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Recent changes in Red Lake (Romania) sedimentation rate determined from depth profiles of 210Pb and 137Cs radioisotopes
Journal of Environmental Radioactivity 100 (2009) 644–648
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Recent changes in Red Lake (Romania) sedimentation rate determined from depth profiles of 210Pb and 137Cs radioisotopes R. Begy*, C. Cosma, A. Timar Faculty of Environmental Science, Babes-Bolyai University, Fantanele Str. 30, RO-400294 Cluj-Napoca, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2008 Received in revised form 13 January 2009 Accepted 20 May 2009 Available online 21 June 2009
This work presents a first estimation of the sedimentation rate for the Red Lake (Romania). The sediment accumulation rates were determined by two well-known methods for recent sediment dating: 210Pb and 137 Cs methods. Both techniques implied used the gamma emission of the above-mentioned radionuclides. The 210Pb and 137Cs concentrations in the sediment were measured using a gamma spectrometer with a HpGe detector, Gamma-X type. Activities ranging from 41 7 to 135 34 Bq/kg were found for 210Pb and from 3 0.5 to 1054 150 Bq/kg for 137Cs. The sediment profile indicates acceleration in sedimentation rate in the last 18 years. Thus, the sedimentation process for the Red Lake can be divided in two periods, the last 18 years, and respectively, the period before that. Using the Constant Rate of 210Pb Supply method values between 0.18 0.04 and 1.85 0.5 g/cm2 year (0.32 0.08 and 2.83 0.7 cm/year) were obtained. Considering both periods, an average sedimentation rate of 0.87 0.17 g/cm2 year (1.17 cm/year) was calculated. Considering an average depth of 5.41 m for the lake and the sedimentation rate estimated for the last 18 years, it could be estimated that the lake will disappear in 195 years. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: 210 Pb 137 Cs Radioisotopes Dating methods Sedimentation rate
1. Introduction In 1837 in the metamorphic-Mesozoic zone of the Oriental Carpathians a relatively rare phenomenon occurred. A sliding of the Ghilcos Mountain blocked the Bicaz River leading to the formation of a natural barrage lake, which was afterwards named the Red Lake. The barrage resisted the vertical and regressive erosion as well as the static water pressure. The lakes formed by the natural barraging of valleys often have a short life, the formed barrage being eroded or quickly silted. In the case of the Red Lake, despite the significant water volume, the geology and consistency of the barrage did not allow the rapid erosion of this. A natural barrage can constitute damage for the natural dynamics of the environment, the sedimentation being a consequence of the tendency for reestablishing equilibrium. In the case of the Red Lake, there is the high sedimentation rate, which is strongly influenced by human activities (Pandi, 2004). One of the most important recent sedimentation dating methods is 210Pb method, which is a natural radionuclide resulted from 238U series (Arnaud, et al., 2006). Disequilibrium between
* Corresponding author. Tel.: þ40 7458 73206; fax: þ40 264307032. E-mail addresses: [email protected] (R. Begy), [email protected] (C. Cosma), [email protected] (A. Timar). 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.05.005
210 Pb and its parent isotope in the series, 226Ra, arises through diffusion of the intermediate gaseous isotope 222Rn. A fraction of the 222Rn atoms, produced by the decay of 226Ra in soil escapes into the atmosphere where they decay through a series of short-lived radionuclides to 210Pb. This is removed from the atmosphere by precipitation or dry deposition, falling on the land surface or into lakes. The lead that falls directly in the lake is transported by the water column and deposited on the bottom of the lake together with the sediment. The ages obtained using this method turned out to be accurate especially in stable environments with constant sedimentation rates, where the calculation model is well defined (Schmidt et al., 2007; Van den Bergh et al., 2007; Tylmann, 2004). The method also provided good results for non-uniform sedimentation environments (where the sedimentation rate is not constant), though in this case the difficulty lies in finding of an appropriate sedimentation model. Two simple models, which are known as the constant rate of 210Pb supply (CRS) (McDonald and Urban, 2007; AlonsoHernandez et al., 2006) and the constant initial concentration (CIC) model (Appleby and Oldfield, 1978; Robbins, 1978) are usually applied. Another possibility for dating the lake sediments is by using the artificial radionuclides. The two sources of these are the nuclear weapon test that took place between 1953 and 1963 and the of Chernobyl atomic plant power station accident from 1986. The
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
645
Fig. 1. Sediment deposition over 171 year (Pandi, 2004) and sampling location.
radionuclides which were deposited and are present nowadays are: 90 Sr, 137Cs and 239-240-241Pu (Jeter, 2000). In the northern hemisphere the deposition reached a significant level until 1954 and grew rapidly later. The Chernobyl accident from 26th April 1986 injected a lot of artificial radionuclides in the atmosphere, this process continued during May, 1986. The main nuclides were 131I, 137Cs, 134Cs and 90Sr.
