Krypton 81: A New Method of Paleogroundwater Dating

Recently, because of Kr’s advantages of steady chemical property, long half-life and no extra source in the movement of groundwater, it is becoming an effective method to date old groundwater (age from 10-10 year). This paper summarizes the principles of Kr in groundwater dating and extraction method, presents the application of Kr in groundwater dating and discusses the problems of recent related researches, makes prospects of studies of radionuclide in hydrogeology chemistry, and provide scientific support for the measurement of Kr and the researches of old groundwater dating.


Introduction
To study the origin, recharge and discharge of groundwater and to quantify the characteristics of groundwater transport, groundwater age dating is an important step. Recently, a large number of radioactive isotopes are used in groundwater dating studies [1]. For example, 3 H, 85 Kr and 3 He/ 3 H can be used to measure the age of young groundwater (about 50 years); 39 Ar is used to measure the age of groundwater from 50 to 1000 years. The method of using 39 Ar as isotope tracer to determine the age of groundwater makes up for the blank stage between the range determined by 3 H/ 3 He (< 50 years) and 85 Kr (>1000 years) [2]; 14 C is used to measure the age of groundwater between 1,000 and 40,000 years old. Although the 14 C dating method is not absolutely accurate, it is an important method in solid phase or mesoscale measurement [3]. It is also very effective to use 4 He for groundwater dating, whose accumulation can be calculated by the amount of U decay [4]. However, there are few isotopes that can be used to measure the age of extremely old groundwater [5]. Although 36 Cl(half-life 3.01×10 5 years) and 129 I(half-life 1.7×10 7 years) are two possible ways to analyze the age of old groundwater, they have complex chemical mechanisms in the process of groundwater movement, which makes the analysis more difficult and the age estimation results inaccurate. With the development of isotopic research and optimization of testing equipment domestic and abroad, as a radioactive nuclide of inert gas, 81 Kr, with its very long half-life ((2.29±0.11)×10 5 years) and stable chemical properties, has become a possibility to measure the ancient groundwater (10 5 -10 6 years).
In 1969, Loosli and Oeschger found 81 Kr in atmosphere, making it an ideal tracer for the measurement of groundwater and glacier ages at the scale of 10 5 to 10 6 years [6]. Low Radiation Counting Technology (LLC) became the first method to measure the content of 81 Kr [7,8], but this method was subsequently phased out due to its slow speed and low efficiency. In 1997, Accelerator Mass Spectrometry (AMS) technology was developed [9], and it was successfully applied to the analysis and detection of 81 Kr groundwater samples in the Great Artesian Basin region of Australia [10,11]. In 1999, Atomic Trap Trace Analysis technology (ATTA-1) was developed, which is an atomic counting method based on laser technology to cool, trap and count atoms [12,13], In 2004, Sturchio et al. [14] applied the second-generation device (ATTA-2) to measure the age of 81 Kr in deep groundwater in the desert of Nubian region in Egypt, which required several tons of groundwater samples, so it could not be widely popularized. As ATTA technology improved and developed, its efficiency and accuracy improved. In 2012, Jiang [15] et al. developed a third-generation device (ATTA-3), which required only 6-8µL of krypton samples to measure, which means about 100-200L groundwater samples or approximately 2L extracted dissolved gas. ATTA-3 is currently the most advanced measurement tool with noble gas as tracer, which has been widely used in recent related researches [15]. It has been successfully applied in many areas and has become a definitely effective method for the measurement of krypton content. While the use of 81 Kr for groundwater dating are relatively mature in some regions, such as the Great Artesian Basin in Australia, New Mexico, the Baltic sea artesian basin, etc., few studies are carried out in the most research field in the world, and the use of 81 Kr for groundwater studies is just the start. Different conditions are required for age measurement by 81 Kr and other methods under different circumstances. For example, some studies classify 81 Kr from different sources, and then determine their content, leading to research results in different directions. For example, J. C. Zappala et al. limited the human's contribution to 81 Kr in the atmosphere by 2.5% [16]. The isotope 85 Kr can be used as an ideal tracer for measuring young surface water [17]. In this paper, by searching relevant literatures on the application of 81 Kr in groundwater dating, summarizing the principle of groundwater dating by using 81 Kr, the measurement method of 81 Kr and its practical application ，providing ideas and research guidance for the application of 81 Kr in the measurement of ancient groundwater age.

