Seasonal variations of 137Cs activities in the Dora Baltea River (Northwest Italy) after the chernobyl accident

J. Environ. Radioactivity 22 (1994) 77 88

Seasonal Variations of 137Cs Activities in the Dora Baltea River (Northwest Italy) after the Chernobyl Accident

P. Spezzano, S. Bortoluzzi, R. Giacomelli & L. Massironi Enea-Area Energia Ambiente e Salute, Dipartimento Analisi e Monitoraggio Dell'Ambiente, Amb-Mon-Masal, CRE Saluggia, 13040 Saluggia, Vercelli, Italy (Received 9 June 1992; revised version received 2 November 1992: accepted 1 December 1992)

ABSTRACT Concentrations o f 137Cs in water were monitoredJrom 1987 to 1991 in the Dora Baltea river (northwest Italy), a river characterized by a catchment area located in a mountainous area which is largely covered with snow and ice in winter. A strong seasonal variation o f water contamination was observed. Increased water concentrations in summer were attributed to caesium deposition and accumulation on snow-covered surfaces in winter and a delayed release in summer during ice and snow melting. An attempt o f relating the observed cyclical variations with estimated tran~sfer rates f r o m the catchment has also been performed.

INTRODUCTION It is generally accepted that radionuclides deposited on to a drainage basin after an atmospheric release are slowly removed from the catchment and transferred towards surface water bodies. Published data (Helton et al., 1985; Bonnett, 1990; Santschi et al., 1990) suggest that only a small fraction of freshly deposited 137Cs' ranging from 0.1 to 2%, is rapidly washed out by runoff from the catchment area within a few days after deposition. In any case, concentrations in streams are expected to decrease exponentially during the first weeks after a single deposition event. The remaining activity is slowly removed at a loss rate ranging from 77 J. Environ. Radioactivity 0265 931X/93/$06-00 ~t~; 1993 Elsevier Science Publishers Ltd, England. Printed in Ireland.

78

P. Spezzano, S., Bortoluzzi, R. Giacomelli, L. Massironi

0.007 to 0.56% of the total inventory per year. As 137Cs is strongly absorbed on to soil particles, especially at the selective frayed edge sites of clay particles like illite (Tamura & Jacobs, 1960; Evans et al., 1983; Cremers et al., 1988), the long-term mobility is due essentially to erosional processes and is mainly related to the transport of eroded particles (Rogowski & Tamura, 1970; Carlsson, 1978; Ritchi & McHenry, 1990). Recent observations (Hansen & Aarkrog, 1990; Hilton et al., 1993; Spezzano et al., 1993) have shown an enhanced transport of soluble 137Cs from acid peaty soils with average loss rates up to 3-7% per annum. The pulse-like shape of the Chernobyl cloud allowed the authors to identify a different source of secondary water contamination having a clearly definite seasonal pattern. In this study, water caesium concentrations in the Dora Baltea river (northwest Italy) were monitored over a fiveyear observation period. Increased water concentrations in summer were attributed to 137Cs previously stored on ice and snow-covered soil and then released as a consequence of the seasonal increase of temperature.

MATERIALS AND METHODS The Dora Baltea river (160 km in length and average flow rate of 110 m3/s) originates from the massif of Mont Blanc (4810m of altitude) and flows into the Po river (Fig. 1). Its basin, of 4322 km 2, collects most of the waters from the western alpine mountains (about 2000mm average annual rainfall) through 16 minor valleys located in a region which is the most intensely covered with glaciers (about 20000 ha) of the Italian side of the Alps. The most important are the Glacier of Rutor (950 ha), the Glacier of Miage ( l l 0 h a ) , the Glacier of Brenva (750ha) and the Glacier of Lys (1070ha). The massif of Mont Blanc and the heads of the valley of La Thuile, Valgrisenche and of the valley of Rhemes receive the winds from the Atlantic, bearers of damp air which gives rise to abundant precipitations, essentially as snow. Several different lithological features are found in the catchment area, including granitic and calcareous rocks, metamorphic schists, gneiss and alluvial deposit and morainic materials along the river valleys. In its lower part, the Dora Baltea river feeds the channels used for the irrigation of the fields, especially rice-fields, during the growth season. Sampling of the flowing water was carried out on a monthly basis starting from January 1987. Samples (201) were collected about 8 km before the confluence with the Po river, oven-dried and counted for 1000 min using a high-purity germanium (HPGe) detector connected to a computerized system for spectral analyses. From 1989 to 1991, measure-

79

1~7Cs activities in the Dora Baltea river, N W Italy

/ ) 0

\

Z 2

52 ,m

~ R A

~

k,.x

BALTEA

Fig. 1. The Dora Baltea river catchment with indications of sampling points (see Tables 1 and 2).

ments were performed on a quarterly basis on composite samples, as the activity levels were often below the detection limit. Flow data were obtained at Mazze (Fig. 1, point 5), about 22 km before the confluence of the D o r a Baltea river with the Po river (G. P. Tricerri, pers. comm., 1992).

