Abstract
Geothermal water samples taken from deep aquifers within the city of Kaifeng at depths between 800 and 1650 m were analyzed for conventional water chemical compositions and stable isotopes. These results were then combined with the deuterium excess parameter (d value), and the contribution ratios of different carbon sources were calculated along with distributional characteristics and data on the migration and transformation of geothermal water. These results included the conventional water chemical group, hydrogen, and oxygen isotopes (δD-δ18O), dissolved inorganic carbon (DIC) and associated isotopes (δ13CDIC). The results of this study show that geothermal water in the city of Kaifeng is weakly alkaline, water chemistry mostly comprises a HCO3-Na type, and the range of variation of δD is between −76.12‰ and −70.48‰, (average: −74.25‰), while the range of variation of δ18O is between −11.08‰ and −9.41‰ (average: −10.15‰). Data show that values of d vary between 1.3‰ and 13.3‰ (average: 6.91‰), while DIC content is between 91.523 and 156.969 mg/L (average: 127.158 mg/L). The recorded range of δ13CDIC was between −10.160‰ and −6.386‰ (average: −9.019‰). The results presented in this study show that as depth increases, so do δD and δ18O, while d values decrease and DIC content and δ13CDIC gradually increase. Thus, δD, δ18O, d values, DIC, and δ13CDIC can all be used as proxies for the burial characteristics of geothermal water. Because data show that the changes in d values and DIC content are larger along the direction of geothermal water flow, so these proxies can be used to indicate migration. This study also shows demonstrates that the main source of DIC in geothermal water is CO2thathas a biological origin in soils, as well as the dissolution of carbonate minerals in surrounding rocks. Thus, as depth increases, the contribution of soil biogenic carbon sources to DIC decreases while the influence of carbonate dissolution on DIC increases.
Highlights
Stable isotope compositions can be used to measure the biogeochemical behavior of groundwater, including determining sources and recharge [1,2,3,4,5], the evolution of water quality [6], and transitions between surface and groundwater [7,8]
The difference, is in growth rate; as depth increases, the growth rate of dissolved inorganic carbon (DIC) gradually becomes smaller, while the growth rate of δ13 CDIC gradually increases. These results indicate that the source of DIC in geothermal water may change with increasing of depth, so it can be inferred that the sources of DIC in geothermal water at different depths in Kaifeng city are different
The speed of water-rock reactions are accelerated, which means that the content of DIC increases; δD, δ18 O, and δ13 CDIC exhibit different enrichment degrees, while d values decrease
Summary
Stable isotope compositions can be used to measure the biogeochemical behavior of groundwater, including determining sources and recharge [1,2,3,4,5], the evolution of water quality [6], and transitions between surface and groundwater [7,8]. Hydrogen and oxygen isotopes in water (i.e., δD-δ18 O), often referred to as “fingerprints” [13,14], can be used to extract information about the water cycle, including the sources [15], migration [16], retention, and excretion of the groundwater [17,18]. Because of the low content of hydrogen in underground rocks, isotope exchange mainly comprises oxygen. The deuterium excess parameter, or d value (d = δD-8δ18 O) [19,20], is often used to study groundwater, because it gives an indication of the degree of water rock oxygen isotope exchange [21], as well as variation within a certain area [22]. Since the main factors restricting groundwater remain essentially the same, the d value for one aquifer will vary according to length of groundwater residence time; a change from high to low can indicate the direction of groundwater flow
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