensions.

The turbidity of the river waters at Aries varies from 10 to 100 mg/l according to river discharge. It decreases to less than 1 mg/1 in the marine waters (Tables 1 and 2).

Fourteen artificial radionuclides covering a large range of half-lives (1 month to several thousand years) have been detected in the river suspensions by gamma-spectrometry, including 241Am because of its relatively high concentration. The average total concentration of artificial emitters in the river suspensions is 1200 Bq/kg and is therefore higher than the total activity of the natural gamma-emitters which were determined (4°K, 228Ac, 226Ra, 7Be: 650 Bq/kg).

Several activity ratios of different isotopes of the same element were determined in the Rh6ne river suspended particulate matter (Tables 1 and

58 60 134 137 238 239+240 2). Co/ Co, Cs/ Cs and Pu/ Pu activity ratios seem to be relatively constant within a factor of 2 during the period of study but the l°3Ru/l°6Ru ratio is much more variable.

At the southern extremity of the surface plume only caesium and plutonium isotopes, 54Mn, l°6Ru and 125Sb were still detected due to difficulty in collecting sufficient quantities of material in that low turbidity area. In the pro-delta deposited sediments, most of the short-lived radio- elements were not detected systematically in all samples.

Total artificial dissolved activities in the river waters (Table 3) show large variations (0.05--0.8 Bq/l) during the same year (1984) despite similar river discharges, ll°mAg dissolved concentrations could not be measured, whilst 241Am, 144Ce, l°3Ru and 95Zr have only been determined in a few samples.

DISCUSSION

Rh6ne river

Origin of radionuclides Fallout of short-lived gamma-emitters originating from the 25th Chinese atmospheric nuclear test of October 1980 was nearly undetectable by the


end of 1982 (Thomas, 1983) and, considering the high activities of 137Cs and transuranic elements in the river particles, it is very unlikely that a significant fraction of the artificial radioactivity in the Rh6ne river at Aries may be attributed to global atmospheric fallout. For instance, 137Cs concentrations in suspended matter sampled in 1984 in French rivers exposed only to atmospheric fallout averaged 15 Bq/kg. Industrial sources must therefore be considered.

As mentioned previously, several nuclear power plants and the reprocessing plant at Marcoule discharge radioactive effluents to the river. Early studies of the 137Cs content of Rh6ne river suspended matter upstream and downstream of the Marcoule plant have shown a 10-20 fold concentration increase, attributed to nuclear effluents (Martin, 1970). A recent comparative study of artificial radioactivity of deposited sediments by Foulquier et al. (1987) has shown an overall 25 fold concentration increase downstream of this plant. Several radionuclides were only detected downstream of the plant and therefore were considered to characte- rize its effluents. Many of these, however, may also be found in power plant effluents (as well as in atmospheric fallout). Although the influence of Marcoule is obvious for certain radionuclides (e.g. 137Cs, l°6Ru), it is not always easy in the case of minor effluent components to discriminate between the inputs from power stations, reprocessing plants and the atmosphere. To a certain extent, simple comparison of concentrations may also be biased by variations in the relative abundance of the clay mineral fraction in the river bed.

We have attempted to examine further this source-term problem, paying particular attention to the suspended matter which is more representative of the riverine radioactive flux and including plutonium isotopes.

Since we did not sample the Rh6ne upstream of Marcoule, we compared the concentrations in suspended sediments collected in the Rh6ne river at Aries to those measured in another large river (the Loire, for which a large data base was available) experiencing similar nuclear industrial influences (e.g. a total maximal output capacity of about 8000 MWe, essentially PWR reactors, in 1982-1983) but with no reprocessing plant.

Since the annual liquid discharge (3H excluded) of the various PWR reactors are comparable in both rivers for a given net electrical output (about 0.22 GBq of liquid releases per MW year in 1982-1985: C61eri, 1985) and as the average compositions of their effluents are relatively constant (Martin & Thomas, 1989), it might be expected that such a comparison would allow us to distinguish the fraction of the artificial radioactivity in the Rh6ne originating from the power stations alone. The above estimates allow us to calculate roughly that the total beta input (3H excluded) in liqui ' effluents from the Rh6ne power plants should be about

Artificial radionucOdes discharged by the RhOne river 119

0-9 TBq/year. This estimate does not seem unrealistic since the total (beta plus gamma) activity released in the liquid effluents of only two power plants (Tricastin and Bugey; see Fig. 2) averaged 0.44 TBq/year in 1980- 1985 (Bidard & Bardin, 1987). These values are obviously almost negligible relative to recent Marcoule discharges (38-48 TBq/year).

