Article
Reference
The Fate of Chernobyl
137Cs in Lake Lugano
DOMINIK, Janusz, SPAN, Daniel Gilbert
Abstract
A rapid removal of Chernobyl ¹³⁷Cs from a number of large lakes has been previously reported. Our measurements of ¹³⁷Cs in water, sediments and pore water in the mono- and meromictic basins of Lake Lugano (Lago di Lugano) reveal generally slower half-removal times of 1.2 and 6.7 yrs, respectively. In the seasonally anoxic southern basin, this is most probably related to an intensive recycling of ¹³⁷Cs between water and sediments. In the permanently stratified northern basin the removal rate is much slower due to an important inventory build up in the deep anoxic part of the basin.
DOMINIK, Janusz, SPAN, Daniel Gilbert. The Fate of Chernobyl
137Cs in Lake Lugano. Aquatic Sciences , 1992, vol. 54, no. 3-4, p. 238-254
DOI : 10.1007/BF00878139
Available at:
http://archive-ouverte.unige.ch/unige:154291
Disclaimer: layout of this document may differ from the published version.
1 / 1
Aquatic Sciences 54, 3/4, 1992 1015-1621/92/040238-17 $1.50+0.20/0
© 1992 Birkh/iuser Verlag, Basel
The Fate of Chernobyl 137Cs in Lake Lugano
J. Dominik and D. Span
Institut F.-A. Forel, Universit~ de Gen~ve 10, route de Suisse, 1290 Versoix
Key words: Chernobyl, Cs-137, sediment, pore water, redox processes, Lake Lugano (Lago di Lugano).
ABSTRACT
A rapid removal of Chernobyl 137Cs from a number of large lakes has been previously reported. Our measurements of 1 a 7Cs in water, sediments and pore water in the mono- and meromictic basins of Lake Lugano (Lago di Lugano) reveal generally slower half-removal times of 1.2 and 6.7 yrs, respectively. In the seasonally anoxic southern basin, this is most probably related to an intensive recycling of 137Cs between water and sediments. In the permanently stratified northern basin the removal rate is much slower due to an important inventory build up in the deep anoxic part of the basin.
Introduction
Radioactive fallout from the Chernobyl accident at the end of April and beginning of May 1986 was very heterogeneous in space and varied in Switzerland by, at least, one order of magnitude. 137Cs fallout in the regions of major prealpine lakes has been estimated to 1.5, 2.8, 17 and 24 kBq/m / for Lake Sempach (Wieland et al., 1992), Lake Geneva (Dominik, 1989), Lake Constance (Mangini et al., 1990) and Lake Lugano (Santschi et al., 1990), respectively. This heterogeneity was mainly related to the differences of rainfall between April 30 and May 4, with the highest rainfall in the Canton of Tessin (25-58 mm) and no rainfall in the Canton of Valais (V61kle et al., 1986). The evolution of 137Cs activities in a number of Swiss lakes has been surveyed during the two years following the Chernobyl accident (Santschi et al., 1990; Mangini et al., 1990). In freshwater environment a rapid uptake of Chernobyl Cs isotopes on settling particles has been observed.
Laboratory experiments and field studies show that 137Cs is removed preferenti- ally by Al-rich clay particles (Robbins et al., 1992), especially by illite (Aston and Duursma, 1973). It is believed that Cs + is involved in exchange reactions with K + in the interlayer sites of illite group minerals. However, 137Cs in sediments from Lake Michigan is not particularly enriched in fine fraction (Alberts and Muller, 1979) and is firmly bound in crystalline fraction of sediments (Alberts et al., 1989). On the other hand, remobilization and recycling of Cs isotopes during anoxic conditions in bottom
water have been reported from Par Pond, located near the Savannah River nuclear plant (Alberts et al., 1979; Evans et al., 1983). Remobilization of Chernobyl Cs from sediments of Lake Sempach has also been postulated by Wieland et al. (1992).
In this paper we report the results of measurements of ~37Cs in lake water, sediment and pore water samples from two basins of Lake Lugano collected 3-4 years after the fallout. These results allow to evaluate a long-term behavior of this nuclide in mono- and meromictic lake with seasonally or permanently anoxic deep water masses.
Material and methods
General setting and physico-chemical characteristics of Lake Lugano are given elsewhere in this volume (Barbieri and Polli; Barbieri and Mosello, this issue). Water samples (20-50 1) were collected at 5 depths on May 26, September 9, December 12, 1989 and on March 6, 1990, at station Melide (southern basin) and on March 6, 1990 at station Gandria (northern basin), spiked with 134Cs ' and sent to the laboratory for further treatment. The activity of spike added exceeded that expected in lake water from Chernobyl fallout by one order of magnitude. Water samples were passed through an exchange resin Cu2Fe(CN)6 according to the method described by Mann and Casso (1984), maintaining a flow rate at about 250 ml/min, and Cs isotopes retained by the resin counted directly by gamma spectroscopy. The activity of spike adsorbed on resin (134Csspr) was obtained as the difference between the total measured a 34Cs activity (~ 34CSmr) and the Chernobyl ~ 34Cs fraction adsorbed from lake water at the time of sampling, assuming the ~34Cs/~37Cs ratio at time of Chernobyl fallout was equal to 0.49___ 0.02. Efficiencies of the Cs isotope recovery were calculated as:
Eft= 134Csspr/134Cssp (1)
134Cssvr = ~34CSmr -- 0.49 13VCsmreXp(--
2t)
(1 a)where t is time elapsed between Chernobyl fallout and the date of measurement, 2 is radioactive decay constant of 134Cs and the subscripts denote: spr - activity of spike retained on resin, mr - total activity measured on resin, and sp - activity of spike added to water samples. This method appeared more precise than the two stage adsorption used by other authors (Mann and Casso, 1984; Santsehi et al., 1990). We obtained an average Cs isotope recovery of 72 %. Water samples were not filtered prior to pumping through the resin, but it seems that particles were not efficiently retained in the resin chamber. This is probably related to a relatively high porosity of resin column due to a coarse grain-size of resin (0.3-1.0ram). After gamma measurements of a few samples, the exchange resins were resuspended in small volume of water, vigorously shaken and passed through a 0.3 mm sieve. When resin grains were retained on the sieve no sediment particles were observed in water. We thus suppose that the measured activities mostly represent "soluble" Cs and, perhaps, some part of the particulate fraction, however more rigorous experiments are necessary to verify this assumption. Our preliminary data obtained from a
240 Dominik and Span measurement of particulate fraction retained on 0.45 ~tm filter in one sample collected from the hypolimnion of the northern basin in 1992 suggest that particulate fraction activity is one order of magnitude lower than the raw water activity retained in the exchange resin.
