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The origin and dynamics of 137Cs discharge from a coniferous forest catchment
.••
Journal
of Hydrology
ELSEVIER
Journal of Hydrology 192 (1997) 338-354
The origin and dynamics of 137Cs discharge from a coniferous forest catchment T o r b j 6 r n N y l 6 n a'*, H a r a l d G r i p b INational Defence Research Establishment, NBC Research Department, S-901 82 Ume,~,Sweden bSwedish Universityof Agricultural Sciences, Department of Forest Ecology, S-901 83 Ume~, Sweden Received 14 September 1995; revised 26 January 1996; accepted 2 February 1996
Abstract The turnover of radioactive caesium was studied experimentally and theoretically in a forested catchment that was covered by snow during the wet deposition of radioactive nuclides from the Chernobyl accident. The study spans from 1 week before to 8 years after the deposition event. A fraction of the catchment is covered by a mire (16%). From the edge of the mire a stream channel runs to the outlet of the catchment. Two phases of decreasing activity concentration in the stream water were found in addition to a positive influence of runoff on the activity concentration in the stream. The half-lives for 137Csin the stream water corresponding to the early and the later phase were estimated by non-linear regression to be 6.5 days and 4 years, respectively. During the first phase, which corresponded to the initial snow melt in 1986, 6.8% of the deposition was lost from the catchment, whereas the slow secondary loss during the following 8 years was 1.8%. The main contribution to the yearly discharge of ~37Cs occurred during spring and autumn when the areal contribution to saturated surface runoff was highest. The remaining deposition in soil a few years after the fallout was significantly lower in the mire than in the surrounding forest. By using the calculated activity concentration of mCs in the stream water together with the remaining deposition in the different biotopes and information on stream flow for the catchment it was concluded that the loss originated from the mire. During the initial phase 44% of the deposition was lost from the mire, and during the following years the yearly loss was 30% from the fraction that constantly undergoes saturated surface runoff and 2% from the drier fractions of the mire. Until the end of the study it was not possible to demonstrate any loss from the recharge areas (podzol and cambisol soils), which means that physical decay will govern the decrease in activity in these areas. © 1997 Elsevier Science B.V.
* Corresponding author. 0022-1694/97/$17.00 © 1997- Elsevier Science B.V. All rights reserved PII S0022-1694(96)03083- !
T. NylOn,H. Grip/Journal of Hydrology 192 (1997) 338-354
339
1. Introduction Investigations after the Chernobyl accident have shown that food such as meat, fish, berries and mushrooms from the boreal forests will give the major contribution to the collective dose for the Swedish population, integrated over longer time periods, from internal exposure by radioactive caesium (Bergman, 1993, 1994; Aarkrog, 1994). Bergman et al. (1993) found that the amount of radioactive caesium in this region will decrease mainly by physical decay and loss via runoff. Further, SaxOn (1994) stated that the loss of 137Csvia runoff also must be considered regarding the contribution to rivers and lakes where it might be the most important factor, besides redistribution of lake sediments, for uptake of radioactive caesium in fish in longer time perspectives. Most of the observations reported on the dynamics of radioactive caesium in catchments suggest a rapid washout of between 0.1 and 5% (Davis et al., 1984; Helton et al., 1985) during the first days after fallout. The remaining annual loss will typically be less than a few per mille, as shown by Bonnett and Appelby (1994). This relatively small loss often fits an exponential decrease in activity concentration with time (Helton et al., 1985; Hilton et al., 1993; SaxOn, 1994), which indicates a retardation of the caesium loss from the system, e.g. owing to long-term redistribution in the living biota and abiotic fixation of radioactive caesium in the catchments. However, the processes behind this behaviour are not yet clearly understood. An often quoted reason for the slowly decreasing activity concentration is the intercalation of radioactive caesium in the clay lattice (Carbol, 1993). Also, in organic soils the fixation of caesium seems to increase with time. Shand et al. (1994) suggested that the mechanism for this was intercalation in a low-level content of clay or chemical binding to organic matter in humin. Several investigations have shown that radioactive caesium is found in the uppermost 20 cm of the soil column (Tobler et al., 1988; Carbol, 1993). Smith et al. (1995) found that the solids-aqueous distribution coefficient (Kd) of 137Cs in these shallow horizons was at least one order of magnitude lower in an organic soil of a saturated peat bog than in mineral soils. Hilton et al. (1993) demonstrated that the potential for release of radioactive caesium via runoff was higher in fibrous organic peat than in mineral soils. Hilton and Spezzano (1994) also found that 137Cswas sorbed to the fibrous peat in flooded peat bogs by ion exchange and that it was easily exchanged for calcium. In the studied region, peat soils show a predominance in discharge areas. During periods of saturated surface runoff in discharge areas there will be contact between the leaking water and the topsoil layers (Bishop, 1992) where 137Cs is found. The stormflow events generating saturated surface runoff are not evenly distributed throughout the year; rates of runoff in northern Sweden are low during winter (November-April) and summer (JuneAugust) and attain their maximum during spring and autumn. Consequently, discharge of radioactive caesium from land to rivers and lakes in northern Sweden is likely to be highest during spring and autumn. This study deals with data that span a period from a few days before to 8 years after the accident at Chernobyl. It concerns the initial and long-term loss of radioactive caesium from a coniferous forest catchment in northern Sweden that was covered by snow during the fallout. The aims of the study were: to quantify the fast initial and slow residual discharge of radioactive caesium during and after the fallout; to quantify the seasonal
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contribution to the release of radioactive caesium from the catchment; to demonstrate the influence of runoff on caesium concentration in stream water; to find the origin of 137Cs discharge from the catchment. The hypothesis was that essentially only the discharge areas contribute to the loss of radioactive caesium from the studied catchment.
2. Materials and methods
2.1. Site description The study was carried out in a 50 ha coniferous forest catchment in Northern Sweden (64°15'N, 19°46'E). The catchment is a part of the Vindeln Forest Experimental Station and serves as a base for long-term reference measurements of climate, water balance and chemistry for the Swedish University of Agricultural Sciences. Between 1983 and 1988 the annual precipitation in the area was 710 mm and the mean runoff 330 mm (Bishop, 1992). The catchment is drained by a perennial stream in the eastern part, and an intermittent stream in the western part (Fig. 1). The two streams were straightened and deepened to improve drainage during the 1930s. The potential discharge areas in the catchment can be divided into two groups: a mire, 16% of the catchment, situated in the northern upstream part of the catchment, and the discharge areas scattered in the coniferous forests along the two streams (Fig. 1). The remaining fractions of the catchment, composed of hill slopes and water divides, are identified as recharge areas ('rech. forest' in Table 4, below). With regard to the floristic composition of the field strata, two groups can be identified in the mire (Malmstrtm and Tamm, 1926). Areas characterised by dwarf shrubs and scattered Scots pine (Pinus sylvestris) on a ground stratum of peat mosses (Sphagnum spp.) forming tufts (13.6% of the catchment, described as 'shrub mire' in the text) and areas characterised by cyperaceous plants with a ground stratum of peat mosses, not forming tufts (2.4% of the catchment; 'open mire'). The latter fraction of the mire is in close vicinity to the origin of the perennial stream. The forested parts are dominated by mature Scots pine predominantly on the higher elevated areas on a ground stratum of lichens and mosses (Cladina spp. and Pleurozium spp.). Norway spruce (Picea abies (L) Karst.) forest of the Vaccinium type, predominates on the lower parts of the recharge areas. Scattered in the lower areas of the spruce forest are discharge areas characterised by Sphagnum, Polytricium and Equisetum silvaticum ('rech. forest' in Table 4, below). The soils on the hill slopes and at the water divides are orthic cambisols and on low-lying areas are carbic podzols (FAO-UNESCO, 1981), all developed on glacial till or wave-wash sediments. The mire is composed by peat.
2.2. Sampling methods Runoff was monitored by means of water stage recorders at 90 ° V-notch weirs at Weir A at the outlet of the catchment (50 ha) and Weir E upstream at the beginning of the perennial stream, draining 19 ha of the catchment (Fig. 1, open circles). The area drained by Weir E is called Subcatchment E in the text. The remaining fraction of the catchment is called Subcatchment A - E (31 ha between Weirs E and A), The stage-discharge relationship
7". Nyl6a, H. Grip~Journal of Hydrology 192 (1997) 338-354
341
0
S t r e m w a t e r weir
•
G r o u n d w a t e r tube
300
Altitude above s e a level (m)
•
Recharge areas Potential dlaeltm'ge a r e a s in the forest
ms
Slnmb mire
ms
O p e n ndre
280
N
260
0
200m
Fig. I. The catchment selected for the study (the location is indicated in the superimposed map of Sweden). Along the perennial stream there are two sites for sampling of stream water (0), Weir E at the head of the stream, Weir A at the end of the catchment and one location for sampling of groundwater (0).
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determined by Bishop (1992) was used to calculate the discharge at the sampling times. Stream water was sampled manually from the weirs in polythene bottles and stored at +4°C. During April-May 1986 stream water was sampled twice a day in 50 ml bottles at Weir A, at approximately 12:00 h and 20:00 h. This sampling was originally done for other purposes (this is the reason for the small volume) but was included in this study. Larger samples of 1 1 were taken with less frequency during the period 1986-1992 (Weirs A and E). The groundwater samples, taken during 1986 and 1994 from a tube with inlet 3 - 4 m below the soil surface (Fig. 1, filled circle), were buffered to pH 3 with a HCl-citrate buffer and preconcentrated from 5 1 to 1 1. This was also the case for stream water taken during 1993. The activity concenWation in groundwater was in all cases below the detection limit (Ld = 0.01 Bq l-l). Soil was sampled during June 1989 by means of special metal augers, down to about 15 cm in the mineral soil and 35 cm in peat, dried at 70°C for 3-5 days to constant weight, and homogenised in a mill. Each observation reported on softs is a composite of five subsamples within 100 m 2. 2.3. Sample preparation and measurements
The 50 ml bottles were washed in 0.1 M HCI and deionised water and analysed without further preparation. Water from the 11 bottles was analysed in 1 1 Marinelli beakers. The beakers were calibrated for 137Csand 134Cswith standard solutions. High-resolution gamma spectrometry was used for detection of 137Cs and 134Cs with either a GeLi (Ptg, 32% efficiency, FWHM = 1.6 keV at 662 keV, 50 ml geometry, detection limit (Lochamy, 1976) Ld = 1.8 Bq 1-I 134Csand 2 Bq 1-I 137Cs) or HpG¢ (ORTEC, Oak Ridge, Tennessee, USA, 55% efficiency, FWHM = 1.5 keV, Marinelli beakers, detection limit Ld = 0.048 Bq 1-1 m34Csand 0.038 Bq 1-I 137Cs) detector type. The equipment was intercalibrated with other radiometry laboratories during the study, as reported by Holm (1994). All reported values of 137Csand 134Cs were corrected for physical decay to the day of the main fallout (29 April 1986). 2.4. Calculations
Part of the deposition of 137Cs originates from the pre-Chernobyl fallout. The radioactive nuclides ejected to the atmosphere during the testing of nuclear weapons resulted in a deposition of 137Cs,mainly during the 1960s. Bergman et al. (1991) and Vintersved et al. (1991) estimated the pre-Chernobyl fallout in the northern parts of the boreal zone to a range of 1.2-1.5 Bq m -2, corrected for physical decay to April 1986. These estimations were based on the latitude effect on deposition of fission products described by UNSCEAR (1977). However, several investigations, such as that by Small (1960), have shown that precipitation accounted for the main fraction of the deposition of long-lived fission products from the nuclear weapons tests and that local variations in annual rainfall accounted for a local and mesoscale variation in nuclear fallout deposition. Lrw and Edvarson (1960) demonstrated this as a strong positive correlation between deposition and annual precipitation at the studied latitudes (60° and 63°N). This must have influenced the cumulative deposition of pre-Chemobyl m37Cspositively in the studied catchment, owing to the rather high mean annual precipitation of 710 ram, and thus the earlier estimations for the region might not be valid for the studied catchment.
