50

45

40

35 Sr, soluble/Cs, soluble

30

25

20

15

10

5

0

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Year

FIG. 4.19. Ratio of 90Sr to 137Cs activity concentration in Pripyat River water in the city of Chernobyl.

spring flood events in 1991, 1994 and 1999 are clearly discernible as sharp peaks when the floodplains became inundated.

Significant reduction in 90Sr release from the CEZ is expected after completion of a whole series of flood control (water protective) measures on the right bank of the river, drying-up of the cooling pond and completion of the water runoff control system on the left bank polder. In particular, during 2001, 10 km of canals of the left bank polder (reclamation) system were dredged and many engineering works such as flow gates, drains and others were restored and repaired. These works allowed better control of runoff from the polder system.

Recently, the water management authorities in Belarus and Ukraine agreed to justify actions concerning construction of a bypass channel along the Belarusian–Ukrainian border in the CEZ. The construction of a 10 km long channel (between the settlements of Krasne and Zimovishche) would direct surface runoff from Belarusian catchments during high water periods straight to the Braginka River, which flows into the Kiev reservoir, thus avoiding the most contaminated lowland areas within the CEZ. This proposal has considerable merit, since it would reduce the downstream 90Sr source term during flood conditions.

4.5.ANALYSIS OF KEY PROCESSES GOVERNING THE LONG TERM DYNAMICS OF RADIOACTIVE CONTAMINATION OF THE DNIEPER WATER SYSTEM

The initial radioactive contamination after the Chernobyl accident resulted from direct fallout on the water surface. Thereafter, the dynamics of radioactivity in river water were controlled by the

redistribution of radionuclides between the water and the bottom sediments. In the following period, which started approximately one year after the fallout [4.37], the contamination of river water depended on inflow of radionuclides from the catchment areas. The model proposed in Ref. [4.37] assumes that the radionuclide concentration in river water during this time period is directly proportional to the average concentration of its exchangeable form in the surface soil layer on the catchment. This means that the key processes responsible for the long term dynamics of radionuclides in river water are vertical migration and exchange of radionuclide species in soils on the catchment. For the case where the bulk of the radionuclides deposited on the catchment are in the form of condensation particles, the time dependence of radionuclide concentration in the dissolved phase can be expressed as follows:

		K	Ê			d ˆ		Ê	Ê	u2	ˆˆ
Cw	=		Á	1	+		˜exp Á		-Á		+lt˜˜
		pDEt Ë				t ¯		Ë	Ë4DE		¯¯

(4.1)

where

K is a site specific parameter (Bq/m2);

DE is the effective diffusion coefficient in sediments (m2/a);

uis the effective velocity of convective transport (m/a);

d is the kinetic parameter of radionuclide fixation in soil (a0.5);

l is the radioactive decay constant (a–1); t is the time (a).

Equation (4.1) predicts with adequate accuracy the long term dynamics of 137Cs in the three rivers (Irpen, Teterev and Uzh) flowing across the southern part of the radioactive trace formed after the Chernobyl accident [4.37]. It also yields satisfactory results for the rivers flowing through other territories with similar soil and fallout characteristics. As an illustration, Fig. 4.20 shows calculated and experimental dependences of the mean annual concentration of 137Cs in the Iput River (at the Dobrush measuring section). Good agreement of the calculated and measured concentrations suggests that the underlying assumptions regarding the physicochemical mechanisms are valid.

49

(Bq/L)137-	2
	1
	1.5
Caesium	0.5
	0

0

2

4

6

8

10

12

14

Time after the accident (years)

FIG. 4.20. Predicted (line) and measured (points) activity concentration of 137Cs in the Iput River (cross-section at Dobrush). Experimental data were taken from Ref. [4.39].

	1
(Bq/L)	0.8
-137	0.6
Caesium	0.4
	0.2
	0
	0	5	10	15	20

Time after the accident (years)

FIG. 4.21. Reconstructed activity concentration of 137Cs in the Snov River (cross-section at Khoromnoe).

