Uranium series isotopes concentration in sediments at San Marcos and Luis L. Leonreservoirs, Chihuahua, Mexico

Uranium series isotopes concentration in sediments at San Marcos and Luis L. Leon reservoirs, Chihuahua, Mexico C. Méndez-García, M. Renteria-Villalobos, R. García-Tenorio, and M. E. Montero-Cabrera Citation: AIP Conference Proceedings 1607, 83 (2014); doi: 10.1063/1.4890707 View online: http://dx.doi.org/10.1063/1.4890707 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1607?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Spatial analysis techniques applied to uranium prospecting in Chihuahua State, Mexico AIP Conf. Proc. 1607, 116 (2014); 10.1063/1.4890711 Potential human health risk by 234,238 U and 210 Po due to consumption of fish from the "Luis L. Leon" reservoir (Northern Mexico) AIP Conf. Proc. 1607, 70 (2014); 10.1063/1.4890705 Thorium isotopes in colloidal fraction of water from San Marcos Dam, Chihuahua, Mexico AIP Conf. Proc. 1544, 25 (2013); 10.1063/1.4813456 Leon M. 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Reina Mercedes s/n, 41012 Seville, Spain. * PhD Graduate Student at CIMAV a)Corresponding author: [email protected] Abstract. Spatial and temporal distribution of the radioisotopes concentrations were determined in sediments near the surface and core samples extracted from two reservoirs located in an arid region close to Chihuahua City, Mexico. At San Marcos reservoir one core was studied, while from Luis L. Leon reservoir one core from the entrance and another one close to the wall were investigated. 232Th-series, 238U-series, 40K and 137Cs activity concentrations (AC, Bq kg-1) were determined by gamma spectrometry with a high purity Ge detector. 238U and 234U ACs were obtained by liquid scintillation and alpha spectrometry with a surface barrier detector. Dating of core sediments was performed applying CRS method to 210Pb activities. Results were verified by 137Cs AC. Resulting activity concentrations were compared among corresponding surface and core sediments. High 238U-series AC values were found in sediments from San Marcos reservoir, because this site is located close to the Victorino uranium deposit. Low AC values found in Luis L. Leon reservoir suggest that the uranium present in the source of the Sacramento – Chuviscar Rivers is not transported up to the Conchos River. Activity ratios (AR) 234U/ 238U and 238U/226Ra in sediments have values between 0.9-1.2, showing a behavior close to radioactive equilibrium in the entire basin. 232Th/238U, 228Ra/226Ra ARs are witnesses of the different geological origin of sediments from San Marcos and Luis L. Leon reservoirs. Keywords: Uranium, 210Pb-137Cs dating, fresh water sediments, radioactivity analysis, Chihuahua. PACS: 91.67.Pq; 91.67.Qr; 91.67.Ty. INTRODUCTION Disequilibrium in uranium and thorium decay series is an important and potential indicators for tracing migration of the these radionuclides in the environment [1]. They can show changes and disturbance in the lithology of a selected site. Usually, (dis)equilibrium is analyzed by means of the activity ratios (AR) between given radioisotopes of the same or different decay series. Unat and Th concentration in the earth crust is around 1.7 and 8.5 mg kg-1 for U nat and Th respectively. They are associated to some oxides as uraninite, thorianite, which contents tend to have an increased value in igneous rocks. The distribution of natural radionuclides in sediment cores and their AR has been studied by several authors [2-7] to know the effects of soil properties in biogeochemical mobilization of these radionuclides and understand their behavior in the environment. The selected site for this study is located in the state of Chihuahua, Mexico's northern border. The Basin of Sacramento-Chuvíscar Rivers, which constitute the last major tributaries to the Rio Conchos, originates in the vicinity of a uranium deposit in San Marcos, Chihuahua, where a reservoir of the same name is located. The last Rio Conchos reservoir before joining the Rio Grande is created artificially by the Luis L. Leon "El Granero" dam [8]. Radiation Physics AIP Conf. Proc. 1607, 83-92 (2014); doi: 10.1063/1.4890707 © 2014 AIP Publishing LLC 978-0-7354-1243-9/$30.00 83 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 In Chihuahua State, several radioanomalies have been found. Some of them are located in San Marcos basin [9]. Here, both surface and ground waters are important resources to the region. The surface water gathered in San Marcos dam reservoir is used for agricultural purposes, whereas groundwater from shallow and deep wells is used for both agricultural and domestic activities. Previous studies in the mentioned site show the