https://orcid.org/0000-0002-8923-2160
Figures
Abstract
This article presents research findings on 3H in abiotic environmental compartments, specifically, the ‘water-soil-air’ system. All of the research areas are located within the Semipalatinsk Test Site (STS): the perimeter of the ‘Degelen’ site, the riverside zone of the Shagan river and the ‘background’ area–the southeastern part of the STS (SEP). As research progressed, numerical values of 3H and its species were revealed in various environmental compartments. The presence of 3H was registered not only in underground nuclear test locations but also in the ‘background’ area–SEP. Maximum 3H tritium concentrations in the water were detected at the ‘Degelen’ site (up to 57000±5000 Bq/kg) and the Shagan riv (up to 61500±6000 Bq/kg), in the air of the ‘Degelen’ site (up to 56±11 Bq/m3), in the soil of the ‘Degelen’ site (up to 5170±500 Bq/kg) and the Shagan riv (4100±400 Bq/kg) in the free water, at SEP (up to 1710±170 Bq/kg) in the organic constituent. Based upon all of the findings, 3H was found to be readily distributed in abiotic environmental compartments depending on certain conditions. Research suggests that water plays a key role in 3H migration processes in the natural system of interest. The second most but equally important constituent is soil and microorganisms of plant and animal origin living there. These assumptions are indirectly proven by research findings that show the HTO and HT air concentration dynamics depending on the sampling location.
Citation: Timonova L, Larionova N, Aidarkhanova A, Lyakhova O, Aktayev M, Serzhanova Z, et al. (2024) Tritium distribution in the ‘water-soil-air’ system in the Semipalatinsk Test Site. PLoS ONE 19(4): e0297017. https://doi.org/10.1371/journal.pone.0297017
Editor: Tim A. Mousseau, University of South Carolina, UNITED STATES
Received: July 16, 2023; Accepted: December 26, 2023; Published: April 4, 2024
Copyright: © 2024 Timonova et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting information files.
Funding: This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP14869391). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
1. Introduction
Due to its nuclear and physical characteristics, 3H, as a pure β-emitter, is one of the least dangerous isotopes from the perspective of external exposure [1]. Its half-life is T1/2 = 12.3 years, and its energy is 18.6 keV. However, because 3H is a hydrogen isotope that is incorporated in many organic compounds, it can be a source of human internal exposure through inhalation and ingestion. Due to the intake of 3H by β-emission, molecular structures and intermolecular bonds become disordered.
Under natural conditions, nuclear reactions with cosmic rays affecting atom nuclei of light chemical elements incorporated in the ambient air are a source of continued synthesis of 3H in the atmosphere. Among other compounds, tritium compounds such as 3H oxide (HTO), molecular hydrogen (HT) and methane (CH3T) are focused on [2].
The atomic industry, namely, nuclear power plants (NPP), currently accounts for the highest fate of releases and discharges of man-made 3H into the environment. 3H produced at NPPs, unlike other radionuclides, enters the environment by bypassing treatment facilities through liquid effluents in the form of tritiated water and gas releases because of its extremely high migration capacity [3]. As shown by predictive estimates, further development of nuclear energy will continue to favor the growth of 3H production and release into the environment [1].
Nuclear tests have been another source of a great deal of 3H produced in the environment. For instance, as a result of numerous tests conducted at the Semipalatinsk Test Site (STS), significant concentrations of 3H are currently also registered in environmental compartments. This radionuclide was detected close to locations of nuclear explosions in surface and ground waters [4, 5], air [6–10], plants [11–13], soil [14–18] and snow cover [19].
The article [1] quotes the analysis of data generalization on the behavior of 3H in different environmental components. Taking into account the relatively short half-life of 3H, its high migration capacity, and the availability of its several physical and chemical forms, modern knowledge about the regularities of 3H behavior in the environment is rather ambiguous. The migration of 3H in environmental components represents complex multiyear and multistage processes that depend on its spatial distribution, seasonal and interannual variability, weather conditions, and nonuniform distribution in the ecosystem. Accordingly, issues concerning the study of environmental 3H migration processes deserve special attention.
Also, from our point of view, it is worth highlighting separately a number of reactor experimental studies with tritium, in which special production of tritium is carried out to solve the problems of choosing materials for fusion reactor blankets. Such studies are carried out in various countries, including in Kazakhstan, at the IVG1.M (Kurchatov) and WWR-K (Almaty) research reactors [20–25].
Thus, the scale of tritium contamination of natural ecosystems located in the zone of influence of hazardous radiation facilities and nuclear installations may be underestimated. Once in the environment, tritium reacts with oxygen and is quickly integrated into numerous cycles of the biosphere as tritiated water (HTO). Due to its chemical properties, it is extremely mobile in biological systems and may be found in all hydrogenated molecules and associated water in the biosphere [1]. To correctly evaluate the content of 3H in the environment, it is important to understand the mechanisms of its distribution. At least three key components–water, soil and air–are the main link in the vital activities of all living things on earth and matter most in studying 3H migration.