However, the comparison between the Rh6ne and Loire rivers can be biased by several factors such as different affinities of their suspended matter for the radionuclides in general, and in 1981 in the Loire river there was a significant contribution via atmospheric fallout of long-lived radionuclides (Martin & Thomas, 1989; Thomas, 1988). In order to discriminate to a first approximation between fallout and industrial sources, and to reduce the uncertainties inherent in concentration comparisons between suspended matter of different compositions, it was considered preferable to normalize the concentrations of the particulate artificial radionuclides to the concentrations of 7Be. This latter ubiquitous radionuclide is a well known natural tracer of atmospheric fallout and has never been mentioned as an artificial component of nuclear effluents. We therefore compared radionuclide/TBe activity ratios in suspended matter from both rivers, instead of comparing absolute concentrations. This approach will be illustrated first using the typical example of 137Cs, which has been studied in the major French rivers (1979-1985 data). The 7Be concentrations in particulate matter are controlled by various factors, the most obvious being the intensity of atmospheric deposition, which is related to the rainfall pattern. This factor also controls the atmospheric input of artificial radionuclides. The characteristic 137Cs/7Be activity ratio resulting from global fallout in river suspended matter collected in the tributaries of the Gironde estuary was 0.11 + 0.04 (n = 7). A similar ratio was measured in the Loire river upstream of the power plants (0.12 in 1983), whereas downstream of the power plants it increased to 0.17 + 0-05 (n = 5). This slight increase is attributed to the power plant effluents and shows the very small impact of the electronuclear industry (Martin & Thomas. 1989; Thomas, 1988). In the Rh6ne downstream of Marcoule, at Arles, this ratio, which reaches 5.5 (Table 1), demonstrates the occurrence of an additional source.

An extended comparison of element/TBe ratios is presented in Fig. 3. It shows that, with the exception of the 6°Co/7Be ratio which is nearly identical in both rivers, contamination of particulate in the Rh6ne river is higher by 1 to 2 orders of magnitude than in the Loire river, confirming the predominant impact of the reprocessing plant at Marcoule. 24~Am concentrations in Loire river suspended matter are poorly known; a single measurement upstream of the power plants gave a 241Am/7Be ratio of 0.000 50, not significantly different from the ratio measured in a sediment


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Fig. 3. Comparison of artificial radioactivity of suspended matter collected in the Rh6ne river (at Aries, 1982-1985) and in the Loire river (at Montjean, 1980--1983). Concentra- tions are normalized to VBe content and the bottom axis is the ratio (element/'Be) Rh6ne/

(element/7Be) Loire (see text for explanation).

sample collected in the vicinity of a power plant at St Laurent on the Loire river (0.000 76). This ratio reaches 0.10 in the Rh6ne at Aries (Table 1), showing a probable 7Be normalized Rh6ne/Loire ratio of about 130. In summary, the impact of the reprocessing plant at Marcoule is particularly noteworthy for ]34Cs and the transuranic elements.

In fact, the only radioelement which originates essentially from the power plants is 6°Co, which is the dominant constituent (about 35 %) of the total beta activity discharged in the liquid effluents of PWR reactors (Luykx & Fraser, 1983). This latter result confirms the lack of impact of the Marcoule plant on radiocobalt concentrations in the sediments (Foulquier et al., 1987).

Variations of Marcoule discharge with time Little is known about the history of Marcoule discharges. Data have been summarized by Luykx & Fraser (1978, 1980, 1983) for the period 1972- 1980 only and global monthly data can be found since 1985 (CEA, 1985, 1986, 1987). They show relatively constant discharges of 137Cs from 1972 to 1977 (0.9 TBq/year) and a slight increase from 1978 to 1980 (3.6 TBq/ year); and an irregular increase in ]°6Ru from 1972 to 1978 (about 9 to 33 TBq/year from 1972 to 1978). Total alpha activity increased sharply from 7-25 GBq/year from 1972 to 1979, to 88 GBq/year in 1980 and did not change much in 1985-1987 (73-113 GBq/year).

We attempted to determine whether variations in the radioactive discharges could be deduced from the sediment record of the lower Rh6ne river and if more recent sedimentary data could help to extrapolate the missing information by summarizing the published results for the river section between 50 and 86 km from the mouth (Murray & Fukai, 1978;


Ballestra etal. , 1979; Fernandez, 1984; Foulquier & Pally, 1984; Foulquier etal. , 1987). After comparison with our recent results, the following points can be observed.

No clear trend in 137Cs concentrations is observed during the period 1979-1985, while ~°fRu increases by a factor of only 2-5 in 1983. On the contrary, an important increase, reaching at least one order of magnitude, is observed for 144Ce, 239+24°pu and 241Am between 1973 and 1985. These data seem to reflect relative constancy of 137Cs and t°6Ru discharges over a number of years, but confirm the important increase, since about 6 years ago, of the less soluble radionuclides, e.g. 144Ce and the transuranics.