Radioactive depositions were made at thousands of km from the accident place in the northern hemisphere, being important in some region as in Poland (Gaca et al., 2006) or Romania (Cosma, 2002). The depositions are estimated to be 1017 Bq compared to 4.3 1017 Bq due to nuclear testing (Cambray et al., 1987). The radionuclide distribution was non-uniform and greatly controlled by winds and rains.
Fig. 2. Physical characteristics, porosity and water content in the core samples.
646
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Table 1 Radionuclide concentration in sediment cores. Core 1 Radionuclide Conc. (Bq/kg)
Core 2 Radionuclide Conc. (Bq/kg)
210
226
137
210
80 14 103 18 114 20 66 11 88 15
57 8 68 10 70 11 67 10 50 8
395 63 60 10 28 4 3 0.5 –
77 19 135 34 80 20
Pb
1 2 3 4 5 6
Ra
Cs
Pb
226
Ra
53 8 50 9 57 9
Core 3 Radionuclide Conc. (Bq/kg)
Core 4 Radionuclide Conc. (Bq/kg)
Core 5 Radionuclide Conc. (Bq/kg)
137
210
226
210
226
137
210
226
137
168 27 1090 175 68 11
101 23 78 21 93 21 69 19 62 16 48 14
68 10 64 9 64 9 56 10 62 9 58 8
67 11 110 19 60 10 41 7 57 10
53 8 54 8 49 7 45 7 67 10
205 33 679 50 91 81 3 0.5
59 10 111 20 98 17 103 20
65 10 52 5 63 10 63 13
228 36 1054 150 38 6 103 16
Cs
Pb
2. Materials and methods 2.1. Study site The Red Lake (25 780 N, 46 780 E) is situated on the north-eastern part of Romania, in the Oriental Carpathian Mountains, at a mean altitude of about 983 m above sea level. The lake has two sectors, a southern part of 1000 m long and a western part of 442 m. The maximum depth is 9.6 m the average depth of the lake being 5.41 m. The surface area and volume of the lake are 116,500 m2 and 606,500 m3 respectively. The annual rainfall in the catchments of the Red Lake ranges from 544 to 1026 mm, the monthly maximum rainfall being about 306 mm in June and the minimum being 35.8 mm in November (Pandi et al., 2004). Fig. 1 shows the shape of the Red Lake and the locations from where the sediment samples were gathered.
Ra
137
Cs
Ra
C
Pb
Ra
Cs
time will result in changes in the initial unsupported 210Pb concentrations, in accordance with equation (3). The dates of deposition for older sediments are not calculated from their present concentration but from the distribution of 210Pb in the sediment record. After decay, the present amount of 210Pb remaining in the record from inputs at time s in ds time interval is Pels ds
(4)
where P denotes the 210Pb supply rate. Assuming P constant, the residual sediments of age t or older is: A ¼
Z
N
Pels ds ¼
t
P
l
210
elt
Pb in the entire record, the The residual in this equation, is
210
Pb inventory, obtained by putting t ¼ 0
P
Að0Þ ¼
(6)
l
It follows that A ¼ Að0Þelt
(7)
The values of A and A(0) can both be calculated by numerical integration of the concentration versus depth profile. If m denotes the depth of the sediment layer of age t, A ¼
Z
N
CðmÞdm m
Að0Þ ¼
Z
N
CðmÞdm
(8)
0
From their values, the age of sediments of depth m is calculated using the formula t ¼
1
l
ln
Að0Þ A
(9)
From equations Cuns ¼ Cuns ð0Þelt
(10)
(10) and (3) for sediments of age t,
2.3. CRS model The total 210Pb activity in sediments has two sources, supported 210Pb which derives from in situ decay of the parent radionuclide 226Ra, and unsupported 210Pb which derives from the atmospheric flux: Ctot ¼ Ctot ð0Þelt þ Csup 1 elt
(1)
210
Pb). (where l is the decay constant of In the case of lakes with a constant sedimentation rate the activity of sediment at depth m decrease in concordance with disintegration law: CðmÞ ¼ Cð0Þelm=r
210
Pb in
(2)
and Cð0Þ ¼ P=r
(3)
is the atmospheric 210Pb activity in the superior part of sediment core and P denotes the mean annual rate of supply of fallout 210Pb to the sediments (Carroll et al., 1995). When the sediment accumulation is not constant throughout the time, the CRS model (constant rate of 210Pb supply) is applied. This model is based on the assumption that the rate of deposition of unsupported 210Pb from the atmosphere is constant. When the CRS model is valid, changes in the sedimentation rate through