The Principle of 81 Kr Dating
The method of measuring the age of groundwater by krypton radioisotope [18] needs to satisfy three conditions: (1) Groundwater and atmosphere are fully balanced in the recharge area; (2) The studied area is completely a closed system, and there is no groundwater mixing in different paths; (3) There is no krypton isotope diffusion exchange between groundwater aquifers and adjacent anhydrous aquifers during groundwater movement. If these conditions are met, groundwater age can be estimated by using the 81 Kr decay equation, as shown in formula (1) and formula (2): 81 Rs is the ratio of 81 Kr abundance ratio of dissolved gas in groundwater to atmosphere; τ is the 81 Kr decay constant; t is groundwater age [Error! Bookmark not defined.]. When the ATTA measurement error of 81 Kr is δ 81 Rs, the age error of groundwater sample is shown in the formula (3):

81 Kr Measuring Technology
The amount of 81 Kr in the atmosphere is constantly changing with months and years. For example, Andreas Bollhfer et al. have shown that 81 Kr in Europe has changed over the past 50 years, and have suggested that its data can be used as a parameter for groundwater dating [19]. Inert gases are distributed uniformly and stably in the atmosphere [20][21]. The abundance of krypton in the atmosphere is (1.14±0.01) PPM [22], krypton has six stable isotopes ( 78 Kr, 80 Kr, 82 Kr, 83 Kr, 84 Kr, 86 Kr) and two extremely low abundance radioisotopes ( 81 Kr, 85 Kr). 81 Kr is generated by the interaction between cosmic rays and atomic nuclei [23], with a half-life of (2.29±0.11)×10 5 years and abundance of 6×10 -13 [9,23,24]. The 81 Kr generated by solar activity, human nuclear industry, nuclide spontaneous and neutron-induced fission is negligible [9,13,[25][26]. As an inert gas, krypton is extremely stable in physical and chemical properties, and its mixed transport in groundwater is extremely simple, so it can be used to measure the age of ancient groundwater and old polar glaciers (10 5 -10 6 years) [11,[27][28]. However, because of the extremely low content of 81 Kr in groundwater, it is more difficult to use 81 Kr in old groundwater dating. 81 Kr extraction and purification experiments become the most important and difficult part of paleogroundwater dating. The 81 Kr trap capture rate is extremely low, the amount of trapped atom 81 Kr is small and its trap lifetime is long enough. W. Jiang et al. used afluorescence signal way to analysis and counted the rare 81 Kr isotope, and successfully counted the 81 Kr abundance. They also used a control isotope 85 Kr to assist their experiment [15]. Generally, to correctly measure the age of groundwater samples, three steps are required: (1) the extraction of dissolved gas from groundwater, (2) the isolation and purification of krypton from dissolved gas, (3) and the measurement of krypton radioactive isotopes. The sample size of 81 Kr required for ATTA measurement is 6-8µL, which means about 100-200L groundwater samples, also means 2L dissolved gas should be extracted. In order to prevent the shortage of krypton samples caused by the diversification of dissolved gas components in water (such as rich of CH 4 or CO 2 ), the gas intake in the field is usually about 5L [29].

Extract Dissolved Gas from Groundwater
According to Henry's law, the solubility of gas in water is positively related to the equilibrium partial pressure of gas under isothermal and isobaric conditions. Therefore, in an environment with extremely low partial gas pressure, the ambient dissolved gas in the saturated dissolved sample will escape, and reach the equilibrium in the environment with lower partial gas pressure, so that the ambient dissolved gas can be extracted. Currently, there are two main methods for field groundwater dissolved gas separation: vacuum atomization method and degassing film method [33]. The idea is to increase the gas-liquid contact area to allow dissolved gas to escape. How these two methods work is shown in figure 1. Because the collection of samples has a big influence to the measuring results in groundwater dating, and the amount of sample in groundwater dating by krypton 81 is larger than using other isotopes, the collection work should be very careful in the field. Considering the actual field condition, there are four requirements in the extraction of dissolved gas from groundwater in field: (1) Before collecting the samples, there should be a simple air tight test, to make sure that the total gas leakage rate in the system is less than 1%; (2) The time of collection should be controlled in 2 hours, and the devices should be effective, with 30L/min of samples generally; (3) Extracting gas should be efficient to reduce the influence of isotope fractionation; (4) When collecting samples, the local water temperature, water pressure, pH, depth, gas pressure and the amount of samples should be recorded.