Special attention must be paid to activity ratios which are often more useful than concentrations in tracing sediment dynamics. The same rate of fixation of different isotopes of a given element by either coarse or fine-grained sediment particles is to be expected if these isotopes have the same speciations in the waters. Although no data have been found concerning radionuclide speciation in industrial nuclear effluents released to natural waters, it is likely that effluent pretreatment and storage result in the same chemical form for all isotopes of a radioelement. This point will be confirmed below. Field evidence of the absence of important grain size control on activity ratios may be found for instance in the Scheldt, where a regular 238pu/239+24°pu distribution is observed despite large variations of sediment composition, from silty sand to clay (Duursma et al., 1985). Also size fractionation of sediments sampled near Marcoule by Schoer & Ffrstner (1985) has resulted in identical distributions of both 134Cs and 137Cs.

From 1979 to 1985, then, the 134Cs/137Cs ratio remained rather stable, ranging from 0.12 to 0-24, with an average of 0.18 + 0.04. However, it must be kept in mind that this ratio may vary by a factor of 2-3 in monthly discharges from Marcoule (Fernandez, 1984). During our two surveys of 1984, this ratio was lower than the average (Table 1). The 58Co/6°Co ratio is more scattered (1.5 + 0-9), probably because of the short half-life of 58Co (71 days). The 238pu/239+24°pu ratio was first determined in 1977, averag- ing 0.25 + 0-13 (Fukai et al., 1981); our more recent ratios are identical (Table 1), and are similar to the ratio in aquatic vegetation (approximately 0.3; Foulquier et al., 1987). The l°3Ru/l°fRu ratio does not seem to have been measured prior to this study. No data allow estimation of the long-term variations of dissolved radionuclides in the Rhfne river.

In summary, no important discharge variations have been found in recent years for a limited number of elements and measurable activity ratios are rather constant. In these conditions, it appears reasonable to assess the radioactive flux to the Mediterranean. It is likely that the particulate phase is the major vector for most of the radionuclides.


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Numerous data have been acquired by other workers on artificial radioactivity in deposited sediments but their representativeness in terms of river transport may be questionable if grain size selection and mixing with old deposits occur. We therefore paid particular attention here to the suspended matter, despite sampling and analytical constraints, and to the distribution of elements between the particulate and dissolved phases.

Radionuclide input to the Mediterranean We have attempted to estimate the dissolved and particulate radioactive discharges by the Rh6ne river using our own dissolved and suspended matter data obtained during 4 surveys from 1982 to 1985 (Tables 1 and 3), and average yearly liquid and solid discharges of 1580 m3/s and 4.3 million tonnes/year respectively (Savey & DelOglise, 1967). The results are re- ported in Table 4.

Comparison of our data with concentrations in riverine deposited sediments downstream of Marcoule sampled before 1986 (Foulquier et al., 1987) shows that concentrations in suspended matter are generally higher, by a factor of 1-4-3-3 (ll°mAg, 58Co, 6°Co, 134Cs, 137Cs, 54Mn). Nearly identical values were obtained for 241Am, 144Ce, l°ORu and t25Sb. In the absence of AI data for the sediments, it is difficult to decide if these differences are due to differential settling of coarse fractions, or to temporal or spatial variability. The average 134Cs/13-7Cs activity ratio is, as expected, identical in sediments and suspended matter. The 5SCo/6°Co ratio tends to be lower in the sediments (about 0.8), possibly because of decay effects (mixing with 'old' sediments due to sampling with a cone- type dredge).

It is clear that the true average flux is difficult to assess with such inhomogeneous data, in particular with respect to short-term variations, but this preliminary estimate will provide a first reference value for use in tracing the Rh6ne terrigenous influence on the deep sea sediment system.

Dissolved radionuclide concentrations downstream of the Marcoule plant are obviously variable in time; concentration ranges published by Foulquier et al. (1987) show a factor of 10-20 between extreme values. Because of the large dissolved variations between our two 1984 surveys, the dissolved flux has been calculated using both values. The total 137Cs and I°6Ru flUX estimates are not very different from the Marcoule discharges in 1978 (Luykx & Fraser, 1983). The sum of the Z41Am and plutonium discharges is relatively high (108 GBq/year) and is very similar to the total alpha liquid discharges at Marcoule in 1985-1987 (CEA, 1985, 1986, 1987).