Bottom sediment cores were taken with the submersible F.-A. Forel at the same stations, on June 6, September 22, December 15, 1989 and March 6, 1990, subsampled and freeze-dried as described by Span et al. (this issue). Pore water samples were obtained by sucking out fluid surface sediments through a polyethylene pipe into nitrogen pre-filled bottles placed inside the submersible. The bottles were equipped with inlet and outlet valves allowing control of the flow rate and were filled till the sediment slurry appeared in the outlet valve. All precautions were taken to minimize the contact of samples with the atmospheric oxygen. In the laboratory, about 20 1 samples of the sediment slurry were forced with N-gas pressure to the Westphalia continuous flow centrifuge (Burrus et al., 1989) operating at 9600 rpm, maintaining the flow rate of about 1 1/min. After three runs in the centrifuge, water was passed through 0.45 lam filter under a nitrogen atmosphere in order to eliminate the remaining, predominantly organic particles. The filtrate, pale yellow in color probably due to the presence of colloidal organic matter, was spiked with 134Cs and the subsequent treatment was the same as for the lake water samples.
All samples were counted for Cs isotopes in well-type, low background detectors (PGT and ORTEC) calibrated with the standard solutions kindly provided by P. Santschi (while at EAWAG, D/ibendorf). This ensured a proper comparison with the former measurements of Cs isotopes in Lake Lugano (Santschi et al., 1990).
Results
137Cs in lake water, sediments and pore water in the southern bas& (station Melide) During the period of stratification, 137Cs activities increase with depth in the hypolimnion of the southern basin (Fig. 1). This gradient disappears after the winter overturn, and in March similar concentrations are observed at all depths. Deep water concentrations increase from June through December and drop between December and March by factors 3.6 and 5 at 75 and 84 m water depth, respectively. The increase of Cs activity is accompanied by a decrease in dissolved oxygen concentrations, whereby the thickness of anoxic bottom water grows from June through December.
A strong increase of ammonium concentrations in anoxic water is also observed, particularly in September and December.
Water column inventories were calculated by multiplying the 137Cs activities at each depth segment with the corresponding water volume obtained from the table given by Barbieri and Polli (this issue). The integrated water column inventories of 137Cs divided by surface area of the southern basin are 2.7, 3.4, 4.0 and 1.8 kBq/m 2 in May, September, December and March, respectively.
The 137Cs activities in the sediment pore water are always higher than in the overlying lake water. Pore water concentrations, decrease from September (721 _+ 7 Bq/m 3) to December (396_+ 19 Bq/m3), to March (256___ 10 Bq/m3). These
o I I s I I lO I I 9,mo/I o ( ,, , 5 I I lO I I O= mg/I 0 I 10 I I 20 I I i Temp. 0 [ .... 10 I l 20 I I I Temp.
2~ I ,." 20
May 26, 1989 tember 9, 1989
4°1 I \
,'\e~ 6ot :/\, \
80 ."" 8 0 ~
0.01 t I 0.05 0.1 0.2 i l I NH: rag/l, 0.01 I 0.05 0.1 0.2 I I I 10 20 30 50 100 I I I I I Cs-137Bq/m s R ~ I 10 20 30 L , I 50 100 I
[MELIDEJ
o I I s I I lO I I ~ mort o I ,,, s I lO I 3, mg~
0 I 10 I I 20 I I I Temp. 0 10 I 20 I I Temp.
.::::.
....20 -'
NH~ img/I Cs-137 Bq/m 3
I I I
0.01, , 0.05, 0.11 0.2, NH~,mg/1 0.01L , 0.05, 0.1, 0.2, 10 ~ ~ ~ 100 I I I Cs-137B¢I/m' 10 20 30 50 100 ~ I I I I 4
NH~"
mg/lC8-137 Bcl(m 3 ... oxygen
temperature ammonium
Cs- 137
Figure 1. Vertical profiles of 137Cs activity in lake water and sediment pore water from the southern basin compared to temperature, oxygen and ammonium profiles
242 Dominik and Span
Cs-137 Chernobyl (Bq/g) Cs-137 Chernobyl (Bq/g)
5 10 15 20 5 10 15 20
0 I I I I I I I 0 I I I I I I I
"7 0= f la.9 kB m' 14 0, r--- la.7 kSq/m'
~o.4 | Inventory 1:o,4 :
0.6 June ~ 0.8 September
0 5 10 15 20 0 5 10 15
E 0.8 December t e 0.6 [- March
MELIDE
Figure 2. 137Cs from Chernobyl fallout in sediments from the southern basins (Melide). Specific activities and inventories at the time of sampling
concentrations, however, may not readily be comparable as the sampling method does not provide a precise depth control. In order to test the influence of sampling depth on the Cs activity, two samples were collected the same day; one sample from the very top sediment layer and the other a few cm deeper. Indeed, the "deeper"
sample shows lower 137Cs activity in dry sediments (3.4+1.1 Bq/g) than the
"surface" sample (5.9+__0.1 Bq/g), but pore water activities were not significantly different (411+19 and 382___19 Bq/m 3, respectively). This result indicates that sampling depth is critical for 137Cs activity in sediment particles but not necessarily in pore water, which apparently have a smaller activity gradient. It also suggests that adsorption equilibrium for 137Cs in sediments is not achieved. The mean sampling depth be can estimated from the specific 137Cs activity in solids retained in centrifuge as compared to the depth integrated mean activity from sediment core profiles at the same site. This yields the sampling interval at station Melide of about 0-5 cm, matching well our visual observations during sampling.
