Stable Isotopes of Deep Groundwater in the Xiongxian Geothermal Field

Stable Isotopes of Deep Groundwater in the Xiongxian Geothermal Field

Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 512 – 515 15th Water-Rock Interaction Interna...

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Available online at www.sciencedirect.com

ScienceDirect Procedia Earth and Planetary Science 17 (2017) 512 – 515

15th Water-Rock Interaction International Symposium, WRI-15

Stable isotopes of deep groundwater in the Xiongxian geothermal field Yanlong Konga,1, Zhonghe Panga, Jumei Panga, Yingchun Wanga, Fengtian Yangb a

Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Key Laboratory of Groundwater Resources and Environment, Ministry of Education, Jilin University, Changchun 130021, China

Abstract Karst groundwater samples were collected in the Xiongxian geothermal field for the δ18O and δ2H data analysis. Both the oxygen (-8.4‰ to -8.6‰) and hydrogen (-72.4‰ to -73.0‰) isotopes were found enriched than the precipitation (-9.4 to -11.7‰ for δ18O and -76‰ to -85‰ for δ2H, respectively) in the late Pleistocene, when the karst groundwater was recharged. The enriched oxygen isotope is attributed to water-dolomite interaction in low-middle temperature geothermal field. The water-dolomite ratio was calculated as 0.3, based on the oxygen isotope exchange. The enriched hydrogen isotope is deduced to be caused by bacterial SO42- reduction. © 2017 2017The TheAuthors. Authors. Published by Elsevier Published by Elsevier B.V.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. Peer-review under responsibility of the organizing committee of WRI-15 Keywords: stable isotopes; deep groundwater cycle; Xiongxian geothermal field, China

1. Introduction With the recent development of deep geological engineering including petroleum, geothermal reservoir exploration and nuclear waste disposal, more chances to know the deep groundwater cycle against all odds are available. Here the deep groundwater is defined as groundwater with depth larger than 1 km, below which both temperature and stress could affect the groundwater flow. At such depths, groundwater flow always happens at the continental scale and complex geochemical processes are generally incorporated1,2. Stable isotopes of deep groundwater in the sedimentary basins are useful in determining the groundwater origins, revealing the mixing processes and tracing the chemical evolution. During the past several years, more progresses have been made in utilizing the stable isotopes composition to characterize the deep groundwater systems. * Corresponding author. Tel.: +86-10-82998611; fax: +86-10-62010846. E-mail address: [email protected]

1878-5220 © 2017 The Authors. 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.2016.12.129

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Especially, some new findings from the low permeable aquifers or aquitards extend the use of stable isotopes in identifying paleo-hydrogeological processes in the sedimentary basins, although those works can also be carried out in shallow groundwater systems. For example3 characterized paleo-groundwater flow and solute transport mechanisms across 384 m of Cretaceous shale (aquitard) in the Williston Basin, Canada, using high-resolution depth profiles of water isotopes (δ18O, δ2H). They conclude molecular diffusion appears to be the dominant solute transport mechanism through the aquitard. Later4 used the numerical simulation of δ18O to indicate that the pore water residence time in the Ordovician sequence approaches the age of the rocks. In this work, we will show the preliminary results on the stable isotopes in the Xiongxian geothermal field with the purpose to identify the factors affecting isotopes exchange in deep groundwater systems. This work will be useful for the future research on the deep groundwater cycle in the Bohaibay Basin. What’s more, it will serve as a basis for the sustainable exploitation of geothermal energy in the Xiongxian geothermal field. 2. Study area and sampling 2.1. Study area Xiongxian lies in the North China Plain (NCP). It is at a distance of 108 km from Beijing and 100 km from Tianjin (Fig. 1). The Xiongxian geothermal field is located in Niutuozhen uplift (Fig. 1), which has a total area of 640 km2, with the area within Xiongxian territory being around 320 km2. The recoverable heat in the Xiongxian geothermal field is estimated to be 2.5 × 1018 J 5. Geothermal water in Xiongxian has been used for more than 30 years, with reservoir temperature ranges from 50 to 95°C. The geothermal energy is mainly used as space heating, bathing and greenhouse agriculture. Space heating starts from November 15th, and ends on March 15th of the next year.

