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An Isotopic Geoindicator in the Hydrological Cycle
Available online at www.sciencedirect.com
ScienceDirect Procedia Earth and Planetary Science 17 (2017) 534 – 537
15th Water-Rock Interaction International Symposium, WRI-15
An isotopic geoindicator in the hydrological cycle Zhonghe Panga,b,1, Yanlong Konga, Jie Lia, Jiao Tiana,b a
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b University of Chinese Academy of Sciences, Beijing, 100049, China
Abstract This paper synthesizes the research findings on the stable isotopes (δ2H and δ18O) of the water molecular, with respect to applications in tracing the natural water cycle. A general purpose diagram of δ2H versus δ18O may serve as a geo-indicator of various circulation processes the water is involved: hydrological, hydro-meteorological and hydro-geochemical. Equations and explanations are provided for a total of 13 lines in the diagram that are illustrated to show the fractionation and changes of stable isotopes when experiencing different processes: GMWL; Moisture recycling line; evaporation line, mixing lines with seawater and andesitic water, exchange lines with H2S, CO2, silicate mineral, hydrocarbon, carbonate and clay minerals. These lines serve as a basis in using water stable isotopes in the hydrological, hydrogeological and paleo-climate research. © 2017 2017Published The Authors. Published B.V. by Elsevier B.V. by ThisElsevier is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Hydrological cycle; stable isotope; geoindicator; δ2H - δ18O diagram
1. Introduction The isotopic ratios of the chemical elements oxygen and hydrogen forming the water molecular (18O/16O and H/1H, respectively) are powerful tools when used in hydrological, hydro-meteorological and hydro-geochemical logical and climatic processes (Fig. 1). Such studies require a good understanding of the isotopic fractionation and mixing in processes controlling the isotopic composition of different water bodies in the hydrological systems like vapor, precipitation, surface water, groundwater and glacier ice in polar and high mountain regions. 2
* Corresponding author. Tel.: +86-10-82998611; fax: +86-10-62010846. E-mail address: [email protected]
1878-5220 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.135
Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
Fig. 1. Stable isotope fractionation of the water molecule in natural water cycle
Since the discovery of Global Meteoritic Water Line (GMWL1), the δ2H versus δ18O diagram has often used to identify whether the water source is originated from precipitation and what atmospheric processes it has undergone. Later on, several other atmospheric processes, such as evaporation from open surface of a water body, mixing of different water masses, various diagenetic water-rock interactions (isotopic exchange with minerals caused by mineral alteration) are found and generally illustrated in the δ 2H - δ18O diagram together with the GMWL. In this paper, we attempt to synthesize isotopic variations in major types of processes in the natural water cycle as well as water-rock interactions that control the stable isotopic composition of the water molecules and show them all in a single δ2H versus δ18O diagram. It has been found useful in learning (and teaching) and even more in practical applications of stable isotopes. 2. The δ2H - δ18O diagram Although it is difficult to exhaust all possibilities causing stable isotopic changes involved in the natural water cycle, those found in the literature can be summarized in a diagram such as Fig.2. New ones maybe added to the diagram later on. The behavior of stable isotopes in these 13 processes are relatively is well understood and can be shown using evolution trend lines in a single δ2H versus δ18O diagram (Fig. 2). A further separation of the diagram is convenient if a closer look at the details of the specific process is needed. Generally speaking, one can find these lines refer to processes in the atmospheric water system, surface water system and groundwater system. 