As the ~ 37 Cs activities in centrifuged sediments (As) remain fairly constant (range 4.1-4.7 Bq/g), while those in pore water (Aw) decrease from September through March (Fig. 1), distribution coefficients Kd =
As/Aw
increase from 0.57, to 1.2 to 1.8 × 10 4 cm3/g for September, December and March, respectively. These coeffi- cients are 5 to 20 times higher then Kd obtained by Comans et al. (1989) from surface sediments of Lake Ketelmeer.Profiles of 137Cs activity from Chernobyl fallout in sediment cores show sharp peaks at about 2 cm depth (Fig. 2) except in the core sampled in December. A
~
0.5I'
~1.5
2.5 Figure 3. Total
0.2
Cs-137 (Bq/g)
0.6 1 5 10 15 20 0.2 0.6 1 5 10 15 20
iii/i
19 _10~ --2s
- 0.5o
1 5"E"
l
~91 Inventory 1964-
15 1.5 Cs-137 nuclear Inventory tests -10~"
"o3.6 (6.4) kBq/m 2 4.2 (7.6) kBq/2
.2o 2
215
JUNE DECEMBER I
MELIDE 2.5
137Cs profiles in sediment cores from station Melide. Pre-Chernobyl 137Cs inventories given in parenthesis are decay corrected to January 1, 1964. Note the different positions of the 1964 maxima, due to surface sediment loss in the core from December
different porosity profile, a low Chernobyl Cs activity and a smaller depth of
137Cs
peak from nuclear explosion fallout in 1963 suggest that surface sediments are missing in this core. From a comparison of the 1963/4 Cs peak in cores recovered in June and December (Fig. 3) we estimate that about 0.22 g/cm 2 or 4.5 cm of surface sediments are missing in the December core. Fitting the porosity profiles from these two cores gives a similar result (0.21 g/cruZ). Although no sediment loss or resuspension were observed during coring, the sampling site could possibly be disturbed during former numerous dives at this station. 137Cs activities from Chernobyl fallout in Fig. 2 are calculated from the total activity using decay corrected
134Cs/137Cs
ratios (the "old" Cs from nuclear tests represents only 4% of the total activity in the upper sediment section). The Chernobyl 137Cs inventories are similar in cores from June and September, and much lower in the core recovered in December in which the surface sediments are lost. The deficit of more than 50 % of 137Cs inventory in March, as compared to June and September, is difficult to explain.It can be attributed to an incomplete core recovery or to a local heterogeneity of sediment and Cs deposition.
Assuming that the position of the Chernobyl Cs peak corresponds to May 1986 the calculated sedimentation rates are 0.042, 0.033 and 0.024 g/cm z yr in the cores recovered in June, September and March, respectively. This is less than the long-term average sedimentation rate of 0.056 g/cm 2 yr in the core recovered in June, as inferred from the position of 137Cs peak originating from nuclear tests in 1963/4. Even higher sedimentation rates of 0.063 g/cm / yr were measured in near-bottom sediment traps in 1985 and 1986 at the same station (Simona, 1988) and in 1975-1976 at another
244 Dominik and Span station in the southern basin (Ravera and Viola, 1977) which, however, likely are overestimating net sedimentation rates due to sediment resuspension and focusing.
137Cs in lake water, sediments and pore water in the northern basin (station Gandria) As the seasonal variability is probably less pronounced in the permanently stratified basin, the 137Cs profile in water column was measured only once in March 1990. This activity profile shows a maximum (Fig. 4 a) in the region of a pycnocline located at 100 m water depth. Below this depth oxygen is absent and ammonium content increases rapidly. 137Cs activity in the epilimnion is lower than in the southern basin.
Near-bottom water activity is also lower than in the southern basin from June through December and similar to that in March. Sampling density in not sufficient to determine the actual concentration maximum, which is probably situated between 75 and 125 m water depth, slightly deeper than the maximum concentration measured earlier by Santschi et al. (1990) at 50 to 80 m depth (Fig. 4b). The estimated 137Cs inventory in the water column is 7.2 kBq/m 2, nearly a factor two higher than the highest value in 1989 in the southern basin.
The activity of 137Cs in pore water at station Gandria is higher than in overlaying lake water but lower than in the southern basin. The estimated mean sampling depth of 1.5-4.0 cm is somewhat different than in the southern basin, probably because of lower porosity of sediment and thus its stronger resistance for the sucking pipe. The distribution coefficient K d of 3.4 × 104 cm3/g is, by a factor of 2, higher than K~ in sediments from the southern basin in the same period.
Northern basin (Gandria) a) o ... 5, ... !o..~..~
0 10 20 Temp.°C
0 ' ... J"-;"J ... ' ... ' ... ' ... '
''" O2"'" ""ARCH 1 99O
~NH:
"::i:i:i:i:ii~:i:iP~'e~i:i:
0.01 0.02 0.05 0.? 0.2 NH'~ mgjl
i i i i
10 20 30 50 100 Cs-137 Bq/m ~
i i i i
50
10C
ID ~D
~150
25O
I I I I I
b) Cs-137 Bq/m 3
10 20 30 50 100
0 ~ ' , ~ L -,' '
•
~ ~ ~.