Since the ratio between the average annual solid and liquid discharges is 0.086 g/l, the riverine particulate and dissolved radionuclide fluxes are


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Fig. 4. Average concentrations of artificial radionuclides in the Rh6ne river at Aries (1982-1985). Concentration ranges are represented by bars; arrows indicate detection limits. Oblique lines are distribution coefficients (Kd). Particulate transport form pre- dominates over dissolved transport if Kd is higher than about 10 000 (points to the left of

the dashed line).

equivalent when the distribution coefficients (Kd) are equal to 11 600. Obviously the suspended matter is the major vector of radioactivity, except for 125Sb and probably the Ru isotopes which are essentially transported in the dissolved form (Fig. 4). The Ru isotopes are difficult to quantify due to large fluctuations in their dissolved activities. Our ~°6Ru average concentration (0.35 Bq/1) is nevertheless close to previous estimates (0.2 Bq/l, Lambrechts & Foulquier, 1987; 0.1-1.4 Bq/l, Foulquier et al., 1987). On the whole, the dissolved concentrations published by these authors often tend to be higher than ours. More detailed comparisons are impossible in the absence of details concerning individual results, sampling locations and dates.

The degree of association of radionuclides with the particulate phase may be further described by their distribution coefficients (Kd). The Kd estimates for the two isotopes of the same element are not very different. This result probably indicates that, despite possibly different modes of production (e.g. activation and fission for 134Cs and 137Cs), pretreatment of the nuclear effluents before release has homogenized the chemical


forms of the radionuclides. A noticeable exception is for l°6Ru in April 1984.

The comparison of radionuclide Kds measured in 1984 with average river Kds of their stable isotopes (Martin & Whitfield, 1983) shows that they are much lower than the stable Kds in April but are more similar in September (Table 4). Furthermore, for a given nuclide, Kd decreases when dissolved concentrations increase. These results indicate that chemical equilibrium between the dissolved and particulate phases has not always been reached a few tens of kilometres downstream from the point of effluent release, or that some changes in the physico-chemical form of the effluents has occurred between the two surveys. Whatever may be the exact explanation, these results show that a steady state distribution of the radionuclides in the waters entering the estuarine mixing zone is not necessarily achieved.

Elemental distributions within the suspended matter Special attention must be paid to the distribution of artificial radionuclides in the particulate phase as compared to their stable homologues. The distributions of stable trace elements and of corresponding radionuclides in 5 chemical fractions of the suspended matter sample collected at Aries in April 1984, as determined by the sequential extraction technique, are summarized in Fig. 5. For convenience, these fractions are commonly referred to as 'exchangeable' (F1), 'carbonate' (F2), 'hydroxide" (F3), 'organic' (F4) and 'residual' (F5), although their exact physico-chemical nature is questionable and these fractions strictly operational (Martin et al., 1987). The sum of fractions 1 to 4 will be called the 'labile' fraction since it is believed to represent the component of the element which is likely to be involved in biogeochemical exchange processes.

As radionuclide concentrations were sometimes below detection limits in some fractions, a complete distribution could not always be established. Mn is the only element essentially found in the 'labile' fraction. The 'hydroxide' fraction seems to play a significant role in the fixation of 58Co, 144Ce and 95Zr. The 125Sb could only be detected in the 'organic' fraction (more than 75%). Only ~34Cs, ~37Cs and 95Zr were found in the 'residual' fraction. Most often, the artificial nuclides appear to be more 'labile' than their stable equivalents (Co, Sb, Ce, Zr, but not Cs). Conversely, the distributions of 54Mn and stable Mn in the clay-size fraction of a deposited sediment collected near Marcoule (determined using a similar method by Schoer & FOrstner, 1985) and that of 54Mn in our sample at Aries are nearly identical. Therefore, with the exceptions of Mn and Cs. it is impossible to deduce the distributions of the radionuclides in the sedimentary phase from those of their corresponding stable elements.


100% -

5 0

Cs

137 134 stable 5 4 *

I - -

° L r / i •

, / i , 971

Mn

54 s t a b l e *

IF t

F2

~/ F3

F5

100%_

50

Sb 125 stable

n

Co 58 stable

F1

F2

F3

F4

F5

1 0 0 % .

5 0

Ce 144 stable

r-! i l Zr stable _9.5_

"~" F2

~ 2 F3

" F5

Fig. 5. Comparison of the distributions of artificial radionuclides and their stable natural isotopes within different fractions of the suspended matter collected in the Rh6ne river at Aries in April 1984, as determined by sequential chemical extractions (Tessier et al., 1979). Blank areas correspond to the 'labile' fractions F1 to F4; hatched areas to the 'residual' fraction F5. *, Complementary results for the clay-sized fraction of deposited sediments collected near the Marcoule plant, after Schoer & F6rstner (1985). Arrows indicate analytical uncertainties when the complete radionuclide distribution could not be deter-

mined.


The mixing zone

Particulate and dissolved concentrations We have attempted to use the first results from the surface plume and the bottom sediments to define the transfer mechanisms from the river mouth to the Mediterranean Sea. It has long been established that, whenever river water meets sea water of much higher ionic strength, colloids and fine-grained particles, along with their associated trace metals, may coagu- late. As summarized by Eisma (1986), there are several mechanisms which can be involved: compensation of negative particle charge by positive counterions which decrease the electrical repulsive forces; particle attach- ment by adsorption onto organic matter which fixes particles by glueing them together; particle collision via different settling velocities; Brownian motion and turbulent motion of the water.