Thus, the goal of the paper is to research the content and distribution of 3H in abiotic environmental compartments–water, soil and air.
All research was conducted at the STS. The major radioactive contamination status was generated by above ground and underground testing. The bulk of radionuclides, formed in aboveground explosions, has remained at the test epicenters, but a fraction, related to relatively small soil particles or molten rocks, were transported by the wind to considerable distances. More large-scale and complex processes affect the venues of underground nuclear testing and their surroundings, with transportation of radionuclides (including the 3H) by surface and underground water streamflows. Owing to the extensive distribution of 3H at individual STS spots, the test site is a unique ‘natural laboratory’ in which due to the presence of a great deal of 3H in the water, soil and air, one can conduct research on the distribution of 3H under natural conditions, which will allow the subsequent application of findings to other nuclear power facilities. At the same time, peculiarities in distribution of tritium in the ‘water-soil-air’ system at different sites of the STS may be different depending on the nature of radionuclide contamination and natural conditions.
2. Materials and techniques
2.1 Study area
The STS territory with 3H being present in environmental compartments and represented both by various natural conditions and different sources of radioactive contamination was chosen for research. Nine spots were selected–so-called biological monitoring sites (BMSs) (Fig 1), at which soil, water and air were sampled.
Research spots were chosen on the border of the ‘Degelen’ site and in the riverside zone of the Shagan with the high content of 3H in environmental compartments of interest as well as in a conventionally ‘background’ area in the southeastern STS part (SEP), which contains 3H in environmental compartments at the level of background values.
The ‘Degelen’ site is located in the southern STS part. Underground nuclear tests were conducted here in mountain adits from 1961 through 1989. Their near-mouth spots contained a high content of 3H in the air [7, 26]. For research, tritium-contaminated water streams were selected–creeks that outflowed beyond the ‘Degelen’ site. The creeks are hydrologically linked to water streams flowing from adits, thereby causing the carry-over of 3H from nuclear test locations to adjacent areas. Values of 3H concentration in adit waters reach 340.000 Bq/kg [27]. Research areas were in the bed of three water streams: the Karabulak cr. (BMS-1), the Baitles cr. (BMS-2) and the Uzynbulak cr. (BMS-3) originating from adit 177 water stream. The location of the study areas is presented in the figure (Fig 1 in S1 Appendix).
Research spots on the ‘Degelen’ site are represented by soils of the meadow series that are thin (the soil thickness in the bed does not exceed 40–45 cm, the one in the riverside zone– 20–25 cm), fairly humous (up to 20%), and well-washed from readily soluble salts and carbonates. The bulk of plant biomass is concentrated in water stream flood plains represented by diverse meadow vegetation due to additional moistening. The projective cover of plants in the flood plain is 90%, and the productivity is 3–5 cwt/ha.
The Shagan riv. is located at the ‘Balapan’ site and is the most extended surface water stream flowing at STS along its eastern boundary. In 1965, at the confluence of the Shagan and Aschysu riv, an excavation fusion explosion was conducted for the purpose of creating an artificial reservoir, which resulted in the formation of the ‘Atomic Lake’ water body and an external reservoir. At a distance of 4.7 to 7.7 km from Shagan, maxima of 3H are registered in the water, reaching dozens and hundreds of thousands Bq/kg. Fracture waters are contaminated due to tritium in this section flowing along channels of tectonic disturbances [28, 29] from the ‘Balapan’ site area in which underground nuclear tests were conducted in boreholes.
3H distribution in environmental compartments of the riverside zone at the Shagan riv. was considered at 3 spots along the riverbed from ‘Atomic Lake’ and downstream. Selected sections were characterized by the maximum concentration of 3H in the water. That is, the BMS-7 spot (the vicinity of kilometer 2 away from the ‘Atomic Lake’, the left bank), the BMS-8 spot (the vicinity of kilometer 5 away from the ‘Atomic Lake’, the left bank) and the BMS-9 spot (the vicinity of the bed outlet beyond the ‘Balapan’ site, the right bank). Location of study areas in the riverside zone at the Shagan riv. is shown in the figure (Fig 2 in S1 Appendix).
Light chestnut soils are characteristic of the Shagan impact zone. The plant cover of the bed is built up by aquatic and aquatic-riverside species. The terrace is represented by steppe cenoses. The transitional scarp between the bed and the terrace showing plant succession is often covered in ruderal species.
