Anthropogenic Radioisotopes in the Topsoil of Qatar: Is it Something to Worry About?
Authors: Amr Mohamed A. 
Published: July 2019

1. Background

Depleted uranium (DU) has been used extensively during weapons testing and recent military conflicts. All three main isotopes of U (235U, 236U, 238U) are radioactive. The firing of depleted uranium (DU) weapons during conflicts and military testing has resulted in the deposition of DU in a variety of sand-rich environments. Iraq, a near neighbor to Qatar, has received extensive (400 tons) DU contamination from the two wars in 1991 and 2003 (Shomar et al., 2013). Radioactive fallout from past accidental releases of radioisotopes during ground nuclear tests has contaminated the globe with radioactive materials. Most of these radio-contaminants are fission products of uranium and plutonium. Among these, 90Sr, 137Cs, 235U, 238U, 238Pu, 239Pu, and 240Pu have been identified in some soils around the world. Therefore, radioisotopes represent long-term health and environmental problems. Knowledge of the concentrations of these radioisotopes and their isotopic compositions in soil provide valuable information concerning nuclear activities in the affected regions.

2. Objectives

The objectives of this work are to (1) determine the occurrence and distribution of the anthropogenic radioisotopes 90Sr, 137Cs, 235U, 236U, 238U, 238Pu, 239Pu, and 240Pu in the topsoil of Qatar; (2) establish a baseline in the soil in Qatar before commissioning the installation of various nuclear machinery in the area; (3) map and assess the fallout radioisotopes of concern in Qatar; and (4) trace the origin of these contaminants.

3. Methodology

3.1. Instrumentation

The measurements were performed using triple quadrupole collision/reaction cell inductively coupled plasma mass spectrometry (CRC-ICP-MS/MS, Agilent 8800), which has been developed recently by Agilent Technologies (Fernandez et al., 2015). The CRC-ICP-MS/MS combines two quadrupole mass filters, Q1 and Q2, before and after the Octopole Reaction System (ORS3) cell, respectively, in a tandem mass spectrometer (MS-MS) configuration. The normal mode of operation of the 8800 is MS-MS mode, where the first quadrupole works as a unit mass filter, restricting the ions entering the reaction cell to a single mass to charge ratio (m/z) at any given time. In this way, ions entering the collision reaction cell are precisely controlled, resulting in the ability to exactly control the reaction chemistry occurring in the cell, even if the sample composition changes. A high performance sample introduction system (AridusTM-II, CETAC) that incorporates a low-flow fluoropolymer nebulizer was coupled to the CRC-ICP-MS/MS instrument. The spectrometer was optimized to provide the highest ion counts of 88Sr, 133Cs, or 238U ions. Argon and reactive gases used in the experiments were grade five (99.999%). The measurements of pure Sr, Cs, and Pu fractions extracted from soil samples were performed using CRC-ICP-MS/MS in single MS mode. Direct measurements of 90Sr, 137Cs, 238Pu, 239Pu, and 240Pu in leached solutions were conducted on the CRC-ICP-MS/MS in MS-MS mode using reactive gases for isobaric separation.

3.2. Quality control/quality assurance (QC/QA)

In order to check the feasibility of the proposed analytical techniques, a set of standard reference materials and proficiency test samples were measured with CRC-ICP-MS/MS as validation experiments.

3.3. Collection of soil and sediment samples and sampling strategy

A systematic sampling plan was followed in the collection of the soil samples in Qatar. Sampling points are located at regular intervals on a 1:10000 square grid. The regular spacing on this grid is 10 km, resulting in approximately 132 soil samples and coastal sediment samples from locations distributed across the country. Sampling locations were demarcated using a Global Positioning System (GPS) and were then positioned in the maps using a Geographic Information System (GIS). The Geostatistics tool in ArcGIS was used to interpolate the concentration and distribution of the radioisotopes in the top soil of Qatar.

3.4. Leaching of Sr, Cs, or Pu from large soil samples

Because of the extremely low levels of 90Sr, 137Cs, 238Pu, 239Pu, and 240Pu in the environmental soil samples, it was necessary to bulk sample extracts to ensure that sufficient analyte is present for an accurate and precise analysis. For this purpose, we applied the procedure developed by Maxwell et al. (2013) with slight modification. The radioisotopes were extracted from1000 g of the Qatari soil samples by concentrated nitric and hydrochloric acids.