In macrotidal estuaries, hydrodynamic processes interfere with these mechanisms; however, in the case of the Rh6ne river, which flows into a non-tidal sea, the situation is much simpler and the sedimentation processes much easier to study. Indeed a deposition belt is observed close to the river mouth itself, corresponding to the coagulation/flocculation of riverine particles.

The results of the two 1984 cruises are shown in Tables 1 to 3 and typical distributions of selected radionuclides are shown in Fig. 6. Variations in total particulate concentrations sometimes reach an order of magnitude. A first simple explanation is that there is a primary control of concentrations by a varying abundance of clay minerals, which in turn can be traced by A1 content. In fact, concentration variations are reduced after normalization to AI, particularly at the marine end-member, where element/A1 ratios (activity per g AI) are often closer to river-estuarine ratios than are absolute concentrations.

The most striking observation is the simultaneous concentration increase of both dissolved and particulate (either absolute or Al-normalized) Cs isotopes during the September cruise (sample D49-09), followed by a decrease in marine waters. In this sample, Cs isotope Kd values are particularly low (6000-7000), being much less than in the other samples (10000-60000). For the two cruises, marine suspensions seem to be enriched in 134Cs relative to the river input, whilst 134Cs dissolved concentrations remain unexpectedly high in the marine waters (there is no 134Cs in recent fallout). The dissolved caesium excess might therefore correspond to a slight mobilization from the particulate phase. However, because the river was sampled a few days before the plume area, this pattern may also correspond to a pulse of dissolved Cs released by the Marcoule plant being diluted in sea water.


D I S S O L V E D

4 0 . 1 m BcVL /

30 tS"" th~e°.~tYca, -

2O-{ ~ ' N "~" ~'~"/~.~"on .

'o t \ , ' - ? ;

P A R T I C U L A T E ¢

i

o8 . o6 _

o4 t - _ _ 0.2 '~

I I I I

20 4/f '°° '\

l l l I I 0 5 10 15 C1%o 20

-- ~ 1"5 1

¢: 1.0 .,m I

2 0 -

~ 10_

5 -

I 1

5 10

I I I

1 5 C ; % c 20

Fig. 6. Dissolved and particulate concentrations of selected radionuclides versus chlorinity in the surface plume off the Rh6ne river mouth. Error bars are one standard deviation.

Particulate activities are normalized to aluminium content.

The occurrence of dissolved ~34Cs in actual marine waters cannot be explained without ambiguity. A first hypothesis is that an exchange process with particles is involved. The problem is that particulate 134Cs is also increasing. This apparent contradiction suggests a two step mechanism, involving first a mobilization from the detrital particles to the water, followed by an uptake by planktonic particles which are predominant in marine waters. Another possibility is advection of strongly fixed particulate 134Cs and diffusion of dissolved 134Cs from the recent pulse observed near the river mouth. A similar process should apply to 137Cs which is also characterized by relatively high dissolved concentrations (3.7 and 6.3 mBq/l) in the sea waters. On the basis of a 134Cs/137Cs activity ratio in Marcoule effluents of c a . 0.18, and an average marine dissolved ratio of 0-07 in our samples, the fraction of dissolved 137Cs due to fallout would amount to 60%, i.e. 2-2-3.8 mBq/l. Thus dissolved 137Cs measured in the sea waters in September 1984 is partly of terrigenous origin.

Few comparative data are available. 137Cs concentrations in surface waters sampled off Monaco in 1973 were 7.0-7.4 mBq/l (Murray & Fukai,

Artificial radionuclides discharged by the Rhrne river 129

1978). In 1975-1978, the north-western Mediterranean average decreased to 4-8 mBq/l (Fukai et al., 1980). In 1980, the recent fallout background in north-east Atlantic waters was estimated at about 3.1 mBq/1 (Holm et al. , 1983). Pre-Chernobyl concentrations measured from February to April 1986 in French Mediterranean coastal waters beyond the direct influence of the Rh6ne discharge also averaged 3.1 (1.0-4.5) mBq/l (Calmet et al., 1988). These concentrations are similar to our estimates here for Mediterranean marine waters. Despite the global persistence of the 137Cs inventory in the Mediterranean water column (Fukai et al., 1980), all these results indicate a two-fold decrease of fallout ~37Cs in north-western Mediterranean surface waters during the last 10 years. This observation is consistent with the 137Cs 'effective half-life' of about 10 years determined in the mixed upper layer of north-east Atlantic ocean water (Aarkrog, 1989).

Particulate 137Cs is either constant or decreasing in the sea water, relative to the river input.