Pb in
(5)
210
2.2. Sampling and analysis Core samples were collected at different location in the lake using a flag-gravity corer, its inner and outer diameter being 10.6 and 11 cm respectively. The length of the cores ranged in size from 24 to 60 cm. The columns were divided in 4–6 layers of 8–10 cm each. Aliquots (2–5 g) of the sliced core sections were used for measuring physical characteristics of the sediment sample, including bulk density, water content and porosity and the remaining of the sliced core section were dried in oven at a temperature of 70 C and stored in the laboratory for the radionuclide measurements (210Pb, 226Ra and 137Cs). The depth profiles of the porosity and water content are shown in Fig. 2. Measures of radioisotope concentrations have been performed using an ORTEC DigiDart spectrometer with a HpGE detector, Gamma-X (GMX) type, with a resolution of 1.92 keV at 1.33 MeV line of 60Co and a relative efficiency of 34.2%. The shielding of the detector is 40 cm in diameter, with an 8 cm lead wall. Inside the lead shielding, there is a Cu layer of 3 mm. The measurement geometry was cylindrical in all cases. The minimal spectrum acquisition time implied was 24 h. The activity of 210 Pb was determined using the 46.5 keV gamma emission with the relative intensity of 4%. 137Cs specific activity was estimated using the 661 keV (89.9%) gamma line (Tanner et al., 2000). 226Ra activities were determined based on the assumption of secular equilibrium with its progenies, using the 214Pb (351.92 keV with relative intensity of 37.2%) and 214Bi (609.4 keV with relative intensity of 46.3%) gamma emissions, after a month of storage (Masque et al., 2002). The detection limit for 210 Pb is 8 2 (2d) Bq/kg (Uyttenhove, 2003).
Pb
229 37 345 55 47 7 67 10 10 2 18 2
Fig. 3. Sedimentation rate calculated with
210
Pb and
137
Cs from Core 3.
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Fig. 4. Radionuclide distribution in sediment core 1, 2, 4, 3, 5 (210Pbtot, P r ¼ elt C
(11)
Since P ¼ lAð0Þ ¼ lAelt
(12)
226
Ra, and
647
137
Cs).
Sediment age, calculated from 137Cs distribution is in good agreement with the values obtained from 210Pb using the CRS model (Fig. 3). The 210Pb concentration profile (Fig. 4) shows an exponential decrease with depth for the lower part of the column, which is typical for lakes with a constant sedimentation rate. In the upper
it follows that the dry mass sedimentation rate at time t in the past can be calculated directly using formula r ¼
lA C
(13)
(Appleby, 2001).
3. Results and discussion The 137Cs, 226Ra and total 210Pb concentration in the surface sediments of the Red Lake are presented in Table 1. 137Cs originating from both sources can be noticed. For example, in the third core, the first peak (Chernobyl) appears at the depth 23 cm and a second peak originating for the nuclear weapon tests from 1963 can observed at 42 cm. After the physical investigation of the subsamples from the sediment core for dry and wet density were obtained values from 0.5 0.05 to 1 0.1 g/cm3 and 0.75 0.07 to 1.5 0.1 g/cm3 respectively. The porosity profile (Fig. 2) shows an exponential decrease with depth, which is characteristic for homogeneous sediments with uniform compaction. In first 10–15 cm the water content is increased which denotes recent sediment (Saravana Kumar, et al., 1999).
Fig. 5. The two periods in the sedimentation process.
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R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Table 2 Sedimentation rates in Red Lake. Sampling station (water depth in m)
1 (3) 2a (4) 3 (2.5) 4 (3.5) 5 (3.5) a
Average sedimentation rate Dry mass sedimentation rate (g/cm2 year)
Linear sedimentation rate (cm/year) First period
Second period
First period
Second period
1.49 0.35 0.95 0.23 1.27 0.3 1.2 0.3 2.77 0.6
0.5 0.1 0.6 0.1 0.42 0.1 0.45 0.1 0.55 0.13
1.35 0.35 0.6 0.1 1.5 0.37 0.93 0.26 1.5 0 .37
0.34 0.08 0.22 0.06 0.5 0.1 0.25 0.06 0.3 0.07
Was not used in calculation because there ware just few subsamples.