50 , ~,
100 "/ "
~: 200- / / ," ,"
250 - //
:i: ~0~i:i:i:i~:i:i:i:i:i ---~--- October 196"; (Santschi et al. 1990 ... ~ ... April 1988 (Santsc~ et al. 1990
• March lSS0 frh~ study)
Figure 4. a) Vertical profiles of 137Cs activity in lake water and sediment pore water from the northern basin compared to temperature, oxygen and ammonium profiles, b) the same x37Cs profile compared to earlier measurements by Santschi et al. (1990)
A 1
o
E._E 2
a).=_
J=~.3
Chernobyl Cs-137 in sediments (Bq/g) Porosity (%)
0 s 10 15 20 90 95
I I I I I I I I I
: i" =.."
Inventory (kBq/m2)11.3 --- GII , i" ...
9.8 + ... G2 /'
11.9 o ... G4
.~i.ii. . L .*
9.1 z= . . . G7 ,:" ."
"'-...
~ ::.,:. ..'::-~:'~:~" ... ... ":'$ '"'"-gi ''1
!
if...": ; /..",/'f: ~,ii: .':"
• :: ,:.
0 0
i Northern basin (Gandria)
100
= 0
1
2
3
4
5
Figure 5. ~37Cs from Chernobyl fallout in sediments from the northern basins (Gandria). All cores were recovered in March 1990. Note a low porosity layer at about 2 cm depth (on the right)
Four sediments cores were recovered in March i990 from the northern basin at station Gandria within the distance of less then 1 m one from each other. The reproducibility of sampling at this station appeared to be quite satisfactory (Fig. 5).
The depths of 137Cs peak vary by only 0.5 cm. The mean inventory is 10.5 kBq/m 2 whereby the extreme values differ from the mean by less than 25%. The mean inventory make 75 % of the inventories found in the southern basin sediments during anoxic conditions in bottom water in June and September. The laTCs profiles in the northern basin are characterized by sharp peaks and by low but constant activities of about 1 Bq/g in the uppermost sediments, contrasting with the sediments from the southern basin, where Cs-peaks are strongly asymmetric with a relatively high activity of about 5 Bq/g in the uppermost sediments.
Sedimentation rate obtained from the mean depth of Chernobyl Cs-peak in sediments from four cores recovered at station Gandria is 0.119 g/cm 2 yr. This is twice as much as the mean rate of 0.056 g/cm 2 yr measured in sediment traps in 1985 and 1986 at the same station (Simona, 1988). A more compact, light gray layer about 1 cm thick (turbidite or slump)just above the laTCs peak, marked by a lower porosity in Fig. 5, is probably responsible for the higher sedimentation rate. If the sediment
246 Dominik and Span weight in this layer is omitted the resulting "pelagic" sedimentation rate of 0.055 g/cm2yr agrees with the measurements obtained from sediment traps.
Numerous thicker turbidite layers were observed in older sediments at this station (Dominik et al. 1991) and the ~37Cs peak from nuclear bomb tests has not been found.
Discussion
Cs budget in Lake Lugano
In the complex hydrological system of Lake Lugano and with relatively few data available only a very crude budget can be achieved. The main purpose of these calculations is to evaluate the role of sediments as the internal source of Cs in the southern basin. As this basin receives water from the northern basin we first estimate the Chernobyl Cs output from the mixed layer of the northern basin.
A comparison of our data from 1990 in northern basin with those of Santschi et al. (1990) from 1987 and 1988 shows a decrease of ~ 37Cs activity in the epilimnion and in the oxic part of the hypolimnion as well as its transfer to the anoxic part of the basin. The loss of Cs from the mixed layer (0-100 m) can be calculated according to equation (2) derived from a general mass balance (Imboden and Schwarzenbach, 1985):
kw (Co kw Cin) exp[_(kw+ks)t]
(2)C(t)=kw+k-C~n + kw+k s
k = kw + ks (2 a)
where Co and C (t) are 137Cs mean concentrations in 0-100 m depth mixed layer at time of Chernobyl fallout and after a time t, Cin is CS concentration in inflows (from soil erosion), k is total removal rate constant, k w is water renewal rate in oxic layer (water output/volume) and ks is a net transfer rate to the deep basin, essentially on settling particles, but also includes diffusional transport across the pycnocline and deposition in sediments above 100 m water depth. Radioactive decay of 137Cs is not included in equation (2) as all measured inventories are decay corrected to May 1st, 1986.
Multiplying both sides of equation (2) with the volume of the oxic layer and dividing by the lake area we obtain a similar expression in terms of inventories per surface area (kBq/m2). After rearrangement:
kw I~n (1 -- exp [-- (kw + ks) t] + Io exp [- (kw + ks) t] (3) I (t) - kw + ks
It can be shown that, for the period considered (4 years), in the right hand of equation (3) the first term of the sum is much smaller than the second one. With the estimated residence time of Cs in the soil of an alpine watershed of about 800 yr (Dominik et al., 1987) and the ratio of watershed/northern basin surface areas of 10, the Ii, would be equal to 0.3 kBqm-Zyr -~. As long as the ks < 0.8 yr -~, the term accounting for
input from the watershed 4 years after Chernobyl fallout will increase the inventory by less then 10%. Fitting the function (3) to measured inventories (with kw = 0.16 yr-1), results in k s less than 0.8 yr -1, and thus the equation (3) simpli- fies to:
I(t) = lo exp (- kt) (4)
From the inventories measured between 1987 and 1990 we obtain k = 0.57 yr -1.