Inert Gas Separation and Purification
Krypton has a volume fraction of 1.14 ppm in the air, and even more in groundwater and polar glaciers, at about 3 ppm, so it needs about 10L of air or 3-5L of dissolved gas from environmental samples to purify 10µL of krypton gas. Inert gas separation and purification are usually carried out by means of low-temperature distillation and enrichment, gas chromatography separation, physical or chemical adsorption and purification, and generally by a combination of these two or three methods.
The design ideas of krypton separation and purification system is: first, activated carbon adsorpts at low temperature to remove N 2 , O 2 and other components, and krypton enriches 100 times; After that, non-inert gas components (N 2 , O 2 , CH 4 , etc.) in residual gas are removed by high temperature sponge iron furnace. Finally, Kr and Ar are separated by gas chromatograph, and Kr is extracted from Ar. The flow chart of krypton separation and purification system is shown in figure  2 [33]: The product from these two steps (the extraction of dissolved gas and the purification of krypton) is then used to measure the content of 81 Kr by ATTA, and finally the decay equation is used to estimate the age of groundwater.

Analytical Instruments and Techniques
The core component of ATTA technology, which is based on laser technology to cool, trap and count atoms, is 3D magneto-optical trap (3D MOT), which captures atoms of different isotopes by changing the frequency of laser in the trap. Recently, Sapam Ranjita Chanu studied the cooling technology of transferring atoms from 3D magneto-optical trap [1]. Differs from the low radiation counting technology (LLC) isotope of radiation energy resolution and accelerator mass spectrometry (AMS) of isotope with the requirements of resolution of isotopes and isobars charge-mass ratio, the capture of ATTA technology only relates to the atomic transition frequency, thus it has a high selectivity, and is not influenced by any non-target atoms or molecules such as similar decay energy elements, isotope, isobars. Prior to the mature development of ATTA technology, accelerator mass spectrometry (AMS) was very inefficient in detecting krypton, requiring several hundred microliters of krypton samples and several tons of groundwater samples, which was extremely difficult to operate. As the sample size and isotope abundance decrease, the relative error of ATTA-3 technique increases. In order to make ATTA measurement error less than 10%, 6-8 microliters of pure krypton samples is required, which means 100-200L groundwater samples. The development of ATTA technology makes it possible to date groundwater by using inert gases.