RESULTS AND DISCUSSION The temporal variation of 137Cs concentrations in the river water from January 1987 to December 1991 is shown in Fig. 2. ~34Cs activities, when detectable, mirrored 137Cslevels and therefore are not reported here. The first impression which results from this graph is the cyclical pattern on a seasonal basis, with elevated caesium concentrations in summer and lower concentrations during all the other seasons. Moreover, peak summer concentrations showed a constant and fast decline, while non-summer concentrations were more or less constant over the five-year observation

P. Spezzano, S., Bortoluzzi, R. Giacomelli, L. Massironi

80 150

JI i

I00~

t--

r,.)

I i

°

Date

Fig. 2. Variation of 137Csconcentrations from 1987 to 1991 in the Dora Baltea river water.

period, with little sample to sample variations. 137Cs concentrations in 1990 and i 991 were comparable to their pre-Chernobyl values (3-5 5.1 Bq m 3 in summer 1985). About 90% of the yearly amount of 137Cs carried by the river was concentrated during summer months in 1987 and 1988, while in 1989 and 1990 this percentage dropped to about 75%. In non-summer months, the a m o u n t of 137Cs carried by the river was quite constant over the observation period, at a value of (8.3 ± 2.8) x l0 s Bq m o n t h -~. The observed behaviour is not strictly related to the average flow rate of the studied river (Fig. 3), as the normally high flow rates observed in autumn as a result of increased atmospheric precipitations did not correspond to high caesium concentrations, so the behaviour should not

~7Cs activities in the Dora Baltea river, N W Italy

81

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150

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Date Fig. 3. V a r i a t i o n o f D o r a Baltea flow rate f r o m 1987 to 1991.

be linked to fluvial sediment discharge. Summer is generally a dry season in northwest Italy and most of the water flowing through the Dora Baltea river in this season is derived from the melting of ice and snow from the mountains where all the streams which contribute to the different inflows originate. It is known that most of the Chernobyl debris were injected in the atmosphere at heights of between 1000 and 2000 m, while some fraction of it was transported at higher altitude in the troposphere (Gavrilas, 1988). Jaworowski and K o w n a c k a (1988) showed that, over Poland, a few weeks after the accident large amounts of activity were found in the troposphere up to 9 km, and caesium was also measurable in the stratosphere at heights of 15 km. Thus, some of the Chernobyl fallout should have been stored on

P. Spezzano, S., Bortoluzzi, R. Giacomelli, L. Massironi

82

ice and snow-covered surfaces up in the mountains and only successively delivered to surface streams during the following summer as the temperature raised. Measurements performed on superficial snow collected in June 1986 within the drainage area (Fig. 1) showed a considerable amount of caesium with an activity ratio consistent with the 134Cs/137Cs Chernobyl ratio of 0.5 (Table 1). It is likely that this caesium activity must have contributed to the river water contamination during the succeeding months. However, summer pulses of activity were clearly detected in all the following years. This behaviour could be the consequence of a progressive release of the activity stored in 1986. Nevertheless, ground deposition (wet and dry) of ~37Cs measured at the Center of Saluggia (Vercelli, northwestern Italy) from 1987 to 1990 (Fig. 4) decreased with a half-time of 335 days tr = -0.82, P > 99.9%), assuming a simple exponential decay. Hotzl et al. (1989) reported a half-time for 137Cs deposition of 280 days in the Federal Republic of Germany during the period August 1986-December 1988. From 1987 to 1990, the amount of J37Cs carried by the river during summer months decreased with a half-time of about 320 days ( r - - 0 - 9 8 , P > 95%). Therefore, it seems that the decrease of the amount of ~37Cs transported by the river in summer reflects the decrease of the 137Cs deposition rate. The authors could explain the experimental data, observing that in winter, part of the caesium deposition does occur on snow-covered surfaces. Only in spring and summer, when snow melts, can t-~TCs accumulated below the snow-line be released and then promptly delivered to streams by surface runoff. 137Cs transfer to the river should be quite complete and fast if we consider that when the underlying soil is partially frozen, the contact of water with soil particles is substantially reduced. If this is true, fallout deposition should be subject to an orographical effect and should accumulate at the bottom of valleys along mountain chains, a process already observed for plutonium in northern Italy (Cigna et al., 1987, 1988). The observed behaviour can lead to interesting considerations. Radio-

Concentrations of

TABLE 1 on Melted Superficial Snow Collected Within the Dora Baltea Drainage Area in June 1986

134Csand 137CsMeasured

Locality

Date of sampling

Altitude (m)

l~4Cs(Bql t)

137Cs(Bql i)