presence of uranium in surface waters, fish, and some plants [10-13]. Data about reservoir’s water radioisotopes content shows high uranium specific activities up to 7.7 Bq·L-1. The high uranium concentrations in the area are attributed to the lixiviation and/or erosion of the uraniferous deposits located in this area [14-16]. Natural uranium and thorium concentration has not been reported before in the Luis L. Leon area. The main purpose of this paper is to verify if Uand Th radioisotopes are transported from San Marcos area to Luis L. Leon reservoir. STUDY AREA, MATERIALS AND METHODS Two sites were selected for this stud, the first is San Marcos reservoir, located approximately 25 kilometers West-Northwest to Chihuahua City, Mexico. It was built in the beginning of the XX century. It has a capacity of 4.45 million m3 of water and is used in flood control and agriculture in the area. The lithology of the study area consists mainly of volcanic and volcanoclastic rocks of rhyolitic composition dikes and resurgent rhyolite domes and dikes of intermediate to basic composition [9]. It also has few outcrops of Cretaceous limestone in the northern boundary of the basin [16]. Uranium deposits at San Marcos, named as Victorino and San Marcos I, and their minerals have been recently described by Reyes-Cortés, et al. [17]. The mineral species found in the San Marcos deposits were: uranophane and metatyuyamunite at San Marcos 1 site; uranophane, uraninite, masuyite and becquerelite at Victorino site [17]. The other is the Luis L. Leon reservoir, located approximately 90 kilometers Northeast of Chihuahua City. It was built in 1968, having a water capacity of 854 million m3. The reservoir is used in the fish hatchery to control the species that there exist, farm, agricultural and domestic use. The rocks exposed in the region are limestone, igneous intrusive and volcanic, whose ages range is from the Upper Jurassic to the Recent. At least three abandoned mines are located near to this reservoir: Carrizalillo, Chorreras and La Verde [8]. Sampling Three sediment cores were collected manually following a well-established protocol with a steel tube (7 cm diameter and 40 cm length). Figure 1 shows the geographic location of Chihuahua State, as well as of the two reservoirs studied. One sediment core was extracted at 20 m depth from San Marcos reservoir in September 2007 (CSM), at a place close to the dam’s wall. The two cores from Luis L. Leon reservoir were taken at 10 m depth, one near to the reservoir’s entrance (ECLLL) and the other at the northern zone (NCLLL) in April 2012. Also some surface sediment samples were taken, close to the sediment cores sampling sites; table 1 shows the details of the location of sediment cores and surface sediments samples. FIGURE 1. Study area showing the location and the sampling points for a) San Marcos and b) Luis L. Leon reservoirs 84 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 TABLE 1. Sampling details of sediment cores and surface sediments collected at San Marcos and Luis L Leon reservoirs’ areas. Reservoir Sample type Sample ID Location (lalitude, longitude) San Marcos. Luis L. Leon. Core Surface sediment Surface sediment Surface sediment Surface sediment Surface sediment Core Core Surface sediment Surface sediment Surface sediment Surface sediment Surface sediment Surface sediment Surface sediment CSM SM1 SM2 SM3 SM4 SM5 ECLLL NCLLL LLL1 LLL2 LLL3 LLL4 LLL5 LLL6 LL7 28.74174, -106.35683 28.77962, -106.35839 28.78555, -106.36757 28.78999, -106.36956 28.79408, -106.36915 28.79525, -106.36228 28.88401, -105.30953 28.95626, -105.29211 28.94499, -105.27856 28.91572, -105.27756 28.92618, -105.29260 28.90981, -105.30293 28.86896, -105.31165 28.94045, -105.31855 28.96748, -105.30624 SM represents San Marcos area. LLL represents Luis L. Leon area. Laboratory analysis The cores were cut into 2 cm thick sections. Sections were dried at 55 ºC to constant weight, ground to a powder with an agate mortar pestle, homogenized by sieving to <63 μm grain size and packed for further analysis. Organic matter (OM) content was estimated by loss on ignition (LOI) at 550ºC according to [18]. 210Pb activity was determined by alpha counting of 210Po, assuming secular equilibrium between 210Pb, 210Bi and 210Po in the sediment after the sampling. The powder was digested with aqua regia (1:2) and 210Po extracted using the technique of liquid-liquid extraction with tributyl phosphate (TBP), autodeposited on cupper disc and measured using an Alpha Analyst TM (CANBERRA) spectrometer. Samples were spiked with 209Po as radiotracer yield [19-22]. 