Distribution characteristics of tritium in the soil in Beishan area of Gansu Province

LI Jie-biao, SU Rui, YANG Jing-zhi, ZHOU Zhi-chao, JI Rui-li,ZHANG Ming, GAO Yu-feng

1 Beijing Research Institute of Uranium Geology, Beijing 100029, China.

2CNNC Key Laboratory on Geological Disposal of High-level Radioactive Waste, Beijing 100029, China.

3Territorial Resources Exploration Center of Hebei Bureau of Geology and Mineral Resources,Shijiazhuang 050081, China.

Abstract: Beishan region in Gansu was the preselected area for China's high-level radioactive waste (HLW) repository. In selecting and evaluating a new dump site, the tritium study is of great significance. The Xinchang-Xiangyangshan preselected area in the Beishan area was taken as an example. This paper selects typical unit and tries to use the distribution characteristics of tritium in the soil to study the atmospheric precipitation infiltration recharge in this area. The results show: In this region, the spatial variability of the tritium content in surface soil is large; it is feasible to use bound tritium tracer method to study the theory of atmospheric precipitation infiltration recharge; the atmospheric precipitation infiltration has close relationship with the soil particle composition, salt content, mineral composition, water content and organic matter content. These results can provide important basis for developing the atmospheric precipitation infiltration recharge, groundwater numerical simulation, nuclide migration study and so on.

Keywords: Tritium; Atmospheric precipitation infiltration; Free water tritium; Bound tritium;High-level radioactive waste

Introduction

High-level radioactive waste (HLW) refers to the high-level radioactive liquid waste and its solidifications generated by the post disposal of spent fuel. As a special hazardous waste, HLW is characterized by strong radioactivity and toxicity,long half life of nuclide, and large thermal values(PAN Zi-qiang and QIAN Qi-hu, 2009). Currently,the worldwide accepted disposal method is the application of deep geological disposal to HLW.Specifically, the HLW is buried inside the geologic body as deeply as about 500-1 000 m under the ground. In this way, HLW is permanently isolated from our human environment. The underground projects with HLW buried inside are usually defined as “HLW repository” (WANG Ju et al.2006). To guarantee the safe disposal of HLW, it's imperative to select the right site for HLW repository. Inside the repository, the HLW's hazardous nuclide gains its access to human environment mainly through the migration of groundwater. In other words, groundwater is the major carrier of hazardous nuclide inside the repository. So the hydrogeological condition is one of the important considerations when the HLM repository is selected (GUO Yong-hai et al. 2008,2013). The study on atmospheric precipitation infiltration recharge is the top concern for the evaluation of hydro-geological conditions.

As a radioactive isotope, tritium is timestamped. In the atmosphere, tritium takes the form of hydrogen, which is oxidized in a short time, and gets involved in the circulation of natural water.Since tritium has the identical behavioural characteristics as water, it is the most relevant isotope to study water movement. Therefore, this isotope is one of the effective ways to study atmospheric precipitation infiltration recharge(Healy R W, 2010). Tritium is also a very crucial nuclide to dispose of HLW and the relevant study is one of key scientific concerns in the study on HLW disposal (IAEA, 1998). Meanwhile, the scientific basis can also be provided for the study on influence and evaluation of tritium pollution,and some reference values can be made available for the studies on other radioactive nuclides like technetium and129I.

Tritium exists inside soil in two forms, namely free water tritium and bound tritium. The former has been widely used and accepted in the hydrogeological field. Its study results mainly include: ① The study on the characteristics of atmospheric precipitation infiltration recharge(Dicer T et al. 1974; Allison G B and Hughes M W, 1978; ZHANG Zhi-gan et al. 1990; Rangarajan R and Athavale R N, 2000; LIN Rui-fei and WEI Ke-qin, 2006; WANG Bing-guo et al. 2008; LIU Jun et al. 2009); ② The study on migration characteristics of soil pollutants (Maxwell R M et al. 2009); ③The migration experiment study on tritium in the soil of aeration zone (WANG Zhi-ming et al. 2001; WANG Jin-sheng et al.2003). The studies on bound tritium are focused on the ecological environment. The study results mainly include the migration and distribution of tritium water (free water tritium and bound tritium)in the simulative aquatic-terrestrial ecosystem(WANG Shou-xiang et al. 1994), the behaviour of bound tritium in tea tree soil, corn soil, soybean soil and green vegetables soil (SHI Jian-jun et al.2001a, b; SHI Jian-jun and CHEN Hui, 2002; SHI Jian-jun and GUO Jiang-Feng, 2002), the migration and circulation process of tritium in plants (Boyer C et al. 2009) and the current knowledge about the role of bound tritium in environment (Kim S B et al. 2013).