Interpretation of these conflicting caesium data is therefore difficult, since reconcentration in biota despite generally very low concentration factors for this element would be surprising and different behaviour of the two isotopes would also be unexpected. A more likely explanation is a lack of equilibrium between the dissolved and particulate phases because of a recent caesium discharge in the river.

The plutonium isotope distribution (Tables 1 to 3) is rather similar to that for caesium. A large dissolved 239+24°pu peak (0.28 mBq/l) and very low Kds (40 000) are also observed near the river mouth during September (D49-09) after which concentrations return rapidly to the riverine value. It must be noted that the dissolved 238pu/239+24°pu activity ratio is still 0.13 + 0.02 in the sea waters in September, thus providing evidence of a partial industrial origin, as was shown above for 137Cs. Taking into account an average activity ratio in river water of 0.20 and the fallout ratio of 0.04, the fraction of dissolved 239+24°pu due to fallout would be about 50% of the measured activity in the September sample, i.e. 0.016 mBq/1, lower than the concentration measured in the sea waters in April (0.043 mBq/1). These results are not very different from the average 239+24°pu concentrations measured in (unfiltered) north-western Mediterranean surface waters in 1973 (0.056 + 0.022 mBq/l; Murray & Fukai, 1978).

At this southerly extremity of the plume, the 239+24°pu/Al ratio in suspended matter equals the riverine ratio in April (130 mBq per g A1) but is the highest of all the mixing zone data in September (414 mBq per g AI). Thus marine suspended matter shows a plutonium enrichment.

The particulate l°rRu distribution (Tables 1 and 2) is characterized by an overall seaward decrease in l°6Ru/Al, from 5.2-6.7 to 2.8-3.7 Bq per g Al,


but once again by a dissolved excess and a low K d (4600) in sample D49-09. l°3Ru could not be followed throughout the mixing zone. Although these data could indicate a mobilization of particulate ~°6Ru, the hypothesis of an unsteady pulse cannot be rejected.

In contrast with the patterns for the above radionuclides, the 54Mn distribution (Fig. 6) is regular and shows a particulate decrease associated with a dissolved excess. It is, however, unlikely that 54Mn, which is identical in chemical form to its stable isotope, is mobilized in oxic conditions so that the 54Mn distribution may also reflect a variation in the riverine input. However, there is some growing evidence that release of dissolved labile forms of manganese can occur through photo-catalyzed chemical reduction of particulate Mn (Sunda et al., 1983; Statham & Chester, 1988). In fact, dissolved stable manganese excesses have already been noticed in the Rh6ne plume (Elbaz-Poulichet et al., 1989).

The particulate ~25Sb/AI ratio (Fig. 6) is characterized by a systematic increase, by a factor of 3-10, in the marine suspensions, whereas dissolved concentrations lie below the theoretical dilution line. The particulate enrichment, which is likely to result from planktonic uptake, does not correspond with maximum removal, but it is hardly possible to make any mass balance assessment because of our ignorance of the particle resi- dence time in the plume. Dissolved 125Sb levels have rarely been measured recently in marine waters. In 1984, the concentrations which we measured off the mouth of the Gironde in the coastal Atlantic were about 1.9- 2-9 mBq/l. The fallout background beyond the influence of the La Hague plant in the Channel can be estimated at 3.7 mBq/l (Gu6gu6niat, pers. comm. 1984). Thus part of the dissolved t25Sb, which reaches 6.5 mBq/I in sea water in September 1984, is also supplied by the Rh6ne river.

Data on other nuclides (cobalt isotopes, 144Ce, 95Zr and 241Am) are more scarce. Particulate concentration variations often parallel each other, but a different pattern may be observed between the two surveys. A peak in dissolved activity is observed near the river mouth in September, corresponding to those observed for caesium, plutonium and ruthenium isotopes (sample D49-09). It is difficult to explain the differences observed between the two cruises. The dissolved increase might correspond to mobilization from particles, as previously mentioned for the other elements.

The Rh6ne mixing zone is thus characterized by a large variability in concentrations in water and suspension near the river mouth, far beyond the analytical uncertainties. Most likely, some changes in the effluent discharge rate occur and these disturb the radionuclide distributions. In such a stratified system, characterized by a rapid transit time of water and particles, even a short pulse-like variation in the riverine input may be


registered in the mixing zone. Peak concentrations observed in the surface plume in September 1984 are likely to result from such an artifact. In this respect, it is interesting to point out that, in samples where a dissolved peak concentration is observed, distribution coefficients are lower by an order of magnitude relative to the other samples. If this pulse-related interpretation is exact, the Kd difference would indicate that the lack of chemical equilibrium between the particulate and dissolved phases, as mentioned already for fresh waters, may also occur in the brackish waters. The unsteady distributions of the particulate radionuclides in this area may thus reflect a different reactivity of the radionuclides with time, controlled by both physicochemical conditions and by their degree of association with the particles in the river input.