3. Atmospheric water systems Lines 1, 2, 10 and 11 are generally used in the atmospheric water system. Among them, line 1 is the Global Meteoric Water Line (GMWL: δ2H = 8δ18O + 10), which is the basic reference line in all of the hydrological studies. Line 2 is the line for recycled moisture, as has been described by3. Although most of the atmospheric water is plotted on the GMWL, some are plotted on this line. As a follow up study, several authors4 estimated the recycled fraction in the Urumqi basin to be 8%, based on the isotope fractionation during the process of moisture recycling. Line 11 is the paleo-meteoric water line with depleted isotopes in the precipitation occurring during the last glacial period2,8. Line 1: G 2 H 8G 18O 10 Line 2: G 2 H 8G 18O d (d ²10) Line 10: G 2 H nG 18O d (n¢8, G 18O¢G 18OGWML ) Line 11: G 2 H 8G 18O d (d ¢10)
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Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
Fig. 2. An isotopic geoindicator for the study on hydrological and water-rock interaction processes
4. Surface water systems Line 7 is the evaporation line, which has different slopes but always smaller than that of the GMWL. Typical slopes are 4 – 6, depending on the temperature and relative humidity of the region in question. Compared with the evaporation from the surface water, evaporation from the soil always has a much smaller slope. Line 10 is condensate line of the evaporated vapour from an open water body. In addition, during the falling process, the evaporation may also occur, which is called sub-cloud evaporation. It is very interesting that the deuterium excess (d = δ2H - 8δ18O) changes numerically between 1.1 and 1.2 per 1% of evaporated fraction in both humid5 and arid regions4. Line 7: G 2 H mG 18O d (m¢8, G 18O²G 18OGWML ) Line 6 is the mixing line with a volcanic vapour, using andesitic water as an example6. These waters are mainly found in the geothermal systems associated with island-arc volcanism, e.g. the Circum-Pasific geothermal belt. Geothermal waters are plotted along this line if they are formed by mixing of deep andesitic water and meteoric water. In extreme cases, mixing ratio could be as high as 25% for that of andesitic water end member 7. Line 6: G 2 Handesiticwater -20f10ă, G 18Oandesiticwater -10f2ă Line 8 is the mixing line with sea water. Sea water close to the seashore is plotted along this line, the same as groundwater found in a coastal aquifer that is affected by seawater intrusion. Line 8: G 2 H seawater 0, G 18Oseawater 0 5. Groundwater systems Water-rock interaction processes are common for groundwater. Lines 3, 4 are line for the exchanges with H2S, CO2. Hydrogen sulphide (Line 3) can be generated in groundwaters where an organic carbon substrate is available to support bacterial sulphate reduction, and lead to the enrichment of 2H in the groundwater because of exchange with H2S. A negative shift in δ18O (Line 4) is expected in systems with high ratios of CO 2 to water, especially in the CO2 geological sequestration8. Lines 5,9,13 are the lines for interactions with minerals at elevated temperatures, and silicate and clay minerals, respectively. The exchange between water and silicates can increase δ2H values and
Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
537
decrease δ18O values of groundwater (Line 9). Line 12 is the line for isotopic exchange with hydrocarbons. In the coal formations, line 12 can be found due to the exchange of hydrogen with hydrocarbon. Various underground processes may alter the isotopic composition of groundwater, which give insights into its subsurface evolution history and geochemical reactions of groundwater with the rocks. Geothermal waters show positive δ18O shifts of varying degrees (Line 5). This is attributed to water-rock interactions usually at high temperatures. Recently, it has been found at low temperature geothermal water in a sedimentary basin, significant oxygen shift is found 9. No effect on δ2H is evident due to the lack of H in most rock-forming minerals. While in the clay formations, the dehydration effect, which is in fact the exchange of water isotopes, may occur, resulting in the line 13. Line 3: 103 ln D 2 H H 2O H 2 S
G 2H H O 2
(106 / TK2 ) 290.498 u (103 / TK ) 127.9
G 2 H H S 103 ln D 2 H H O H S 2
2
2
Line 4: 103 ln D 18OCO2( g ) H 2O
G OH O 18
2
G OCO 18
2( g )
0.0206 u (106 / TK2 ) 17.9942u (103 / TK ) 19.97 103 ln D 18OCO2( g ) H 2O
