# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 4 Discussion
### Hydrochemical Characteristics of the Hot Springs
#### 4.1.3 Origin of the Hot Spring Waters  
[PERSON] [48] proposed the Global Meteoric Water Line (GMWL) by providing the range of variation in the \(\delta\)D and \(\delta^{18}\)O values in atmospheric precipitation and the relationship between them for various regions worldwide. The corresponding mathematical expression was as follows: \(\delta\)D = \(8\times\delta^{18}\)O + 10. This provided a basis for the isotopic exploration of groundwater recharge sources. However, due to the differences in the sources of water vapor and evaporation patterns in the circulation systems among different regions, the GMWL of the study region deviated from the regional atmospheric precipitation line (LMWL). In the present study, the northwest region was selected as the precipitation isotope reference site for the study region. The LMWL equation for the northwestern region was as follows: \(\delta\)D = \(7.05\times\delta^{18}\)O \(-2.71\)[49]. The figure depicting the \(\delta\)D-\(\delta^{18}\)O relationships for the hot springs in the study region revealed that the isotope data for these hot springs were mainly distributed close to the regional atmospheric precipitation line (LMWL) of the northwestern region, indicating that the water of these hot springs originated primarily from meteoric water and interacts with geothermal fluids from confined aquifers that flow to the surface through fractures (Figure 5). A geothermal water study in the Sabalan volcano, NW Iran, discovered that the different degrees of \(\delta^{18}\)O displacement of water samples resulted from an isotopic exchange between the geothermal fluid and the host rock in the geothermal reservoir [50]. AK springs exhibited positive displacement along the \(\delta^{18}\)O axis from LMWL, indicating a relatively high degree of water-rock response in AK springs compared to other hot springs. Since altitude varies widely in the Qinghai region, the maximum altitude difference at the sampling site was 633.26 m. Considering that the hydrogen and oxygen isotopes are affected by this altitude variation effect, the altitude effect of isotopes was utilized for the preliminary estimation of the water recharge altitude of the hot springs. The following equation was used:  
\[\mathrm{H}=(\delta\mathrm{G}-\delta\mathrm{P})/\mathrm{k}+\mathrm{h} \tag{2}\]  
Figure 4: Distribution of aqueous samples shown on a Na/1000-K/100-Mg1/2 ternary diagram [45]. In Equation (2), H denotes the elevation of the recharge region, h denotes the elevation of the sampling point, \(\delta\)G is the \(\delta^{18}\)O (\(\delta\)D) value of carbonate springs, \(\delta\)P is the \(\delta^{18}\)O (\(\delta\)D) value of atmospheric precipitation close to the sampling point, and k is the height gradient of the \(\delta^{18}\)O (\(\delta\)D) value of atmospheric precipitation (\(\%\)/100 m). The global average \(\delta^{18}\)O elevation gradient (k (\(\delta^{18}\)O) = \(-\)0.25\(\%\)/100 m) was used as the elevation gradient of the \(\delta^{18}\)O (\(\delta\)D) values of atmospheric precipitation. The isotopically weighted average of atmospheric precipitation in Qinghai (\(\delta^{18}\)O = \(-\)7.19) was used as the \(\delta^{18}\)O value of atmospheric precipitation [51]. The recharge elevation of the hot springs in the study region was calculated using Equation (2). The results revealed that the recharge elevations for the YTBLK, AK, LMY, and GS springs were 3535 m, 3676 m, 3477 m, and 3390 m, respectively, among which the recharge elevation of the AK spring was relatively high (Table 3). [PERSON] et al. discovered a clear spatial correlation between hot springs and active fractures in the western Peruvian Cordillera, and that the tectonics provide pathways for hydrothermal solutions and gases that contribute to the heating of atmospheric water. Therefore, tectonic features play a significant role in the formation of hot spring water [52].