part of the sediment, the 210Pb concentration is decreasing due to the dilution of the atmospheric fallout and the increasing sedimentation rate. In the age depth, profile of the cores two separate sedimentation periods can be observed: a regulated sedimentation period followed by an interval characterized by an increased sedimentation rate (Fig. 5). We can consider that until approximately eighteen years ago, the Red lake sedimentation was uniform. This may be due to due to a regulated woodfelling and to the two settle dams built on Red Brook and Licas Brook. In the last eighteen years, more sediment was deposited in the lake because a considerable part of the suspended sediment from the water passed through the dams. Another reason may be the increased woodfelling in the drainage basin, which lead to a powerful soil erosion. In Table 2 the values for the calculated sedimentation rates are presented. The calculated dry mass sedimentation rates of all five cores range between 0.25 0.06 and 0.5 0.1 g/cm2 year with an average of 0.35 0.06 g/cm2 year from until eighteen years ago. Considering only the last eighteen years the calculated dry mass sedimentation rate is between 0.93 0.26 and 1.5 0.37 g/cm2 year with an average of 1.32 0.31 g/cm2 year (for all considered cores), which is a value four times higher. The values calculated for the sedimentation rates for the regulated sedimentation period (until approximately 18 years ago) are quite reproducible from one core to another (0.42 0.1– 0.55 0.13 cm/year) with an average value of 0.48 0.11 cm/year. As for the recent sedimentation period the sedimentation rates reach values between 1.2 0.3 and 2.77 0.6 cm/year with an average of 1.68 0.43 cm/year. If we consider an average depth of 5.41 m, and this sedimentation rate is maintained, the lake will disappear approximately 195 45 years. It worth being mentioned that based on stratigraphic methods, in 2004 G. Pandi et al., predicted that this would happen in 257 years. Comparing our results to this value, we can conclude that both values are realistic, as uncertainty limits cannot be ascertained to the stratigraphic method. 4. Conclusions Our study shows that the application of radio-chronological methods can provide valuable information on the sedimentation process for the Red Lake that can be used for gaining a better understanding of the environmental changes in the drainage basin. Five cores were studied, and it was concluded that all could be dated using the 210Pb method. All investigated cores presented a well-defined 137Cs peak with high concentration due to the high contamination of this region from Romania after the Chernobyl accident. 210Pb and 137Cs dating methods gave chronological information for all the cores, the values obtained using the two different techniques being in good agreement. The results obtained showed that two separate sedimentation periods, characterize sediment deposition in the Red Lake. A regulated sedimentation period until eighteen years ago, followed by a period of more
intense sediment deposition. If the higher sedimentation rate determined for the recent period is maintained or grows, serious changes in the lake ecosystem can occur leading to the aggrading of the lake in a relatively short time.
References Alonso-Hernandez, C.M., Diaz-Asencio, M., Munoz-Caravaca, A., Delfanti, R., Papucci, C., Ferretti, O., Crovato, C., 2006. Recent changes in sedimentation regime in Cinefuegos Bay, Cuba, as inferred from 210Pb and 137Cs vertical profiles. Continental Shelf Research 26, 153–167. Appleby, P.G., Oldfield, F., 1978. The calculation of lead-210 dates assuming a constant rate of supply of unsupported 210Pb to the sediments. Catena 5, 1–8. Appleby, P.G., 2001. Chronostratigraphic techniques in recent sediments. In: Last, W.M., Smol, J.P. (Eds.), Tracking Environmental Change Using Lake Sediments 1. Springer Netherlands, pp. 171–203. Arnaud, F., Magand, O., Chapron, E., Bertrand, S., Boes, X., Charlet, F., Melieres, M.A., 2006. Radionuclide dating (210Pb, 137Cs, 241Am) of recent lake sediments in a highly active geodynamic setting (Lakes Puyehue and IcalmadChilean Lake District). Science of the Total Environment 366, 837–850. Cambray, R.S., Cawse, P.A., Garland, J.A., Gibson, J.A.B., Johnson, P., Lewis, G.N.J., Newton, D., Salmon, L., Wade, B.O., 1987. Observation of radioactivity from the Chernobyl accident. Nuclear Energy 26, 77–101. Carroll, J., Abraham, J.A., Cisar, D.J., Lerche, I., 1995. Model-determined sediment ages from Pb-210 profiles in un-mixed sediments. Nuclear Geophysics 9, 553–565. Cosma, C., 2002. Some aspects of radioactive contamination after Chernobyl accident in Romania. Journal of Radioanalytical and Nuclear Chemistry 251, 221–226. Gaca, P., Skwarzec, B., Mietelski, J.W., 2006. Geographical distribution of 90Sr contamination