238U and 234U ACs were determined for all samples, except in surficial sediments from San Marcos reservoir. To obtain 238U and 234U ACs, all samples were spiked with 232U and put under the radioanalytical analysis procedure. Total sample dissolution was performed by atmospheric acid digestion using HFc and HNO3c (10:10). The uranium extraction was carried out by chromatography using UTEVA resins. Then, uranium was electrodeposited on stainless steel disc. Radiochemical yield was determined by the 232U counting rate [23-26]. The 238U, 234U ACs were also determined by alpha activity measurements, using the same Alpha spectrometer. All sections from the sediment cores and surface samples were analyzed for 238Uand 232Th-series isotopes (reported here as 226Ra and 228Ra), as well as 40K and 137Cs activities, by high resolution gamma spectrometry using a Canberra HPGe detector. Prior to the analysis, aliquots around 20 g were stored into petri dishes (15mm height per 75 diameter capacity), and hermetically sealed for at least 21 days in order to reach radioactive equilibrium between 226Ra and 222Rn, and their progenies. The detector was calibrated using RGU, RGTh and RGK IAEA certified reference materials in the same geometry as the measured samples. Counting time was 48 h; the analytical precision of the measurements was around 10%. Considering the appropriate corrections for laboratory background, the activity of 226Ra (from 238U-series) was determined from the 609 keV line of 214Bi; 40K activity from its 1461 keV line, 228Ra (from 232Th-series) activity was estimated from the 911 keV line of 228Ac and 137Cs from its 662 keV line. 85 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 To carry out the sediments’ dating procedure, total 210Pbtot activities were estimated by measuring the activity of 210Po, where secular equilibrium from 226Ra to 210Pb, 210Bi and 210Po in the sediment after packing is assumed [27] The sediment accumulation rate was obtained by the activity in excess 210Pbexc, as calculated from the difference of total and supported 210Pb. Supported 210Pb activity is equal to 226Ra activity, by the secular equilibrium with its progenies attained before the measurement. Dating 210Pbexc (unsupported) was calculated in all cores in order to know which model would be applied [19]. ࡼ࢈ ૛૚૙ ࢋ࢞ࢉ ൌ ࡼ࢈ ૛૚૙ ࢚࢕࢚ࢇ࢒ െ ࡼ࢈ ૛૚૙ ࢙࢛࢖ (1) The Constant Rate of Supply model was selected; this model assumes that the supply of 210Pbexc to the sediment is constant through time [28], although the profile of this isotope may reflect the interaction of the sedimentation rate (SR) and radioactive decay. Then, 210Pbexc and SR can be variable through the time. In this model the age t of each section at depth z is estimated by the following expression: ࢚ൌ ૚ ࣅࡼ࢈ ࢒࢔ቀࡵሺ૙ሻ ࡵሺࢠሻቁ (2) Where I(0) (Bq m-2) is the total 210Pbexc inventory of the sediment core and I(z) is the total 210Pbexc in the sediment layer below depth z. The sedimentation rate R (kg m-2 y-1) is obtained directly from: ࡾൌࣅࡼ࢈ࡵሺࢠሻ ࡭ሺࢠሻ (3) Estimated ages obtained by applying this method were confirmed by comparison of the value derived from the 137Cs activity measurements. Fallout of 137Cs introduce this radioisotope in the water, and then in sediments. 137Cs AC in sediment profiles has a peak in the vicinity of 1962-64 years associated to nuclear bomb testing in the atmosphere [29, 30], and in northern hemisphere, it has another peak related with year 1986 from the radionuclide dispersion produced by the Chernobyl accident [31-33]. RESULTS AND DISCUSSION Sampling site of sediments affects both lithology and the environmental factors that may alter the weathering of the elements; consequently, sediments’ radionuclides concentrations will be affected accordingly. The San Marcos reservoir has a narrowing that divides it into two vessels (West and East), and this may influence the sediment transport downstream along the lake (Figure 1). The sediment core CSM was collected in the vessel nearer to the dam’s wall. This location is close to a spring located at the bottom of the San Marcos reservoir. The San Marcos dam collects water that runs from the West at the birth of the creek of the same name, near the Victorino uranium deposit during the rainy seasons. During these periods turbid water occurs frequently and it shows high organic matter content. During periods of drought in the lake the water level drops and the water looks very clean one. Total 210Pbtot activities ranged from 84 to 313 Bq kg−1, and the supported 210Pbsup activity varied from 60 to 275 Bq kg−1. 