There are many study results on tritium, but the studies on distribution laws of atmospheric precipitation tritium in soil are rather limited.Therefore, with the Xinchang-Xiangyangshan preselected area in the Beishan area as an example,the site for HLW repository in Gansu Beishan preselected area was studied on migration laws of atmospheric precipitation tritium in soil. This study is designed to explore the atmospheric precipitation infiltration characteristics in this area, to provide the data support for the circulation and alternation characteristics of groundwater and create a useful basis for the selection of preselected area and the performance assessment.

1 Materials and methodology

1.1 Overview of study area

The study area is located to the north of Hexi Corridor in Gansu Province, and 70 km away from Yumen township to the south. In terms of administrative division, the study area falls within the jurisdiction of Subei county and Yumen County, Gansu Province (as indicated in Fig. 1).Generally speaking, the study area can be defined as the hilly topography. In the north is a continuous chain of highlands, and from the west to the east lie Xinchang Mountain, Hongliujing South Mountain, Jinmiaogou South Mountain,Qianlouzi East Mountain and Hongliuquan South Mountain, which have the precipitous slopes. In the south, from the west to the east lie Xinchang South Mountain, Hongqi Mountain, Yuejin Mountain and Zongkouzi South Dahei Mountain,which have low altitude and poor continuation and mainly consist of metamorphic rock. In the middle is mostly the lowland with an average altitude of 1 400-1 500 m and 1 600-1 700 m for some hills,and with granite as the main materials. There is a valley with the width of 1-2 km and the east-west length of 40 km between the southwest of Sishilijing and the east of Dizhijing, which can be defined as Gobi landform.

The study area, which lies in the hinterland of Eurasian Continent, is far away from the ocean. So the study area features high latitude and altitude,small precipitation, large evaporation, long sunshine time, dry wind, scarce vegetation, cold winter and hot summer, and falls within the typical continental climate. The annual precipitation averages about 70 mm, and rainfall shows some noticeable season-related characteristics. More than 60% precipitation happens during the period from June to August. The annual evaporation averages 3 200 mm, the annual temperature averages 4-7 ℃, and the annual wind speed averages 4 m/s. There is no perennial river, but the dry riverbed and valley as the result of flood are well developed. The groundwater system can be divided into three types like bedrock fissure water,gully depression pore-fissure water and basin pore-fissure water when the conditions capable of controlling groundwater flow like topography,landform and structure are taken into consideration.

Fig. 1 Location map of the study area

1.2 Study methodology

To gain an insight into the characteristics of atmospheric precipitation infiltration recharge, we should do as follows. Above all, it is necessary to analyze the difference of tritium contents in surface soil and the spatial difference of atmospheric precipitation infiltration recharge.Then, we must analyze the vertical distribution characteristics of free water tritium and bound tritium in soil and understand the changes in the process of atmospheric precipitation infiltration recharge. The tritium contents in surface ground are mainly based on the results of previous survey.This study selects four typical profiles in Sishilijing Basin to explore the vertical distribution law of atmospheric precipitation tritium in soil (the location of sampling site is indicated in Fig. 2).

This research collects the soil sample of aeration zone through manual excavation and the sampling groove has a dimension of about 0.8×1.5×(1-3 m) (width×length×depth) (depth is based on the geological conditions of sampling sites and in most cases as deep as the level of groundwater of bedrock). The sampling space is 10-20 cm. Inside reach sampling depth, the circular knife with the size of 80×75 mm2is used to collect three original samples and the electronic balance with the precision of 0.01 g is used to weight and calculate the natural density of soil sample. Meanwhile, the small spade is used to collect the soil sample of about 2 kg with the same depth. The stones, grass roots and other impurities are moved and rapidly placed into the sealing bag.The bag is properly tied with a piece of white cloth and arranged in the shade. Later, the soil sample is sent to lab for analysis and testing. The testing items include tritium content in soil, weighting water, granularity composition, pH, content of organic matters and soil mineral composition.

Fig. 2 Locations of soil sampling sites

The soil granularity analysis and pH testing are completed in the China Industry Minerals & Rock Test Center, the content of organic matters is analyzed at Beijing Center for Physical and Chemical Analysis, the soil mineral composition is tested at Analytical Laboratory Beijing Research Institute of Uranium Geology, and weighting water is completed independently in lab through oven-drying method.