Fig. 1 Locations of Niutuozhen uplift and Xiongxian geothermal field in China. The red block scheme illustrates the Niutuozhen uplift with faults as boundaries. The orchid polygon shows the extent of Xiongxian. The cross section across Xiongxian can be found in Fig.2.

The Niutuozhen uplift is located north of the Jizhong Depression as part of Bohai Bay Basin (BBB). The BBB was formed in the Tertiary on the basement of the North China Platform, and consists of many separate Paleogene faulted depressions6. The strata of the Xiongxian geothermal system and surroundings include Quaternary and Tertiary formations in Cainozoic, Jixian System and Changcheng System in Proterozoic and Archaeozoic (Fig. 2).

Fig. 2 The cross section across the Xiongxian geothermal field in Niutuozhen uplift

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2.2. Sampling and analyses Water samples were collected in the Xiongxian geothermal field in 2013. Samples were analyzed in the Water Isotopes and Water-Rock Interaction Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences. δ2H and δ18O were measured on a laser absorption water isotope spectrometer analyzer (Picarro L2120-i). All δ2H and δ18O values are expressed related to Vienna Standard Mean Ocean Water (V-SMOW) in ‰, and the measurement precision was 0.5 and 0.1‰ for δ2H and δ18O, respectively. 3. Results and discussion The δ18O and δ2H values of Jixian groundwater in the Xiongxian geothermal field change very little and have a range from -8.4‰ to -8.6‰ and from -72.4‰ to -73.0‰, respectively. Taking consideration of hydro-chemical characters between each sample7, we can deduce that groundwater in the Jixian aquifer is well mixed in the Xiongxian geothermal field. This is also supported by the geological evidence. As noted, the Neocene formation is directly deposited on the Jixian carbonate formations, which allows the long term exposing of this karst aquifer and leads to the similarity of water geochemical features 6,7. The δ18O values of Jixian groundwater in the Xiongxian geothermal field are similar with the weighted average precipitation δ18O value (-8.1‰) while the δ2H values are more depleted than the weighted average precipitation δ2H value (-56.8‰) in the Shijiazhuang station (Fig.3). According to Kong et al. (2015), the Jixian groundwater was recharged by precipitation in the late Pleistocene, which has ranges of -9.4 to -11.7‰ for δ18O and -76‰ to -85‰ for δ2H values8. It can be seen that the δ18O values of Neocene (Nm) groundwater are larger than this range while the δ2H values are close to it. In this way, the oxygen isotope shows a clear shift trend in both Neocene and Jixian groundwater, while the hydrogen isotope is only enriched in the Jixian groundwater. The oxygen shift is attributed to exchange of oxygen in the water molecule with oxygen in silicate and carbonate minerals in the rocks. It is mainly related to water-rock ratio and sub-surface temperature. As suggested by Kharaka and Thordsen9, the water-rock ratio can be calculated by the oxygen shift using the law of conservation of mass. In a closed system, and for water reacting with a single mineral only, the equation is: (1) Wδiw + RδiR = Wδfw + RδfR where i and f are the initial and final δ values for water and rock, respectively, and W and R are the mole fractions of oxygen atoms in water and rocks, respectively. The water rock ratio is the W/R, and we know that W+R=1 (2) Assuming isotopic equilibrium between water and the mineral results in: (3) αR-W = (1000 + δfR) / (1000 + δfW) where αR-W is the fractionation factor between water and the mineral at the temperature of reaction. Equation (2) can be approximated to: (4) δfR - δfW = 1000lnαR-W

Fig. 3 Stable isotopes in different water bodies in the Xiongxian geothermal field and Niutuozhen Uplift

The surrounding rock in the Xiongxian region is mainly dolomite, which has an initial average δ18O value of 20.43‰10. Giving the measured temperature of about 80 °C᧨ we can calculate the water-rock ratio as 0.3 based on