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
343
Table 1 The measured ratio of m37Csto a~Cs in the Chernobyl fallout corrected for physical decay to 29 April 1986 Sample
L~TCs/LUCs
SD
n
Reference
Snow
1.72 i.65 1.64 1.71 1.72 1.7 1.67 1.7
0.06 0.08 0.09 0.01 0.01 NE 0.02 0.1
3 13 7 1 1 NE I1 37
This investigation This investigation This investigation This investigation a Vintersved et al. (1991) Edvarson ( 199 I) b
Wet deposition
Lichen Soil (3.5 MBq m -2) Soil (0.2 MBq m -2) Gamma spectrometry in situ Air filter Average ratio
• Froma local hot spOt 200 km fromthe catchment. bFrom air filter measurements 60 km from the catchment at 29 April 1986 (L.-E. de Geer, personal communication, 1990). NE, Not evaluable. The annual deposition of 'old' 137Csby the time of the Chernobyl accident was, according to De Geer et al. (1987), no longer of significance. Persson (1970) found that the ratio of 134Cs to 137Cs originating from the nuclear fallout during the period 1950-1960 amounted to 0.02 in 1961. Thus the 134Csisotope originating from the Chernobyl accident was used to unfold the fraction of 'old' 137Csoriginating from the nuclear global fallout. The caesium activity ratio in the Chernobyl fallout was determined in rainwater and air filters collected during the fallout at the National Defence Research Institute in Ume~ on 29 April 1986, and in snow samples collected from the catchment on 30 April 1986. These ratios were compared with other measurements of the ratio in northern Sweden (Table 1). The average ratio from this table, JO = 1.7 - 0.1 (SD) or _+0.02 (SE) (corrected for physical decay to 29 April 1986), was used to unfold the fraction of 13~Cs originating from the nuclear global fallout ('old 137Cs') from the fraction originating from the Chernobyl fallout ('Chern mCs') in soil samples: 'old 137Cs' ----1 3 7 C s - (134Cs xfO)
(la)
'Chern 137Cs'= 134Cs x f 0
(lb)
Parameters to fit a curve to the time development of ~37Cs in the stream water was calculated by non-linear regressions. Except for two components, which each fitted an exponential decrease of ~37Cs activity concentration in stream water at Weir A with time, runoff influenced the activity concentration positively at higher runoff rates. The best fit to the increase in activity as a function of runoff was obtained by regressions between the natural logarithm of the activity concentration of 137Cs, In (Bq l-I), and runoff (1 s-J). The influence of time and runoff on the activity concentration in the stream was solved by a non-linear expression:
A(t) =Afast x exp[ - t(~ - fast) + kfast x runoff] +Aslow x exp[-t(/~ -- slow) + kslow x runoff]
(2)
where t is elapsed time from 29 April 1986 (days), A(t) is the activity concentration of
344
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354 2000 1500 -
c
1000 r |
500
20
b
15
"7,
10
~.~ ~ . ~
g 5o 00
10
20
30
Time (days) Fig. 2. (a) Observed runoff (line; symbols indicate water sampling), (b) activity concentration (0) and (c) loss of ~37Csin stream water (0), during the snow melt of 1986 in a coniferous forest catchment in northern Sweden. Time indicates elapsed time (days) after the Chernobyl fallout (29 April 1986). The uncertainty (I SD) is also indicated. The curves in (b) and (c) were calculated from the mathematical model described in the text (eqn (2)) and observed daily runoff.
137Cs at time t (Bq 1-~), Afast and Aslow are the fast and slow component of A at time t = 0 (Bq l-I),/3-fast and/3-slow are the decrease rate for the fast and slow component of A (day-t), runoff is stream runoff at Weir A (0.1-90 1 s -l) (1 s-I), and kfast and kslow are constant for exponential influence of runoff during the fast and slow component (s l-l). The ecological half-lives, Tirz, were calculated using values of X or/3 in Eq. (3). The physical decay of 137Cso(Tfys = 30.06 years) was included in the effective ecological halflives, Tlraff (Eq. (3)) (Ahman and Ahman, 1994):
TI/2=
In 2 (X or/~)
1
1
Ti/2eff
Tl/2
(3a)
+
1
(3b)
Tfys
The discharge of 137Cs from the catchment was calculated from Eq. (2) using recorded runoff (1 s -I) at Weir A or by multiplying the measured activity concentration by the corresponding runoff at Weirs A and E. The discharge of ~3~Cs from Subcatchment A - E was calculated by subtracting the discharge of m3~Cs at Weir E from that at Weir A. The monthly contribution to discharge was studied by relating the discharge of 13~Cs during each month to the annual discharge of 137Cs. The relative loss of 137Cs from the catchment, X-fast and X-slow, was calculated by assuming exponential loss
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354
100.0~0
.
.
.
.
.
.
345
.
lO.O0 •
¢,1
1.00
o.lo 0.01 0.00
o ' 1.00
2.00
I 3.00
, 4.00
I S.00
, 6.00
L 7.00
8.00
Time after fallout (year) Fig. 3. Activity concentration of ~37Cs in stream water at Weir A as calculated from the mathematical model described in the text (curve). e , Observed activity concentrations during the first month; O, observed activity concentrations for the rest of the period. The uncertainty (I SD) is in all cases less than 20% of the mean value.
of radioactive caesium over time (years) from the catchment or a specified discharge al'~a:
h= (Dep,)/t
(Dep~)"
(4)
where Dep~ is the remaining deposition at the beginning of the considered period and Dep, is the remaining deposition by the end of the considered period The uncertainties associated with the calculations were described as standard deviations of the mean values (SD), calculated from the error propagation formula (e.g. Knoll, 1989). The parameter values for the equations and statistics were solved with the SYSTAT (1992) software.
3. Results
3.1. Stream w a t e r
Water samples (50 ml) from the week before the Chernobyl accident to 08:00 h, 29 April, were below the detection limit for ]3VCsand m34Cs(Ld = 2 Bq l-m). The initially recorded concentration of m37Csoriginating from the Chernobyl accident in the stream was 20 +_ 4 Bq 1-~ (n = 1) at 16:00 h, 29 April. The activity concentration subsequently declined until the snow melt started. On 5 May, the day of spring flood maximum, a new peak was detected containing 21 _+ 6 Bq 1-~ of ]37Cs (n = 1) (Fig. 2(a) and Fig. 2(b)). After this second peak, the content of radioactive caesium decreased rapidly until June, when it had reached levels of activity that were less than 0.5 Bq l-k A semi-logarithmic plot of the activity concentration against time is given in Fig. 3. The apparently linear relationship for the steeply declining concentration during the first 30 days and the trend in the data for the rest of the studied period indicate that a model
346
T. Nylin, H. Grip~Journal o f Hydrology 192 (1997) 338-354
Table 2 Parameter values for the linear regression In (137Cs) = m + kQ between the natural logarithm of 13~Csin the stream (Bq I-') and runoff (Q, I s-') at various times after the Cbernobyl fallout Time after the Chernobyl fallout (years)
m
0.00-0.08 2.7-4.7 4.7-6.7 6.7-7.7
1.16 -2.4 -2.9 -3.5
k
r2
0.011 0.016 0.022 0.026
0.1 0.3 0.4 0.7
P
< < < <
0.001 0.001 0.001 0.05
n
Q range
34 43 6 5
6-92 1.5-72 0.5-40 1.5-39
with two components may be adequate. The influence of runoff intensity on activity concentration of 137Cs was best described by a linear regression between the logarithm of the activity concentration (Bq 1-I) and runoff (1 s -I) (Table 2). The studied period was, in Table 2, subdivided into five periods to minimise the influence of the decreasing activity concentration with time. In all cases, this regression was significant at the P < 0.1 level. The influence of time and runoff on activity concentration in the stream was solved in Eq. (2), which gave the parameters shown in Table 3 and predicted activity concentrations in Fig. 2(b) and Fig. 3 (continuous lines). The ecological half-life Tit2 (Eq. (3) and Table 3) for the activity concentration during the first month was 6.5 days. This fast decrease in activity concentration was probably caused by the effective sorption of 13~Cs on mosses and soil, shown by B unzl et al. (1988), together with the decreased runoff from superficial aquifers (Degermark, 1987-1994). The second phase gave an ecological half-life of 4 years for activity concentration in the stream, which is in the same range as the 2.99.7 years reported by Saxtn (1994) for the period 1989-1992 in Finnish surface waters and the ecological half-life of 3 years for the inlet to lake Sitlgsjtn in central Sweden during 1987-1990 found by Sundblad et al. (1991). In Fig. 2(c) the calculated and observed daily caesium discharge during the first month after the Cbernobyl fallout are compared. The cumulative discharge during this period was 0.673 _.+ 0.13 (SD) GBq and contributed as much as 80% of the total loss of 0.845 _+ 0.17 (SD)GBq during the period studied (1986-1993). The initial discharge of 137Cs during the period 29 April-31 May 1986 (Fig. 2(c)) corresponds to a uniform loss of 1.35 _ 0.270 kBq m -2 from the catchment, or 6.8 - 3% of the deposition in the catchment (19.8 kBq m -2, Table 4). This is equivalent to a total washout of 137Cs within a hypothetical 10 m leakage zone lining the sides of the two main streams. The slow component of 13~Csdischarge from the catchment was calculated (Eq. (4) and Table 3 Parameter values and statistics for Eq. (2), describing the development of activity concentration in the stream during the first 8 years after the Chernobyl fallout event Component
A
~
k
r2
Fast Slow
18 0.I6
0. I I 0.00049
0.0033 0.019
0.93 0.52
P < 0.0001 < 0.0001
n 34 70
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354
347
Table 4 The average remaining deposition of radioactive caesium on 19 June 1989 in different types of environments; each observation (n) is a composite of five subsamples Biotope
n
Area (ha)
t37Cs (kBq m -2)
I~Cs (kBq m -2)
Old t37Cs (kBq m -2)
Chemobyl "~Cs (kBq m -2)
Chernobyi "~TCs(GBq) (total)
Rech. forest Disch. forest Shrub mire Open mire
20 13 12 11
37.5 4,5 6.8 !.2
22.7 (9) 17.6 (7) 10.4 (7) 4.79 (3)
11.7 (4) 10.0 (4) 6.0 (3) 2.6 (0.2)
2.84 (2) 0.52 (I) 0.16 (0.3) 0.37 (0.3)
19.8 (9) 17.1 (7) 10.3 (7) 4.4 (3)
7.42 (3) 0.77 (0.3) 0.70 (0.5) 0.053 (0.04)
Numbers in parentheses are standard deviations.