The continuous extraction method (the equipment of extraction experiment is indicated in Fig. 3) created by CNNC Beijing Study Institute of Uranium Geology is used to test the specific activity of tritium in soil. The operation procedure goes as follows. A balance is used to weigh quartz boat, soil sample (about 50-70 g) + quartz boat, as well as two3H collection bottles and exhaust collection bottle. The transfer liquid gun is used to add the bottom water of 20 ml into two3H collection bottles, and then the collection bottles are properly assembled and connected with quartz tube. The quartz boat with weighed soil sample is placed inside oxidation oven. The heating process lasts for two hours at the temperature of 200 ℃.When the heating process is completed, the solution inside two3H collection bottles is poured inside measuring flask until it's mixed evenly. A balance is used to weigh two3H collection bottles and exhaust collection bottle. At this time, free water tritium has been extracted. After this process is completed, a balance is used to weigh two new3H collection bottles and the bottom water of 20 ml is added into either bottle, and then the collection bottles are connected with quartz tube. The temperature of oxidation oven is raised to 700 ℃and soil sample is added for about two hours. After the oxidation process is completed, the preceding steps are repeated to collect the bound tritium inside measuring flask. The extracted water sample is sent to Analytical Laboratory Beijing Study Institute of Uranium Geology for analytical testing.The measuring instrument is the U. S. Quantulus 1220-003 low-background liquid scintillation counter, and the measuring period lasts for 500-900 min. The count rate of bottom sample water is 0.454 cpm and the count rate of instrument is 23.41%. The lower detection limit is about 8.14×10-2Bq/kg. To prevent the influence of gravity water content in soil on analysis, this study uses the specific activity of tritium in soil for analysis.

Fig. 3 Schematic diagram of experimental device of tritium extraction

2 Result and analysis

2.1 Distribution characteristics of tritium content in surface ground

When the past scientific results are considered(QIU Guo-hua, 2013), this test result shows that the specific activity of free water tritium, bound tritium and overall tritium in the surface soil (with the sampling depth of 10-30 cm) averages 3.84 Bq/kg, 3.45 Bq/kg and 7.28 Bq/kg, which all reach the level of environmental background. In the variability analysis, the variability coefficients are 44.05%, 55.75% and 45.66%, implying a moderate variability. In general,3H's migration rate and groundwater rate are consistent. So the distribution law of tritium in aeration zone can be deemed as the result of atmospheric precipitation infiltration process. The spatial variability reveals that atmospheric precipitation infiltration has a substantial spatial variability. With the changes in depth, the specific activity of free water tritium in four testing sites has the variability coefficient of 28.50%, 43.82%, 84.22% and 41.51%, suggesting the moderate variability. In fact, the specific activity of bound tritium and general tritium also falls into the category of moderate variability.

The aforesaid test results show that the tritium contents in soil vary considerably within the study area. Since the human activity and permanent rivers are both absent inside the study area, the atmospheric rainfall is the sole source of recharge for groundwater. In this case, the difference of tritium content in soil is a direct indicator of heterogeneity in the retention, migration and accumulation of atmospheric rainfall in soil within different segments. This phenomenon can be attributed to such causes as the extremely uneven rainfall distribution of arid area, the spatial variability of soil texture and the difference of vegetation distribution. When we consider the geological and landform characteristics of different sampling sites and the previous study results of soil infiltration within aeration zone (LI Jie-biao et al. 2015), it is easy to find that the soil texture is the leading contributor to the difference of tritium contents in surface soil within the study area(which can also be found from CYD-01-CYD-04),which includes clay, fine sand, medium sand,coarse sand and gravel. The difference of soil texture results in different precipitation infiltration recharges so that the soil tritium contents are different.

2.2 Sectional distribution characteristics of tritium

The free water tritium in soil migrates downward with the atmospheric precipitation infiltration and witnesses a significant influence of atmospheric precipitation infiltration within a short period. Thus, the distribution characteristics of free water tritium content in soil show how the atmospheric precipitation infiltration recharge recently happens. The distribution characteristics of bound tritium in soil are attributed to the long-term effects of atmospheric precipitation infiltration. In contrast, the study on atmospheric precipitation infiltration has a longer time scale.The previous studies on atmospheric precipitation infiltration recharge are mainly focused on the peak value displacement method of nuclear explosion tritium. This method is based on free water tritium. There're some previous studies on the migration of bound tritium. However, these studies turn attention to the migration and distribution of bound tritium in the plant-soil system (Diabat S and Strack S, 1993; Evenden W G et al. 1998), but the bound tritium has not been used to study the atmospheric precipitation infiltration recharge. So the studies on bound tritium still remain in an early development stage. The research results of Koch-Steindl H and Koch-Steindl H and Pröhl G (2001) show that any changes in the soil's physical (like the composition of particle diameter in soil) and chemical conditions (like temperature, pH and Eh value) as well as organic matters and microbe contents will affect the migration of nuclides. In this case, this paper uses the soil's particle composition, mineral contents, pH, specific activity of tritium, gravity water content, soil Cl-content and organic matter content to examine its relevance to the distribution of tritium.