A site in the STS SEP was chosen as a background area at a distance from nuclear test locations. This area is located between the ‘Degelen’ site in the west and the ‘Balapan’ site in the northeast. No nuclear tests were conducted in this territory. The radiological situation in SEP is attributed both to the global fallout and the fallout from aboveground nuclear tests conducted at the ‘Experimental Field’ and ‘Telkem’ [30]. The 3H concentration in this area is close to background values. However, low numerical values of 3H in the air and vegetation were registered in a number of places within the territory selected in the course of previous research [31].
In the southeastern STS part, research spots were near the ‘Balapan’ site (BMS-4, 5, 6) where water bodies are located (Fig 3 in S1 Appendix). The plant cover of SEP is reckoned among dry steppes on light chestnut soils and is characterized by the presence of saline soils. The vegetation is represented by a complex of gramineous-sandy needle grass-absinthial associations.
2.2 Environmental sampling
All soil, water and air sampling activities were carried out in the summertime.
Water was sampled (Fig 4 in S1 Appendix) monthly (June, July, August) by one sample from creek surfaces at the ‘Degelen’ site and from the Shagan bed. Water in SEP was sampled from surface lakes once (in July). These lakes are recharged by precipitation. In a dry season, lakes dry up, producing dry closed hollows that are covered in white crusts of saline soils and salts. In this regard, no water sampling seemed possible every month.
For transportation and storage, each water sample was collected in plastic canisters.
Soil over the entire research period was sampled once in July. To do so, test pits were drilled at research spots as deep as possible, and depending on the area selected, the depth varied from approximately 100 to 400 cm (Fig 5 in S1 Appendix). Deeper drilling was impossible due to the outcropping bedding rock.
For transportation and storage, each collected soil sample was packed in a triple polyethylene bag. The weight of one sample was about 1 kg.
Air was sampled monthly (June, July, August) for one sample. To that end, a special sampling device was used–a tritium collector ‘OS1700’ (‘ORTEC’, USA), which allows sampling of 3H from the air in two forms: in the form of aqueous vapors, HTO, and incorporated in HT gaseous compounds (Fig 6 in S1 Appendix).
The main point of device operation is based upon the sequential capture of 3H aqueous vapors with an absorbent placed in special containers: the first cascade captures HTO, and the second cascade, following air oxidation (when passing through a palladium spiral heated up to 450 °C), captures gaseous 3H compounds. Distilled water was used as a 3H absorbent.
The time of taking one sample was 2 hours. The volume of pumped air for 2 hours was 0.06 m3. During air sampling, temperature and humidity were measured using a hygrometer of psychrometric type VIT-2 (JSC ‘Thermopribor’, Russia). During the studies, the air temperature for the whole summer period ranged from 12 to 38.5 °C, humidity—from 10 to 67%.
One of the reasons for conducting studies only in summer time is the limitations of the operational characteristics of the air sampling device used. Since air sampling is possible only at temperatures from + 2 to 45 °C and humidity from 5 to 90%. Also, for such period of time as late fall, winter and early spring there are incomplete conditions for determination of free forms of 3H in soil—the soil is too dry.
2.3 Laboratory activities
2.3.1 Soil sample preparation.
According to the literature data, 3H in soil is known to exist in free water and bound forms [17, 32, 33]. Forms such as free water include 3H in surface-adsorbed water (TSAW) and 3H in interlayer water (TIW) [34]. Organically bound 3H (OBT), hydroxyl 3H (HT) and crystalline 3H (CBT) are bound forms.
Soil samples to determine the content of all 3H forms were prepared as per the procedure described in the articles [17, 35] and working instructions [36, 37].
A conventional approach to the determination of radionuclide species is a step-by-step extraction of each form [38].
The articles [17, 35] present methodological approaches for the study of 3H forms in soil. The quality and efficiency of the distillation and autoclave decomposition methods used to determine the forms of 3H in free water and to isolate bound forms of 3H, respectively, are evaluated. The isotope indicator with known specific activity of 3H was used during the research. The efficiency of determination of 3H content in soil was up to 98%.
3H in the surface-adsorbed water was isolated from the sample when heated to 150 °C. 3H isolation from the interlayer water requires heating up to 400 °C. Therefore, 3H species contained in the form of free water were extracted by distillation [36] with gradually rising temperature. That said, aliquots were collected at 150 °C and 400 °C.
The distillation process (Fig 7 in S1 Appendix) ran in a specially prepared unit consisting of a heating mantle, backflow condenser, and three-necked round-bottomed flask that served as a reactor. A thermometer was mounted in one of the flask necks to control the temperature, and a dry air feed system was mounted in the other. The dry air feed system involved an air feed compressor and a Drexel flask that used calcium chloride as a desiccant. Dried air was fed in order to extract 3H completely and accelerate the production of a condensate.