Assuming the annual water output from the northern basin of 1.05 x 10 6 m3/day, kw is equal to 0.16 yr -1 and thus, from equation (2b), ks = 0.41 yr -1. In other words, about 30 % of the a 37Cs output from the 0-100 m mixed layer is transferred from the northern basin to the southern basin and 70 % (about 12.4 kBq/m 2 till March 1990) to the deep stagnant basin and to sediments. This downward flux may be underestimated, as the decay corrected inventories of 137Cs in the deep basin (5.3 kBq/m 2 ) and in bottom sediment (11.5 kBq/m 2 ) point to a higher removal rate or a higher initial fallout. The latter is assumed to be 24 kBq/m 2 (Santschi et al., 1990), but the regression to inventories in the oxic water layer results in an initial input of 20 kBq/m 2. The decay corrected inventory of 137Cs in water of the deep basin makes nearly a half of calculated downward flux from the oxic part of the basin till March 1990. The other half should be found in the sediments. The observed proportions of the ~ 3VCs inventory in the northern basin are close to 70 % in sediments and 30 % in deep water. While the inventories measured in bottom sediments are most probably enhanced by focusing and a distal turbidite, this discrepancy may also partly result from the assumption of a well mixed upper 100 m of water, which is not strictly true, especially during thermal stratification of the lake. A three box model would be more adequate but it cannot reasonably be tested with the available data.
From the change of the measured inventories in water of the southern basin (dI/dt) and an estimated fluvial input and output we can calculate the seasonal Cs recycling between water and sediments (Table 1):
dI/dt = Inb + I w -Iout + I~ea
(5)
In the southern basin the 137Cs inventory increases linearly from May through December. An extrapolation of this increase back to March 1989 gives an inventory of 2.25 kBq/m / and thus the average concentration in the completely mixed lake of about 40 Bq/m 3. Using this value for the first time interval and the measured concentrations in the epilimnion for the other periods until March 1990 we calculate the output of 137Cs by the outlet (/out). Input to the southern basin (Inb) is equal to the output from the oxic water layer of the northern basin (as calculated above), plus the fluvial input from the watershed. This time, the input from the watershed (Iw) is included in the budget, as the ratio of watershed/southern basin surface area is larger than for the northern basin. As shown in Table 1, the inputs are nearly balanced by the fluvial output.
Recycling of 137Cs between water and sediments is thus responsible for important variations of the 13 V Cs inventory in the southern basin of Lake Lugano from March 1989 through March 1990. From input, output and inventory changes we obtain a
248
Table 1. la7Cs budget for southern basin of Lake Lugano (1989/1990)
Dominik and Span
Period in Bq m- 2 period - 1
from to /out Inb Iw dI/dt Inb + I w I s
--/out
6/3/89 26/5/89 262 163 94 494 - 5 499
26/5/89 9/9/89 240 183 122 645 65 580
9/9/89 12/12/89 234 138 107 575 11 564
12/12/89 6/3/90 256 106 95 -2117 -54 -2063
in Bqm-2yr -1
6/3/89 6/3/90 992 589 418 - 403 16 - 419
in Bqm-Zd -1
6/3/89 26/5/89 3.23 2.01 1.16 6.10 -0.06 6.2
26/5/89 9/9/89 2.26 1.72 1.15 6.08 0.61 5.5
9/9/89 12/12/89 2.49 1.46 1.14 6.12 0.11 6.0
12/12/89 6/3/90 3.05 1.27 1.13 -25.20 -0.65 -24.6 Symbols are explained in text. Initial inventory in lake water on 6/3/89 was obtained by extrapolation
relatively constant net flux of 137Cs from sediment to water of 6.2, 5.5 and 6.0 Bq/m 2 day in spring, summer and autumn (Table 1). Budget calculations for winter, from December 1989 to March 1990, show that the net flux of 137Cs from the water to the sediment is close to 25 Bq/m 2 day. The substantial decrease of the 137Cs inventory during this period is difficult to explain otherwise than by a rapid scavenging of Cs by particles and surface sediments after the lake overturn and recharging with oxygen.
A strong gradient of 137Cs in the hypolimnion and between pore water in sediments and lake bottom water during stratification confirms the budget calcula- tions. We cannot calculate the upward flux across sediment-water interface because the pore water value represents the mean activity from a few uppermost centimeters of sediment. However the flux from the sediments can be estimated from the 137Cs gradient in deep part of the hypolimnion according the first Fick's law. From the gradient between 84 and 75 m water depth and the vertical eddy diffusion coefficient taken as 0.3 × 10-4 m2/s for stagnant near-bottom water (the lowest value obtained from deep waters of the northern basin by Wriest et al., this issue), the x37Cs flux from sediments would be 4, 11, and 13 Bq/m 2 day in May, September and December, respectively. These values are similar or, at most, higher by a factor of 2 than the net fluxes obtained from the input-output budget.
137Cs in sediments
Examination of 137Cs in bottom sediments can be considered as a complement to the lake water study. However, with only one sampling station in each basin and considering the spatial heterogeneity of the sediments, the comparison may not
necessarily be very conclusive. At the station Melide, at least one out of four cores collected suffers from incomplete surface recovery (December). A low inventory in sediments at this station found in March, just when the water column inventory decreased by a factor of 2 is puzzling. We have no final explanation for this observation, but it may result from a small scale sediment heterogeneity (cf. Dominik et al. 1991).
A clear asymmetry of Chernobyl peak in the sediments at the station Melide (Fig. 2), with a relatively high activity in the top layer, may suggest that a part of
~3VCs returns to the surface sediments after a transit through the water column.
