Journal
APPLIED GEOCHEMISTRY
Volume 89, Issue -, Pages 49-58Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.apgeochem.2017.11.011
Keywords
Geothermal system; Stable isotopes; Hydrogeochemistry; Triassic carbonate reservoir; Chongqing; China
Categories
Funding
- national key research and developmental program of China [2016YFC0502306]
- National Natural Science Foundation of China [41302213, 41472321]
- Guangxi Science Foundation [2013GXNSFBA019221]
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Chongqing, located in the southwestern China, was named as A spa city of the world. It's most geothermal reservoirs are developed in carbonate rock aquifers, and those thermal waters are characterized by high concentrations of sulfate and low HCO3-. However, little was known about the origin and genesis of those thermal waters, which is necessary for their protection and determination of regime of exploitation. Therefore, thermal and surface waters were sampled seasonally and analyzed for hydrogeochemical and isotopic compositions (delta O-18/delta H-2; delta S-34-SO4; delta O-18-SO4) to decipher the origin of the thermal waters in the Triassic carbonate aquifers in Chongqing area. Key questions include the relative significance of hydrothermal processes and solute supply. The results showed that: (1) the major chemical composition of analyzed the thermal waters from Chongqing area was characterized by Ca-SO4; (2) the delta H-2 and delta O-18 values of the thermal waters ranged from -48.6% to -63.1% with an average value of -54.2%, and from -6.5% to -9.2% with an average value of -8.0%, respectively, indicating that the thermal waters originated from the local rain water with an elevation of 415 m to 1453 m above mean sea level; (3) the estimated geothermal temperatures varied from 63.8 degrees C to 78.3 degrees C (Quartz), indicating that the depth of the geothermal reservoir varied from 411 m to 1728 m, which is located in the Lower Triassic Jialingjiang formation; (4) the delta S-34-SO(4)(2-)and delta O-18-SO(4)(2-)in the geothermal waters ranged from 29.7% to 34.1% with a mean value of 32.1%, and from 12.5% to 16.5% with a mean value of 15.2%, respectively, suggesting that the high SO(4)(2-)concentrations resulted mainly from the dissolution of gypsum in the second part of the Lower Triassic Jialingjiang Formation; (5) three processes of water-rock interactions (gypsum dissolution, carbonate dissolution by carbonic acid and carbonate dissolution by sulfuric acid) in the thermal system were unveiled by the hydrogeochemical and isotopic models. Among these three processes, gypsum dissolution seems to be primarily responsible for solute composition of the thermal water. This study also indicates the integration of hydrogeochemical and isotopic data is very useful tool to decipher the origin and genesis of the carbonate thermal waters.
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