The distillate collected at each temperature was prepared for measurements.
The determination of bound forms requires complete soil digestion, i.e., with the mineral’s crystalline lattice destroyed, which is achievable by autoclave digestion with variations in physical and chemical conditions.
The principle of the autoclave digestion technique consists of decomposing soil samples (with the matrix destroyed) affected by temperature and pressure in the pressurized enclosed volume [37].
The sequential determination of bound forms is impossible. Therefore, the determination process of 3H bound forms ran in three stages.
At stage 1 of the autoclave digestion, the total content of 3H-bound forms was determined. At stage 2, organically bound 3H was removed from the sample by calcination. Calcination was performed in a muffle furnace at 700 °C for 120 min. Next, a calcinated sample was extracted by autoclaving to determine the content of crystalline bound 3H. At stage 3, the content of organically bound 3H was determined by calculation from the difference in the total content and the content of crystalline bound 3H.
Stage I Determination of the total content of bound forms
Stage II Removal of OBT by calcination
Stage III Determination of OBT content from the difference in the total content of bound forms and CBT
Autoclave digestion was accomplished as follows (Fig 8 in S1 Appendix). A 2 g subsample that was quantitatively transferred to a Teflon reaction chamber (the inner autoclave insert) was collected from a dry sieved soil sample (1 mm). It was treated in concentrated nitric and hydrofluoric acids at a ratio of 3 and 7 ml. The Teflon reaction chamber was placed in the outer autoclave case. Autoclaves were placed in a steel clamping mechanism (bed) to keep them still. Autoclaves were aged in the order of 2.5 hours ion the baker at 160 ± 5 °C. Once autoclaving was complete, a resulting sample was removed from the baker, aged to cool to room temperature and filtered. Insert walls during filtration were flushed in concentrated nitric and hydrofluoric acids to ensure the best wipe sampling.
After autoclave decomposition the obtained sample was a mixture of acids, which made it difficult to measure by liquid scintillation method. For this reason, this called for sample neutralization followed by distillation. To do so, a resulting sample was neutralized with a 50% sodium hydroxide solution to pH = 7–8 and was further distilled. To remove highly volatile products, the first distillate portion (one-fifth of a sample) was discarded. The remaining solution was distilled until dry salts were produced. A resulting condensate was prepared in full for the measurement.
Hydroxyl 3H was not isolated. This form was expected to be removed together with organically bound 3H, i.e., in calcination.
2.3.2 Water and air sample preparation.
Because 3H sampling from the air was accomplished by capturing with a liquid absorbent–distilled water, samples were prepared similarly to water sample preparation [39] (Fig 9 in S1 Appendix). To determine the 3H concentration both in the water and the air, all of the samples collected were filtered through an ashless filter (for quantitative and qualitative analyses, TC 2642-001-68085491-2011) to remove coarse suspensions.
Once filtered, each sample was placed in a round-bottomed flask and mounted on the heating mantle to remove salts by distillation. A resulting distillate was used as an aliquot for the analysis.
2.3.3 Spectrometric analysis.
3H in all samples (soil, water and air) was determined as per the procedure [39]. A 5 cm3 aliquot was collected from each sample prepared and placed in a 20 cm3 plastic vial. A 15 cm3 ‘Ultima Gold’ (‘PerkinElmer’, USA) scintillator was added to each vial. The vial was then plugged, and the resulting solution was thoroughly shaken until it became homogeneous.
Samples prepared were measured by liquid scintillation method using ‘TRI-CARB 2900 TR’ (‘PerkinElmer’, USA) spectrometric equipment (Fig 10 in S1 Appendix). A set of 3H calibration sources—quenched standards (Packard, Canberra Company, USA) with known 3H activity was used for equipment calibration.
The time from water and air sampling to measurement of 3H concentrations was on the order of 3 days, all soil samples were on the order of 10 days.
2.3.4 Processing of results.
The specific activity of 3H in soil was calculated using the following formula:
where:
CPM—number of pulses per minute registered by the spectrometric equipment ‘TRI-CARB 2900 TR’ for the measured sample, imp/min;
CPMscint—number of pulses per minute, registered by spectrometric equipment ‘TRI-CARB 2900 TR’ for ‘Ultima Gold’ scintillator, imp/min;
1 000—conversion of the value from ml (mg) to l (kg);
Vsample—volume of the measured sample (5 ml);
60—conversion from minutes to seconds (from imp/min to imp/s), since 1 Bq is one decay per second;
Eff– 3H registration efficiency, determined by the formula:
where:
Aknown—known activity of the calibration 3H source, Bq;
Vdist—volume of soil distillate;
Msubsample—mass of soil suspension (for bound forms 2 g, for free forms 200 g).
