Analytical methods
The chemical analyses were measured at the laboratory of Beijing Research Institute of Uranium Geology (Table 1). K, Na, Mg and Ca were measured by 22.2 ion chromatogram using 883 Basic IC pulse ion chromatograph based on the Standard Test Method of Drinking Water-Metal Indicator (GB/T 5750.6-2006). Cl, F, NO3 and SO4 were analyzed by ion chromatogram using ICS-1100 ion chromatograph based on the Test Method of the Quality of Groundwater (DZ/T 0064.51-1993). HCO3 and CO3 were analyzed by titration method using AT-510 automatic titration analysis meter based on the Test Method of the Quality of Groundwater (DZ/T 0064.49-1993). Si were determined by plasma mass spectrometry using NexION300D plasma mass spectrometer based on the Test Method of the Quality of Groundwater (DZ/T 0064.80-1993). H2SiO3 was measured by spectrophotometer at the Laboratory of the Beijing Brigade of Hydrogeology and Engineering Geology. The concentration of SiO2 was calculated according to the concentration of Si and H2SiO3. δ 2H andδ 18O were detected by zinc reduction method for hydrogen isotopes (DZ/T 0184.19-1997) and carbon dioxide–water equilibrium method for oxygen isotopes (DZ/T 0184.21-1997).
The saturation index (SI) value is the logarithm of the ratio of the ion activity product (IAP) to the equilibrium solubility product (K) at sample temperature is described as:
SI=log (IAP/K) (1)
The value of the saturation index (SI) indicates whether the solution is in a state of undersaturation (negative SI), saturation (positive SI), or at the state of equilibrium (SI=0) with respect to a mineral (Lorah and Herman, 1988; Shen et al., 1993). The value of the saturation index with respect to anhydrite, aragonite, calcite, dolomite, gypsum and halite of the water samples were calculated with PHREEQC (Parkhurst and Appelo, 1999) and using Aq•Qa carbonate equilibria database to obtain their partial pressure of CO2 (PCO2). The concentrations of the hydrochemical compositions of the water samples were checked by charge balance error (Freeze and Cherry, 1979). The absolute values of relative error calculated for all the water samples from the balance between cations and anions range from 0.06% to 1.7%.
Table 1 Field data and chemical constituents of the hot water samples