runoff data) for two periods. The first period was from June 1986 to June 1989 (3.06 years), when the soil samples were taken. During this period, 0.117 +_- 0.02 GBq was discharged from the catchment. The second period, from June 1989 to December 1993 (4.5 years), generated a 137Csdischarge of 0.055 _+ 0.01 GBq from the catchment. The low secondary loss corresponds to a uniform loss of 0.23 - 0.04 kBq m -2, or 1.2%, from the catchment during June 1986-June 1989 (3.06 years) and 0.11 -+ 0.02 kBq m -2, or 0.6%, during the rest of the period (4.5 years).
3.2. Seasonal contribution to the discharge of 137Csfrom the catchment The monthly contribution to the annual discharge of 137Cs from the catchment during 1986-1993 is presented in Fig. 4. It can be seen in the figure that, except for 1986, snow melt at the end of April and beginning of May contributes about 50% of the annual discharge of 137Cs. One exception is the year 1987, which almost lacked a spring flood and therefore had only a minor loss of 137Csduring spring. Except for the generally clear dominance of contributions during spring, considerable discharge of 137Cs also occurs during autumn. In December-March, when the surface soil is frozen and precipitation is intercepted in the catchment as snow, runoff is low, and thus the discharge of m37Csreaches its minimum.
3.3. Discharge of 137Cs f r o m the subcatchments The contribution to discharge of 137Cs from the subcatchments above Weir E and between Weir E and Weir A was studied during June-December 1986, and in May and September 1993, as described above. The results show that the activity concentration of 137Cs at Weir E (head of the stream) was higher than at Weir A, except during late autumn, when the two weirs gave similar values (Fig. 5(a)). Fig. 5(b) shows the discharge of 137Cs (Bq s -l) from the catchment (Weir A, white bars), Subcatchment E (Weir E, grey bars) and Subcatchment A - E (by subtracting the discharge of 137Csat Weir A from that at Weir E; black bars). The discharge of ~37Cs from Subcatchment E was high in June 1986, low during the dry period of July, elevated during autumn, and again decreasing at the beginning of the winter season in October-December. in 1993, the discharge of 137Cs was
348
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
~oo
i!
•s
19s6
3O 25
8O
3oi 15'
20 o
,
I0' $.
,
0..,-.~
,
Month
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~i
1991
1988
60 • 50.
1992
15' lO" $. O'
~0'
70
40. 40. 30, 20. I0 0
SO.
3O 2O .I.
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25, 3O 2O
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Fig. 4. The monthly contribution to the annual loss of °3~Csfrom the catchment calculated from Eq. (2) during 8 years after the Chernobyl fallout event. higher during the spring flow peak (13 1 s -I, at W e i r E) than during a rainstorm event in October (4 1 s -I, at Weir E). The discharge o f 137Cs from the forested Subcatchment A - E showed the same pattern d u r i n g s u m m e r 1986 but decreased to negative values during A u g u s t - O c t o b e r , whereupon it was elevated to approximately the same value as for
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
349
1.2
i
1 °
|
0.8 -
r,. 0.6
I
•
II
D
li
li
I
I
I
0.4
0.2
a I
ilii
a !
a I
B I
B I
i !
[] I
I
3.5
2
1, r.I
0 -0.5 -1
I< I
..
T
,i,,.,_.,,.,....Pl.,.I,r I,rl,ru,. -= I i! '" Ji|||||lllii
Date
Fig. 5. (a) Activity concentrationof 137Csin stream, at the top of the perennial stream (Weir E, m) and at the end of the catchment (Weir A, El). The uncertainty (1 $D) is also indicated. (b) The discharge of 13~Cs(Bq s -s) from the total catchment (Weir A, white bars), $ubcatchment E (Weir E, grey bars) and Subcatchment A-E (by subtracting the dischargeof L~TCsat Weir E from the correspondingvalue at Weir A, black bars). The uncertainty (l $D) is also indicated. Subcatchment E. The negative values during the periods August-September 1986, and May and September 1993 mean that a fraction of the activity that passed Weir E was intercepted in the stream between the two weirs. During 1986, the observed amount of 137Cs intercepted in the stream bank was nearly the same as the loss of 137Cs from Subcatchment A - E during the following autumn (Fig. 5(b)). It is thus possible that the origin of ~37Cs discharge from the forested Subeatchment A - E was loss from the stream bank. High activity of t37Cs in stream sediments was observed in an alpine catchment by Hongve et al. (1994), and might constitute a temporal sink for a fraction of the loss o f 137Cs from catchments.
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3.4. Remaining deposition in soil It is likely that the main contribution to loss of radioactive caesium during the early phase took place in the saturated discharge areas, which then should be relatively depleted of the initial deposition in comparison with the recharge areas by the time of the soil sampling. The remaining activity in soil during June 1989, was analysed in four categories (Fig. 1). The results of average calculated deposition density of 'Chernobyr and 'old' caesium in the different types of environments are presented in Table 4. The remaining deposition is in both cases highest in the recharge areas. A test of variance (Tukey's test) gives the relation between the content of 'old' m37Csin the biotopes (ascending order): 'shrub mire' = 'open mire' = 'disch. forest' < 'rech. forest'(P = 0.001); for the fraction of Chernobyl deposition: 'open mire' < 'shrub mire'(P = 0.001) < 'disch. forest'(P = 0.05) = 'rech. forest'. The calculated content of ~34Cs in the recharge areas (11.7 kBq m -2, Table 4) is in the same range as found by Arntsing et al. (1991) for the region. This, together with the lower remaining activity on the mire, suggests that the loss of the Chernobyl fraction of 137Cs mainly originates from the mire. The remaining deposition on the mire is lowest in the open fraction, which indicates that the 137Cs discharge during 1986-1989 was more effective from this fraction than from the shrub type of the mire.
4. Discussion and conclusions The fraction of the total Chernobyl deposition of 137Cs in the catchment lost via runoff was 7 -+ 3%. It is not reasonable that the different parts of the catchment contributed equally to the loss of radioactive caesium during spring 1986. The mire, which constitutes the main fraction of the discharge areas, was saturated by water during the weeks after the fallout, and thus generated a loss of radioactive caesium via melt water runoff. Potential discharge areas in the coniferous forest lining the stream, on the other hand, probably only gave a minor contribution to the discharge of radioactive caesium via melt water, as a large fraction of the wet deposited fallout intercepted in the forests still was held in the tree canopies (Nyltn and Grip, 1989; Bunzl et al., 1989; Sombre et al., 1990; Melin and Wallberg, 1991). During the following years, the main fraction of fallout intercepted in tree canopies was transferred to the forest floor (Melin and Wallberg, 1991). The hypothesis is thus that the origin of the observed initial activity in the stream was the mire. As the deposition appears to have been uniform over the whole catchment, the sum of the discharge of J37Cs from the catchment and the remaining deposition in 1989 on the mire should give about the same mean deposition (kBq m -2) on the mire as on the recharge areas. The fraction of Chernobyl ~37Csresiding in the mire during 1989 (Table 4) added to the cumulative loss from April 1986 to June 1989 (Eq. (2)) was 1.54 -+ 0.3 GBq, or 19.3 _ 4 kBq m -2 if evenly distributed on the mire, which is about the same as the calculated Chernobyl deposition in recharge areas, 19.8 -+ 9 kBq m -2. This corroborates well with the conditions predicted from the hypothesis and thus is a strong indication that the origin of 137Cs loss during spring 1986 was the mire. The amount of caesium that discharged during
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351
60
¢I
50
50
40
40
30
30
20
dg
10 00
.-q
10
"% ' oi
10 20 30 4 0, 50, 60 70, 8 0 90, 100 Runoff 0/s)
Fig: 6. The relation between the fraction of discharge areas in the catchment and the maximum discharge intensity for different storm events during 1981-1982. @, Snow melt events during spring; O, rainfall events. The distribution of maximum runoff intensities during the study is indicated by the continuous line. Data are from Rodbe (1987) and Degermark (1987-1994).