The specific activity of 3H in water was determined by the standard formula:
Vsample—volume of measured water sample (5 ml);
The minimum detectable activity (MDA) for the determination of 3H in soil and water was calculated by the formula:
where:
CPMfon—number of pulses per minute registered by spectrometric equipment ‘TRI-CARB 2900 TR’ for pure distilled water, imp/min;
t—measurement time, min.
Volumetric activity of 3H in air was calculated by the following formula:
where:
Vair—volume of pumped air (0.06 m3).
MDA for the determination of 3H in air was calculated by the formula:
The uncertainty was calculated taking into account the standard deviation of the dialed pulses in the spectrum of the measured sample and taking into account the errors of the laboratory measuring utensils used at all stages of sample preparation. On average, the uncertainty amounted to 20%.
3H is able to migrate in environmental compartments both horizontally and vertically and downward and backward. After HTO enters the atmosphere, it becomes mixed with air humidity, followed by transfer to or deposition onto the soil. HTO settles in the soil as a result of airborne wet and dry depositions. Thus, the HTO concentration that settled in the soil moisture as per [40–42] is calculated by the following expression:
where CRs-a–a reference value (empirical constant);
Cam – HTO concentration in the air humidity, Bq/kg.
where Cair – HTO concentration in the air, Bq/m3;
Ha–the absolute humidity, kg/m3 (calculated given the air temperature, °C and air humidity, %; shows the maximum amount of water to be contained in 1 m3 of air).
Values of CRs-a corresponding to IAEA’s recommendations depend on a number of local factors and may vary from 0.23 to 1 [40]. This is determined by the ratio of the HTO concentration in the soil to that in the air humidity.
where CSHTO − HTO concentration in the soil, Bq/kg.
To calculate CRs-a values, values of 3H activity concentration in the free form (HTO, 3H in the surface-adsorbed water) in the soil and values of HTO concentration in the air humidity were used. It is worth noting that all results obtained from research were used for the calculation in simultaneous soil and air sampling in July.
3. Results and discussion
3.1 The results
A comparative analysis was carried out to quantitatively assess the content of 3H in environmental compartments.
Table 1 lists the values of the 3H activity concentration in the water for the entire study area.
As shown in the table, throughout the summertime, the 3H activity concentration in the water of the study area at the ‘Degelen’ site varies from 16 500 to 57 000 Bq/kg. At the BMS-2 site, a decrease in the 3H concentration is noted from June through August at 54 000 to 16 500 Bq/kg. Most likely, this is related to the seasonal pattern of the surface water stream in Baitless Creek [43]. For this reason, variations in the water level occur throughout the summertime in the riverside zone of the Shagan river. The Shagan river has 3H activity concentrations varying from 100 to 61 500 Bq/kg. At the same time, consistently elevated 3H values in the water are characteristic of ‘Degelen’ water streams. The vicinity of the Shagan exhibits the interval of locally growing and dropping 3H activity concentrations (BMS-8), which may indicate that surface waters may most likely become contaminated due to the entry of ground waters in this research area [29]. At spots of the conventionally ‘background’ area in SEP of STS, 3H in the water was not discovered, and its activity concentration value was below MDA (<6 Bq/kg).
Data on HTO and HT volumetric activities in the air are listed in Table 2.
According to the findings, each summer month exhibits an elevated concentration of HTO in the air of the ‘Degelen’ site compared to other research sites. The volumetric activity of HTO was up to 56 Bq/m3, and that of HT was up to 16 Bq/m3. The maximum HTO volumetric activity in the air of the ‘Degelen’ site is noted in the vicinity of Karabulak (BMS-1) and Uzynbulak cr. (BMS-3) beds, minima–in the vicinity of the Baitles cr. (BMS-2) bed. The maximum HT concentration in the air was registered virtually at every spot only in June.
Values of HTO volumetric activities in the riverside zone of the Shagan are traceable virtually at all research spots throughout the summertime up to 5.7 Bq/m3. HT volumetric activity for this area was only revealed in June up to 6.4 Bq/m3. The MDA value for determining the volumetric activity of HTO and HT in air is 0.5 Bq/m3.
Within SEP, numerical values of HTO and HT were only recorded in June (up to 4.5 Bq/m3).
Activity concentration values obtained for 3H species in the soil are listed in Table 3.