in Poland. Radiochimica Acta 94, 175–179. Jeter, H.W., 2000. Determining the ages of the recent sediments using measurements of trace radioactivity. Terra et Aqua 78, 21–28. Masque, P., Isla, E., Sanchez-Cabeza, J.A., Palanques, A., Bruach, J.M., Puig, P., Guillen, J., 2002. Sediment accumulation rates and carbon fluxes to bottom sediment at the Western Bransfield Strait (Antarctica). Deep-Sea Research II 49, 921–933. McDonald, C.P., Urban, N.R., 2007. Sediment radioisotope dating across a stratigraphic discontinuity in a mining-impacted lake. Journal of Environmental Radioactivity 92, 80–95. rtxii de S¸tiintx Pandi, G., 2004. Lacul Ros¸u. Studiu de hidrogeografie. Casa Ca a, Cluj. (in Romanian). Pandi, G., Horvath, Cs., Vigh, M., 2004. Importanta cunoasterii timpului de schimbare a volumului de apa in unutatiile lacustrine. In: Conferinta Stiintifica anuala a institutului National de hidrologie si Gospodarirea apelor (in Romanian). Robbins, J.A., 1978. Geochemical and geophysical applications of radioactive lead. In: Nriagu, J.O. (Ed.), Biogeochemistry of Lead in the Environment. Elsevier Scientific, Amsterdam, pp. 285–393. Saravana Kumar, U., Navadaa, S.V., Raoa, S.M., Nachiappanb, R.P., Kumarb, Bhishm, Krishnamoorthyc, T.M., Jhac, S.K., Shuklac, V.K., 1999. Determination of recent sedimentation rates and pattern in Lake Naini, India by 210Pb and 137Cs dating techniques. Applied Radiation and Isotopes 51, 97–105. Schmidt, S., Jouanneau, J.M., Weber, O., Lecroart, P., Radakovitch, O., Gilbert, F., Jezequel, D., 2007. Sedimentary processes in the Thau Lagoon (France): from seasonal to century times scales. Estuarine, Costal and Self Science 72, 534–542. Tanner, P.A., Pan, S.M., Mao, S.Y., Yu, K.N., 2000. g-ray spectrometric and a-counting method comparison for the determination of Pb-210 in estuarine sediments. Applied Spectroscopy 54, 1443–1446. Tylmann, W., 2004. Estimating recent sedimentation rates using 210Pb on the example of morphologically complex lake (upper lake Radunskie, N Poland). Geochronometria 23, 21–26. Uyttenhove, J., 2003. Capabilities and limitations of high-resolution gamma spectrometry in environmental research. In: Seventh International Symposium on the Natural Radiation Environment 7, pp. 445–455. Van den Bergh, G.D., Boer, W., Schaapveld, M.A.S., Duc, D.M., Van Weering, Tj.C.E., 2007. Recent sedimentation and sediment accumulation rates of the Ba Lat prodelta (Red River, Vietnam). Journal of Asian Sciences 29, 545–557.
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Recent changes in Red Lake (Romania) sedimentation rate determined from depth profiles of 210Pb and 137Cs radioisotopes R. Begy*, C. Cosma, A. Timar Faculty of Environmental Science, Babes-Bolyai University, Fantanele Str. 30, RO-400294 Cluj-Napoca, Romania
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 September 2008 Received in revised form 13 January 2009 Accepted 20 May 2009 Available online 21 June 2009
This work presents a first estimation of the sedimentation rate for the Red Lake (Romania). The sediment accumulation rates were determined by two well-known methods for recent sediment dating: 210Pb and 137 Cs methods. Both techniques implied used the gamma emission of the above-mentioned radionuclides. The 210Pb and 137Cs concentrations in the sediment were measured using a gamma spectrometer with a HpGe detector, Gamma-X type. Activities ranging from 41 7 to 135 34 Bq/kg were found for 210Pb and from 3 0.5 to 1054 150 Bq/kg for 137Cs. The sediment profile indicates acceleration in sedimentation rate in the last 18 years. Thus, the sedimentation process for the Red Lake can be divided in two periods, the last 18 years, and respectively, the period before that. Using the Constant Rate of 210Pb Supply method values between 0.18 0.04 and 1.85 0.5 g/cm2 year (0.32 0.08 and 2.83 0.7 cm/year) were obtained. Considering both periods, an average sedimentation rate of 0.87 0.17 g/cm2 year (1.17 cm/year) was calculated. Considering an average depth of 5.41 m for the lake and the sedimentation rate estimated for the last 18 years, it could be estimated that the lake will disappear in 195 years. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: 210 Pb 137 Cs Radioisotopes Dating methods Sedimentation rate
1. Introduction In 1837 in the metamorphic-Mesozoic zone of the Oriental Carpathians a relatively rare phenomenon occurred. A sliding of the Ghilcos Mountain blocked the Bicaz River leading to the formation of a natural barrage lake, which was afterwards named the Red Lake. The barrage resisted the vertical and regressive erosion as well as the static water pressure. The lakes formed by the natural barraging of valleys often have a short life, the formed barrage being eroded or quickly silted. In the case of the Red Lake, despite the significant water volume, the geology and consistency of the barrage did not allow the rapid erosion of this. A natural barrage can constitute damage for the natural dynamics of the environment, the sedimentation being a consequence of the tendency for reestablishing equilibrium. In the case of the Red Lake, there is the high sedimentation rate, which is strongly influenced by human activities (Pandi, 2004). One of the most important recent sedimentation dating methods is 210Pb method, which is a natural radionuclide resulted from 238U series (Arnaud, et al., 2006). Disequilibrium between