1: Courmayeur 2: Valtournenche 3: Gressoney

6 June 1986 9 June 1986 I0 June 1986

3584 2050 Not available

34-8 < 2-1 .1.6

64.4 1.8 3-7

137Csactivities

83

in the Dora Baltea river, N W Italy

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Fig. 4. Monthly deposition of ~3VCs to ground measured at Saluggia from 1987 to 1991.

nuclide concentrations in river waters are simply related to the sum of the a m o u n t of fresh fallout washed off from the catchment and the yearly transfer of the accumulated activity retain in the soil store (Jacobi, 1972; Carlsson, 1978; Helton et al., 1985; Queirazza et a., 1988) by means of the relationship:

Cw

Aagc"I ( t )

÷

~,bSc

•O

(1)

where C w is the radionuclide concentration in the water (Bq m 3); Sc is the catchment area (m2); Q is the river flow rate (m3y 1); I(t) is the timedependent deposition flux ( B q m - 2 y 1); D is the cumulated soil activity ( B q m 2); 2, is the fraction of freshly deposited radionuclide which is removed by runoff (dimensionless); and 2b is the fraction of the cumulative deposit removed per year (y '). On an annual basis, we could consider the summer levels of ~37Cs as a direct transfer from the catchment. If we make the simplifying assumption that in 1990 the a m o u n t of 137Cs which is deposited within the catchment and rapidly washed out of it is small, if compared with the activity removed from the soil store (137Cs fallout in 1990 was only 0-08% of the deposition which occurred in May 1986), then the rate constant 2b can be calculated from measured water concentrations and flow rates, knowing the mean deposited activity in the catchment:

84

P. Spezzano, S., Bortoluzzi, R. Giacomelli, L. Massironi

2b -- C w Q

(2)

DSc

An estimate of the mean deposition in May 1986 was performed starting from activities measured on grass samples collected along the river valley (Fig. 1) in the weeks following the Chernobyl fallout (Spezzano, 1988) and using c o m m o n parameters for the mass interception factor and the effective constant decay on grass. The results (Table 2) indicate that the average ~3VCs deposition on the whole catchment should have not been very different from the value of 11 kBq m 2 recorded at Saluggia (Spezzano & Giacomelli, 1990), from direct measurements of soil and fallout samples. With this value of accumulated soil activity and from water concentrations measured in non-summer months, an average loss rate of 0.0094% was calculated. If we consider this "~b value of 0.000094 (y ~) as representative of the delayed erosional component from the soil store for all the years from 1987 onwards (even if it is recognized that transfer rates generally show a decrease with elapsed time), then the mean ~37Cs activity measured in the river water could be explained if, on average, about 28% (range 1444) of the yearly caesium deposition on the catchment was transferred to streams. This seems to be a very large value. However, it is not impossible since most of the catchment is located in a very mountainous area, with altitudes ranging from 3000 to 4000 m and with several areas covered with snow for more than 200-250 days a year. Obviously, the fine structure observed for caesium activities in the Dora TABLE 2

Deposition of 137Cs Along the Dora Baltea River Valley after Chernobyl and Estimated From Measurements of Grass Samples Collected in May 1986 Locality

4: Saluggia 6: lvrea 7: Andrate 7: Andrate 8: Verres 8: Verres 9: St. Cristophe 10: Pollein 10: Pollein 11: Morgex 11: Morgex

Date o['sampling

13 May 1986 9 May 1986 9 May 1986 10 May 1986 12 May 1986 14 May 1986 7 May 1986 12 May 1986 14 May 1986 12 May 1986 14 May 1986

Grass concentrations (kBq kg i jresh weight)

Estimated ground deposition ,tkBq m ~-)

1~4Cs.

WCs.

1~7C~

0.48 1.40 1-34 0-82 0.98 0.84 0.37 0.26 0-19 0.20 0.16

0.86 2.60 2-44 1.56

12 24 23 16 25 25 6.5 6.4 6,4 4.7 47

1.95 1.60

0.78 0.51 0.41 0.37 0.30

7o!

SX7Cs activities in the Dora Baltea river, N W Italy

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Fig. 5.

Measured

and calculated

137Cs

concentrations

in t h e D o r a

Baltea river water

from

1987 to 1991.

