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Radiochemistry of sediments from the southern Dead Sea, Jordan N. S. Abu-Jaber 7 B. A. Al-Bataina 7 A. Jawad Ali
Abstract A sediment core from the southern Dead Sea was analyzed using gamma spectroscopy as well as 210Pb dating in order to ascertain if any radioactive contamination could be detected and to determine the sedimentation rates in the area. Results of this study show no presence of man-made radionuclides in the area. Sedimentation rates were determined to be between 0.25 and 0.4 g/cm 2/year. (F0.5 cm/year), which is in line with what would be expected assuming carbonate layers are annual varves.
fission products and uranium daughters have been incorporated into the sediments of the area throughout the length of time represented by the sediment core. This time was evaluated by radiochemical techniques, specifically by 210Pb dating.
210-Pb dating
The use of 210Pb to date sediments ranging from a few years up to 100 years is a common tool in modern oceanographic (Nittrouer and others 1979) as well as limnological studies (Bollhofer and others 1994). Sediments in Key words Dead Sea 7 Dimona 7 Jordan 7 modern marine and lacustrine environments accumulate Radiochemistry 7 210Pb dating 210 Pb in two ways. In part 210Pb is derived from the 238U decay series, due to the presence of the parent isotopes, specifically 226Ra within the sediment. 210Pb from this source quickly reaches secular equilibrium with the 226Ra Introduction in the sediment, and remains constant at the time scale of interest to this type of study. 210Pb from this source is The issue of radioactive contamination in southern Jortermed “supported”, as opposed to the second source, dan has recently become a focus of popular concern. This which is derived due to the decay of 222Rn in the atmosphere. As 222Rn decays, 210Pb is formed. Subsequently is largely due to the proximity of the Dimona Nuclear the 210Pb adheres to clay particles present in soils or in Power Plant (NPP) in southern Israel. The secrecy sursuspended sediments in water bodies. This 210Pb is “unrounding this site and press reports highlighting safety supported”, and subsequently decays at a rate proporproblems associated with the storage of the waste products has caused speculation about leakages from the reac- tional to its half-life of about 22.5 years. This unsuptor. Additionally, high concentrations of naturally occur- ported 210Pb is commonly termed excess 210Pb and is the basis of dating sediments and determining sedimentation ring radon in the Jordan Valley Rift area have recently rates in modern marine and lacustrine environments. been documented (Steinitz and others 1992). The source Differentiating between supported and excess 210Pb is of this gas is believed to be from deep-seated faults. The purpose of this study is to evaluate the nature of the simply a matter of monitoring where 210Pb becomes conradioisotopes present in the study area near the Dimona stant with depth in a core. This constant rate of 210Pb activity is the supported 210Pb, and the excess 210Pb is thus NPP. A sediment core from the southern Lisan Bay area the difference between total and supported 210Pb activity. in the southeastern corner of the Dead Sea was taken in order to evaluate whether measurable concentrations of Received: 31 January 1997 7 Accepted: 11 March 1997 N. S. Abu-Jaber (Y) 7 A. Jawad Ali Department of Earth and Environmental Sciences, Yarmouk University, Irbid 21163, Jordan E-Mail: abujaber6yu.edu.jo B. A. Al-Bataina Department of Physics, Yarmouk University, Irbid 21163, Jordan
Study area The Dead Sea occupies the lowest point in the Jordan Valley Rift system (JVR). The JVR is a major left-lateral transform fault which separates the Arabian and African crustal plates, and constitutes the continuation of rifting of the Red Sea. This system has led to the formation of a series of downfaulted blocks, thus leading to the descrip-
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tion of the system as a “leaky” transform fault (Garfunkel 1981). Sedimentation in the rift system has continued since the Miocene. The study area is in the Lisan Bay area in the southeastern coast of the Dead Sea, about 2 km north of Ghor Haditheh (Fig. 1). This location was deemed the most suitable for the study because it lies at the mouth of Wadi Ibn Hammad, which drains a significant portion to the Karak Mountains adjacent to the Dimona NPP. Release of radionuclides from Dimona would surely be seen in the sediments here. The susceptibility of sediments to erosion here is minimal, and thus the core represents an almost complete time sequence. The abiotic nature of the Dead Sea ensures the lack of bioturbation, which is confirmed by the preservation of the primary sedimentary structures. Such structural preservation would help determine the date of release of fission products. Thus the location seems ideal for this work.
Materials and methods A sediment core was taken using a 2 inch diameter polyethylene pipe which was previously split down the middle and subsequently taped back together. After drying the core for about three days, the tube was opened and the sediment exposed, showing excellent preservation of the sedimentary structures (Fig. 2). The sample was divFig. 2 The sediment core in three segments. Scale is in centimeters
Fig. 1 Location map of the core. The drainage basin of Wadi Ibn Hammad is marked. Inset is a view of the area
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ided into 18 near-equal sections and dried. The total length of the sampled core was about 65 cm. Each sample was scanned for gamma radiation using an Ortec GeLi detector coupled with a computer for control, data reduction and analysis. Each sample was scanned for at least 8 h. Excess 210Pb was determined using the extraction technique described by Nittrouer and others (1979). This technique is based on the extraction of 210Po, a daughter of 210Pb. About 9 g of dry sample was weighed and treated with an equal mixture of nitric acid and perchloric acid. The mixture was heated to near-dryness and subsequently treated with hydrochloric acid. Again the sample was heated to near-dryness and brought to volume with dilute hydrochloric acid. The sample was then centrifuged for about 1 h, and the resulting separated solution was placed in a beaker with a silver plate for autoelectrodeposition. The silver plate was later placed in an alpha counting system for at least 24 h. The counting system used consisted of a surface barrier detector in a vacuum tight chamber. The data was recorded using a multichannel analyzer. Because 208Po was not available as a tracer as prescribed in Nittrouer and others (1979), the extraction was conducted without it, and alternative methods were used to evaluate the efficiency of the extraction, as we will explain.