* Corresponding author. Tel.: þ40 7458 73206; fax: þ40 264307032. E-mail addresses: [email protected] (R. Begy), [email protected] (C. Cosma), [email protected] (A. Timar). 0265-931X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2009.05.005
210 Pb and its parent isotope in the series, 226Ra, arises through diffusion of the intermediate gaseous isotope 222Rn. A fraction of the 222Rn atoms, produced by the decay of 226Ra in soil escapes into the atmosphere where they decay through a series of short-lived radionuclides to 210Pb. This is removed from the atmosphere by precipitation or dry deposition, falling on the land surface or into lakes. The lead that falls directly in the lake is transported by the water column and deposited on the bottom of the lake together with the sediment. The ages obtained using this method turned out to be accurate especially in stable environments with constant sedimentation rates, where the calculation model is well defined (Schmidt et al., 2007; Van den Bergh et al., 2007; Tylmann, 2004). The method also provided good results for non-uniform sedimentation environments (where the sedimentation rate is not constant), though in this case the difficulty lies in finding of an appropriate sedimentation model. Two simple models, which are known as the constant rate of 210Pb supply (CRS) (McDonald and Urban, 2007; AlonsoHernandez et al., 2006) and the constant initial concentration (CIC) model (Appleby and Oldfield, 1978; Robbins, 1978) are usually applied. Another possibility for dating the lake sediments is by using the artificial radionuclides. The two sources of these are the nuclear weapon test that took place between 1953 and 1963 and the of Chernobyl atomic plant power station accident from 1986. The
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
645
Fig. 1. Sediment deposition over 171 year (Pandi, 2004) and sampling location.
radionuclides which were deposited and are present nowadays are: 90 Sr, 137Cs and 239-240-241Pu (Jeter, 2000). In the northern hemisphere the deposition reached a significant level until 1954 and grew rapidly later. The Chernobyl accident from 26th April 1986 injected a lot of artificial radionuclides in the atmosphere, this process continued during May, 1986. The main nuclides were 131I, 137Cs, 134Cs and 90Sr.
Radioactive depositions were made at thousands of km from the accident place in the northern hemisphere, being important in some region as in Poland (Gaca et al., 2006) or Romania (Cosma, 2002). The depositions are estimated to be 1017 Bq compared to 4.3 1017 Bq due to nuclear testing (Cambray et al., 1987). The radionuclide distribution was non-uniform and greatly controlled by winds and rains.
Fig. 2. Physical characteristics, porosity and water content in the core samples.
646
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Table 1 Radionuclide concentration in sediment cores. Core 1 Radionuclide Conc. (Bq/kg)
Core 2 Radionuclide Conc. (Bq/kg)
210
226
137
210
80 14 103 18 114 20 66 11 88 15
57 8 68 10 70 11 67 10 50 8
395 63 60 10 28 4 3 0.5 –
77 19 135 34 80 20
Pb
1 2 3 4 5 6
Ra
Cs
Pb
226
Ra
53 8 50 9 57 9
Core 3 Radionuclide Conc. (Bq/kg)
Core 4 Radionuclide Conc. (Bq/kg)
Core 5 Radionuclide Conc. (Bq/kg)
137
210
226
210
226
137
210
226
137
168 27 1090 175 68 11
101 23 78 21 93 21 69 19 62 16 48 14
68 10 64 9 64 9 56 10 62 9 58 8
67 11 110 19 60 10 41 7 57 10
53 8 54 8 49 7 45 7 67 10
205 33 679 50 91 81 3 0.5
59 10 111 20 98 17 103 20
65 10 52 5 63 10 63 13
228 36 1054 150 38 6 103 16
Cs
Pb
2. Materials and methods 2.1. Study site The Red Lake (25 780 N, 46 780 E) is situated on the north-eastern part of Romania, in the Oriental Carpathian Mountains, at a mean altitude of about 983 m above sea level. The lake has two sectors, a southern part of 1000 m long and a western part of 442 m. The maximum depth is 9.6 m the average depth of the lake being 5.41 m. The surface area and volume of the lake are 116,500 m2 and 606,500 m3 respectively. The annual rainfall in the catchments of the Red Lake ranges from 544 to 1026 mm, the monthly maximum rainfall being about 306 mm in June and the minimum being 35.8 mm in November (Pandi et al., 2004). Fig. 1 shows the shape of the Red Lake and the locations from where the sediment samples were gathered.