The advantage of Eq. (4.1) is that a prediction (or reconstruction) of the long term dynamics of radionuclide concentration in river water can be made based on measurements made over a relatively short period of time. In particular, for the Snov River, along which transboundary transport of radionuclides from the Russian Federation to Ukraine occurs, only fragmentary results of measurements of 137Cs specific activity made in 1998 are available [4.13]. Based on these data, and using Eq. (4.1), the dynamics of the contamination of this river over the period starting from 1987 to the present day can be reconstructed (see Fig. 4.21).

Equation (4.1) can also, most probably, be used for estimating the distribution of radionuclides over the whole length of the river, provided data for one or several measuring sections and a catchment contamination map are available. Earlier studies suggest that the radionuclide concentration in a measuring section is directly proportional to the average contamination density of that part of the river catchment upstream of the given measuring section (see Fig. 4.22).

	0.3
(Bq/L)		y = 1.2271x
	0.2	R2 = 0.9406
-137
Caesium	0.1
	0
	0	0.05	0.1	0.15	0.2
		Contamination density (MBq/m2)

FIG. 4.22. Concentration of 137Cs in the Iput River in 1991– 1993 as a function of upstream watershed contamination density. Experimental data were taken from Ref. [4.40].

During the years after the Chernobyl accident the radiocaesium activity concentration in most contaminated aquatic ecosystems decreased markedly. Lakes with no permanent inflows and outflows (closed lakes), however, still present a radioecological problem that is expected to continue for some time. This is explained by the fact that the main mechanism underlying the reduction of the radionuclide concentration in the water of such lakes is a fairly slow migration to the lower layers of bottom sediments. Given negligible runoff and sedimentation, the dynamics of radiocaesium in lake water is described by a simple equation with only one unknown parameter [4.38]:

	Cw (t) =		s	exp(-lt) =	A	exp(-lt)
		Kd	pDEt
					t
				(4.2)
where
Cw	is the radionuclide activity concentration in
	the water layer (Bq/m3);
A	is a constant for a given lake and radionuclide;
Kd	is a dimensionless distribution coefficient that
	is equal to the ratio of radionuclide activity
	concentration in the water to that in the
	surface of the sediments (on a volume basis).

Equation (4.2) was tested against 137Cs activities measured between 1993 and 1999 in Lake Svyatoe in the Bryansk region of the Russian Federation (see Fig. 4.23). It can be seen that the 137Cs concentration in the lake is actually equal to the intervention level effective in the Russian Federation today (11 Bq/L) and will remain high for many years. Given the fact that the 137Cs

50

30

(Bq/L)	25
water	20
137in	15
	10
-
Caesium	5
	0

6

8

10

12

14

Time after the accident (years)

FIG. 4.23. Measured versus modelled 137Cs activity concentration in Lake Svyatoe water. Modelled activity dynamics are given by the solid line, measured values by circles. Standard deviations of the mean for measured values are also shown.

concentration in fish in these lakes is also quite high, it may be concluded that, as a result of the Chernobyl accident, the lifestyle of the local inhabitants has been affected for a considerable time. Hence, the lakes without outflow, in a sense, can be considered to be significant hot spots (see Section 9.3.5).

4.6.TRANSBOUNDARY FLUXES OF RADIONUCLIDES IN THE DNIEPER RIVER BASIN

The transboundary movement of radionuclides in rivers is determined by water discharge rates and radionuclide volumetric activity at the border crossing. Table 4.17 gives the average the

TABLE 4.17. AVERAGE TRANSBOUNDARY FLOW OVER MANY YEARS OF THE MAJOR RIVERS OF THE UPPER DNIEPER RIVER BASIN MOST CONTAMINATED BY THE CHERNOBYL ACCIDENT [4.29]

