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-δ 18 O), dissolved inorganic carbon (DIC) and associated isotopes (δ 13 C DIC ). The results of this study show that geothermal water in the city of Kaifeng is weakly alkaline, water chemistry mostly comprises a HCO 3 -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 δ 18 O 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 δ 13 C DIC 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 δ 18 O, while d values decrease and DIC content and δ 13 C DIC gradually increase. Thus, δD, δ 18 O, d values, DIC, and δ 13 C DIC 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 CO 2 thathas 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.

Dissolved organic matter (DOM) plays an important role in the chemical evolution of groundwater. Thus, in order to understand the composition and characteristics of DOM in groundwater, analyzed 31geothermal water samples from five aquifers (i.e., between 600 m and 1600 m) in the city of Kaifeng were analyzed and the results were compared in order to clarify their spatial distribution, characteristics, sources, and environmental influences. Results show that as the depth of a thermal reservoir increases, the ultraviolet absorption (UV 254 ) of geothermal water does not change significantly, the concentration of dissolved organic carbon (DOC) gradually increases with depth, and the fluorescence intensity of DOM remains weak. Some differences are also evident with regard to the location and intensity of geothermal water sample DOM fluorescence peaks depending on thermal reservoir. The results of this study show that the main source of DOM in geothermal water is endogenous, derived from high stability organic matter derived from sedimentary processes and associated microbial activity. Within the three geothermal reservoir depth ranges, 600 m to 800 m, 800 m to 1000 m, and 1000 m to 1200 m, DOM components were mainly protein-like as well as soluble microbial metabolites. However, at deeper depths, within the 1200 m to 1400 m and 1400 m to 1600 m thermal reservoirs, the proportion of protein-like components in DOM decreased, while the ratio fulvic-like and humic-like components increased, leading to changes in the positions of fluorescence peaks. Finally, our results demonstrate a close relationship between the intensity of fluorescence peaks, suggesting that a number of fluorescent components may share a common source.

The deep geothermal water found within Kaifeng City, Henan province, China, is mainly contained within a loose-pore geothermal reservoir in the Minghuazhen Formation (Neogene Period). To understand the role and composition of Dissolved Organic Matter (DOM) in geothermal water, water samples collected from 13 geothermal wells at different depths were studied using three-dimensional (3D) excitation-emission matrix-parallel factor (EEM-PARAFAC) analysis. Fluorescent components were analyzed according to depth, and DOM in geothermal water between 800 m and 1600 m was classified. The results show that the fluorescence index (FI), biological index (BIX), and humification index (HIX) of DOM differ among geothermal water from different thermal reservoirs. Based on these three indices, the humification degree of DOM in deep geothermal water in Kaifeng City is low and is mainly derived from an endogenous source, which is closely related to microbial activities in thermal reservoirs. The fluorescent components of DOM in geothermal water from depths less than 1200 m are mainly tryptophan, tyrosine, and fulvic acid-like. The fluorescent components of DOM in geothermal water from depths greater than 1200 m are more complex, with tryptophan, tyrosine, humic acid, and fulvic acid-like components. Therefore, the characteristics of DOM composition in the geothermal water from different reservoirs in Kaifeng can also be used to infer and explain that the quality of deep geothermal water has not been affected by human activities, and there is no obvious hydraulic connection between the geothermal water of each thermal reservoir.