Yanlong Kong et al. / Procedia Earth and Planetary Science 17 (2017) 512 – 515

Fig. 3 and the four equations above. It should be noted that errors could be involved in the quantitative process of water-rock ratio due to the variable dolomite oxygen isotopes and subsurface temperature. Other minerals may also be involved in isotopic exchange ratios. But the calculation is valid to support that oxygen shift could happen even in the low-middle (90 - 150 °C) geothermal system when a lower water ratio is provided. The δ2H values of Jixian groundwater in Xiongxian fit between the modern precipitation and late Pleistocene precipitation. The 2H isotope exchange in the sedimentary basins is generally found between water and clay minerals. But it should not be the case in the Jixian limestone aquifers. In addition, several authors7 found that the SO42- of Jixian groundwater is quite lower than that of shallow Quaternary groundwater. Several authors11 found that bacterial SO42- reduction (BSR) under low geo-temperature during the burial digenesis is the likely origin of H 2S in Jinxian Sag near our study region. Considering that, the process of sulfate reduction could enrich the 2H isotope by releasing H2S and the subsurface temperature is less than 100 °C, we deduce that it is the process of BSR that leads to the alteration of both SO42- and δ2H. 4. Conclusions The stable isotopes (18O and 2H) of deep groundwater in the Xiongxian geothermal field were investigated. The oxygen shift found in Jixian groundwater is due to the water-dolomite interaction. Based on this phenomenon, the water-dolomite ratio was calculated as 0.3. The deuterium content within Jixian groundwater is more enriched when compared with the isotopic composition of precipitation in the late Pleistocene. This 2H enrichment was deduced to be caused by the bacterial SO42- reduction. Acknowledgements This study is supported by the National Natural Science Foundation of China (Grant 41372257 and 41430319). The authors appreciate the International Postdoctoral Exchange Fellowship Program China (20130019) and Project funded by China Postdoctoral Science Foundation (2015M581168) for the financial support of this work. References 1.Garven G. Continental-scale groundwater flow and geologic processes. Annu Rev Earth Planet Sci. 1995; 23: 89–117 2.Kong Y, Pang Z, Shao H, Hu S, Kolditz O. Recent studies on hydrothermal systems in China: a review. Geothermal Energy 2014; 2: 19. 3.Hendry, M., Barbour, S., Novakowski, K., Wassenaar, L.. Paleohydrogeology of the Cretaceous sediments of the Williston Basin using stable isotopes of water. Water Resources Research 2013; 49: 4580-4592. 4.Al, T., Clark, I., Kennell, L., Jensen, M., Raven, K. Geochemical evolution and residence time of porewater in low-permeability rocks of the Michigan Basin, Southwest Ontario. Chemical Geology 2015; 404: 1-17. 5.Huang J. Assessment and management of sedimentary geothermal resources, Report in: Geothermal training in Iceland 2012; UNU-GTP, Iceland, p. 1-63. 6.Pang Z., Kong, Y., Li, Y. and Li, J. Water-Rock Interaction in CO2 Sequestration in a Depleted Oil Reservoir Pilot Test. Procedia Earth and Planetary Science 2013; 7: 656-659. 7.Kong Y, Pang Z, Pang J, Luo L, Luo J, Shao H, Kolditz, O. Deep groundwater cycle in Xiongxian geothermal field. Proceedings World Geothermal Congress 2015; Melbourne, Australia, 2015, April, 19 -25. 8.Chen Z., Qi Ji., Xu J. et al. Paleoclimatic interpretation of the past 30,000 yr from isotopic studies of the deep confined aquifer of the North China Plain. Applied Geochemistry 2003, 18: 997-1009. 9.Kharaka and Thordsen. Stable isotope geochemistry and origin of waters in sedimentary basins. In: Clauer, Chaudhuri editors. Isotopic Signatures and Sedimentary Records, Heidelberg: Springer Berlin; 1992. P. 411-466. 10.Vasconcelos, C., McKenzie, J., Warthmann, R., Bernasconi, S. Calibration of the δ18O paleothermometer for dolomite precipitated in microbial cultures and natural environments. Geology 2005; 33: 317 – 320. 11.Cai, C., Worden, R., Wolff, G., Bottrell, S., Wang, D., Li, X.. Origin of sulfur rich oils and H2S in Tertiary lacustrine sections of the Jinxian Sag, Bohai Bay Basin, China. Applied Geochemistry 2005; 20: 1427 – 1444.

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