The Application of 81 Kr in
Groundwater Dating 81 Kr can be used to estimate the age of old groundwater in deep aquifers (up to 5000m deep) and as a temperature indicator of the palaeoclimate. As a reference isotope, 81 Kr also has many applications. Compared with 14 C whose results are usually very accurate, 81 Kr can be used to date the age in its time range (10 5 -10 6 years), as the verification of the results of 4 He [2]. The University of Science and Technology of China, the institute of hydrogeology and environmental geology, Chinese academy of geological sciences, and the International Atomic Energy Agency (IAEA) cooperated and collected seven deep groundwater samples from the west to the east region in the North China Plain and used the vacuum extraction and the membrane to extract the dissolved gas, and successfully separated and purified the krypton and measured. The results show that the groundwater is getting older from west to east in the North China Plain, and the most ancient groundwater is aged from 0.85 to 1.15 Ma. In Guanzhong Basin by applying the method of degassing membrane extraction from eight groundwater water samples (each sample 100L to 160L) and the separation and purification of krypton, as well as the atomic trap trace analysis technique (ATTA), Pang Zhonghe [3] measured the content of 81 Kr and estimated the age of groundwater in Guanzhong basin in the depth of 5000m, and compared the results with 14 C, 4 He and 36 Cl, indicating that the estimated age of groundwater by 81 Kr is basically consistent with other methods. He also illustrated that the age characteristics of groundwater estimated by 81 Kr can reveal the rules of groundwater movement in Cenozoic rift basin, and to some extent, it can reflect the temperature of paleoclimate on the scale of millions of years. Matsumoto [35] dated groundwater in the North China Plain (NCP) in combination with 81 Kr and 4 He: In coastal areas, the age scale of groundwater samples measured by 81 Kr is between 0.5 and 1Ma years [36], 4 He in the underground water samples in the central and coastal areas of the North China Plain comes from the atmosphere, continental crust and mantle pool, which reflects the active tectonic activities in the continental crust of the North China Plain. The ages of 4 He and 81 Kr measured in the crust can be used to assess the patterns of 4 He inflowing into the aquifer and its vertical diffusivity in the aquifer. In addition, the age measured by 4 He is consistent with that of 81 Kr, which proves the feasibility of the dating method of 81 Kr. Therefore, 4 He can be used as a verification approach to assist 81 Kr dating. 81 Kr was used to study the groundwater flow process and chemical solute transport rules, and 81 Kr concentration was measured from groundwater samples from two local monitoring Wells in Sturchio at the nuclear waste treatment plant (WIPP) in New Mexico, and the results were compared with the reverse hydrogeochemical simulation results [4]. 81 Kr can be used as a method to measure the age of old groundwater in aquifers, but the diffusion and exchange with surrounding aquifers complicate the estimation. If enough information about the hydrogeochemical interaction of groundwater with the surrounding aquifer, as well as other isotope results (such as δ 2 H, δ 18 O, 36 Cl, etc.) as the complement and validation, the 81 kr can be used as an effective way of the movement law of groundwater simulation and age. Gerber [5] analyzed the content of 81 Kr and inert gas to study the groundwater age in the deep aquifer of the Baltic Sea Artesian Basin (BAB) and revealed the groundwater flow pattern at the scale of millions of years. Analysis shows that the groundwater flow system consists of three parts: Holocene and Pleistocene interglacial precipitation, glacial meltwater, and ancient high-salinity salt water. Interglacial precipitation and glacial meltwater are based on the time scale of hundreds of thousands of years, while 4 He and 40 Ar, as qualitative supporting evidence of 81 Kr results, prove that the residence time of high-salinity salt water components exceeds 1~5Ma years, and the high-salinity salt water components come from the evaporation enrichment of prequaternary seawater. In addition, isotope measurements of inert gas and stable environment isotopes can reveal the replenishment mechanism of glacier.

Conclusion and Recommendation
Atomic Trap Trace Analysis technology (ATTA) has experienced through the development of the first, second and third generations. For now, the measurement demand for krypton samples is 6-8µL, corresponding to the collection of groundwater samples of about 100-200L and the extraction of dissolved gas of about 2L. Although it is better than the previous equipment method, in fact, the collection amount of 100-200L deep aquifer groundwater in the field is still very large. The research and improvement of test equipment and measurement technology still need to be continued, so as to the improvement of work efficiency and the reduction of work difficulty. 81 Kr for dating ancient deep aquifer groundwater aged from 10 5 to 10 6 is an effective way, but due to the errors in measuring instruments, improper operation result error such as the dissolved gas leakage, and the spread and exchange of krypton with weak permeable layer or aquiclude layer, the incalculable complex hydrogeochemical evolutions, 81 Kr cannot be the only way to date the ancient underground water. Other methods (such as 4 He, 36 Cl, reverse hydrogeochemical simulation, etc.) should be used as reference and validation when using 81 Kr to measure the groundwater age, and also the specific hydrogeochemical conditions should be considered and combined, to comprehensively and scientificly analyze the age and movement rules of local groundwater. The latest progress in 81 Kr dating is Aeschbachhertig W. et al. which has been used to measure very old groundwater [6]. Aggarwal et al. used 81 Kr as a constraint factor and 4 He as a combination to measure the age of paleogroundwater [7].
Besides ancient groundwater, 81 Kr dating samples can also be seawater, polar glaciers, etc. In addition to dating, it can also be used to study groundwater movement process and explore geological tectonic activities. It is also a prospect of 81 Kr in hydrogeology and paleoclimology to be used as a temperature indicator to reconstruct paleoclimate characteristics, and combine meteorology and atmospheric dynamics to simulate climate change since ancient times. In addition, using 81 Kr to study the age, origin, characteristics of recharge and discharge, and movement rules of groundwater can provide certain scientific basis for nuclear waste treatment and underground pollution in nuclear science and radiochemistry.
In summary, combined with the above prospects, this paper will serve as a suggestion guidance to provide a scientific basis for the measurement of 81 Kr in the study of paleogroundwater dating.