Alternatively, the asymmetry can be interpreted as a result of resuspension and focusing of sediments deposited originally on the slopes of the basin. Remobilization and diffusion of X37Cs from sediments postulated on the basis of budget calculations and a relatively low distribution coefficient (Kd) in sediments should, in principle, result in a considerable increase of the width of Chernobyl peaks in sediments. Using a diffusional formalism (cf. Lerman and Lietzke, 1975; Crank, 1975; Robbins et al., 1979; Berner, 1980) one can relate the Kd to the width of peak (FWHM) of Cs in sediments, considered initially as plane source, after a time elapsed between Cs deposition and sediment sampling. Using the measured Kd (0.57-1.8 x 104 cm3/g), the porosity (0.97), and assumed dry sediment density (2.5 g/cm3), a diffusion coefficient at 5°C (1.1 x 10 -5
cruZ/s,
Li and Gregory, 1974), we obtained the expected FWHM in a range 3.1-5.5 cm. The observed FWHMs are between 1.25 and 2.5 cm, clearly lower than the calculated values. Similarly, the expected spread for the Cs peak from nuclear tests would be in the range of 6-11 cm while the measured FWHMs are about 5.5 cm (Fig. 3). These theoretical considerations are, however, only valid in a homogeneous medium, and FWttMs are very sensitive to porosity variations. For example, a 7% decrease in porosity would decrease the calculated FWHM by a factor of 2. In laminated sediments, consisting of thin layers of different mineral composition, grain-size and porosity, the diffusional transport in pore water may be markedly modified (cf. phosphorus migration in Lake Lugano sediments, Span et al., this issue). In the northern basin, the Chernobyl peak is quite sharp (FWHM = 0.7 cm as compared to the expected value of 1.6 cm) and symmet- ric (Fig. 5). This is most probably due to the deposition of a low porosity layer, just above the Chernobyl peak, which rapidly cut off the direct contact with bottom water and may decrease the upward diffusion and loss rate of Cs. Moreover, the lateral delivery of sediment increase the Cs inventory at this site.The Chernobyl ~3VCs inventory in the sediments of the southern basin (Melide, June and September cores) make up roughly 60 % of the estimated fallout. Similarly, decay corrected inventories of Cs from nuclear tests (6.4 and 7.6 kBq/m 1, June and December cores) correspond to 60-70% of the fallout between 1958 and 1966 measured in Locarno (H. V61kle, KUR, pets. comm.), if the cbntribution of Cs eroded from soil is taken into account.
Overview
The increase of the water column inventories from June through December in the southern basin can be explained by an input from bottom sediments; a strong
250 Dominik and Span decrease of the inventory between December and March suggests removal of ~37Cs to the sediments and marginally an enhanced loss by the outlet after the winter overturn.
In principle, the input of Cs from sediment to overlaying water may result from sediment resuspension or chemical remobilization and diffusion. The former process has been well documented in the Great Lakes (Robbins and Eadie, 1991) but chemical mobility of Cs and its migration to the water column is still disputed.
Contrary to many trace elements, Cs is not directly involved in redox diagenetic reactions but may be released into pore water as a result of exchange reactions (Sholkovitz et al., 1983), in particular with chemically similar NH~- ions (Evans et al., 1983). Cremers et al. (1988) showed that Cs is specifically adsorbed on the frayed- edge sites of illite and the decrease of Kd for ~37Cs with the increase of NH~- concentration in water (Comans et al., 1988) may suggests that ammonium ions can successfully compete with Cs for these sites. It can be postulated that the extent of Cs fixation and remobilization depends on: a) sediment composition, especially with respect to clay minerals type and content, b) concentration of NH2 ions in water, and c) chemical form and equilibration of Cs-isotopes in water-sediment system. In Lake Lugano sediments mica type minerals are probably abundant (Niessen, 1987), but little is known about the mineral composition of the clay fraction. Ammonium concentrations in pore water of near-surface sediments are about 0.2-0.3 mM (Lazzaretti, personal, comm.). This ion is also present in relatively high con- centrations in the bottom waters during the stagnation period in the southern basin (Fig. 1) and throughout the year in the northern basin (Fig. 4). Although there is a relation between increasing concentrations of 137Cs and NH~- in bottom waters, it may be coincidental. However, several arguments suggest chemical rather than physical mobilization processes as responsible for the Cs inventory increase in Lake Lugano water. Physical resuspension is expected to be most effective at the lake overturn and decreases during the stagnation period, which is just the contrary to the observed a 37Cs inventory evolution in the southern basin. Moreover, to account for the observed increase of Cs activity to about 150 Bq/m 3 in near-bottom water, the particle concentration of 10 to 30 rag/1 would be required, assuming a specific activity on particles similar to these found in bottom sediments, i.e. of 5 and 15 Bq/g. Such a particles concentration is very unlikely in bottom waters of Lake Lugano.
Additionally, a high efficiency of particle retention in the exchange resin column would be required. Furthermore, at least in the studied area of the southern basin, Cs inventories in sediments are not particularly enhanced by a focusing effect, as discussed above.
Our data suggest that Chernobyl Cs is remobilized from anoxic sediments and is not rapidly scavenged by particles in anoxic lake water. This is supported by a strong gradient between pore water and bottom lake water, by increasing concentrations of 37Cs with depth in the southern basin, persisting until the winter overturn, and by a substantial build up of the Cs inventory in the hypolimnion from spring to winter, in parallel to the oxygen depletion and ammonium formation. In the northern basin, a gradient of 137Cs activities between sediment pore water and overlaying bottom water also occurs but the maximum of 13VCs concentration is observed near the permanent pycnocline, suggesting a different transport mechanism. This maximum can be explained by a rapid release of 13 V Cs from settling particles and/or sediments
20--~ _
~15- 0~
.N
2 L.
r-.
e- l
Regression lines measured
southern basin (Melide) "
... northern basin (Gandrla)
. ~ northem ' ' _
... (without the first point) u
~
... Fallout from Chemobyl accident- t, Cyoarsl
....-....=...-7 6.7
'" ... 2.5
e ~ 1.2
Santschi et al. 1990 this study
lll=t,Jt,,tl,,,,,,li,,,liiJ=iitiiiii=Jiiiii=iiiiiiiiiii,iii I
1986 1987 1988 1989 199o
_•_.•_30
25- 20 15 10
Figure 6. Evolution of the normalized, decay corrected 137Cs inventory in Lake Lugano water from 1986 through 1990. The time necessary to remove half of the inventory (tl/2) is shown on the right
into water at the level corresponding to the depth of the oxycline oscillations. It seems that a fraction of 137Cs is replaced rapidly at certain exchangeable sites of particulate matter as soon as the 13VCs bearing particles reach the anoxic zone at 100 m water depth.