Activities in the surface layer of sediments deposited off the mouth are generally lower than those in the plume suspended matter. The aluminium content of these sediments is not significantly lower than in the suspended matter (Table 2). Thus differential settling of clay minerals does not seem to account for most of these activity differences. Some degree of decay effect, resulting from mixing of recent particulate material with subsur- face, older deposits, is likely to explain the lower concentrations of the short-lived nuclides (e.g. l°3Ru, 58Co) but the present data do not allow us to distinguish these decay effects from those of radionuclide mobilization from bottom deposits.

A meaningful comparison between suspended matter and deposited sediments is, however, difficult to carry out, since sediments integrate radionuclide input variations over a finite time and we have seen that concentrations in riverine suspended matter were not constant.

It is interesting to note that the deposited radionuclide distribution shows concentrations tending to decrease in samples collected on either side of the north-south transect. This indicates that, on a 20 km scale, the major fingerprint of the terrigenous influence of the Rh6ne can be traced in a southward direction, a point which will be further examined by consideration of activity ratios.

Activity ratios in sediments and suspended matter To give greater insight into transfer processes between the Rh6ne river and the Mediterranean Sea, we shall focus here on the activity ratios measured in the river and marine suspended matter and in bottom sediments throughout the area (Fig. 7).

In the river and pro-delta area near the river mouth (between 330 and 333 km), the 134Cs/137Cs and 238pu/239+24°pu ratios are rather similar in the suspended material and deposited sediments, although slightly lower 134Cs/137Cs ratios can be found in the bottom deposits. 7Be concentrations


40

:50

20

IC

M a rcou le

i

Vo l lobr~gue O o m a/el

=... ,~LES ~ _

rT ~

134 Cs, % 137 Cs

~1 SCPRI

I I

Z 5 8 Pu . %

239+240 Pu

f a l l o u t r a t i o

J i i 250

I I

% 6ot

"%t se

A , I I ~ l I I

I

, . . . . . . . 1 1

I I % ~ " . . . l s J / ~ I t ~ .m. 4 / 8 4 I

I ~ . s e d i m e n t s I

, , z , , ; I , , 500 30 ~40 550 Km

s u r f o c e

p l u m e

s u r f a c e

p lume

Fig. 7. Longitudinal distributions of caesium and plutonium activity ratios in suspended matter (s.m.) and deposited sediments in the Rh6ne delta (1984). The kilometre scale is enlarged in the marine section. Error bars are one standard deviation. Caesium ratios at

Vallabr~gue dam after SCPRI bulletins.

are lower in the sediments 'than in the suspensions, indicating partial reworking of slightly older river sediments.

Sedimentation rates in the pro-delta area are very high and may reach a few tens of cm/year in the immediate vicinity of the river mouth (Roustan area) (Fernandez, 1984). In this area, the bottom sediments originate mainly from the flocculation and settling of river particles, with a possible input of river sediment due to bed-load supply. Artificial radionuclide concentrations decrease rapidly away from the mouth in all directions (Table 2). 7Be concentrations are lower than in the suspended matter of the plume.

Moving seawards (Fig. 7), the activity ratios remain almost constant in the surface plume but decrease significantly in bottom sediments from 0.25


- -; ~ ~ ° ~"

N ,~, \ o.~ . ° ii

C~ '~ I I

_>eJ ~

I /

I I

0

~ °

f ° t O V ' ~

t~

\

\ p.,,,.

~o~

=

~r

0

=

~r

0

~r

E

.--,_ . . -

+ 0

~.'_=

~ E

r~ L.

O ~

-~=

7.


to 0.07 for 238pu/239+24°pu, and from 0.12 to 0-04 for 134Cs/137Cs (see Fig. 8). Thus these bottom sediments are not representative of the present Rh6ne particulate load and have either a different age or origin. If we assume that these bottom sediments represent 'old' Rh6ne sediment, the 3-fold decrease of the 134Cs/137Cs ratio would correspond to a period of 3 years and 7Be would have disappeared, while the plutonium activity ratio, which does not seem to have varied since at least 1977, would not be affected by radioactive decay. It is necessary to invoke an older source of low activity sediments.

Sedimentation rates in that area (about 20 km from the river mouth) are much lower than those near the mouth, at less than 1 cm/year, according to Fernandez (1984). This value, established on the basis of 137Cs penetration depths without fallout corrections, might be overestimated. It is therefore possible that the sediment layer which was sampled by ourselves represents a mixture of a few millimetres of recent Rh6ne sediment with older sediments deposited before the first significant radioactive releases by the Marcoule plant in the 1960s and thus contaminated only by atmospheric fallout.