4. Results and Discussion

The developed methods were applied to measure the 90Sr, 137Cs, 235U, 236U, 238U, 238Pu, 239Pu, and 240Pu concentrations in the topsoil samples collected from the 132 sites in Qatar. The concentrations of 90Sr in the collected Qatari soil samples vary from 0.18–0.99 fg/g (1.00–5.49 Bq/kg) with a mean value of 0.606 fg/g (3.364 Bq/kg) and a median value of 0.610 fg/g (3.390 Bq/kg). The average atomic concentrations and equivalent activities of 90Sr in the Qatari topsoil samples are presented in Table 1. A comparison with 90Sr activities in other countries are presented in Table 1. The concentrations of 137Cs vary from 0.030–1.210 fg/g (0.098–3.993 Bq/kg) with a mean value of 0.619 fg/g (2.038 Bq/kg) and a median value of 0.620 fg/g (2.051 Bq/kg) (Table 2). The corresponding distribution map for the 137Cs activities is given in Fig. 1. The U concentrations range from 0.05 to 4.7 mg/kg and the 235U/238U isotopic signatures are in the range 0.007–0.008, i.e. comparable to the isotopic ratio in natural uranium (NU). The concentrations of 238Pu vary from < 0.026–0.058 fg/g ( < 0.016–0.027 Bq/kg) with a mean value of 0.034 fg/g (0.0195 Bq/kg) and a median value of 0.032 fg/g (0.0195 Bq/kg). The concentrations of 239Pu fall in the range 18.31–113.85 fg/g (0.042–0.261 Bq/kg) with a mean value of 65.59 fg/g (0.150 Bq/kg) and a median value of 66.16 fg/g (0.152 Bq/kg). The concentrations of 240Pu fall in the range 3.12–30.35 fg/g (0.027–0.258 Bq/kg) with a mean value of 12.06 fg/g (0.103 Bq/kg) and a median value of 10.78 fg/g (0.092 Bq/kg). The combined concentrations of 239+240Pu in environmental soil samples from Qatar and other countries are presented in in Table 3. A thematic maps were built using the Geographic Information System (GIS) software. The concentration and distribution trends of 90Sr are 137Cs, were found to be similar. On the other hand, The concentration and distribution trends of U are Pu were found to be similar. The results showed that residential areas, including the capital Doha, had the lowest concentrations of the radioisotopes in the country, while the western part of Qatar exhibited the highest values. More importantly, due to the low concentration of organic matter (OM) in Qatari soil, the very limited P-fertilization, the alkaline nature of the soil (pH 8), and the low Fe/Mn content, the U and Pu concentrations in the soil are slightly low compared to those of 90Sr and 137Cs. The isotopic and activity concentration ratios of 238Pu/239Pu, 240Pu/239Pu, and 238Pu/239+240Pu can be used to identify the source of these materials. The mean238Pu/239Pu isotope ratio in Qatari soils is (3.674 ± 1.053) ×  10− 4 (Table 4). The 238Pu/239Pu isotope ratio from the reported global fallout and Chernobyl fallout are 1.77 ×  10− 4 and 4.3 ×  10− 3, respectively. The mean isotope ratio of 240Pu/239Pu in Qatari soils is 0.179 ±  0.035. The mean 240Pu/239Pu isotope ratios from the reported global and Chernobyl fallouts are 0.18–0.19 and 0.34–0.57, respectively. The average isotopic and activity ratios of 238Pu/239,240Pu in Qatari soils are (3.061 ±  0.879) ×  10− 4 and 0.052 ±  0.004, respectively. The activity ratio 238Pu/239+240Pu in releases from nuclear fuel reprocessing plants, nuclear tests, nuclear weapons, and the Chernobyl fallout are approximately 0.25, 0.026, 0.014, and 0.47, respectively, (Bu, et al., 2015). Therefore, it is difficult to identify the source of the Pu, but it may be due to the contribution of more than one source. The most probable sources are both the Chernobyl fallout of Pu isotopes and several decades of fallout plutonium accumulation due to nuclear weapons testing.

5. Conclusions

In general, no anomalous results were recorded. The concentrations of U observed in soils collected throughout the State of Qatar were well within the normal background levels. 235U/238U activity ratios do not indicate DU contamination, within statistical detectability, anywhere in the country. The data confirm that the source of the 90Sr, 137Cs, 238Pu, 239Pu, and 240Pu is the global fallout. The concentrations of these anthropogenic radioisotopes are extremely low and do not pose threats to the environment or to human health.

6. Novelty

New data bank was established for (1) the concentrations of the radioisotopes 90Sr, 137Cs, 235U, 238U, 238Pu, 239Pu, and 240Pu in topsoil of Qatar, and (2) the isotopes ratios of U (235U/238U) and Pu (240Pu/239Pu, 238Pu/239Pu, and 238Pu/239, 240Pu).

7. Recommendations

This work provides a basis for monitoring the concentration of anthropogenic radioisotopes, which may be affected by events connected with any nuclear activity and/or accidents occurring in the future. It is recommended to establish a monitoring program to provide a rapid warning system in the event of excessive radioisotopes production and fallout in the region. In particular, it would be especially advisable to continue isotopic monitoring of the most sensitive regions of Qatar on an approximately yearly basis.

Acknowledgment

This article was made possible by NPRP award [NPRP4-1105-1-173 and NPRP08-187-1-034] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the author.