210Pbexc activities were obtained by eq. (1). The 210Pbexc inventory in CSM was estimated to be 207.5±4 Bq m−2, the mean 210Pbexc AC was 34.8 Bq kg-1 and its distribution did not show a monotonic behavior whereby the CRS model dating was applied (Figure 2). The 210Pbexc distribution is characterized by a near exponential decline, behavior typical of undisturbed soils. The model employed was corroborated by 137Cs activities, where its maximum concentration were found at 36 cm depth, corresponding to 1963 weapons fallout peak, and at 16 cm depth, relevant to Chernobyl accident in 1986. The results obtained by 86 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 CRS method were accepted, as there is no discrepancy with the 137Cs maker. The sedimentation rate calculated by eq.(3) shows high variability with time. These results were compared with data of rainfall in the region and a high correlation (r = 0.89) was observed. Consequently, low values in sedimentation rate belong to drought seasons and conversely high sedimentation rates are associated with periods of high rainfall contribution to the reservoir. FIGURE 2. 210Pb, 137Cs concentration activities and sedimentation rate profiles (with uncertainty about 3 years) in sediment core in San Marcos reservoir (CSM). All radionuclide concentration values were different if compare them in surface and core sediments (Table 2). Radionuclide concentrations were higher in surface sediments than in the sediment core, except for 137Cs, because it corresponds to the two mentioned fallout events. TABLE 2. Comparison of radionuclide concentrations among core and surface sediments at San Marcos reservoir. Different letter means that significant difference exists. Letter A means higher concentration. 228Ra 238U 226Ra 40K 137Cs CSM B B B B A SMS A A A A B 40K, 228Ra (232Th) , 226Ra and 238U AC mean values in surface sediments were 1164, 377, 856, and 893.5 Bq kg-1respectively (table 3); these results are alike those found in parent rocks in a previous study of the San Marcos – Victorino area [34] and correspond to their sampling locations, as pointed out above. On the contrary, core sediments’ ACs are lower. These concentrations suggest that adsorption in core sediments correspond to the characteristics of the water column, provided in part by the spring located at the bottom of the reservoir, near the point where the sediment core was extracted. Activity concentrations in both surface and core sediments are higher than the reported by UNSCEAR (2000), where reference values for earth crust are 35, 35, 30 and 400 Bq kg-1 for 226Ra, 238U, 228Ra and 40K, respectively. In core sediments 234U and 238U content shows a quite constant distribution (coefficient of variation =10%). Activity ratios (AR) of U-series isotopes help in analyzing (dis)equilibrium and they may serve to reveal the relationship between factors such as weathering and adsorption [35]. The 234U/238U AR in core sediments were between 1.1 – 1.4 with a mean of 1.2, values close to the equilibrium (Table 3). High AR values were found in layers close to the water interface. This observation is consistent with values reported for San Marcos reservoir water in [14], that were obtained in a sampling performed at 2005, when the water level in reservoir was very low and most water in the vessel probably was supplied by the above mentioned spring. 210 137 87 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 40K, 228Ra (232Th) and 226Ra ACs show a coefficient of variation >20% in the sediment core profile (Figure 3); 40K AC shows some increasing behavior with depth, finding the highest at the bottom of the core. 228Ra (232Th) shows variability in the profile due to high AC found in the first and last layers; if these values were considered as outliers, the behavior of the 228Ra (232Th) would be almost constant with depth, which could be explained by the low solubility of thorianite (ThO2) and Th strong sorption onto the sediments. 226Ra AC shows a non-monotonic behavior in the core profile. The values of 226Ra ACs divide the layers into two main groups, one for the layers closest to the surface, with ACs between 83 and 105 Bq kg-1, and the other group for the deepest layers, with ACs between 57 and 71 Bq kg-1. This behavior suggests that in deep strata reducing conditions prevail where 226Ra is more soluble, while in shallow layers oxidation conditions could be found. TABLE 3. Statistics of radionuclide concentrations and AR in San Marcos area. CSM 40K 226Ra 228Ra 238U 234U 228Ac/238U 238U/226Ra 226Ra/228Ra 234U/238U Mean 971.9 80.4 147.6 92.7 112.7 1.6 1.2 0.6 1.2 S.D 181.9 16.4 30.4 9.6 10.5 0.3 0.3 0.2 0.1 Range 749.9 - 1257.6 57 - 104.8 106.9 - 210.6 78.7 - 106.7 99.5 - 125.9 1.1 - 2-4 0.8 - 1.7 0.3 - 0.7 1.1 - 1.4 C.V (%) 18.7 20.4 20.6 10.4 9.3 20.8 24.0 30.3 6.5 SSM Mean 1163.7 856.1 376.8 S.D 26.2 68.7 71.6 Range 1123.8 - 1185.9 738.2 - 897.6 321.3 - 501.9 C.V (%) 2.3 8.0 19.0 238U-and 232Th-series isotopes commonly occur together in nature; the 232Th/238U mass ratio is around 3.5 in almost all natural systems [36]. AR mean value in earth crust is 1.2 [37]. The 228Ra (232Th) /238U AR mean value in the sediment core was higher than 1.2. 