Another hypothesis is that lateral advection occurs from the Gulf of Lyon involving transport of sediments contaminated only with atmospheric fallout and subsequent mixing with recent Rh6ne sediments. Marine suspended matter collected between 100 and 325 m depth in the western Gulf of Lyon during the Ecomarge program in 1983 has t37Cs/AI ratios ranging from 350 (100 m) to 170 (325 m) mBq per g AI, only slightly less than those observed here in these sediments (296-592 mBq per g AI). A contribution by sediments supplied by general circulation on the continen- tal shelf may therefore also be envisaged.

In both cases, plutonium and caesium activity ratios allow calculation of the fraction of Rh6ne material in these samples. The percentages of 137Cs and 239+24°pu originating from the Rh6ne river are thus 30 and 16% respectively, as calculated from the average activity ratios in the suspended matter at Aries (Table 1) and in atmospheric fallout (no 134Cs; 238pu/239+24°pu = 0"04). If the sediments sampled at about 20 km southwards from the river mouth include older sediments, the geographical extension of the Rh6ne 'print' along the bottom would be larger than that indicated in Fig. 8 by the low activity ratios, but would be restricted to a very thin surface layer. If a contribution of lateral advection of sediments is envisaged, it would dominate, by a factor 3 to 6, the deposition of Rh6ne material at this distance from the mouth.

Whatever may be the exact explanation, it is clear that deposition of the Rh6ne suspended sediments is very limited as soon as one moves seawards


and it is likely that sedimentation rates decrease by 2 or possibly 3 orders of magnitude along a 20 km southerly transect.

CONCLUSIONS

This study represents the first comprehensive assessment of artificial radionuclides discharged by the Rh6ne river to the Mediterranean Sea. Fourteen different artificial radionuclides, mainly originating from the Marcoule reprocessing plant, have been measured in the particulate and dissolved phases.

The total activity discharged averages 71 GBq/day, i.e. one order of magnitude or more less than from the La Hague reprocessing plant. A large fraction of this activity is due to ~°6Ru (72%) and 137Cs (11%).

Because of less efficient dispersion, total concentrations of artificial radionuclides in suspended matter still remain higher at the river mouth (1200 Bq/kg), so that the Rh6ne river's suspended matter represents the most active samples encountered thus far in French riverine and estuarine environments (Thomas, 1988). In the marine environment, activities in the coarser sediments deposited in the immediate vicinity of the La Hague release point in 1976-1980 were comparable (Gu6gu6niat et al., 1979; Jeandel et al., 1981), but concentrations in the marine suspended matter near La Hague were not measured and may possibly be higher.

Total dissolved concentrations of artificial nuclides range from 0.05 to 0.8 Bq/l near the mouth.

This radioactive discharge represents a sufficient flux for use in tracing the Rh6ne sediment discharge to the Mediterranean Sea and remotely from the river mouth and it may be used to discriminate between the Rh6ne terrigenous supply and other sediment sources.

However, such an applicaton, which requires some knowledge of estuarine exchange processes, may be biased by yearly and particularly short-term fluctuations in the Rh6ne radioactive discharge. In fact, the distributions of particulate and dissolved radionuclides in the mixing zone could not be interpreted unambiguously in terms of geochemical ex- changes because of the probable daily variations in the input signal to the estuarine system.

Other difficulties in interpreting the estuarine behaviour of the radionuclides may arise from large K d variations in the river waters. Such K d variations were only determined for two samples but the observed differences may indicate that equilibrium radionuclide association with the particulate phase is not always achieved at the entrance of the estuarine zone.


Other aspects are that radionuclide behaviour cannot simply be deduced from the behaviour of the corresponding stable trace elements since, as shown by our results and those of Schoer & F6rstner (1985), near Marcoule the distributions of the stable and radioactive elements in the particulate phase are sometimes different.

In view of the yearly and shorter-term fluctuations, it is likely that activity ratios, which appear to be much more constant with time and independent of sediment nature, will be more useful than absolute concentrations in tracing geochemical processes. The most promising tracers are the radioactive isotopes of Cs, Pu and perhaps Co, whose activity ratios have remained almost constant over the past 10 years. However, caution must be exercised because of the different specific forms of the stable and radioactive isotopes.

Most particulate radionuclides settle within a few kilometres of the river mouth. Complementary analyses of box core sediments as well as nepheloid layer suspended sediment are now being performed to close the radionuclide mass balance of the Rh6ne delta and to define the actual significance of the nepheloid layer as a transport medium for particles into the Mediterranean Sea.

A C K N O W L E D G E M E N T S

We thank J. C. Guary, G. Corbierre and P. Prat for their assistance in analytical work. We thank the Commission of the European Communit ies for supporting this work. Other support came from CNRS ( G R E C O Intrract ion Con t inen t - -Ocran) and I A E A .

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origins, concentrations and distributions of artificial radionuclides discharged by the rhône river...

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