In the course of the analysis, 3H in the soil of every research site was found to be mostly in the form of surface-adsorbed water, which is one of the free forms. No other free form– 3H in the interlayer water–was observed at any of the research spots. Maxima of 3H activity concentrations in the form of surface-adsorbed water are noted for the soil of the ‘Degelen’ site and the riverside zone of the Shagan riv. The concentration of this form varied from 120 to 5 170 Bq/kg and from 55 to 4 100 Bq/kg, respectively. That said, the distribution in the soil profile is nonuniform. For example, at the ‘Degelen’ site (BMS-2), the maximum value is noted in the 100–200 cm layer and in the vicinity of Shagan in the 80–160 cm (BMS-7) and 0–120 cm (BMS-8) layers. The ‘background’ area of SEP is notable for the lower level of 3H activity concentration– 4 to 30 Bq/kg.
3H-bound forms were revealed in the soil of the ‘Degelen’ site and in the ‘background’ area of SEP. Values of 3H activity concentrations in the hydroxyl and organically bound forms for the ‘Degelen’ site were up to 340 Bq/kg, and those for the ‘background’ area of SEP were up to 1 700 Bq/kg. For the riverside zone of the Shagan, 3H in this form was below MDA—<50 Bq/kg. 3H in the crystalline bound form was detected only in one case in the ‘background’ area of SEP at BMS- 6 with an activity concentration value of 1.250 Bq/kg.
3.2 Discussion
The figure (Fig 2) graphically presents the dynamics of the average 3H content in the water by months of summertime observations.
a) Dynamics of 3H concentration in the water of the ‘Degelen’ site, b) Dynamics of 3H concentration in the water of the riverside zone of the Shagan river.
As seen from the graphs presented, as a whole, no seasonal variation in 3H activity concentration is observed on the boundary of the ‘Degelen’ site. Some decrease is only characteristic of BMS-2 (Table 1). Unlike the ‘Degelen’ site, an increase in the 3H concentration from June to August is characteristic of Shagan water. The exception is BMS-7, where the 3H concentration over the observation period is noted to be 1 000–1 300 Bq/kg, which is within the determination uncertainty of 3H (Table 1). As opposed to ‘Degelen’, in general, a much wider range of 3H concentration values is observed during the observation period. At the BMS-8 spot, an interesting dynamic of 3H content (Table 1) can be traced. The 3H activity concentration from a minimum in June of 100 Bq/kg increased to a maximum of 61 500 Bq/kg in July. In August, the 3H concentration decreased to 20 000 Bq/kg (Table 1). Such dynamics of 3H concentration at this spot may be due to tectonic disturbances revealed earlier by geophysical research [28, 29]. These disturbances are the main channel through which 3H-contaminated fracture waters discharge into surface waters of the Shagan river in this particular bed section. It is worth noting that the recharge zone of ground waters is in the northern ‘Balapan’ site approximately 20 km away from point BMP-8. That said ground waters at the ‘Balapan’ site are artesian with a head value of up to 80 meters, for which reason 3H-contaminated waters enter the Shagan riv. with a certain head. For example, the higher a head value the more contaminated waters enter the river. At the same time, the maximum total discharge of streams falls behind the onset of intense precipitation event by more than 2 months. That period can be taken as the infiltration time of precipitation through fracture systems at the ‘Balapan’ site from the recharge zone to the discharge area. Early in July, it reaches maxima and by the end of the month, it begins to decrease. In this regard, in June we measure low values of 3H concentration (100 Bq/kg) and a sharp increase in 3H activity concentration up to 61 500 Bq/kg in July. In August 3H concentration is recorded to decrease down to 20 000 Bq/kg due to a lower ground water head (Table 1).
Figures (Fig 3) below depict the dynamics of the average concentrations of HTO and HT in the air of research sites versus sampling months.
a) Dynamics of HTO concentration in the air of the ‘Degelen’ site, b) Dynamics of HT concentration in the air of the ‘Degelen’ site, c) Dynamics of HTO concentration in the air in the riverside zone of the Shagan, d) Dynamics of HT concentration in the air in the riverside zone of the Shagan, e) Dynamics of HTO concentration in the air of SEP, f) Dynamics of HTO concentration in the air of SEP, MDA: minimum detectable activity.
As mentioned above, air was sampled along water streams–creeks of the ‘Degelen’ site and the Shagan river Air is closely linked to open tritium sources, specifically, soil and water, in which the process of nonuniformly emanated 3H is sure to occur from surfaces of sources throughout the year depending on the time period. Vegetation growing at research spots can be an additional open source of 3H –the transpiration process may also affect the mechanism by which 3H is produced in the air [6].
Maxima of HTO and HT volumetric activity in the air were registered at the ‘Degelen’ site, 56 and 16 Bk/m3, respectively. It is worth noting that at this site, maxima of 3H activity concentration were also registered in the water and soil. The dynamics of the HTO concentration in the air show that the values of the volumetric activity decrease toward August, which in specific cases is characteristic of surface waters–BMS-2 (Table 1, Fig 2). Such a variation in HTO in the air may be affected by a decrease in the temperature at this point in time followed by a reduced evaporation of 3H from water, its emanation from soil and transpiration from vegetation.