the first month (0.67 +_ 0.13 GBq or 8.4 _ 1.7 kBq m -2 from the mire) subtracted from a deposition of 19.3 kBq m -2 on the 8 ha mire then gives an initial loss of 44 _ 11% from the mire. As the whole mire contributed to the snowmelt runoff in 1986 the areal distribution of the remaining 56% was probably uniform. The different observations of the dynamic behaviour of 137Csdischarge from the studied catchment (Fig. 4), together with the remaining deposition in the different biotopes (Table 4) and earlier investigations on the origin of stream water, suggest that the fraction of the catchment contributing to discharge of ~37Cs is dynamically changing during the year: 1. the fraction of the studied catchment involved in saturated surface discharge was investigated by Rodhe (1987), who found that during nine rain and snowmelt events, between 0.4 and 33% of the studied catchment contributed to saturated surface discharge and that the fraction of discharge area was positively correlated with the maximum stream flow during the runoff event (Fig. 6, circles). A frequency plot of the maximum stream flow of runoff events during the studied period (Fig. 6, histogram) shows a clear dominance of low values. Fig. 6 thus shows that only small fractions of the potential discharge areas are frequently involved in saturated surface discharge. Regarding this, it is likely that the main part of the leached ~37Csoriginates from the 'open mire', which, except during winter when the soil surface is frozen, constantly undergoes saturated surface discharge. This discussion also agrees with the seasonal behaviour of 137Cs discharge from the catchment in Fig. 4. 2. In Table 2 the intercept of the natural logarithm of the activity concentration in the stream decreases with time, which indicates a decreasing pool of J37Cs in the fraction of the discharge area that is engaged even at low rates of runoff. The slope, on the other hand, increases with time, indicating that the pool of 137Cs that discharges at higher rates of runoff is not as affected by time. Further, in Table 4 the remaining deposition (kBq m -2) during 1989 is lower in the 'open mire' than in the 'shrub mire', which also
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agrees with a more effective loss from the wetter fraction of the mire during the phase of slow leakage. The conditions described above lead to a second hypothesis on the discharge of 137Cs from the mire: the loss of 137Cs from the mire is controlled by the dynamically changing discharge areas. Areas that frequently undergo saturated surface discharge hence will empty their pool of n37cs more rapidly than areas that seldom are engaged. As the 'open' mire is constantly wet, the loss should be greater from this fraction than from the 'shrub' mire. This hypothesis was tested by using (1) the calculated remaining deposition on the mire after the spring flood in 1986 (June 1986) and the measured deposition 3 years later in June 1989, and (2) the calculated discharge of 137Cs at Weir A during the 3 years, as follows: 1. if the hypothesis is correct, the total deposition on the mire during 1989 subtracted from the corresponding deposition in 1986 should be equal to the discharged 137Csduring the elapsed time 3.06 years. From Table 4, the loss of 137Cs from the two fractions of mire was calculated. A comparison with the corresponding loss calculated from Eq. (4) and recordings of runoff was also made: a hypothetical uniform distribution of the 56 - 20% remaining on the mire after the initial snow melt in 1986 gives a uniform deposition of 11 __ 2 kBq m -2, or 0.88 + 0.2 GBq. The remaining activity in 1989 was 0.7 __ 0.5 GBq on the shrub mire and 0.053 _ 0.04 GBq on the open mire (Table 4), i.e. 0.75 _ 0.5 GBq in weighted total, The discharge during the elapsed time should therefore be 0.13 ± 0.6 GBq (0.75 _ 0.2 GBq subtracted from 0.88 +-- 0.5 GBq). 2. The amount of 137Csdischarged via Weir A, calculated from Eq. (4) and recordings of runoff for the 3.06 years was 0.12 ± 0.05 GBq, which is very close to the estimate above. The two ways of calculating the loss of 137Csfrom the mire agree well, which supports the hypothesis that the loss of 137Cs is controlled by the dynamically changing discharge areas on the mire. The decrease in remaining deposition during the 3.06 years gives an annual uniform loss of 30 +_-7% for the open mire and 2.1 ___0.4% for the shrub mire (Eq. (4)). The effective ecological half-life for 137Csis then about 2.3 years in the 'open mire', and about 16 years in the 'shrub mire', corresponding to ecological half-lives of 2.5 years and 34 years, respectively, for stable caesium introduced to the mire by wet deposition. Further, the exponential decline in activity concentration in the stream is, until the end of this study, to a large extent explained by a decreasing pool of 137Csin the open fraction of the mire. The loss of ~37Csfrom discharge and recharge areas in the forest must be orders of magnitude lower than from the 'open' mire, as there was no way of identifying any loss of Chernobyl m37Cs from these areas with the methods used. Nevertheless, there is an indication of a future discharge of radioactive caesium from the forested discharge areas in Table 4 where the remaining activity of 'old' 13~Csis in the same range as in the mire. The vertical percolation Of 137Csin recharge areas also seems to be very low. If the low activity concentration of 0.01 Bq 1-I (the limit of detection for the measurements in the groundwater tube) constitutes the upper limit for the concentration in the column of groundwater in the catchment, then the loss from the recharge areas during the 8 years is at most 30 Bq m -2. This corresponds to an annual loss of less than 0.02% of the deposition on
T. NylOn, H. Grip/Journal of Hydrology 192 (1997) 338-354
353
the recharge areas during the 8 years since the Chernobyl accident. Under these assumptions, the effective half-life for 137Csin the recharge areas is 30 years, corresponding to an ecological half-life exceeding 4200 years for stable caesium introduced by wet deposition.
Acknowledgements This work has been supported by the National Radiation Protection Institute, the Centre for Ecological Research in Ume~t and the CEC (Radiation Protection Programme). We would also like to thank Dr. Ronny Bergman for helpful criticism, and Elon Manfredsson and Kenneth Lidstrtm for help with sampling and analyses.
References Am-krog, A., 1994. Doses from the Chemobyl accident to the Nordic populations via diet intake. In: H. Dahlgaard (Editor), Nordic Radioecology: The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 433-456. ~dlman, B. and Ahman, G., 1994. Radiocesium in Swedish reindeer after the Chemobyl fallout: seasonal variations and long-term decline. Health Phys., 66(5): 503-512. Arntsing, R., Bjurman, B., de Geer, L-E., Edvardsson, K., Finck, R., Jakobsson, S. and Vintersved, I., 1991. Field gamma ray spectrometry and soil sample measurements in Sweden following the Chernobyl accident. A data report. FOA Rep. D 20177-4.3, Stockholm. Bergman, R., 1993. Products from forest and lake: food resource or exposure problem, after radioactive fallout. FOA Pep. C 40315-4.3, Umel. Bergman, R., 1994. The distribution of radioactive caesium in boreal forest ecosystems. In: H. Dahigaard (Editor), Nordic Radioecology. The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 335-379. Bergman, R., NylOn, T., Palu, T. and Lidstrtm, K., 1991. The behaviour of radioactive caesium in a boreal forest ecosystem. In: L. Moberg (Editor), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 425-456. Bergman, R., NylOn, T., Nelin, P. and Palo, T., 1993. Caesium-137 in a boreal forest ecosystem: aspects on the long time behaviour. FOA Rep. C 40284-4.3, Umel. Bishop, K., 1992. Episodic increases in stream acidity, catchment runoff pathways and hydrograph separation. PhD Thesis, University of Cambridge. Bonnett, P.J.P. and Appelby, P.G., 1994. Rates of removal of sediment associated radiocaesium from the Plynlimon experimental catchments, Powys, UK. Environ. Pollut., 83: 327-334. Bunzl, K., Schimmack, W., Kreutzer, K. and Schierl, R., 1988. The migration of fallout Cs- 134, Cs- 137 and Ru106 from Chemobyl and of Cs-137 from weapons testing in a forest soil. Z. Pflanzenntihr. Bodenkd., 152: 3944. Bunzl, K., Schimmack, W., Kreutzer, K. and Schierl, R., 1989. Interception and retention of Chemobyl derived Cs-134, Cs-137 and Ru-106 in a spruce stand. Sci Total Environ., 78: 77-87. Carboi, P., 1993. Speciation and transport of radionu¢lides from the Chernobyl accident within the Gide~ Site. PhD Thesis, CTH, Gtteborg. Davis, R.B., Hess, C.T., Norton, S.A., Hanson, D.W., Hoagland, K.D. and Anderson, D.S., 1984. Cs-137 and Pb2 i 0 dating of sediments from soft water lakes in New England and Scandinavia, a failure of Cs- 137 dating. Chem. Geol., 44: 151-185. De Oeer, L.-E., Arutsing, R., Vintersved, I., Sisefky, J., Jakobsson, S. and Engstrtm, J-A., 1987. Particulate radioactivity, mainly from nuclear explosions in air and precipitation in Sweden mid-year 1975 to mid-year 1977. FOA Pep. C 40089-T2, Stockholm.
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Degerrnark, C. (Editor), 1987-1994. Climate and Chemistry of Water at Svartherget, Reference Measurements 1986-1993. Swedish University of Agricultural Sciences, Ume~. Edvarson, K., 1991. Fallout over Sweden from the Chernobyl accident. In: L. Moberg (Editor), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 47-66. FAO-UNESCO, 1981. Soil Map of the World, Vol. V, Europe. UNESCO, Paris, pp. 172-173. HeRon, J.C., Muller, A.B. and Bayer, A., 1985. Contamination of surface-water bodies after reactor accidents by the erosion of atmospherically deposited radionuclides. Health Phys., 48:757-771. Hilton, J. and Spezzano, P., 1994. An investigation of possible processes of radiocaesium release from organic upland soils to water bodies. Water Res., 28: 975-983. Hilton, J., Livens, F.R., Spezzano, P. and Leonard, D.R.P., 1993. Retention of radioactive caesium by different soils in the catchment of a small lake. Sci. Total Environ., 129: 253-266. Holm, E., 1994. Intercalibration of gamma-spectrometric equipment. In: H. Dahlgaard (Editor), Nordic Radioecology. The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 335-379. Hongve, D., Blakar, I.A. and Brittain, J., 1994. Radiocaesium in the sediments of Ovre Heimdalsvatn, a Norwegian subalpine lake. J. Environ. Radioact., 26. Knoll, G.F., 1989. Radiation Detection and Measurement. Wiley, New York. Lochamy, J.C., 1976. The minimum detectable activity concept. NBS SP456. National Bureau of Standards, Gaithersburg. Ltw, K. and Edvarson, K., 1960. Content of caesium-137 and (zirconium + niobium)-95 in Swedish soils. Nature, 187(4739): 736-738. Malmstrtm" C. and Tamm" O., 1926. The experimental forests of Kulblicksliden and Svartberget in North Sweden. I. Tana-n, O.: Geology, II. MalmstriSm"C.: Vegetation. Skogsf'Orstksanstaitens exkursionsledare XI, Uppsala. Melin, J. and Wallberg, L., 1991. Distribution and retention of caesium in Swedish boreal forest ecosystems. In: L. Moberg (Editor), The Chemohyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 467-476. Nyltn, T. and Grip, H., 1989. Transport of caesium-137 in a forest catchment. In: W. Feldt (Editor), The Radioecoiogy of Natural and Artificial Radionuclides. Fachverband fur Strahlenschutz e. V., Ktln, pp. 221-226. Persson, R.B.R., 1970. Fe-55, Sr-90, Cs-134 and Pb-210 in the biosphere. Radiological health aspects of the ecological contamination from radioactive material in northern Sweden. PhD Thesis, University of Lund. Rodhe, A., 1987. The origin of streamwater traced by oxygen-18. PhD Thesis, Uppsala University, Rep. Ser. A, No. 41. Saxtn, R., 1994. Transport of Cs-137 in large Finnish drainage basins. In: H. Dahlgaard (Editor), Nordic Radioecology. The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 63-78. Shand, C.A., Cheshire, V.M., Smith, S., Vidal, M. and Rauret, G., 1994. Distribution of radiocaesium in organic soils. J. Environ. Radioact., 23: 285-302. Small, S., 1960. Wet and dry deposition of fallout materials at Kjeller. Tellus, 12: 308-314. Smith, J.T., Hilton, J. and Comans, R.N.J., 1995. Application of two simple models to the transport of Cs-137 in an upland organic catchment. Sci. Total Environ., 168: 57-61. Sombre, L., Vanhouche, M., Thiry, Y., Ronneau, C., Lambotte, J.M. and Myttenaere, C., 1990. Transfer of radiocaesium in forest ecosystems resulting from a nuclear accident. In: E. Desmet, P. Nassimbeni and M. Belli (Editors), Transfer of Radionuclides in Natural and Semi-Natural Environments. Elsevier, Amsterdam. Sundblad, B., BergstrOm" U. and Evans, S., 1991. Long term transfer of fallout nuclides from the terrestrial to the aquatic environment. In: L. Moberg (Editor), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 207-238. SYSTAT, 1992. SYSTAT for Windows: Statistics, Version 5 edn. SYSTAT, Inc., Evanston, IL. Tobler, L., Bajo, S. and Wyttenbach, A., 1988. Deposition of 134,137Cs from Chernobyl fallout on Norway spruce and forest soil and its incorporation into spruce twigs. J. Environ. Radioact., 6: 225-245. UNSCEAR, 1977. Sources and Effects of Ionizing Radiation. UN Scientific Committee on the Effects of Atomic Radiation 1977. Report to the General Assembly, United Nations, New York. Vintersved, I., Arntsing, R., Bjurman, B., de Geer, L.-E. and Jakobsson, S., 1991. Resuspension of radioactive caesium from the Chemobyl accident. In: L. Moberg (Editor), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 85-106.