6. Conclusions In this paper, a general purpose diagram with totally 13 trend lines to illustrate stable isotopes in the water molecular for the purpose of hydrological studies. It can be used to study the isotopic evolution of the average meteoric water in a study area, its evolution through the surface water and groundwater systems as part of the hydrological cycle. They are useful to trace the water origin, reveal the various processes, including hydrological, hydro-meteorological and hydrogeochemical ones, and to quantify the mixing of different water sources. The δ2H versus δ18O diagram developed in this paper can be used as a basic reference for the application of stable isotopes for practical applications as well as learning and teaching. Acknowledgements This study is carried out during the teaching of isotope hydrology at the University of Chinese Academy of Sciences over the last ten years. The support of the UCAS and students of the classes are thanked for their discussions in the teaching process. References 1. Craig, H. Isotopic variations in meteoric waters. Science 1961; 133: 1702-1703. 2. Clark, I. and Fritz, P. Environmental Isotopes in Hydrogeology, Lewis Publishers, New York; 1997. 3. Pang Z, Kong Y, Froehlich K, Huang T, Yuan L, Li Z, Wang F. Processes affecting isotopes in precipitation of an arid region. Tellus, 63B 2011; 352-359, doi: 10.1111/j.1600-0889.2011.00532.x. 4. Kong, Y., Pang, Z. and Froehlich, K. Quantifying recycled moisture fraction in precipitation of an arid region using deuterium excess, Tellus, 65B 2013; 19251, doi: 10.3402/tellusb.v65i0.19251. 5. Froehlich, K., Kralik, M., Papesch, W., Rank, D., Scheifinger, H. and co-authors. Deuterium excess in precipitation of Alpine regions moisture recycling, Isot. Environ.Healt. S. 2008; 44(1): 61-70. 6. Giggenbach,W.F., 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth and Planetary Science Letters 113, 495–510. 7. Pang Z., 2006, pH dependant isotope variations in arc-type geothermal waters, J of Geochem. Exploration 89, 306–308 8. Li J., Pang Z. Environmental isotopes in CO2 geological sequestration. Greenhouse Gas Sci Technol 2015; 5(4):374–388. 9. Qin et al, 2005, Hydrogeochemistry and groundwater circulation in Xi’an Geothermal Field,China, Geothermics 34(4):471-494
ScienceDirect Procedia Earth and Planetary Science 17 (2017) 534 – 537
15th Water-Rock Interaction International Symposium, WRI-15
An isotopic geoindicator in the hydrological cycle Zhonghe Panga,b,1, Yanlong Konga, Jie Lia, Jiao Tiana,b a
Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b University of Chinese Academy of Sciences, Beijing, 100049, China
Abstract This paper synthesizes the research findings on the stable isotopes (δ2H and δ18O) of the water molecular, with respect to applications in tracing the natural water cycle. A general purpose diagram of δ2H versus δ18O may serve as a geo-indicator of various circulation processes the water is involved: hydrological, hydro-meteorological and hydro-geochemical. Equations and explanations are provided for a total of 13 lines in the diagram that are illustrated to show the fractionation and changes of stable isotopes when experiencing different processes: GMWL; Moisture recycling line; evaporation line, mixing lines with seawater and andesitic water, exchange lines with H2S, CO2, silicate mineral, hydrocarbon, carbonate and clay minerals. These lines serve as a basis in using water stable isotopes in the hydrological, hydrogeological and paleo-climate research. © 2017 2017Published The Authors. Published B.V. by Elsevier B.V. by ThisElsevier is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Hydrological cycle; stable isotope; geoindicator; δ2H - δ18O diagram
1. Introduction The isotopic ratios of the chemical elements oxygen and hydrogen forming the water molecular (18O/16O and H/1H, respectively) are powerful tools when used in hydrological, hydro-meteorological and hydro-geochemical logical and climatic processes (Fig. 1). Such studies require a good understanding of the isotopic fractionation and mixing in processes controlling the isotopic composition of different water bodies in the hydrological systems like vapor, precipitation, surface water, groundwater and glacier ice in polar and high mountain regions. 2
* Corresponding author. Tel.: +86-10-82998611; fax: +86-10-62010846. E-mail address: [email protected]