Similar mechanism can be involved at the sediment-water interface in the southern basin. During the summer and autumn, in the anoxic conditions at the interface, NH~- ions can possibly replace Cs at exchangeable sites, following by rapid migration of Cs into the overlaying water. The more firmly bound fraction of Cs will then be buried with particles and replaced at a much slower rate. This could possibly explain the relatively sharp and asymmetric peaks found in sediments of the southern basin sediments.
The long-term evolution of the Chernobyl 13VCs inventory in Lake Lugano is presented in Fig. 6. If 137Cs removal from the lake water can be considered as proportional to its inventory, the reciprocal of the slope of regression line represents the residence time (z) of this nuclide. The time necessary to eliminate half of the inventory
(tl/2
= zln2) is shown on the right side of the diagram. This time is shorter in the southern (1.2 years) than in the northern (2.5 years) basin. Despite the seasonal fluctuations of the t3VCs inventories in the southern basin, the data points follow fairly well the exponential decrease with time. The ~37Cs removal in the northern basin is a more complex process. If the regression is done for the measured inventories from 1987 thought 1990 (neglecting the estimated fallout value), the resulting half- removal time is as high as 7 year. This is mainly due to a relatively rapid initial252 Dominik and Span Table 2. Half removal time (tl/2) of Chernobyl 137Cs from prealpine lakes according to (1) Santschi et al. 1990, (2), Wieland et al., 1992, and (3) this work
Lake tl/2 (years) Remarks Source
Lugano anoxic bottom water
southern basin 1.2 during 8 months (3)
Lugano below 100 m depth
norther basin 6.7 permanently anoxic (3)
northern basin 1.2 oxic from 0 to 90 m (3)
Zurich 0.4 anoxic during summer (1)
only in near-bottom water
Constance 0.3 oxic (1)
Sempach 0.9 anoxic in summer but (1,
artificially mixed
Soppen 0.5 no ~outlet (1)
2)
removal of this nuclide from the surface water, followed by a slower removal and even accumulation of Cs in the deep anoxic water.
The removal time of the Cs depends on a number of parameters such as lake dimensions, water renewal rate, particle character, path and flux, and, as shown by Robbins et al. (1992), may be highly variable in time. However, in the lakes with seasonally anoxic hypolimnion, removal rate seems to be slowed down. Some remobilization of Chernobyl Cs from bottom sediments occurs in Lake Zurich (Schuler et al., 1991) and Lake Sempach (Wieland et al., 1992). In Lake Lugano, this process seems to be even more pronounced, as illustrated in Table 1, which compiles previous estimations of the half removal times from a number of prealpine lakes and the results obtained in the present study.
Conclusions
The measurements of 137Cs activity originating from Chernobyl fallout in Lake Lugano water, sediment pore water demonstrate an intensive recycling of this nuclide. This process is related to redox conditions. In the southern basin, the 137Cs inventory build up in the water starts in spring and continues till the winter overturn, when the inventory decreases by a factor of 2. In the permanently stratified northern basin a part of Cs has been rapidly removed from epilimnion to bottom sediments, but a substantial fraction of fallout is transferred to and retained in the deep anoxic waters. Due to these redox related processes, the overall removal rate of Chernobyl Cs is slower in Lake Lugano than in the majority ofprealpine lakes. A more extensive study of bottom sediments is necessary to define the spatial distribution of 13VCs in the sediments and to determine the areas where sediments serve as a source or sink for this nuclide. Further work is also necessary to determine the ~37Cs distribution between "soluble" and particulate phases in water column.
ACKNOWLEDGEMENTS
The field work of this study was done as a part of integrated research program on Lake Lugano
"Endoceresio 89", financially supported by Banca Unione di Credito. We thank G. Keller, J. Piecard and the crew of submersible "F. A. Forel" for their precious collaboration and A. Barbieri and M. Simona (Laboratorio Studi Ambientali, Lugano) for supplying unpublished profiles of physico- chemical parameters. The help in sampling, in laboratory work and the scientific collaboration of R. Peduzzi (Istituto Cantonale Batteriosierologico, Lugano), K. Hanselmann, H. Brandl, M. Lazzaretti (Universit/it Ziirich), B. Galerini, H. Howa, M. Dulinski and J.-P. Vernet (Institut Forel, Universit6 de Gen6ve) is highly appreciated. We thank the reviewers for their comments, in particular J. A. Robbins for his pertinent remarks and criticism. A part of program was financially supported by the Founds National Suisse de la Recherche Scientifique.
REFERENCES
Alberts, J. J., L. J. Tilly and T. J. Vigerstad, 1979. Seasonal cycling of 137Cs in a reservoir. Science 203:649-651.
Alberts, J. J. and R. N. Muller, 1979. The distribution of 239'24°pu, 238pu, and 137Cs in various particle size classes of Lake Michigan sediments. J. Environ. Qual. 8:20-22.
Alberts, J.J., M.A. Wahlgran, K. A. Orlandini and C. A. Durbahn, 1989. The distribution of 239, z4Opu ' 138pu ' Z41Am and 13VCs among chemically-defined components of sediments, settling particulates and net plankton of Lake Michigan. J. Environ. Radioactivity 9:89-103.
Aston, R.S. and E.K. Duursma, 1973. Concentration effects on 137Cs, 65Zn, 6°Co and l°6Ru sorption by marine sediments, with geochemical implications. Neth. J. Sea. Res. 4:225-240.
Berner, R. A., 1980. Early diagenesis. Princeton University Press, Princeton, 241 pp.