Journal
of Hydrology
ELSEVIER
Journal of Hydrology 192 (1997) 338-354
The origin and dynamics of 137Cs discharge from a coniferous forest catchment T o r b j 6 r n N y l 6 n a'*, H a r a l d G r i p b INational Defence Research Establishment, NBC Research Department, S-901 82 Ume,~,Sweden bSwedish Universityof Agricultural Sciences, Department of Forest Ecology, S-901 83 Ume~, Sweden Received 14 September 1995; revised 26 January 1996; accepted 2 February 1996
Abstract The turnover of radioactive caesium was studied experimentally and theoretically in a forested catchment that was covered by snow during the wet deposition of radioactive nuclides from the Chernobyl accident. The study spans from 1 week before to 8 years after the deposition event. A fraction of the catchment is covered by a mire (16%). From the edge of the mire a stream channel runs to the outlet of the catchment. Two phases of decreasing activity concentration in the stream water were found in addition to a positive influence of runoff on the activity concentration in the stream. The half-lives for 137Csin the stream water corresponding to the early and the later phase were estimated by non-linear regression to be 6.5 days and 4 years, respectively. During the first phase, which corresponded to the initial snow melt in 1986, 6.8% of the deposition was lost from the catchment, whereas the slow secondary loss during the following 8 years was 1.8%. The main contribution to the yearly discharge of ~37Cs occurred during spring and autumn when the areal contribution to saturated surface runoff was highest. The remaining deposition in soil a few years after the fallout was significantly lower in the mire than in the surrounding forest. By using the calculated activity concentration of mCs in the stream water together with the remaining deposition in the different biotopes and information on stream flow for the catchment it was concluded that the loss originated from the mire. During the initial phase 44% of the deposition was lost from the mire, and during the following years the yearly loss was 30% from the fraction that constantly undergoes saturated surface runoff and 2% from the drier fractions of the mire. Until the end of the study it was not possible to demonstrate any loss from the recharge areas (podzol and cambisol soils), which means that physical decay will govern the decrease in activity in these areas. © 1997 Elsevier Science B.V.
* Corresponding author. 0022-1694/97/$17.00 © 1997- Elsevier Science B.V. All rights reserved PII S0022-1694(96)03083- !
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1. Introduction Investigations after the Chernobyl accident have shown that food such as meat, fish, berries and mushrooms from the boreal forests will give the major contribution to the collective dose for the Swedish population, integrated over longer time periods, from internal exposure by radioactive caesium (Bergman, 1993, 1994; Aarkrog, 1994). Bergman et al. (1993) found that the amount of radioactive caesium in this region will decrease mainly by physical decay and loss via runoff. Further, SaxOn (1994) stated that the loss of 137Csvia runoff also must be considered regarding the contribution to rivers and lakes where it might be the most important factor, besides redistribution of lake sediments, for uptake of radioactive caesium in fish in longer time perspectives. Most of the observations reported on the dynamics of radioactive caesium in catchments suggest a rapid washout of between 0.1 and 5% (Davis et al., 1984; Helton et al., 1985) during the first days after fallout. The remaining annual loss will typically be less than a few per mille, as shown by Bonnett and Appelby (1994). This relatively small loss often fits an exponential decrease in activity concentration with time (Helton et al., 1985; Hilton et al., 1993; SaxOn, 1994), which indicates a retardation of the caesium loss from the system, e.g. owing to long-term redistribution in the living biota and abiotic fixation of radioactive caesium in the catchments. However, the processes behind this behaviour are not yet clearly understood. An often quoted reason for the slowly decreasing activity concentration is the intercalation of radioactive caesium in the clay lattice (Carbol, 1993). Also, in organic soils the fixation of caesium seems to increase with time. Shand et al. (1994) suggested that the mechanism for this was intercalation in a low-level content of clay or chemical binding to organic matter in humin. Several investigations have shown that radioactive caesium is found in the uppermost 20 cm of the soil column (Tobler et al., 1988; Carbol, 1993). Smith et al. (1995) found that the solids-aqueous distribution coefficient (Kd) of 137Cs in these shallow horizons was at least one order of magnitude lower in an organic soil of a saturated peat bog than in mineral soils. Hilton et al. (1993) demonstrated that the potential for release of radioactive caesium via runoff was higher in fibrous organic peat than in mineral soils. Hilton and Spezzano (1994) also found that 137Cswas sorbed to the fibrous peat in flooded peat bogs by ion exchange and that it was easily exchanged for calcium. In the studied region, peat soils show a predominance in discharge areas. During periods of saturated surface runoff in discharge areas there will be contact between the leaking water and the topsoil layers (Bishop, 1992) where 137Cs is found. The stormflow events generating saturated surface runoff are not evenly distributed throughout the year; rates of runoff in northern Sweden are low during winter (November-April) and summer (JuneAugust) and attain their maximum during spring and autumn. Consequently, discharge of radioactive caesium from land to rivers and lakes in northern Sweden is likely to be highest during spring and autumn. This study deals with data that span a period from a few days before to 8 years after the accident at Chernobyl. It concerns the initial and long-term loss of radioactive caesium from a coniferous forest catchment in northern Sweden that was covered by snow during the fallout. The aims of the study were: to quantify the fast initial and slow residual discharge of radioactive caesium during and after the fallout; to quantify the seasonal
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contribution to the release of radioactive caesium from the catchment; to demonstrate the influence of runoff on caesium concentration in stream water; to find the origin of 137Cs discharge from the catchment. The hypothesis was that essentially only the discharge areas contribute to the loss of radioactive caesium from the studied catchment.
2. Materials and methods
2.1. Site description The study was carried out in a 50 ha coniferous forest catchment in Northern Sweden (64°15'N, 19°46'E). The catchment is a part of the Vindeln Forest Experimental Station and serves as a base for long-term reference measurements of climate, water balance and chemistry for the Swedish University of Agricultural Sciences. Between 1983 and 1988 the annual precipitation in the area was 710 mm and the mean runoff 330 mm (Bishop, 1992). The catchment is drained by a perennial stream in the eastern part, and an intermittent stream in the western part (Fig. 1). The two streams were straightened and deepened to improve drainage during the 1930s. The potential discharge areas in the catchment can be divided into two groups: a mire, 16% of the catchment, situated in the northern upstream part of the catchment, and the discharge areas scattered in the coniferous forests along the two streams (Fig. 1). The remaining fractions of the catchment, composed of hill slopes and water divides, are identified as recharge areas ('rech. forest' in Table 4, below). With regard to the floristic composition of the field strata, two groups can be identified in the mire (Malmstrtm and Tamm, 1926). Areas characterised by dwarf shrubs and scattered Scots pine (Pinus sylvestris) on a ground stratum of peat mosses (Sphagnum spp.) forming tufts (13.6% of the catchment, described as 'shrub mire' in the text) and areas characterised by cyperaceous plants with a ground stratum of peat mosses, not forming tufts (2.4% of the catchment; 'open mire'). The latter fraction of the mire is in close vicinity to the origin of the perennial stream. The forested parts are dominated by mature Scots pine predominantly on the higher elevated areas on a ground stratum of lichens and mosses (Cladina spp. and Pleurozium spp.). Norway spruce (Picea abies (L) Karst.) forest of the Vaccinium type, predominates on the lower parts of the recharge areas. Scattered in the lower areas of the spruce forest are discharge areas characterised by Sphagnum, Polytricium and Equisetum silvaticum ('rech. forest' in Table 4, below). The soils on the hill slopes and at the water divides are orthic cambisols and on low-lying areas are carbic podzols (FAO-UNESCO, 1981), all developed on glacial till or wave-wash sediments. The mire is composed by peat.
2.2. Sampling methods Runoff was monitored by means of water stage recorders at 90 ° V-notch weirs at Weir A at the outlet of the catchment (50 ha) and Weir E upstream at the beginning of the perennial stream, draining 19 ha of the catchment (Fig. 1, open circles). The area drained by Weir E is called Subcatchment E in the text. The remaining fraction of the catchment is called Subcatchment A - E (31 ha between Weirs E and A), The stage-discharge relationship
7". Nyl6a, H. Grip~Journal of Hydrology 192 (1997) 338-354
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0
S t r e m w a t e r weir
•
G r o u n d w a t e r tube
300
Altitude above s e a level (m)
•
Recharge areas Potential dlaeltm'ge a r e a s in the forest
ms
Slnmb mire
ms
O p e n ndre
280
N
260
0
200m
Fig. I. The catchment selected for the study (the location is indicated in the superimposed map of Sweden). Along the perennial stream there are two sites for sampling of stream water (0), Weir E at the head of the stream, Weir A at the end of the catchment and one location for sampling of groundwater (0).