1878-5220 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2016.12.135
Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
Fig. 1. Stable isotope fractionation of the water molecule in natural water cycle
Since the discovery of Global Meteoritic Water Line (GMWL1), the δ2H versus δ18O diagram has often used to identify whether the water source is originated from precipitation and what atmospheric processes it has undergone. Later on, several other atmospheric processes, such as evaporation from open surface of a water body, mixing of different water masses, various diagenetic water-rock interactions (isotopic exchange with minerals caused by mineral alteration) are found and generally illustrated in the δ 2H - δ18O diagram together with the GMWL. In this paper, we attempt to synthesize isotopic variations in major types of processes in the natural water cycle as well as water-rock interactions that control the stable isotopic composition of the water molecules and show them all in a single δ2H versus δ18O diagram. It has been found useful in learning (and teaching) and even more in practical applications of stable isotopes. 2. The δ2H - δ18O diagram Although it is difficult to exhaust all possibilities causing stable isotopic changes involved in the natural water cycle, those found in the literature can be summarized in a diagram such as Fig.2. New ones maybe added to the diagram later on. The behavior of stable isotopes in these 13 processes are relatively is well understood and can be shown using evolution trend lines in a single δ2H versus δ18O diagram (Fig. 2). A further separation of the diagram is convenient if a closer look at the details of the specific process is needed. Generally speaking, one can find these lines refer to processes in the atmospheric water system, surface water system and groundwater system. 3. Atmospheric water systems Lines 1, 2, 10 and 11 are generally used in the atmospheric water system. Among them, line 1 is the Global Meteoric Water Line (GMWL: δ2H = 8δ18O + 10), which is the basic reference line in all of the hydrological studies. Line 2 is the line for recycled moisture, as has been described by3. Although most of the atmospheric water is plotted on the GMWL, some are plotted on this line. As a follow up study, several authors4 estimated the recycled fraction in the Urumqi basin to be 8%, based on the isotope fractionation during the process of moisture recycling. Line 11 is the paleo-meteoric water line with depleted isotopes in the precipitation occurring during the last glacial period2,8. Line 1: G 2 H 8G 18O 10 Line 2: G 2 H 8G 18O d (d ²10) Line 10: G 2 H nG 18O d (n¢8, G 18O¢G 18OGWML ) Line 11: G 2 H 8G 18O d (d ¢10)
535
536
Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
Fig. 2. An isotopic geoindicator for the study on hydrological and water-rock interaction processes
4. Surface water systems Line 7 is the evaporation line, which has different slopes but always smaller than that of the GMWL. Typical slopes are 4 – 6, depending on the temperature and relative humidity of the region in question. Compared with the evaporation from the surface water, evaporation from the soil always has a much smaller slope. Line 10 is condensate line of the evaporated vapour from an open water body. In addition, during the falling process, the evaporation may also occur, which is called sub-cloud evaporation. It is very interesting that the deuterium excess (d = δ2H - 8δ18O) changes numerically between 1.1 and 1.2 per 1% of evaporated fraction in both humid5 and arid regions4. Line 7: G 2 H mG 18O d (m¢8, G 18O²G 18OGWML ) Line 6 is the mixing line with a volcanic vapour, using andesitic water as an example6. These waters are mainly found in the geothermal systems associated with island-arc volcanism, e.g. the Circum-Pasific geothermal belt. Geothermal waters are plotted along this line if they are formed by mixing of deep andesitic water and meteoric water. In extreme cases, mixing ratio could be as high as 25% for that of andesitic water end member 7. Line 6: G 2 Handesiticwater -20f10ă, G 18Oandesiticwater -10f2ă Line 8 is the mixing line with sea water. Sea water close to the seashore is plotted along this line, the same as groundwater found in a coastal aquifer