SEDIMENT ACCUMULATION RATE IN THE RED LAKE (ROMANIA) DETERMINED BY Pb-210 AND Cs-137 RADIOISOTOPES

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1 EARTH AND ENVIRONMENTAL PHYSICS SEDIMENT ACCUMULATION RATE IN THE RED LAKE (ROMANIA) DETERMINED BY Pb-210 AND Cs-137 RADIOISOTOPES R. BEGY, C. COSMA, Z. HORVATH Faculty of Environental Science, Babes-Bolyai University, Fantanele Str, 30, RO , Cluj-Napoca, Roania, E-ail: Received July 25, 2008 This work presents the first estiates of the Red lake s sedientation rate. The deterination of the sedient accuulation rate was estiated by using two ethods of recent sedient dating: the Pb-210 and the Cs-137 ethod. Both ethods are based on using gaa eission radionuclides. The Pb-210 and Cs-137 concentration in the sedient were easured with a gaa spectroeter with a Hp-Ge detector, GMX type. There has been found activities between Bq/kg for Pb-210 and Bq/kg for Cs-137 respectively. Using the CIC ethods for the calculation of sedientation rate, we obtained a c/year value, which is an average for this lake. Key words: Pb-210, Cs-137, radioisotopes, dating ethods, sedientation rate. INTRODUCTION In 1837 in the etaorphico-esozoic zone of the Oriental Carpathians a relatively rare phenoenon happened, the slope sliding of the Ghilcos Mountain blocked the Bicaz River and fored a natural barrage lake which received the Red nae later. The barrage resisted the vertical and regressive erosion as well as the static water pressure. The lakes fored by natural barrage of the valley have often a short life, because the fored barrage erodes or are quickly silted. The Red Lake is a special case, because the accuulated water s volue after the valley s barrage was significant and the geology and consistency of the barrage did not allow the quick erosion of this. We don t have to forget that the barrage, even natural, constituted daage for the natural dynaic of that side. The sedientation is a consequence of the reestablishing tendency of this equilibriu. In the case of the Red Lake there is the high sedientation rate, which is highly influenced by en ade activities (G. Pandi, 2004). Ro. Journ. Phys., Vol. 54, Nos. 9 10, P , Bucharest, 2009

2 944 R. Begy, C. Cosa, Z. Horvath 2 In the process of the phenoenon s analyze we have to consider that because of the foration of the Red Lake, it is considered like an accuulation lake regarding the sedientation. One of the ost iportant recent sedientation dating ethod is the Pb-210 ethod, which is the natural radionuclide result fro U-238 series. In any cases, the ethod turned out to be accurate, especially in stable environent with constant sedientation rate, where the calculation odel is well defined. The ethod also provides accurate results for non-unifor sedientation (where the sedientation rate is not constant), the difficulty is the finding of an appropriate sedientation odel. Two siple odels, which are called the CSR and CIC odels (Appleby & Oldfield, 1978, Robbins, 1978), are usually applied. Disequilibriu between Pb-210 and its parent isotope in the series, Ra-226, arises through diffusion of the interediate gaseous isotope Rn-222. A fraction of the Rn-222 atos, produced by the decay of Ra-226 in soil, escapes into the atosphere where they decay, through a series of short-lived radionuclide to Pb-210. This is reoved fro the atosphere by precipitation or dry deposition, falling on the land surface or into lakes. The lead which falls directly in the lake is transported by the water colun and deposited to the botto of the lake together with the sedient. Another possibility for dating the lake sedients is the artificial radio nuclides. The two sources of this are the nuclear weapon test between 1953 and 1963, and the accident of Chernobyl atoic plant power station fro The radio nuclides which were deposited and are present nowadays are: Sr-90, Cs-137, Pu In the northern heisphere the deposition reached a significant level until 1954 and grew rapidly later. In 1958 it was noticed a drop and it was followed by an increase because of the restarting of the testing. After the agreeent fro 1963, the Cs-137 deposition diinished, but at the beginning of the 70's it increased again, caused by the states which did not sign the agreeent. The Chernobyl accident fro 26 th April 1986 injected a lot of artificial radionuclides in the atosphere, this process continued during May, The principal radionuclides were I-131, Cs-137, Cs-134. The depositions were at thousands of k fro the accident place, in the northern heisphere, these depositions are estiated to Bq copared to 4,3x10 17 Bq which was provided fro nuclear testing (Cabray et al., 1987). The radionuclides distribution was non-unifor and greatly controlled by winds and rains. In soe parts of northern Europe (Scandinavia, Great Britain) and central Europe the deposition in the first days of Cs-137 was higher than the total depositions provided by nuclear weapon.

3 3 Sedient accuulation rate in the Red lake 945 MATERIALS AND METHODS Fig. 1 shows the shape of the Red Lake and the places where the sedient saples were gathered fro. The profile depth is also arked. The coluns were divided in 4 6 layers of 8 10 c each. Fig. 1 Soe lake characteristics and sapling points. Excess Pb-210 in sedient, over that in equilibriu with the in situ Ra-226 in sedients, decays in accordance with the radioactive decay law, t Pb Pb Ra t C C (0) e C (1 e ) (1) (where is the disintegration constant of Pb-210). This equation can be used for sedientation dating. In the lakes where the erosion process fro catchents area and residence tie in water colun are constant, the result is a constant sedientation rate. It can be supposed that each sedient layer has the sae quantity of atospheric Pb-210 activity (excess Pb-210) at the sedientation oent. Therefore the sedient fro a certain depth (dry deposition in g/c 2 ) will be of the age: t (2) r where r is sedientation rate (g/c 2 y) and atospheric Pb-210 activity will vary with the depth after: C C e / (3) ( ) (0) r where is the constant for Pb-210 (0,03114 year 1 ) and C ( 0) P / r (4)