Ra
137
Cs
Ra
C
Pb
Ra
Cs
time will result in changes in the initial unsupported 210Pb concentrations, in accordance with equation (3). The dates of deposition for older sediments are not calculated from their present concentration but from the distribution of 210Pb in the sediment record. After decay, the present amount of 210Pb remaining in the record from inputs at time s in ds time interval is Pels ds
(4)
where P denotes the 210Pb supply rate. Assuming P constant, the residual sediments of age t or older is: A ¼
Z
N
Pels ds ¼
t
P
l
210
elt
Pb in the entire record, the The residual in this equation, is
210
Pb inventory, obtained by putting t ¼ 0
P
Að0Þ ¼
(6)
l
It follows that A ¼ Að0Þelt
(7)
The values of A and A(0) can both be calculated by numerical integration of the concentration versus depth profile. If m denotes the depth of the sediment layer of age t, A ¼
Z
N
CðmÞdm m
Að0Þ ¼
Z
N
CðmÞdm
(8)
0
From their values, the age of sediments of depth m is calculated using the formula t ¼
1
l
ln
Að0Þ A
(9)
From equations Cuns ¼ Cuns ð0Þelt
(10)
(10) and (3) for sediments of age t,
2.3. CRS model The total 210Pb activity in sediments has two sources, supported 210Pb which derives from in situ decay of the parent radionuclide 226Ra, and unsupported 210Pb which derives from the atmospheric flux: Ctot ¼ Ctot ð0Þelt þ Csup 1 elt
(1)
210
Pb). (where l is the decay constant of In the case of lakes with a constant sedimentation rate the activity of sediment at depth m decrease in concordance with disintegration law: CðmÞ ¼ Cð0Þelm=r
210
Pb in
(2)
and Cð0Þ ¼ P=r
(3)
is the atmospheric 210Pb activity in the superior part of sediment core and P denotes the mean annual rate of supply of fallout 210Pb to the sediments (Carroll et al., 1995). When the sediment accumulation is not constant throughout the time, the CRS model (constant rate of 210Pb supply) is applied. This model is based on the assumption that the rate of deposition of unsupported 210Pb from the atmosphere is constant. When the CRS model is valid, changes in the sedimentation rate through
Pb in
(5)
210
2.2. Sampling and analysis Core samples were collected at different location in the lake using a flag-gravity corer, its inner and outer diameter being 10.6 and 11 cm respectively. The length of the cores ranged in size from 24 to 60 cm. The columns were divided in 4–6 layers of 8–10 cm each. Aliquots (2–5 g) of the sliced core sections were used for measuring physical characteristics of the sediment sample, including bulk density, water content and porosity and the remaining of the sliced core section were dried in oven at a temperature of 70 C and stored in the laboratory for the radionuclide measurements (210Pb, 226Ra and 137Cs). The depth profiles of the porosity and water content are shown in Fig. 2. Measures of radioisotope concentrations have been performed using an ORTEC DigiDart spectrometer with a HpGE detector, Gamma-X (GMX) type, with a resolution of 1.92 keV at 1.33 MeV line of 60Co and a relative efficiency of 34.2%. The shielding of the detector is 40 cm in diameter, with an 8 cm lead wall. Inside the lead shielding, there is a Cu layer of 3 mm. The measurement geometry was cylindrical in all cases. The minimal spectrum acquisition time implied was 24 h. The activity of 210 Pb was determined using the 46.5 keV gamma emission with the relative intensity of 4%. 137Cs specific activity was estimated using the 661 keV (89.9%) gamma line (Tanner et al., 2000). 226Ra activities were determined based on the assumption of secular equilibrium with its progenies, using the 214Pb (351.92 keV with relative intensity of 37.2%) and 214Bi (609.4 keV with relative intensity of 46.3%) gamma emissions, after a month of storage (Masque et al., 2002). The detection limit for 210 Pb is 8 2 (2d) Bq/kg (Uyttenhove, 2003).
Pb
229 37 345 55 47 7 67 10 10 2 18 2
Fig. 3. Sedimentation rate calculated with
210
Pb and
137
Cs from Core 3.
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Fig. 4. Radionuclide distribution in sediment core 1, 2, 4, 3, 5 (210Pbtot, P r ¼ elt C
(11)
Since P ¼ lAð0Þ ¼ lAelt
(12)
226
Ra, and
647
137
Cs).