The following part conducts the preliminary analysis of distribution characteristics only in four profiles.

Fig. 4-Fig. 7 describes the particle composition,mineral composition, specific activity of tritium,pH, gravity water content, Cl-content and organic matter content. As these figures show, the tritium contents in soil have not reached their peak values at four testing sites, which may be relevant to the excessively small thickness of aeration zone in the selected profile, the high infiltration coefficient,and the decaying of tritium generated by thermonuclear explosion to the scope of environmental background (half-life is only 12.32 years).

CYD-01's specific activity of tritium is very low, and free water tritium and bound tritium both have the limited depth changes but no clear laws have been discerned. Yet, the bound tritium contents see a noticeable decline at the depth of 100-130 cm. When compared to the mineral composition, the calcined gypsum content decreases less significantly. The preliminary study shows that calcined gypsum has some effects on the migration of bound tritium.

CYD-02's free water tritium and bound tritium experience a change in consistency with the changes in Cl-content, organic matter content and pH, but in contrast to the changes in gravity water content.

CYD-03's free water tritium and bound tritium show a vastly different change from pH in soil,which may be related to the rock characteristics in two profiles. The specific reasons need future explorations. Where there're some noticeable changes to tritium content and pH at 50-60 cm, the particle composition is compared to find that the particle constituents have the great growth potentials. The calcined gypsum is hardly included as follows. So we can infer that the calcined gypsum and the particle composition in soil have a substantial effect on the distribution of tritium.

CYD-04's free water tritium content have significant depth-based changes, and the law of changes is consistent with the changes in Clcontents of soil water and organic matter contents,but in contrast to the changes in bound tritium content, gravity water contents and pH. The maximum value happens near the ground surface and at the depth of 70 cm. The recent precipitation influence and evaporation role are the direct contributor to what happens near the ground surface. When we compare the particle composition with mineral composition at the depth of 70 cm, we learn that mineral composition experiences limited changes, but particle composition changes significantly. This is attributable to the changes of particle composition lead to changes in soil permeability.

The above-stated analysis means that CYD-01 reveals nothing like a definite pattern. For CYD-02 and CYD-03, the free water tritium content and bound tritium content in soil have a basically same change. In contrast, CYD-04's free water tritium content and bound tritium content have an opposite change, which may be related to the particle composition. Since CYD-04 is located at the upper reach of the basin, the strong hydrodynamic conditions and the big particle diameter in soil mainly feature fine sand, medium-sized sand,rough sand. The specific reasons also need our future research.

3 Conclusions

This paper uses the distribution characteristics of tritium and especially bound tritium in soil for the first time to study the atmospheric precipitation infiltration recharge. It's a valuable academic attempt. Currently, the studies on tritium migration mechanism are far from well developed, and in particular there are no theories about the migration mechanism of bound tritium. Moreover, the testing sites are rather limited. So this test has not identified some definite laws, nor gained valuable knowledge about atmospheric precipitation infiltration recharge. In spite of these defects, this tentative study can at least achieve the results and knowledge as follows:

(1) In the extra-arid desert, the bound tritium concentration in soil of aeration zone at profile can change and get effectively detected. This indicates that it's feasible to explore the atmospheric precipitation infiltration theory within aeration zone through the bound tritium tracer method. It's advisable to continue this research in the future.When the related work is conducted, the testing methods in ecological environment field are used as reference. The manual tracer method is used and the related researches are conducted inside the lab to make comprehensive comparison and analysis.

(2) We can infer from the spatial variability of tritium distribution that the tritium content in soil has the considerable spatial variability within the arid area. This is closely related to the uneven precipitation within desert area and the spatial variability of soil texture. So it's necessary to highlight the exemplary value of the selected profile and select different soil textures when the related work is conducted within the study area.What's more, the peak value of tritium doesn't happen at four profiles. Therefore, the nuclear explosion tritium is not the best tracer when we conduct a study on atmospheric precipitation infiltration recharge.

(3) The above-stated study may conclude that the process of atmospheric precipitation infiltration recharge has much to do with the particle composition, mineral composition, salt content in soil,gravity water content and organic matter content.This probably because these are the main factors capable of affecting soil infiltration and the soil infiltration within the aeration zone plays a decisive role in atmospheric precipitation infiltration recharge, but the specific relationship requires our future study.

Acknowledgements

This study is supported by special project on decommission of nuclear facilities and disposal of radioactive wastes (SASTIND No.2 Department No. [2014]305).