4 946 R. Begy, C. Cosa, Z. Horvath 4 is the atospheric Pb-210 activity in the superior part of sedient core and P denotes the ean annual rate of supply of fallout Pb-210 to the sedients. When it s represented in logarithical scale the activity for Pb-210 versus depth gives a linear profile. The sedientation rate average r can be deterined fro the graphic (J. Carrol et al., 1999). The easures were ade using an ORTEC Digidart spectroeter with a HpGE GMX type detector, it has 1.92 kev resolution at 1.33 MeV line of Co-60 and the relative efficiency of 34.2%. The lead shielding of the detector with 40 c diaeter, with 8 c wall, inside the lead shielding is a Cu layer of 3. The shielding is very iportant especially in the case of environental saples when the saple activity is saller than the background activity. The used geoetry was of Sarpagan type, a cylindrical box with adequate diensions. The inial tie of acquisition was of 24 hours. The activity concentration of lead 210 is deterined using 46.5 kev gaa line with relative intensity of 4% and the Cs-137 fro 661 kev peak (Tanner et al., 2000). Ra-226 is deterined using the radon peaks after a onth of storage, when it reaches the equilibriu between Ra and Rn progeny. The detection liit is calculated using the forula below (J. Uyttenhove, 2003): f t A MDA( E ) 2 br B A / 4 A /2 where A = 5 for an error of 20% detector efficiency; f relative intensity of peak at 46 kev; t easuring tie (s); b shape factor = approx. 2 for syetrical Gaussian peaks; R energy resolution of 46 kev peak (channels); B background (counts on channels). For Pb-210, in our conditions, this value is 8 3(2 ) Bq/kg. 2 RESULTS AND DISCUSSION The values obtained for the radionuclide concentration in sedient layers can be seen in Figs In these figures we can notice the proble of the first layer. This eans that during the collection of the saples a big proble is to aintain the first layer in the initial for. In any cases there are losses of aterial fro the first layer of the sedient. Fro each sedient colun was calculated an average sedientation rate, at the sapling point. The sedientation rates were calculated using CIC and CRS odels. We applied both odels and each gave the sae results. With CRS odel it can be seen that in the last 7 8 years the sedientation process increased, in soe cases the values reached double.

5 5 Sedient accuulation rate in the Red lake 947 In CIC odel, for Pb-210 and Cs-137 concentrations we used two atheatical fittings. The sedientation rate was calculated fro the fittings and fro the atheatical odel and it gave between c/year. Figs show the sedient dates versus depth, and the sedientation rate of each sapling point. Fig. 2 Radionuclide concentration in sedient-point 1. Fig. 3 Radionuclide concentration in sedient-point 2. Fig. 4 Radionuclide concentration in sedient-point 3. Fig. 5 Radionuclide concentration in sedient-point 4.

6 948 R. Begy, C. Cosa, Z. Horvath 6 Fig. 6 Radionuclide concentration in sedient fro point 5. Fig. 7 Sedientation rate in point 1. Fig. 8 Sedientation rate in point 2. Fig. 9 Sedientation rate in point 3. The results for the sedient layer age obtained fro Pb-210 were copared to the ages resulted fro Cs-137, and this gave very appropriate results as Pb-210. For the Red Lake sedientation rate we can calculate an average value fro all five cores c/year. For the calculation of the total fill up of the lake were used the average values of the two furthest points (point 1 and 3). If we consider an average depth of the lake of 5.41, the total fill up will be in approxiately year (G. Pandi, 2004). Deterinations by other ethods predict 257 years (G. Pandi, Cs Horvath). If the accentuated

7 7 Sedient accuulation rate in the Red lake 949 Fig. 10 Sedientation rate in point 4. Fig. 11 Sedientation rate in point 5. sedientation is aintained to increase, the lake can disappear in less than years (CIC odels for point 1). REFERENCES 1. G. Pandi, Cs. Horvath, Riscuri asociate colatãrii Lacului Roºu, Studia Universitas G. Pandi, Lacul Roºu. Studiu de hidrogeografie, Ed. Casa Cãrþii de ªtiinþã, Cluj J. Uyttenhove, Capabilities and liitations of high-resolution gaa spectroetry in environental research, Radioactivity in the environent, Volue 7, J. Carrol et al., Sedient ages and flux variations fro depth profiles of Pb-210: lake and arine exaples, Applied Radiations and Isotopes 50, , P. A. Tanner, S. M. Pan, K. N. Yu, g-ray Spectroetric and a-counting Method Coparison for the Deterination of Pb-210 in estuarine Sedients, Applied Spectroscopy, Volue 54, Nuber 10, P. G. Appleby, Chronostratigraphic Techniques in Recent Sedients, Tracking environental change using lake sedients, Volue 1, P. G. Appleby, F. Oldfield, Application of Pb-210 to sedientation studies. In Ivanovich, M. & R.S. Haron (eds.) Uraniu Series Disequilibru: Application to Earth, Marine & Environental Sciencies, Oxford University Press, Oxford: , R. S. Cabray, P. A. Cawse, J. A. Garland, J. A. B. Gibson et al., Observation of radioactivity fro the Chernobyl accident, Nuclear Energy 26, W. Hewitt, et al., Deterining the Ages of the Recent Sedients using Measureents of Trace Radioactivity, Terra et Aqua, Nuber 78, 2000.