Baltea river cannot be represented by a model dealing with annual average concentrations and annual transfer rates. On a monthly basis, the amount of 137Cs transported in summer should be conversely regarded as a delayed input, as it depends on fallout deposited during previous winter months. It is clear that a single pair of 2~ and 2b values cannot fit the authors' experimental data. The simplest approach seems to be that which works on a quarterly basis, introducing a third parameter, say kc, which should be linked to fallout values recorded the previous winter. Average caesium concentrations should be related to transfer constants by means of the relationship:

86

P. Spezzano, S., Bortoluzzi, R. Giacomelli, L. Massironi

K•S

Cw = K ~ S ~ c ' I ( t ) + ~ D + Q

K~Sc

Q "ZI

(3)

where k~ and kb have the same meaning as 2~ and ),b but on a quarterly basis: Q and I(t) assume the dimensions of m 3 quarter ~ and Bqm 2 quarter ~, respectively; and ZI is the sum of the activities deposited on the catchment during the previous three quarters. For the first, second and fourth quarter kc is set equal to zero and it is different from zero for the third quarter. Obviously, the kc value should present a large variability as it is strongly dependent on climatic and meteorological conditions, e.g. the percentage of precipitations which occur as snow-fall, the fraction of the catchment area which is covered with snow and the seasonal temperature fluctuations. The concentrations of ~37Cs in the water of the Dora Baltea river, calculated using eqn (3) with a kc value of 0-28 as previously evaluated, are reported in Fig. 5 for the period 1987 1991. The results show a reasonable agreement with measured concentrations for the first two years while for successive years the agreement is less satisfying. This could be explained observing that the seasonal variability could have exerted a great influence on caesium storing and release from snow-covered surfaces, so the disagreement could be the result of the unusual mildness of the last winters coupled with a premature summer; this is also suggested by the early peak of both river flow rate and caesium activities and by the lower summer flow rates observed from 1989 to 1991. Moreover, local resuspension could have played an important role on ground deposition measurements (Hotzl et al., 1989), especially some years after the Chernobyl event, so that the values detected from 1989 onwards could be largely overestimated.

CONCLUSIONS This long-term study of caesium transport by the Dora Baltea river after a radioactive deposition within the catchment area, such as the Chernobyl event, has pointed out a strong seasonal variation of water contamination if flowing water originates from high mountains usually covered with snow and ice. These cyclical variations have been ascribed to caesium deposition and accumulation on snow-covered surfaces in winter and a delayed release in summer during ice and snow melting. The most important consequence of this behaviour could be an inaccurate estimate of removal rates from drainage basins which are usually covered with snow in winter, if they are extrapolated from short-

t-~TCs activities in the Dora Baltea river. N W Italy

87

term measurements, and this should be true especially in the presence of a high deposition rate. Many other atmospheric pollutants (e.g. heavy metals) can behave in this manner, so that if they accumulate on snow-covered basins or on lakes covered with ice, then shock loads of contaminants can result in spring and summer when the snow and ice melt.

ACKNOWLEDGEM ENTS Professor A. A. Cigna is gratefully acknowledged for stimulating discussions and suggestions during the development of the manuscript.

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water. In Radioecology Applied to the Protection of Man and his Environment. Commission of the European Communities, Luxembourg, EUR 4800, Vol. 2, pp. 1153-65. Jaworowski, Z. & Kownacka, L. (1988). Tropospheric and stratospheric distributions of radioactive iodine and cesium after the Chernobyl accident. J. Environ. Radioactivity, 6, 145 50. Queirazza, G., Guzzi, L. & Cigna, A. A. (1988). Radionuclides behaviour in the Po river ecosystem (N. Italy) after the Chernobyl accident. Report RT/PAS/88/29, ENEA, Rome, Italy. Ritchie, J. C. & McHenry, J. R. (1990). Application of radioactive fallout cesium137 for measuring soil erosion and sediment accumulation rates and patterns: A review. J. Environ. Qual., 19, 215-33. Rogowski, A. S. & Tamura, T. (1970). Erosional behaviour of cesium-137. Health Phys., 18, 467-77. Santschi, P. H., Bollhalder, S., Zingg, S., Luck, A. & Farrenkothen, K. (1990). The self-cleaning capacity of surface waters after radioactive fallout. Evidence from European waters after Chernobyl, 1986-1988. Environ. Sci. Technol., 24, 519 27. Spezzano, P. (1988). Risultati delle misure effettuate da: Servizi0 di Fisica Sanitaria, Sicurezza e Medicina del lavoro CRE Saluggia (Vercelli). In Misure efJ'ettuate nell'anno 1986 dai laboratori dell'ENEA su campioni ambientali e della catena alimentare in seguito al'incidente di Chernobyl. A cura di F. Giorcelli. ENEA, Rome, Italy. Spezzano, P. & Giacomelli, R. (1990). Radionuclide concentrations in air and their deposition at Saluggia (northwest Italy) following the Chernobyl nuclear accident. J. Environ. Radioactivity, 12, 79-91. Spezzano, P., Hilton, J., Lishman, J. P. & Carrick, T. R. (1993). The variability of Chernobyl Cs retention in the water column of lakes in the English Lake District, two years and four years after deposition. J. Environ. RadioactiviO,, 19, 213-32. Tamura, T. & Jacobs, D. G. (1990). Structural implications in cesium sorption. Health Phys., 2, 391-8.