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determined by Bishop (1992) was used to calculate the discharge at the sampling times. Stream water was sampled manually from the weirs in polythene bottles and stored at +4°C. During April-May 1986 stream water was sampled twice a day in 50 ml bottles at Weir A, at approximately 12:00 h and 20:00 h. This sampling was originally done for other purposes (this is the reason for the small volume) but was included in this study. Larger samples of 1 1 were taken with less frequency during the period 1986-1992 (Weirs A and E). The groundwater samples, taken during 1986 and 1994 from a tube with inlet 3 - 4 m below the soil surface (Fig. 1, filled circle), were buffered to pH 3 with a HCl-citrate buffer and preconcentrated from 5 1 to 1 1. This was also the case for stream water taken during 1993. The activity concenWation in groundwater was in all cases below the detection limit (Ld = 0.01 Bq l-l). Soil was sampled during June 1989 by means of special metal augers, down to about 15 cm in the mineral soil and 35 cm in peat, dried at 70°C for 3-5 days to constant weight, and homogenised in a mill. Each observation reported on softs is a composite of five subsamples within 100 m 2. 2.3. Sample preparation and measurements
The 50 ml bottles were washed in 0.1 M HCI and deionised water and analysed without further preparation. Water from the 11 bottles was analysed in 1 1 Marinelli beakers. The beakers were calibrated for 137Csand 134Cswith standard solutions. High-resolution gamma spectrometry was used for detection of 137Cs and 134Cs with either a GeLi (Ptg, 32% efficiency, FWHM = 1.6 keV at 662 keV, 50 ml geometry, detection limit (Lochamy, 1976) Ld = 1.8 Bq 1-I 134Csand 2 Bq 1-I 137Cs) or HpG¢ (ORTEC, Oak Ridge, Tennessee, USA, 55% efficiency, FWHM = 1.5 keV, Marinelli beakers, detection limit Ld = 0.048 Bq 1-1 m34Csand 0.038 Bq 1-I 137Cs) detector type. The equipment was intercalibrated with other radiometry laboratories during the study, as reported by Holm (1994). All reported values of 137Csand 134Cs were corrected for physical decay to the day of the main fallout (29 April 1986). 2.4. Calculations
Part of the deposition of 137Cs originates from the pre-Chernobyl fallout. The radioactive nuclides ejected to the atmosphere during the testing of nuclear weapons resulted in a deposition of 137Cs,mainly during the 1960s. Bergman et al. (1991) and Vintersved et al. (1991) estimated the pre-Chernobyl fallout in the northern parts of the boreal zone to a range of 1.2-1.5 Bq m -2, corrected for physical decay to April 1986. These estimations were based on the latitude effect on deposition of fission products described by UNSCEAR (1977). However, several investigations, such as that by Small (1960), have shown that precipitation accounted for the main fraction of the deposition of long-lived fission products from the nuclear weapons tests and that local variations in annual rainfall accounted for a local and mesoscale variation in nuclear fallout deposition. Lrw and Edvarson (1960) demonstrated this as a strong positive correlation between deposition and annual precipitation at the studied latitudes (60° and 63°N). This must have influenced the cumulative deposition of pre-Chemobyl m37Cspositively in the studied catchment, owing to the rather high mean annual precipitation of 710 ram, and thus the earlier estimations for the region might not be valid for the studied catchment.
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Table 1 The measured ratio of m37Csto a~Cs in the Chernobyl fallout corrected for physical decay to 29 April 1986 Sample
L~TCs/LUCs
SD
n
Reference
Snow
1.72 i.65 1.64 1.71 1.72 1.7 1.67 1.7
0.06 0.08 0.09 0.01 0.01 NE 0.02 0.1
3 13 7 1 1 NE I1 37
This investigation This investigation This investigation This investigation a Vintersved et al. (1991) Edvarson ( 199 I) b
Wet deposition
Lichen Soil (3.5 MBq m -2) Soil (0.2 MBq m -2) Gamma spectrometry in situ Air filter Average ratio
• Froma local hot spOt 200 km fromthe catchment. bFrom air filter measurements 60 km from the catchment at 29 April 1986 (L.-E. de Geer, personal communication, 1990). NE, Not evaluable. The annual deposition of 'old' 137Csby the time of the Chernobyl accident was, according to De Geer et al. (1987), no longer of significance. Persson (1970) found that the ratio of 134Cs to 137Cs originating from the nuclear fallout during the period 1950-1960 amounted to 0.02 in 1961. Thus the 134Csisotope originating from the Chernobyl accident was used to unfold the fraction of 'old' 137Csoriginating from the nuclear global fallout. The caesium activity ratio in the Chernobyl fallout was determined in rainwater and air filters collected during the fallout at the National Defence Research Institute in Ume~ on 29 April 1986, and in snow samples collected from the catchment on 30 April 1986. These ratios were compared with other measurements of the ratio in northern Sweden (Table 1). The average ratio from this table, JO = 1.7 - 0.1 (SD) or _+0.02 (SE) (corrected for physical decay to 29 April 1986), was used to unfold the fraction of 13~Cs originating from the nuclear global fallout ('old 137Cs') from the fraction originating from the Chernobyl fallout ('Chern mCs') in soil samples: 'old 137Cs' ----1 3 7 C s - (134Cs xfO)
(la)
'Chern 137Cs'= 134Cs x f 0
(lb)
Parameters to fit a curve to the time development of ~37Cs in the stream water was calculated by non-linear regressions. Except for two components, which each fitted an exponential decrease of ~37Cs activity concentration in stream water at Weir A with time, runoff influenced the activity concentration positively at higher runoff rates. The best fit to the increase in activity as a function of runoff was obtained by regressions between the natural logarithm of the activity concentration of 137Cs, In (Bq l-I), and runoff (1 s-J). The influence of time and runoff on the activity concentration in the stream was solved by a non-linear expression:
A(t) =Afast x exp[ - t(~ - fast) + kfast x runoff] +Aslow x exp[-t(/~ -- slow) + kslow x runoff]
(2)
where t is elapsed time from 29 April 1986 (days), A(t) is the activity concentration of
344
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354 2000 1500 -
c
1000 r |
500
20
b
15
"7,
10
~.~ ~ . ~
g 5o 00
10
20
30
Time (days) Fig. 2. (a) Observed runoff (line; symbols indicate water sampling), (b) activity concentration (0) and (c) loss of ~37Csin stream water (0), during the snow melt of 1986 in a coniferous forest catchment in northern Sweden. Time indicates elapsed time (days) after the Chernobyl fallout (29 April 1986). The uncertainty (I SD) is also indicated. The curves in (b) and (c) were calculated from the mathematical model described in the text (eqn (2)) and observed daily runoff.
137Cs at time t (Bq 1-~), Afast and Aslow are the fast and slow component of A at time t = 0 (Bq l-I),/3-fast and/3-slow are the decrease rate for the fast and slow component of A (day-t), runoff is stream runoff at Weir A (0.1-90 1 s -l) (1 s-I), and kfast and kslow are constant for exponential influence of runoff during the fast and slow component (s l-l). The ecological half-lives, Tirz, were calculated using values of X or/3 in Eq. (3). The physical decay of 137Cso(Tfys = 30.06 years) was included in the effective ecological halflives, Tlraff (Eq. (3)) (Ahman and Ahman, 1994):
TI/2=
In 2 (X or/~)
1
1
Ti/2eff
Tl/2
(3a)
+
1
(3b)
Tfys
The discharge of 137Cs from the catchment was calculated from Eq. (2) using recorded runoff (1 s -I) at Weir A or by multiplying the measured activity concentration by the corresponding runoff at Weirs A and E. The discharge of ~3~Cs from Subcatchment A - E was calculated by subtracting the discharge of m3~Cs at Weir E from that at Weir A. The monthly contribution to discharge was studied by relating the discharge of 13~Cs during each month to the annual discharge of 137Cs. The relative loss of 137Cs from the catchment, X-fast and X-slow, was calculated by assuming exponential loss
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354
100.0~0
.
.
.
.
.
.
345
.
lO.O0 •
¢,1
1.00
o.lo 0.01 0.00
o ' 1.00
2.00
I 3.00
, 4.00
I S.00
, 6.00
L 7.00
8.00
Time after fallout (year) Fig. 3. Activity concentration of ~37Cs in stream water at Weir A as calculated from the mathematical model described in the text (curve). e , Observed activity concentrations during the first month; O, observed activity concentrations for the rest of the period. The uncertainty (I SD) is in all cases less than 20% of the mean value.
of radioactive caesium over time (years) from the catchment or a specified discharge al'~a:
h= (Dep,)/t
(Dep~)"
(4)
where Dep~ is the remaining deposition at the beginning of the considered period and Dep, is the remaining deposition by the end of the considered period The uncertainties associated with the calculations were described as standard deviations of the mean values (SD), calculated from the error propagation formula (e.g. Knoll, 1989). The parameter values for the equations and statistics were solved with the SYSTAT (1992) software.