Sediment age, calculated from 137Cs distribution is in good agreement with the values obtained from 210Pb using the CRS model (Fig. 3). The 210Pb concentration profile (Fig. 4) shows an exponential decrease with depth for the lower part of the column, which is typical for lakes with a constant sedimentation rate. In the upper
it follows that the dry mass sedimentation rate at time t in the past can be calculated directly using formula r ¼
lA C
(13)
(Appleby, 2001).
3. Results and discussion The 137Cs, 226Ra and total 210Pb concentration in the surface sediments of the Red Lake are presented in Table 1. 137Cs originating from both sources can be noticed. For example, in the third core, the first peak (Chernobyl) appears at the depth 23 cm and a second peak originating for the nuclear weapon tests from 1963 can observed at 42 cm. After the physical investigation of the subsamples from the sediment core for dry and wet density were obtained values from 0.5 0.05 to 1 0.1 g/cm3 and 0.75 0.07 to 1.5 0.1 g/cm3 respectively. The porosity profile (Fig. 2) shows an exponential decrease with depth, which is characteristic for homogeneous sediments with uniform compaction. In first 10–15 cm the water content is increased which denotes recent sediment (Saravana Kumar, et al., 1999).
Fig. 5. The two periods in the sedimentation process.
648
R. Begy et al. / Journal of Environmental Radioactivity 100 (2009) 644–648
Table 2 Sedimentation rates in Red Lake. Sampling station (water depth in m)
1 (3) 2a (4) 3 (2.5) 4 (3.5) 5 (3.5) a
Average sedimentation rate Dry mass sedimentation rate (g/cm2 year)
Linear sedimentation rate (cm/year) First period
Second period
First period
Second period
1.49 0.35 0.95 0.23 1.27 0.3 1.2 0.3 2.77 0.6
0.5 0.1 0.6 0.1 0.42 0.1 0.45 0.1 0.55 0.13
1.35 0.35 0.6 0.1 1.5 0.37 0.93 0.26 1.5 0 .37
0.34 0.08 0.22 0.06 0.5 0.1 0.25 0.06 0.3 0.07
Was not used in calculation because there ware just few subsamples.
part of the sediment, the 210Pb concentration is decreasing due to the dilution of the atmospheric fallout and the increasing sedimentation rate. In the age depth, profile of the cores two separate sedimentation periods can be observed: a regulated sedimentation period followed by an interval characterized by an increased sedimentation rate (Fig. 5). We can consider that until approximately eighteen years ago, the Red lake sedimentation was uniform. This may be due to due to a regulated woodfelling and to the two settle dams built on Red Brook and Licas Brook. In the last eighteen years, more sediment was deposited in the lake because a considerable part of the suspended sediment from the water passed through the dams. Another reason may be the increased woodfelling in the drainage basin, which lead to a powerful soil erosion. In Table 2 the values for the calculated sedimentation rates are presented. The calculated dry mass sedimentation rates of all five cores range between 0.25 0.06 and 0.5 0.1 g/cm2 year with an average of 0.35 0.06 g/cm2 year from until eighteen years ago. Considering only the last eighteen years the calculated dry mass sedimentation rate is between 0.93 0.26 and 1.5 0.37 g/cm2 year with an average of 1.32 0.31 g/cm2 year (for all considered cores), which is a value four times higher. The values calculated for the sedimentation rates for the regulated sedimentation period (until approximately 18 years ago) are quite reproducible from one core to another (0.42 0.1– 0.55 0.13 cm/year) with an average value of 0.48 0.11 cm/year. As for the recent sedimentation period the sedimentation rates reach values between 1.2 0.3 and 2.77 0.6 cm/year with an average of 1.68 0.43 cm/year. If we consider an average depth of 5.41 m, and this sedimentation rate is maintained, the lake will disappear approximately 195 45 years. It worth being mentioned that based on stratigraphic methods, in 2004 G. Pandi et al., predicted that this would happen in 257 years. Comparing our results to this value, we can conclude that both values are realistic, as uncertainty limits cannot be ascertained to the stratigraphic method. 4. Conclusions Our study shows that the application of radio-chronological methods can provide valuable information on the sedimentation process for the Red Lake that can be used for gaining a better understanding of the environmental changes in the drainage basin. Five cores were studied, and it was concluded that all could be dated using the 210Pb method. All investigated cores presented a well-defined 137Cs peak with high concentration due to the high contamination of this region from Romania after the Chernobyl accident. 210Pb and 137Cs dating methods gave chronological information for all the cores, the values obtained using the two different techniques being in good agreement. The results obtained showed that two separate sedimentation periods, characterize sediment deposition in the Red Lake. A regulated sedimentation period until eighteen years ago, followed by a period of more
intense sediment deposition. If the higher sedimentation rate determined for the recent period is maintained or grows, serious changes in the lake ecosystem can occur leading to the aggrading of the lake in a relatively short time.
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