If the dynamics of the HTO concentration in the air are considered for the riverside zone of the Shagan, one can trace an increasing trend of the HTO concentration from June to August, which is consistent with an increase in the 3H concentration in the water (Fig 2). The exception is BMS-9, which in this case may be attributed to the location of this site on the right bank with the inflow of purer ground waters [44]. That said, the nonuniform entry of HTO into the air of BMS-7 and BMS-8 may be impacted by the capillary rise of 3H-contaminated ground waters coming from the ‘Balapan’ site followed by evaporation from the soil surface.
The study of HTO concentration in the ‘background’ area of SEP and HT concentration at all other research spots revealed the same picture of their dynamics. All of the numerical values of 3H volumetric activity were registered in June, followed by a sharp decrease in the concentration down to the detection limit of 0.5 Bq/m3. At the same time, no quantitative values of 3H in the water of this territory were revealed (Table 1). Such a change in HTO for SEP is attributable to one of the reasons. This area is mainly characterized by saline soils. July is believed to be the hottest month when a drought season occurs. By that time, all of the sites available, such as lakes, plants and soil, dry up in this area for which reason 3H emanation from the soil and its transpiration from plants cease. Hence, in SEP, numerical values of HTO volumetric activity in the air are only registered in July.
The dynamics of HT concentration in the air of all research spots demonstrates a unified behavior of distribution–maxima of the volumetric activity at all research spots were noted in July. At the same time, these are only quantitatively recorded throughout the research period on the boundary of the ‘Degelen’ site. Perhaps this is due to intrazonal conditions of the mountain range–the presence of ample meadow vegetation, which produces up to 500 g/m2 [7] and soils with high humus content–up to 20% [27]. That said, the entry and production of HT in the air in the first summer month may be affected by vital activities of microorganisms in the soil and the plant-animal organic mass containing 3H [9]. The ratio between heat and moisture is known to be a determinant of the maximum number and vital activities of microorganisms in the soil [45]. Most likely, late spring and early summer account for such an optimal ratio. In that period, the temperature is not high with rainfall, which is necessary for maintaining vital activities of microorganisms in the soil and the plant-animal organic mass. The appearance and production of the maximum HT concentration in June is most likely to fall just on when complex organic matter contained in the soil is split into more elementary matter capable of being emanated, producing volatile gaseous compounds, which, among others, contain gaseous 3H compounds [9]. As soon as the hot spell comes, the number and vital activities of microorganisms in the soil are reduced and slowed down. For this reason, the HT concentration in the air decreases by midsummer.
Because soil over the entire research period was only sampled once, any graph constructions to trace the dynamics of 3H content in the soil seem impossible. For a more detailed concept of a 3H form in the soil, its species have been studied. The diagrams (Fig 4) depict the average percentage of 3H species detected in the soil of all research spots.
a) ‘Degelen’ site, b) ‘Background’ area of SEP, c) Riverside zone of the Shagan.
Diagrams derived show the percentage of 3H species in the soil. The soil of the ‘Degelen’ site is dominated by free water forms, namely, by 3H in the surface-adsorbed water. The average percentage of 3H in the surface-adsorbed water all over the ‘Degelen’ site was on the order of 88%. The total content of bound forms is insignificant and does not exceed 12%.
The soil of the SEP ‘background’ area is dominated by 3H bound forms, in which the average percentage is on the order of 65% for the entire ‘background’ area. Of these, at one spot (BMS–6), the content of crystalline bound 3H is observed in the soil, whereas other spots (BMS-4 and BMS-5) contain no form like this but are dominated by organically bound 3H. The average content of 3H in the surface-adsorbed water was 35%.
Determinations of 3H species in the soil of the Shagan riverside zone showed that 3H at all BMSs is only contained in the surface-adsorbed water.
Based upon the results presented, it is worth noting that free water 3H dominates in the soil of the research spots. This applies specifically to the ‘Degelen’ site and the Shagan riverside zone with research spots located in the bed of 3 water streams (Karabulak, Baitless and Uzynbulak cr.) and along the Shagan bed, respectively. 3H domination in the form of free water is most likely to be related to the inflow of ground waters in the soil. 3H domination, according to hydrogeological maps of the terrain, occur at shallow depths on the order of 2–5 m along water streams. The presence of rock fragments along the bed from underground nuclear explosions in mountain adits may affect 3H bound forms fixed in the soil of research spots at the ‘Degelen’ site [27]. Rock is a mineral in which hydroxyl 3H (chemically bound water) can be incorporated [17]. The presence of 3H in bound forms is more difficult to explain in the ‘background’ area, which is at a distance from nuclear test locations. The radiological situation in this territory is attributed to the fallout from aboveground nuclear tests conducted at the ‘Experimental Field’ and ‘Telkem’ sites. While the 3H concentration in the soil in the form of free water is available in the source, the concentration in the organic constituent may reflect 3H historical levels in the environment years or decades ago [46, 47]. It is worth noting that in the course of earlier research undertaken in the SEP ‘background’ area, the presence of 3H could be recorded in organic constituents of plants [31].