3. Results
3.1. Stream w a t e r
Water samples (50 ml) from the week before the Chernobyl accident to 08:00 h, 29 April, were below the detection limit for ]3VCsand m34Cs(Ld = 2 Bq l-m). The initially recorded concentration of m37Csoriginating from the Chernobyl accident in the stream was 20 +_ 4 Bq 1-~ (n = 1) at 16:00 h, 29 April. The activity concentration subsequently declined until the snow melt started. On 5 May, the day of spring flood maximum, a new peak was detected containing 21 _+ 6 Bq 1-~ of ]37Cs (n = 1) (Fig. 2(a) and Fig. 2(b)). After this second peak, the content of radioactive caesium decreased rapidly until June, when it had reached levels of activity that were less than 0.5 Bq l-k A semi-logarithmic plot of the activity concentration against time is given in Fig. 3. The apparently linear relationship for the steeply declining concentration during the first 30 days and the trend in the data for the rest of the studied period indicate that a model
346
T. Nylin, H. Grip~Journal o f Hydrology 192 (1997) 338-354
Table 2 Parameter values for the linear regression In (137Cs) = m + kQ between the natural logarithm of 13~Csin the stream (Bq I-') and runoff (Q, I s-') at various times after the Cbernobyl fallout Time after the Chernobyl fallout (years)
m
0.00-0.08 2.7-4.7 4.7-6.7 6.7-7.7
1.16 -2.4 -2.9 -3.5
k
r2
0.011 0.016 0.022 0.026
0.1 0.3 0.4 0.7
P
< < < <
0.001 0.001 0.001 0.05
n
Q range
34 43 6 5
6-92 1.5-72 0.5-40 1.5-39
with two components may be adequate. The influence of runoff intensity on activity concentration of 137Cs was best described by a linear regression between the logarithm of the activity concentration (Bq 1-I) and runoff (1 s -I) (Table 2). The studied period was, in Table 2, subdivided into five periods to minimise the influence of the decreasing activity concentration with time. In all cases, this regression was significant at the P < 0.1 level. The influence of time and runoff on activity concentration in the stream was solved in Eq. (2), which gave the parameters shown in Table 3 and predicted activity concentrations in Fig. 2(b) and Fig. 3 (continuous lines). The ecological half-life Tit2 (Eq. (3) and Table 3) for the activity concentration during the first month was 6.5 days. This fast decrease in activity concentration was probably caused by the effective sorption of 13~Cs on mosses and soil, shown by B unzl et al. (1988), together with the decreased runoff from superficial aquifers (Degermark, 1987-1994). The second phase gave an ecological half-life of 4 years for activity concentration in the stream, which is in the same range as the 2.99.7 years reported by Saxtn (1994) for the period 1989-1992 in Finnish surface waters and the ecological half-life of 3 years for the inlet to lake Sitlgsjtn in central Sweden during 1987-1990 found by Sundblad et al. (1991). In Fig. 2(c) the calculated and observed daily caesium discharge during the first month after the Cbernobyl fallout are compared. The cumulative discharge during this period was 0.673 _.+ 0.13 (SD) GBq and contributed as much as 80% of the total loss of 0.845 _+ 0.17 (SD)GBq during the period studied (1986-1993). The initial discharge of 137Cs during the period 29 April-31 May 1986 (Fig. 2(c)) corresponds to a uniform loss of 1.35 _ 0.270 kBq m -2 from the catchment, or 6.8 - 3% of the deposition in the catchment (19.8 kBq m -2, Table 4). This is equivalent to a total washout of 137Cs within a hypothetical 10 m leakage zone lining the sides of the two main streams. The slow component of 13~Csdischarge from the catchment was calculated (Eq. (4) and Table 3 Parameter values and statistics for Eq. (2), describing the development of activity concentration in the stream during the first 8 years after the Chernobyl fallout event Component
A
~
k
r2
Fast Slow
18 0.I6
0. I I 0.00049
0.0033 0.019
0.93 0.52
P < 0.0001 < 0.0001
n 34 70
T. Nyldn, H. Grip~Journal of Hydrology 192 (1997) 338-354
347
Table 4 The average remaining deposition of radioactive caesium on 19 June 1989 in different types of environments; each observation (n) is a composite of five subsamples Biotope
n
Area (ha)
t37Cs (kBq m -2)
I~Cs (kBq m -2)
Old t37Cs (kBq m -2)
Chemobyl "~Cs (kBq m -2)
Chernobyi "~TCs(GBq) (total)
Rech. forest Disch. forest Shrub mire Open mire
20 13 12 11
37.5 4,5 6.8 !.2
22.7 (9) 17.6 (7) 10.4 (7) 4.79 (3)
11.7 (4) 10.0 (4) 6.0 (3) 2.6 (0.2)
2.84 (2) 0.52 (I) 0.16 (0.3) 0.37 (0.3)
19.8 (9) 17.1 (7) 10.3 (7) 4.4 (3)
7.42 (3) 0.77 (0.3) 0.70 (0.5) 0.053 (0.04)
Numbers in parentheses are standard deviations.
runoff data) for two periods. The first period was from June 1986 to June 1989 (3.06 years), when the soil samples were taken. During this period, 0.117 +_- 0.02 GBq was discharged from the catchment. The second period, from June 1989 to December 1993 (4.5 years), generated a 137Csdischarge of 0.055 _+ 0.01 GBq from the catchment. The low secondary loss corresponds to a uniform loss of 0.23 - 0.04 kBq m -2, or 1.2%, from the catchment during June 1986-June 1989 (3.06 years) and 0.11 -+ 0.02 kBq m -2, or 0.6%, during the rest of the period (4.5 years).
3.2. Seasonal contribution to the discharge of 137Csfrom the catchment The monthly contribution to the annual discharge of 137Cs from the catchment during 1986-1993 is presented in Fig. 4. It can be seen in the figure that, except for 1986, snow melt at the end of April and beginning of May contributes about 50% of the annual discharge of 137Cs. One exception is the year 1987, which almost lacked a spring flood and therefore had only a minor loss of 137Csduring spring. Except for the generally clear dominance of contributions during spring, considerable discharge of 137Cs also occurs during autumn. In December-March, when the surface soil is frozen and precipitation is intercepted in the catchment as snow, runoff is low, and thus the discharge of m37Csreaches its minimum.
3.3. Discharge of 137Cs f r o m the subcatchments The contribution to discharge of 137Cs from the subcatchments above Weir E and between Weir E and Weir A was studied during June-December 1986, and in May and September 1993, as described above. The results show that the activity concentration of 137Cs at Weir E (head of the stream) was higher than at Weir A, except during late autumn, when the two weirs gave similar values (Fig. 5(a)). Fig. 5(b) shows the discharge of 137Cs (Bq s -l) from the catchment (Weir A, white bars), Subcatchment E (Weir E, grey bars) and Subcatchment A - E (by subtracting the discharge of 137Csat Weir A from that at Weir E; black bars). The discharge of ~37Cs from Subcatchment E was high in June 1986, low during the dry period of July, elevated during autumn, and again decreasing at the beginning of the winter season in October-December. in 1993, the discharge of 137Cs was
348
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
~oo
i!
•s
19s6
3O 25
8O
3oi 15'
20 o
,
I0' $.
,
0..,-.~
,
Month
1987
~i
1991
1988
60 • 50.
1992
15' lO" $. O'
~0'
70
40. 40. 30, 20. I0 0
SO.
3O 2O .I.
1o
.m.m.n._.
..... l.~---
0
1989
"
•
I
"
3•]
1993
25, 3O 2O
'
I
10 0
0
m
.
.
.
I .
.
Im . . = .
Fig. 4. The monthly contribution to the annual loss of °3~Csfrom the catchment calculated from Eq. (2) during 8 years after the Chernobyl fallout event. higher during the spring flow peak (13 1 s -I, at W e i r E) than during a rainstorm event in October (4 1 s -I, at Weir E). The discharge o f 137Cs from the forested Subcatchment A - E showed the same pattern d u r i n g s u m m e r 1986 but decreased to negative values during A u g u s t - O c t o b e r , whereupon it was elevated to approximately the same value as for
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
349
1.2
i
1 °
|
0.8 -
r,. 0.6
I
•
II
D
li
li
I
I
I
0.4
0.2
a I
ilii
a !
a I
B I
B I
i !
[] I
I
3.5
2
1, r.I
0 -0.5 -1
I< I
..
T
,i,,.,_.,,.,....Pl.,.I,r I,rl,ru,. -= I i! '" Ji|||||lllii
Date
Fig. 5. (a) Activity concentrationof 137Csin stream, at the top of the perennial stream (Weir E, m) and at the end of the catchment (Weir A, El). The uncertainty (1 $D) is also indicated. (b) The discharge of 13~Cs(Bq s -s) from the total catchment (Weir A, white bars), $ubcatchment E (Weir E, grey bars) and Subcatchment A-E (by subtracting the dischargeof L~TCsat Weir E from the correspondingvalue at Weir A, black bars). The uncertainty (l $D) is also indicated. Subcatchment E. The negative values during the periods August-September 1986, and May and September 1993 mean that a fraction of the activity that passed Weir E was intercepted in the stream between the two weirs. During 1986, the observed amount of 137Cs intercepted in the stream bank was nearly the same as the loss of 137Cs from Subcatchment A - E during the following autumn (Fig. 5(b)). It is thus possible that the origin of ~37Cs discharge from the forested Subeatchment A - E was loss from the stream bank. High activity of t37Cs in stream sediments was observed in an alpine catchment by Hongve et al. (1994), and might constitute a temporal sink for a fraction of the loss o f 137Cs from catchments.
350
7".Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
3.4. Remaining deposition in soil It is likely that the main contribution to loss of radioactive caesium during the early phase took place in the saturated discharge areas, which then should be relatively depleted of the initial deposition in comparison with the recharge areas by the time of the soil sampling. The remaining activity in soil during June 1989, was analysed in four categories (Fig. 1). The results of average calculated deposition density of 'Chernobyr and 'old' caesium in the different types of environments are presented in Table 4. The remaining deposition is in both cases highest in the recharge areas. A test of variance (Tukey's test) gives the relation between the content of 'old' m37Csin the biotopes (ascending order): 'shrub mire' = 'open mire' = 'disch. forest' < 'rech. forest'(P = 0.001); for the fraction of Chernobyl deposition: 'open mire' < 'shrub mire'(P = 0.001) < 'disch. forest'(P = 0.05) = 'rech. forest'. The calculated content of ~34Cs in the recharge areas (11.7 kBq m -2, Table 4) is in the same range as found by Arntsing et al. (1991) for the region. This, together with the lower remaining activity on the mire, suggests that the loss of the Chernobyl fraction of 137Cs mainly originates from the mire. The remaining deposition on the mire is lowest in the open fraction, which indicates that the 137Cs discharge during 1986-1989 was more effective from this fraction than from the shrub type of the mire.