238U/226Ra ARs show disequilibria (AR ≠1) in the 238U chain at the first layer and the deepest four in the sediment core, where were found AR values between 1.3 and 1.7. This behavior may be explained by their differential ion mobility [6]. 228Ra (232Th) /226Ra AR mean value in the core was 1.8, higher than AR=1.1 reported by (Evans 1997) in most environmental samples. Usually this AR is applied to assess the conservation of the proportion within 232Th and 238U decay series. Thereby, in this sediment core the expected proportion could not been observed. FIGURE 3. Distribution of radionuclides in sediment core in San Marcos reservoir. 88 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 Luis L Leon reservoir collects water that comes from the Rio Conchos basin. Its elongated shape has a length of approximately 12 km. Its relatively recent construction from a rough relief area but with a steady stream of water, produces a reservoir bottom of abundant vegetation including trees, which are now submerged. The vegetation and the topography may play the role of filtering the solutions and suspended material in the water flowing along the reservoir. Both entrance and northern sediment cores (ECLLL and NCLLL) extracted from Luis L. Leon reservoir show a non-monotonic distribution of 210Pbexc concentration with depth, but they have an exponential decreasing. The CRS model dating was applied for them. 210Pbtot activities ranged from 48 to 59 Bq kg−1, and 41 to 55 Bq kg−1 for ECLLL and NCLLL, respectively (Figure 4). The 210Pbexc inventory in ECLLL and NCLLL were estimated to be 194 ± 5 and 179 ± 3 Bq m−2, respectively. The age calculated for the deepest layer of the ECLLL was down to the year 1976, whereas for the NCLLL it was 1971. The maximum 137Cs activity concentration was found around 1989 in both cases, which suggest the acceptance of the dating model applied in this reservoir by the radioisotope concentration of the Chernobyl accident fallout. The sedimentation rates in ECLLL are correlated with rainfall (0.708 p<0.001) but not in the case of NCLLL. This could be related to the variability of the annual water discharge from the dam, producing changes in the sedimentation rates near dam wall. Unat concentrations were one order of magnitude higher in SMS and CSM than in sediments from the Luis L. Leon reservoir area. Activity concentrations of Unat isotopes were higher in ECLLL than in NCLLL and SLLL (Table 4), suggesting that radionuclides are transported by the different streams reaching the reservoir. This feature suggests that radioisotopes are filtered from the water by the reservoir accidents, preventing their further transport downstream to the Rio Conchos. 232Th, 238U, 226Ra ACs did not show difference between NCLLL and SLLL, suggesting that the core taken in this point could retain the features of the Luis L. Leon surface lithology; the difference in 40K concentration in these samples is due to low content of feldspar in NCLLL. FIGURE 4. 210Pb, 137Cs concentration activities and sedimentation rate profiles in sediment core in Luis L. Leon reservoir. ECLLL means entrance and NCLLL Northern Core. 89 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52 TABLE 4. Comparison of radionuclide concentrations among core and surface sediments at Luis L Leon reservoir area. Different letter means that significant difference exists. Letter A means higher concentration. Only ACs values of 40K were above the reference values established by [38] (400 Bq kg-1) (Table 5). The high concentration of potassium corresponds to the feldspar appearing in the diffraction patterns. Activity concentrations were homogenous in both sediment cores, the lower ACs were found in NCLLL. 228Ra(232Th)/238U AR in ENLLL, NCLLL and surface sediments were about one, as in the upper earth crust. 234U/238U ARs are alike in both sediment core (1.1 – 1.2) indicating equilibrium between isotopes. The same may be observed for 226Ra/238U AR. The radionuclide concentrations in both sediment cores did not show high temporal variability (C.V <20%). 238U and 234U showed almost the same behavior in both sediment cores, the concentrations of these radionuclides from certain depth to the bottom of the cores is almost constant (Figure 5). FIGURE 5. Distribution of radioisotopes in sediment core in Luis L. Leon reservoir. 228Ra 238U 226Ra 40K 137Cs ECLLL A A A A A NCLLL B B A B A SLLL B B A A B 90 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 54.152.109.166 On: Thu, 24 Sep 2015 17:03:52