# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 4 Discussion
### Hydrochemical Characteristics of the Hot Springs
#### 4.1.5 Thermal Storage Temperature and Circulation Depth  
Thermal storage temperature is an important consideration when assessing geothermal resources. Thermal storage temperature scaling is commonly accomplished using two methods: cation temperature scaling and SiO\({}_{2}\) temperature scaling. In general, the cation temperature scaling method is used to calculate the thermal storage temperature of partial-equilibrium water [60]. The water samples in the present study region were in the 'immature water' state. Therefore, the SiO\({}_{2}\) temperature scale was adopted in the present study, and the calculations were performed using the following equations [61]. \[\mathrm{T}=1309/(5.19-\mathrm{lgS})-273.15 \tag{3}\]  
\[\mathrm{T}=1302/(4.69-\mathrm{lgS})-273.15 \tag{4}\]  
\[\mathrm{T}=1000/(4.78-\mathrm{lgS})-273.15 \tag{5}\]  
Equation (3) represents the quartz silica temperature scale, Equation (4) represents the chalcedony temperature scale, and Equation (5) represents the \(\alpha\)-cristobalite temperature scale. \(\mathrm{T}\) denotes the estimated thermal storage temperature (unit: \({}^{\circ}\mathrm{C}\)) and \(\mathrm{S}\) denotes the SiO\({}_{2}\) concentration (unit: mg\(\cdot\)L\({}^{-1}\)). The following order was revealed by the calculation results: chalcedony temperature scale > quartz silica temperature scale > \(\alpha\)-cristobalite temperature scale. The quartz silica temperature scale estimates ranged from 29.1 to 103.8 \({}^{\circ}\mathrm{C}\), the chalcedony temperature scale estimates ranged from 66.7 to 164.9 \({}^{\circ}\mathrm{C}\), and the \(\alpha\)-cristobalite temperature scale estimates ranged from \(-\)18.1 to 53.4 \({}^{\circ}\mathrm{C}\). The result values from all three methods revealed that the AK springs had the highest thermal storage temperature (Table 3). The estimated temperature obtained using the \(\alpha\)-cristobalite temperature scale had negative values, while the estimated temperature obtained using the quartz silica temperature scale estimates had relatively low values. Therefore, these two methods were ineffective for estimating thermal storage temperature in the study region. Furthermore, because the chalcedony temperature scale is frequently used to estimate thermal storage temperature, the estimated value obtained using the chalcedony temperature scale may reflect the thermal storage temperature in the study region. The circulation depth was estimated based on the local geothermal gradient and the estimated thermal storage temperature of the chalcedony temperature scale [62] using Equation (6) provided below:  
\[\mathrm{H}=(\mathrm{T}-\mathrm{T}_{0})/\mathrm{g}+\mathrm{h} \tag{6}\]  
In Equation (6), \(\mathrm{H}\) denotes the circulation depth (km), \(\mathrm{T}\) denotes the thermal storage temperature (\({}^{\circ}\mathrm{C}\)), \(\mathrm{T}_{0}\) denotes the temperature of the constant temperature zone (the value used was 4.1 \({}^{\circ}\mathrm{C}\)), \(\mathrm{h}\) denotes the depth of the normal temperature zone (value used: 20 m), and \(\mathrm{g}\) denotes the geothermal gradient (value used: 60 \({}^{\circ}\mathrm{C}\)/km) [63]. The final circulation depth of the hot springs in the study region ranged from 1043 to 2679 m, while the AK spring water circulated at a deeper depth. The thermal storage temperatures and the circulation depths of the hot springs in the study region demonstrated spatially high west and low east characteristics. Due to geological factors in eastern Turkey (e.g., a relatively thick crust, low surface heat flux, and a lack of ideal cover units), the thermal storage temperature and depth of circulation of hot spring water were discovered to be different at different locations during a geothermal water study in eastern Turkey [64].