Additional Remarks on the Rehai Geothermal Springs As Well As Sampling

Additional remarks on the Rehai geothermal springs as well as sampling

The water temperatures of geothermal springs from Rehai, including both neutral springs and acid springs, range from 44°C to more than 96 °C - the local boiling point corresponding to an average altitude of 1260 m above sea level. Most neutral springs at Rehai are discharged from irregular ventswith small areas. Acid geothermal water samples, however, werecollected frompoolswith areas ranging from around 0.1 m2 to more than 10 m2. Nevertheless, the discharge rates of acid springs, although not stagnant, are generally less than those of neutral springs. The Rehai acid springs are distributed mainly in the Laogunguo area, the Diretiyan area and the Dagunguo area located in the northeastern part of the field, around 1 kmaway from the area where the neutral springs are discharged most intensively. Almost allfumaroles are distributed in the Huangguaqing area that lies in the southern part of Rehai.

It is worth noting that the geothermal springs numbered as RHZZQ (RH02), RHDGG (RH04), RHHTJ-L (RH05), RHGMQ (RH06) and TCLPRST (RH09) were sampled in both 2012-2013 and 2009, and the numbers outside and inside the parentheses are for the 2012-2013 samples and the 2009 samples respectively (see the details for the 2009 samples inGuo and Wang (2012)). The concentrations of most constituents in the 2012-2013 samples are somewhat higher than those in the 2009 samples. Inaddition, the pHs of the Zhenzhu spring (RHZZQ) measured in this study (4.50) and in Zhang et al. (2008)’s study (3.50) are evidently lower than that in our study in 2009 (6.42). There was a heavy rain during our sampling in 2009, while it was sunny on the sampling days of this study. Thus,the mixing of rainwater in the 2009 samples is the reason for their differences in terms of water chemistry. The Zhenzhu spring has a small discharge (0.2 L/s) and a comparatively large surface area of around 10 square meters (it is actually an acid pool below which there are numerous very small spring vents), and therefore its pH value and chemical composition were affectedmost by the weathers on the different sampling days.

Calculation of H- and O-isotopic fractionation as a result of subsurface boiling

Taking account that the isotopic equilibrium between liquid and vapor phases can generally be reached in case of subsurface boiling of geothermal fluid, we assume that thedistribution of Dand 18Obetween geothermal water and separated vapor within the Rehai hydrothermal system is at or very close to equilibrium. Moreover, although vapor separation may theoretically be a single step or continuous process, boiling occurring in the subsurface is often intermediate between these two extremes with the former being predominant. Hence,an equation put forward byTruesdell et al. (1977)was used for calculation of H and O isotopic fractionation resulting froma boiling process:

(1)

where and are the isotopic compositions of geothermal waterbefore and after boiling respectively. is the fractionation coefficient of 18OorD between liquid and vapor phases at separation pressure (or temperature), and indicates theratio of vaporin flashing geothermal fluid expressed as below:

(2)

, and denote the enthalpy values of geothermal waterbefore and after vapor separationas well as geothermal vapor, respectively.

References

Guo Q, Wang Y (2012) Geochemistry of hot springs in the Tengchong hydrothermal areas, Southwestern China. J Volcanol Geoth Res 215:61-73

Truesdell AH, Nathenson M, Rye RO (1977) The effects of subsurface boiling and dilution on the isotopic compositions of Yellowstone thermal waters. Journal of Geophysical Research 82(26):3694-3704

SI Fig. 1. Plot of pH value vs. SO42- concentration for the geothermal springs recorded by “Thermal springs in Tibet (Tong et al., 2000)” (a) and “Thermal springs in Traverse Mountains region (Tong and Zhang, 1994)” (b) as well as those sampled by the authors of this paper (c). Legend of sample symbols in Fig. 1-c: ○Rehai samples (collected in 2012 - 2013); ⊕Rehai samples (collected in 2009) (Guo and Wang, 2012); △Tengchong samples (not including Rehai)(Guo and Wang, 2012); ◇Longling samples (unpublished data); ＋Yangbajing samples(Guo et al., 2007); ×Yangyi samples(Guo et al., 2009). Note that Longling is a hydrothermal area located in western Yunnan, and Yangbajing and Yangyi are two high-temperature geothermal fields in southern Tibet.

SI Fig. 2. Relative Cl-, SO42- and HCO3- concentrations of all samples.

SI Fig. 3. Box and whisker plots of pH and characteristic constituents of the neutral (group A) andacid geothermal water samples (group B and C). The boxes show the mean value minus standard error, the mean value, and the mean value plus standard error. Thesmallest and largest values are indicated by the small horizontal bars at the end of the whiskers. ○represents outlier values.

SI Fig. 4.Triangular diagram of Na-K-Mg1/2 for all samples.

SI Fig. 5.Plot of enthalpy relative to chloride for neutral geothermal waters from Rehai. ○: hot spring water; △: geothermal fluid cooling conductively to form hot springs; ●: geothermal fluid cooling adiabatically to form hot springs; ■: parent geothermal liquid; ◆: cold groundwater; ▲: steam separated from geothermal fluid.

SI Fig. 6.A model scheme for the geneses of geothermal waters at Rehai.

SI Table 1.Chemical compositions of some representative water samples from Yellowstone (USA), Nevado del Ruiz (Colombia) and Miravalles (Costa Rica) (in mg/L).They are used to be compared with the acid water samples from Rehai.