Cases and solutions
Since the sediment consists of a mixture of clay minerals and calcium carbonate, it became necessary to determine the ratio of each component in each sample. This is due to the fact that the radionuclides are almost exclusively tied to the clay particles, and this was taken into account when analyzing the data. Calcium carbonate content was determined using a titration method.
Results
Table 2 Results of 210Pb analyses of sediment samples Sample
dph
Sample weight (g)
% Det.
Detrial wt.
dph/gdet
1 5 6 10 11 12
21.3 20.5 25.4 15.2 18.1 13.6
8.920 8.920 9.020 9.020 9.410 9.020
55.0 34.0 45.0 70.0 35.0 79.0
4.91 3.03 4.06 6.31 3.29 7.13
4.34 6.76 6.27 2.41 5.50 1.91
The data obtained from the GeLi detector were analyzed and the suspected corresponding radionuclides are presented in Table 1. Results of 210Po plating are shown in Table 2 and in Fig. 3. Since it is apparent that the point where excess 210Pb disappears in the core is not reached, it is concluded that supported 210Pb activity is between 0 and 1.8 disintegration per hour per gram of detrital material (dph/gdet). The top sample has a lower total activity than samples deeper within the core. This is probably because the top sample is reworked, as is seen from the sedimentary structure within the core (Fig. 2). This debris-flow depositional surface probably represents one winter flood event. Given the slope of the 210Pb decay and assuming supported 210Pb of between 0 and 1.8 dph/ gdet, a sedimentation rate of between 0.25–0.4 g/cm 2/year is concluded for this site. Thus the bottom of the core (at Fig. 3 65 cm) is between 6 and 7.5 210Pb half-lives, i.e., between Cumulative sedimentation versus Table 1 Peaks identified in the gamma energy spectra of the core samples Centroid energy (keV)
Suspected radionuclide
Half-life
45.6 74.4 76.8 84.4 86.6 92.4 185.9 238.4 241.85 295.1 351.9 583.4 609.5 768.5 911.6 934.4 1120.7 1238.4 1377.9 1408.2 1461.0 1509.5 1729.3 1764.2
Am-243 Bi-210m Ba-142 Th-228 Sn-126 Th-234 Ra-226 Pb-212 Ru-103 Pb-214 Pb-214 Tl-208 Bi-214 Bi-214 Ac-228 Bi-214 Bi-214 Bi-214 Bi-214 Eu-152 K-40 Bi-214 Bi-214 Bi-214
7.36!10 3 years 3!10 6 years 10.6 min 1.91 years 10 5 years 24 days 1.59!10 3 years 10.6 h 39.4 days 26.8 min 26.8 min 3.07 min 19.9 min 19.9 min 19.9 19.9 19.9 19.9
min min min min
1.28!10 9 years 19.9 min 19.-9 min 19.9 min
210 Po activity per gram of detritus, which is defined as the total weight minus the carbonate weight. Error associated with the counting technique is about 4%. Line represents least squares fit
132 and 165 years old. If we assume that carbonate lamina represent annual summer season deposition, counting of the lamina yields an age of between 120 and 130 years.
Discussion and conclusions The assemblage of gamma emitting radionuclides present in the sediment core includes a number of daughters of 238 U; i.e., 234Th, 226Ra, 214Pb, 214Bi, and 210Bi. The presence of these radionuclides is consistent with growth of these daughters from their parent, which is readily available in the geological environment of the area. Additionally, daughters of 226Ra may be out of equilibrium with uranium due to the decay of radon which emanates from the faults in the area (Steinitz and others 1992). Thorium daughters in the area also present include 228Th, 228Ac, and 212Pb. Another natural radioactive source in the sediment is 40K, which is not surprising due to the abundance of potassium in the Dead Sea of interest are the man-made radionuclides found in the sediments. There is no indication of the presence of any gamma emitting fission or neutron activation products in the sediment core. 210 Pb dates suggest that this core represents sediments
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going back to 130–160 years and thus represents the entire span of time where humans have produced manmade radionuclides. This indicates that, barring releases during periods of hiatuses, no measurable releases of radionuclides have occurred from the Dimona NPP.
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References Bollhofer A, Mangini A, Lenhard A, Wessels M, Giovanoli F, Schwarz B (1994) High resolution 210Pb dating of Lake Constance sediments: stable lead in Lake Constance. Environ Geol 24 : 267–274 Garfunkel Z (1981) Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80 : 101–117 Nittrouer C, Sternberg R, Carpenter R, Bennett J (1979) The use of Pb-210 as a sedimentological tool: application to the Washington continental shelf. Mar Geol 31 : 297–316 Steinitz G, Vulkan U, Lang B, Gilat A, Zafrir H (1992) Radon emanation along border faults of the rift in the Dead Sea area. Isr J Earth Sci 41 : 9–20