Burrus, D., R. L. Thomas, J. Dominik and J.-P. Vernet, 1989. Recovery and concentration of suspended solids in the upper Rhone River by continuous flow centrifugation. Hydrol. Processes 3:65-74
Comans, R. N. J., J. J. Middelburg, J. Zonderhuis, J. R. W. Woittiez, G. J. De Lange, H. A. Das and C. H. Van Der Weijden, 1989. Mobilization of radiocaesium in pore water of lake sediments.
Nature 339: 367- 369.
Crank, J., 1975. The mathematics of diffusion. Clarendon Press, Oxford, 414 pp.
Cremers, A., A. Elsen, P. De Preter and A. Maes, 1988. Quantitative analysis of radiocaesium retention in soil. Nature 355:247-249.
Dominik, J., 1989. Application des radio-isotopes naturels et anthropogbnes pour l'6tude des processus s~dimentaires dans le L~man. Cahier Fac. Sci. Universit6 de Gen~ve 19:83-95.
Dominik, J., J.-L. Loizeau and D. Span, 1992. Radioisotopic evidence of perturbations of recent sedimentary record in lakes: a word of caution for climate studies. Clim. Dynamics 6:145 /,52 Evans, D.W., J. J. Alberts and R. A. Clark III, 1983. Reversible ion-exchange fixation of cesium-137
leading to mobilization from reservoir sediments. Geochim, Cosmochim. Acta 47:1041-1049.
Imboden, D. M. and R. P. Schwarzenbach, 1985. Spatial and temporal distribution of chemical substances in lakes: modeling concepts. In: Chemical Processes in Lakes (W. Stumm, ed.), Wiley, N.Y., 1-30.
Lerman, A. and T.A. Lietzke, 1975. Uptake and migration of tracers in lake sediments. Linmol.
Oceanogr. 20: 497- 510.
Li, Y.-H. and S. Gregory, 1974. Diffusion of ions in seawater and deep-sea sediments. Geochim.
Cosmochim. Acta 38:703-714.
Mangini, A., U. Christian, M. Barth, W. Schmitz and H. H. Stabel, 1990. Pathways and residence times of radiotracers in Lake Constance. In: M. Tilzer and C. Serruya (eds.), Large Lakes, Springer, Berlin, pp. 245-264.
Mann, D. R. and S. A. Casso, 1984. In-situ chemisorption of radiocesium from seawater. Mar.
Chem. 14:307-3t8.
Ravera, O. and M. Viola, 1977. Sedimentation-rate in a basin (Agno) of Lake Lugano. In: H. L.
Golterman (ed.), Interactions between sediments and freshwater, The Hague, pp. 174-178.
254 Dominik and Span Robbins, J. A., P. L. McCall, J. B. Fisher and J. R. Krezoski, 1979. Effect of deposits feeders on
migration of ~37Cs in lake sediments. Earth Planet. Sci. Let. 42:277-287.
Robbins, J. A. and B. J. Eadie, 1991. Seasonal cycling of trace elements ~ 37Cs, 7Be, and 239 + 24Opu in Lake Michigan. Journ. Geophys. Res. 96:17081-17104.
Robbins, J. A., G. Lindner, W. Pfeiffer, J. Kleiner, H. H. Stabel and P. Frenzel, 1992. Epilimnitic scavenging of Chernobyl radionuclides in Lake Constance. Geochim. Cosmochim. Acta 56:
2339-2361.
Santschi, P. H., M. Amuderer, D. Adler, P. O'Hara, Y.-H. Li and P. Doering, 1987. Relative mobility of radioactive trace elements across the sediment-water interface in the MERL model ecosystems of Narraganset Bay. Journ. Mar. Res. 45:1007-1048.
Santschi, P. H, S. Bollhalder, S. Zingg, A. Liick and K. Ferrenkothen, 1990. The self-cleaning capacity of surface waters after radioactive fallout. Evidence from European waters after Chernobyl, 1986-1988. Environ. Sci. Technol. 24: 519- 527.
Schuler, C., E. Wieland, P. H. Santschi, M. Sturm, A. Lueck, S. Bollhalder, J. Beer, G. Bonani, H. J.
Hofmann, M. Sutter and W. Wolfli, 1991. A multitracer study of radionuclides in Lake Zurich, Switzerland 1: Comparison of atmospheric and sedimentary fluxes of 7Be, 1°Be, 21°pb, 21°po, and ~ 37Cs" Journ. Geophys. Res. 96:17 051 - 17 065.
Sholkovitz, E. R., J. K. Cochran and A. E. Carey, 1983. Laboratory studies of the diagenesis and mobility of 239.24Opu and ~ 37Cs in near-shore sediments. Geochim. Cosmochim. Acta 47:1369-
1379.
Simona, M., 1988. Sedimentazione. In: Ricerche sull'evoluzione del Lago di Lugano; aspetti limnologiei. Campagna 1986. Commissione Internzaionale per la Protezione delle Acque Italo- Svizzere, 71-78.
Wieland, E., P. H. Santschi, P. H6hener and M. Sturm, 1992. Chernobyl laTCs and natural 21°pb, in Lake Sempach, Switzerland: removal processes and remobilization from sediments. Submitted to Geochim. Cosmochim. Acta.
V61kle, H., L. Baeriswyl, G. Ferreri, M. Gobet, A. Gurtner, C. Murith, St. Nowak, L. Ribordy, H.
Surbeck, F. Wicht, S. Bollhalder, K. Farrenkothen, A. Liick, P. H. Santschi and C. Weber, 1986.
Radioaktivit~it in Luftfllter-Niederschlagsproben und Ablagerung auf dem Gras. In: Radioac- tivitaetsmessungen in der Schweiz nach Tschernobyl und ihre wissenschaftliche Interpretation, Tagungsbericht, Bern, 72-83.
Received 14 February 1992;
Revised manuscript accepted 1 September 1992.