				Annual runoff (km3)
	Flow from	Flow to
			Mean		Probability
				75%	90%	95%
Dnieper	Russian	Belarus	3.65	2.96	2.51	2.25
	Federation
Dnieper	Belarus	Ukraine	18.6	15.0	12.5	11.2
Pripyat	Belarus	Ukraine	12.2	9.16	7.25	6.29
Braginka	Belarus	Ukraine	0.09	—	—	—
Sozh	Russian	Belarus	1.84	1.41	1.14	1.00
	Federation
Iput	Russian	Belarus	1.57	1.19	0.94	0.81
	Federation
Besed	Belarus	Russian	0.52	0.41	0.33	0.29
		Federation
Besed	Russian	Belarus	0.76	0.60	0.49	0.43
	Federation
Snov	Russian	Ukraine	0.35	0.27	0.22	0.20
	Federation
Seim	Ukraine	Russian	0.27	0.21	0.16	0.14
		Federation
Seim	Russian	Ukraine	2.61	2.03	1.64	1.44
	Federation
Desna	Russian	Ukraine	5.11	4.24	3.61	3.30
	Federation

51

characteristics over many years of the transboundary water flow of major rivers in the regions most contaminated after the Chernobyl accident and of rivers flowing through the areas surrounding the Smolensk and Kursk nuclear power plants.

The average values in Table 4.18 can be used to forecast transboundary fluxes of radionuclides. For retrospective estimates it is better to use data from routine observations for a given time period.

Radionuclide concentration in rivers is normally measured in cross-sections lying several tens of kilometres from the border. The exceptions are two measuring sections on the Belarusian– Ukrainian border (Belaya Soroka on the Pripyat River and Gden on the Braginka River) and the Vyshkov measuring section on the Iput near the Russian–Belarusian border. For the latter crosssection, however, only limited data are available. Systematic monitoring of radioactive contamination in rivers flowing through the areas around the Kursk and Smolensk nuclear power plants (the Desna and Seim Rivers) is carried out only at measuring sections in the immediate proximity of the nuclear power plants. Therefore, in most cases to estimate radionuclide concentrations at the border, extrapolation methods are required, such as the concentration dependence on catchment contamination density (see Fig. 4.22). In those cases when limited observational series are available, extrapolation of concentration over time can be used.

Estimates of the transboundary transport of radionuclides in the Dnieper River basin have been

made starting from the early phase of the accident. It was shown that, in 1987, the transport of 137Cs from the Sozh catchment was about 80% of its flow to the Kiev reservoir with the water of the Dnieper River [4.28]. In turn, about half of the 137Cs was transported to the Sozh River via the Iput River, whereas the input from the Besed River was only about 8%.

The concentration of radionuclides in rivers has decreased significantly with time. The dependence of annual outflow of 137Cs from the territory of the Russian Federation to Belarus via the Iput River suggests that most of the radionuclides were transported across the border in the first few years after the accident. Table 4.18 contains estimates of transboundary flows of 137Cs and 90Sr for the period 1987–1999 based on the data derived from the measuring sections closest to the borders. It can be seen that over the 12 year period the transboundary transport of 137Cs did not exceed 1% of the amount of radionuclides deposited on the catchment area in 1986. The transboundary movement of 90Sr was somewhat higher, but did not exceed 5% for the same period.

Figures 4.24 and 4.25 show the annual fluxes on the Pripyat, Iput and Besed Rivers near the borders. The transboundary migration of 137Cs has decreased markedly with time. However, the transboundary migration of 90Sr has fluctuated from year to year depending on the rainfall and extent of flooding (see Figs 4.24(b) and 4.25(b)).

The above analysis indicates that the existing system of monitoring of radioactive contamination

TABLE 4.18. ESTIMATES OF TRANSBOUNDARY TRANSPORT OF CAESIUM-137 AND STRONTIUM-90 IN MAJOR RIVERS FLOWING THROUGH CHERNOBYL AFFECTED REGIONS [4.26, 4.41]

	Years	Iput River	Besed River	Pripyat River	Dnieper River
Section		Dobrush	Svetilovichi	Belaya Soroka	Nedanchichi
Border		Russian	Russian	Belarus–	Belarus–
		Federation–	Federation–	Ukraine	Ukraine
		Belarus	Belarus
Caesium-137 outflow	1987–1999	9.1	1.9	31.3	43.1
(TBq)
Caesium-137 (per cent		0.4	0.1	0.7	0.56
catchment inventory)
Strontium-90 outflow	1990–1999	0.9	0.7	52.6	34.3
(TBq)
Strontium-90 (per cent		2.2	1.9	4.3	3.6
catchment inventory)

52