4. Discussion and conclusions The fraction of the total Chernobyl deposition of 137Cs in the catchment lost via runoff was 7 -+ 3%. It is not reasonable that the different parts of the catchment contributed equally to the loss of radioactive caesium during spring 1986. The mire, which constitutes the main fraction of the discharge areas, was saturated by water during the weeks after the fallout, and thus generated a loss of radioactive caesium via melt water runoff. Potential discharge areas in the coniferous forest lining the stream, on the other hand, probably only gave a minor contribution to the discharge of radioactive caesium via melt water, as a large fraction of the wet deposited fallout intercepted in the forests still was held in the tree canopies (Nyltn and Grip, 1989; Bunzl et al., 1989; Sombre et al., 1990; Melin and Wallberg, 1991). During the following years, the main fraction of fallout intercepted in tree canopies was transferred to the forest floor (Melin and Wallberg, 1991). The hypothesis is thus that the origin of the observed initial activity in the stream was the mire. As the deposition appears to have been uniform over the whole catchment, the sum of the discharge of J37Cs from the catchment and the remaining deposition in 1989 on the mire should give about the same mean deposition (kBq m -2) on the mire as on the recharge areas. The fraction of Chernobyl ~37Csresiding in the mire during 1989 (Table 4) added to the cumulative loss from April 1986 to June 1989 (Eq. (2)) was 1.54 -+ 0.3 GBq, or 19.3 _ 4 kBq m -2 if evenly distributed on the mire, which is about the same as the calculated Chernobyl deposition in recharge areas, 19.8 -+ 9 kBq m -2. This corroborates well with the conditions predicted from the hypothesis and thus is a strong indication that the origin of 137Cs loss during spring 1986 was the mire. The amount of caesium that discharged during
T. Nyldn, H. Grip/Journal of Hydrology 192 (1997) 338-354
351
60
¢I
50
50
40
40
30
30
20
dg
10 00
.-q
10
"% ' oi
10 20 30 4 0, 50, 60 70, 8 0 90, 100 Runoff 0/s)
Fig: 6. The relation between the fraction of discharge areas in the catchment and the maximum discharge intensity for different storm events during 1981-1982. @, Snow melt events during spring; O, rainfall events. The distribution of maximum runoff intensities during the study is indicated by the continuous line. Data are from Rodbe (1987) and Degermark (1987-1994).
the first month (0.67 +_ 0.13 GBq or 8.4 _ 1.7 kBq m -2 from the mire) subtracted from a deposition of 19.3 kBq m -2 on the 8 ha mire then gives an initial loss of 44 _ 11% from the mire. As the whole mire contributed to the snowmelt runoff in 1986 the areal distribution of the remaining 56% was probably uniform. The different observations of the dynamic behaviour of 137Csdischarge from the studied catchment (Fig. 4), together with the remaining deposition in the different biotopes (Table 4) and earlier investigations on the origin of stream water, suggest that the fraction of the catchment contributing to discharge of ~37Cs is dynamically changing during the year: 1. the fraction of the studied catchment involved in saturated surface discharge was investigated by Rodhe (1987), who found that during nine rain and snowmelt events, between 0.4 and 33% of the studied catchment contributed to saturated surface discharge and that the fraction of discharge area was positively correlated with the maximum stream flow during the runoff event (Fig. 6, circles). A frequency plot of the maximum stream flow of runoff events during the studied period (Fig. 6, histogram) shows a clear dominance of low values. Fig. 6 thus shows that only small fractions of the potential discharge areas are frequently involved in saturated surface discharge. Regarding this, it is likely that the main part of the leached ~37Csoriginates from the 'open mire', which, except during winter when the soil surface is frozen, constantly undergoes saturated surface discharge. This discussion also agrees with the seasonal behaviour of 137Cs discharge from the catchment in Fig. 4. 2. In Table 2 the intercept of the natural logarithm of the activity concentration in the stream decreases with time, which indicates a decreasing pool of J37Cs in the fraction of the discharge area that is engaged even at low rates of runoff. The slope, on the other hand, increases with time, indicating that the pool of 137Cs that discharges at higher rates of runoff is not as affected by time. Further, in Table 4 the remaining deposition (kBq m -2) during 1989 is lower in the 'open mire' than in the 'shrub mire', which also
352
7",NylOn, H. Grip/Journal of Hydrology 192 (1997) 338-354
agrees with a more effective loss from the wetter fraction of the mire during the phase of slow leakage. The conditions described above lead to a second hypothesis on the discharge of 137Cs from the mire: the loss of 137Cs from the mire is controlled by the dynamically changing discharge areas. Areas that frequently undergo saturated surface discharge hence will empty their pool of n37cs more rapidly than areas that seldom are engaged. As the 'open' mire is constantly wet, the loss should be greater from this fraction than from the 'shrub' mire. This hypothesis was tested by using (1) the calculated remaining deposition on the mire after the spring flood in 1986 (June 1986) and the measured deposition 3 years later in June 1989, and (2) the calculated discharge of 137Cs at Weir A during the 3 years, as follows: 1. if the hypothesis is correct, the total deposition on the mire during 1989 subtracted from the corresponding deposition in 1986 should be equal to the discharged 137Csduring the elapsed time 3.06 years. From Table 4, the loss of 137Cs from the two fractions of mire was calculated. A comparison with the corresponding loss calculated from Eq. (4) and recordings of runoff was also made: a hypothetical uniform distribution of the 56 - 20% remaining on the mire after the initial snow melt in 1986 gives a uniform deposition of 11 __ 2 kBq m -2, or 0.88 + 0.2 GBq. The remaining activity in 1989 was 0.7 __ 0.5 GBq on the shrub mire and 0.053 _ 0.04 GBq on the open mire (Table 4), i.e. 0.75 _ 0.5 GBq in weighted total, The discharge during the elapsed time should therefore be 0.13 ± 0.6 GBq (0.75 _ 0.2 GBq subtracted from 0.88 +-- 0.5 GBq). 2. The amount of 137Csdischarged via Weir A, calculated from Eq. (4) and recordings of runoff for the 3.06 years was 0.12 ± 0.05 GBq, which is very close to the estimate above. The two ways of calculating the loss of 137Csfrom the mire agree well, which supports the hypothesis that the loss of 137Cs is controlled by the dynamically changing discharge areas on the mire. The decrease in remaining deposition during the 3.06 years gives an annual uniform loss of 30 +_-7% for the open mire and 2.1 ___0.4% for the shrub mire (Eq. (4)). The effective ecological half-life for 137Csis then about 2.3 years in the 'open mire', and about 16 years in the 'shrub mire', corresponding to ecological half-lives of 2.5 years and 34 years, respectively, for stable caesium introduced to the mire by wet deposition. Further, the exponential decline in activity concentration in the stream is, until the end of this study, to a large extent explained by a decreasing pool of 137Csin the open fraction of the mire. The loss of ~37Csfrom discharge and recharge areas in the forest must be orders of magnitude lower than from the 'open' mire, as there was no way of identifying any loss of Chernobyl m37Cs from these areas with the methods used. Nevertheless, there is an indication of a future discharge of radioactive caesium from the forested discharge areas in Table 4 where the remaining activity of 'old' 13~Csis in the same range as in the mire. The vertical percolation Of 137Csin recharge areas also seems to be very low. If the low activity concentration of 0.01 Bq 1-I (the limit of detection for the measurements in the groundwater tube) constitutes the upper limit for the concentration in the column of groundwater in the catchment, then the loss from the recharge areas during the 8 years is at most 30 Bq m -2. This corresponds to an annual loss of less than 0.02% of the deposition on
T. NylOn, H. Grip/Journal of Hydrology 192 (1997) 338-354
353
the recharge areas during the 8 years since the Chernobyl accident. Under these assumptions, the effective half-life for 137Csin the recharge areas is 30 years, corresponding to an ecological half-life exceeding 4200 years for stable caesium introduced by wet deposition.
Acknowledgements This work has been supported by the National Radiation Protection Institute, the Centre for Ecological Research in Ume~t and the CEC (Radiation Protection Programme). We would also like to thank Dr. Ronny Bergman for helpful criticism, and Elon Manfredsson and Kenneth Lidstrtm for help with sampling and analyses.
References Am-krog, A., 1994. Doses from the Chemobyl accident to the Nordic populations via diet intake. In: H. Dahlgaard (Editor), Nordic Radioecology: The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 433-456. ~dlman, B. and Ahman, G., 1994. Radiocesium in Swedish reindeer after the Chemobyl fallout: seasonal variations and long-term decline. Health Phys., 66(5): 503-512. Arntsing, R., Bjurman, B., de Geer, L-E., Edvardsson, K., Finck, R., Jakobsson, S. and Vintersved, I., 1991. Field gamma ray spectrometry and soil sample measurements in Sweden following the Chernobyl accident. A data report. FOA Rep. D 20177-4.3, Stockholm. Bergman, R., 1993. Products from forest and lake: food resource or exposure problem, after radioactive fallout. FOA Pep. C 40315-4.3, Umel. Bergman, R., 1994. The distribution of radioactive caesium in boreal forest ecosystems. In: H. Dahigaard (Editor), Nordic Radioecology. The Transfer of Radionuclides through Nordic Ecosystems to Man. Elsevier, Amsterdam, pp. 335-379. Bergman, R., NylOn, T., Palu, T. and Lidstrtm, K., 1991. The behaviour of radioactive caesium in a boreal forest ecosystem. In: L. Moberg (Editor), The Chernobyl Fallout in Sweden. Swedish Radiation Protection Institute, Stockholm, pp. 425-456. Bergman, R., NylOn, T., Nelin, P. and Palo, T., 1993. Caesium-137 in a boreal forest ecosystem: aspects on the long time behaviour. FOA Rep. C 40284-4.3, Umel. Bishop, K., 1992. Episodic increases in stream acidity, catchment runoff pathways and hydrograph separation. PhD Thesis, University of Cambridge. Bonnett, P.J.P. and Appelby, P.G., 1994. Rates of removal of sediment associated radiocaesium from the Plynlimon experimental catchments, Powys, UK. Environ. Pollut., 83: 327-334. Bunzl, K., Schimmack, W., Kreutzer, K. and Schierl, R., 1988. The migration of fallout Cs- 134, Cs- 137 and Ru106 from Chemobyl and of Cs-137 from weapons testing in a forest soil. Z. Pflanzenntihr. Bodenkd., 152: 3944. Bunzl, K., Schimmack, W., Kreutzer, K. and Schierl, R., 1989. Interception and retention of Chemobyl derived Cs-134, Cs-137 and Ru-106 in a spruce stand. Sci Total Environ., 78: 77-87. Carboi, P., 1993. Speciation and transport of radionu¢lides from the Chernobyl accident within the Gide~ Site. PhD Thesis, CTH, Gtteborg. Davis, R.B., Hess, C.T., Norton, S.A., Hanson, D.W., Hoagland, K.D. and Anderson, D.S., 1984. Cs-137 and Pb2 i 0 dating of sediments from soft water lakes in New England and Scandinavia, a failure of Cs- 137 dating. Chem. Geol., 44: 151-185. De Oeer, L.-E., Arutsing, R., Vintersved, I., Sisefky, J., Jakobsson, S. and Engstrtm, J-A., 1987. Particulate radioactivity, mainly from nuclear explosions in air and precipitation in Sweden mid-year 1975 to mid-year 1977. FOA Pep. C 40089-T2, Stockholm.
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