Table 4 lists the CRs-a values and HTO concentrations in the soil moisture (Csw) for all BMSs of interest.
The table lists reported HTO concentrations in the soil moisture Csw calculated given CR s- a values and HTO concentrations in the air humidity Cam. From the results, one can see a quite low but additional contribution by HTO to the soil from the air.
CRs-a ratios that were calculated became certain values for each specific research spot. The value interval is 0.1 to 7.1. The spread of values can be attributed to the variable pattern of the source of 3H entry into different environmental compartments over the entire summertime of research efforts. It was noted that many reported values of CRs-a are within the value limit as per IAEA’s recommendations (from 0.23 to 1). Exceptions are the values of 1.9 and 7.1 calculated for the BMS-7 and BMS-8 spots. Because the ratio of the HTO concentration in the soil to that in the air humidity CRs-a may depend on a number of factors [40], the resulting values are most likely associated with the presence of a developed zone of groundwater containing 3H in the coastal zone of the Shagan riv. (the area of the 2nd and 5th km from the ‘Atomic Lake’, respectively). The mechanism of 3H distribution in such a system can be caused by the processes of capillary rise of polluted groundwater followed by evaporation from the soil surface. The maximum value of 7.1 may additionally be because in July, the specific activity of 3H in water at the BMS-8 study site was 61 500 Bq/kg (Table 1). This is the highest 3H value for water over this research period, which is related to the discharge of 3H-contaminated fracture waters into surface waters of the Shagan river in this bed section as described above. The most stable CRs-a values are characteristic of the SEP ‘background’ area, which is apparently related to no direct impact by an open water source. This fact proves that the aquatic medium matters greatly in the migration and distribution of 3H in various environmental compartments.
4. Conclusion
Activities carried out at STS showed the presence of 3H in different forms in environmental compartments of interest. 3H was recorded not only in locations of underground nuclear tests but also in the ‘background’ area–STS’s SEP. Maximum 3H tritium concentrations in the water were detected at the ‘Degelen’ site (up to 57 000±5 000 Bq/kg) and the Shagan riv (up to 61 500±6 000 Bq/kg), in the air of the ‘Degelen’ site (up to 56±11 Bq/m3), in the soil of the ‘Degelen’ site (up to 5 170±500 Bq/kg) and the Shagan riv (4 100±400 Bq/kg) in the free water, at SEP (up to 1 710±170 Bq/kg) in the organic constituent. Research undertaken suggests that water plays a key role in migration processes of 3H in the natural environment of interest. The second most but equally important constituent is soil and all microorganisms of plant and animal origin living there. These assumptions are indirectly proven by research findings showing the dynamics of HTO and HT concentrations in the air versus a sampling month. In the soil of research spots, the bulk of 3H is contained in the form of free water. This form of 3H is predominant where 3H migrates with surface waters (creeks of the ‘Degelen’ site and the Shagan riv.). 3H bound forms were also revealed in the soil of the ‘Degelen’ site and in the ‘background’ area. Accordingly, rock fragments following underground nuclear explosions conducted at the ‘Degelen’ site can be mechanisms by which these forms are produced, and the fallout from aboveground nuclear tests conducted at the ‘Experimental Field’ and ‘Telkem’ sites can be the ‘background’ area.
As all efforts progressed, 3H in the STS area was found to be readily distributed in abiotic environmental compartments depending on several factors. Not only the radiation effect after nuclear testing but also the following can be the major contributors to 3H distribution: weather conditions (air temperature/humidity), seasonal changes, the type and state of the land cover, hydrogeological changes leading to the variation in 3H behavior in surface waters, transpiration from plants, vital activities of microorganisms in the soil and others [1].
Ratios of the HTO concentration in the soil to that in the air humidity (CRs-a) were calculated. These, in most cases, correspond to IAEA values. The transfer of 3H to the soil from the air in the form of HTO through airborne wet and dry depositions was determined as an additional contribution.
Findings must be considered in monitoring areas, for instance, near NPPs and other radiation-hazardous objects, to assess the impact of 3H releases on the public and environment to avoid underestimation.
Acknowledgments
The authors are grateful to K.E. Tomilov for translating the article into English (Institute of Radiation Safety and Ecology, National Nuclear Center of the Republic of Kazakhstan, Kurchatov).
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