that is affected by seawater intrusion. Line 8: G 2 H seawater 0, G 18Oseawater 0 5. Groundwater systems Water-rock interaction processes are common for groundwater. Lines 3, 4 are line for the exchanges with H2S, CO2. Hydrogen sulphide (Line 3) can be generated in groundwaters where an organic carbon substrate is available to support bacterial sulphate reduction, and lead to the enrichment of 2H in the groundwater because of exchange with H2S. A negative shift in δ18O (Line 4) is expected in systems with high ratios of CO 2 to water, especially in the CO2 geological sequestration8. Lines 5,9,13 are the lines for interactions with minerals at elevated temperatures, and silicate and clay minerals, respectively. The exchange between water and silicates can increase δ2H values and
Zhonghe Pang et al. / Procedia Earth and Planetary Science 17 (2017) 534 – 537
537
decrease δ18O values of groundwater (Line 9). Line 12 is the line for isotopic exchange with hydrocarbons. In the coal formations, line 12 can be found due to the exchange of hydrogen with hydrocarbon. Various underground processes may alter the isotopic composition of groundwater, which give insights into its subsurface evolution history and geochemical reactions of groundwater with the rocks. Geothermal waters show positive δ18O shifts of varying degrees (Line 5). This is attributed to water-rock interactions usually at high temperatures. Recently, it has been found at low temperature geothermal water in a sedimentary basin, significant oxygen shift is found 9. No effect on δ2H is evident due to the lack of H in most rock-forming minerals. While in the clay formations, the dehydration effect, which is in fact the exchange of water isotopes, may occur, resulting in the line 13. Line 3: 103 ln D 2 H H 2O H 2 S
G 2H H O 2
(106 / TK2 ) 290.498 u (103 / TK ) 127.9
G 2 H H S 103 ln D 2 H H O H S 2
2
2
Line 4: 103 ln D 18OCO2( g ) H 2O
G OH O 18
2
G OCO 18
2( g )
0.0206 u (106 / TK2 ) 17.9942u (103 / TK ) 19.97 103 ln D 18OCO2( g ) H 2O
6. Conclusions In this paper, a general purpose diagram with totally 13 trend lines to illustrate stable isotopes in the water molecular for the purpose of hydrological studies. It can be used to study the isotopic evolution of the average meteoric water in a study area, its evolution through the surface water and groundwater systems as part of the hydrological cycle. They are useful to trace the water origin, reveal the various processes, including hydrological, hydro-meteorological and hydrogeochemical ones, and to quantify the mixing of different water sources. The δ2H versus δ18O diagram developed in this paper can be used as a basic reference for the application of stable isotopes for practical applications as well as learning and teaching. Acknowledgements This study is carried out during the teaching of isotope hydrology at the University of Chinese Academy of Sciences over the last ten years. The support of the UCAS and students of the classes are thanked for their discussions in the teaching process. References 1. Craig, H. Isotopic variations in meteoric waters. Science 1961; 133: 1702-1703. 2. Clark, I. and Fritz, P. Environmental Isotopes in Hydrogeology, Lewis Publishers, New York; 1997. 3. Pang Z, Kong Y, Froehlich K, Huang T, Yuan L, Li Z, Wang F. Processes affecting isotopes in precipitation of an arid region. Tellus, 63B 2011; 352-359, doi: 10.1111/j.1600-0889.2011.00532.x. 4. Kong, Y., Pang, Z. and Froehlich, K. Quantifying recycled moisture fraction in precipitation of an arid region using deuterium excess, Tellus, 65B 2013; 19251, doi: 10.3402/tellusb.v65i0.19251. 5. Froehlich, K., Kralik, M., Papesch, W., Rank, D., Scheifinger, H. and co-authors. Deuterium excess in precipitation of Alpine regions moisture recycling, Isot. Environ.Healt. S. 2008; 44(1): 61-70. 6. Giggenbach,W.F., 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth and Planetary Science Letters 113, 495–510. 7. Pang Z., 2006, pH dependant isotope variations in arc-type geothermal waters, J of Geochem. Exploration 89, 306–308 8. Li J., Pang Z. Environmental isotopes in CO2 geological sequestration. Greenhouse Gas Sci Technol 2015; 5(4):374–388. 9. Qin et al, 2005, Hydrogeochemistry and groundwater circulation in Xi’an Geothermal Field,China, Geothermics 34(4):471-494