Hydrogeochemistry and environmental impact of geothermal waters from Yangyi of Tibet, China

The Yangyi geothermal field, located 72 km northwest to Lhasa City, capital of Tibet, has a high reservoir temperature up to at least 207.2 °C. The geothermal waters from both geothermal wells and hot springs belong to the HCO3 (+CO3)–Na type. Factor analysis of all the chemical constituents shows that they can be divided into two factors: F1 factor receives the contributions of SO42−, Cl−, SiO2, As, B, Na+, K+, and Li+; whereas F2 factor is explained by HCO3−, F−, CO32−, Ca2+, and Sr2+. The F1 factor can be regarded as an indicator of the reservoir temperature distribution at Yangyi, but its variable correlation with the results of different geothermometers (Na–K, quartz and K–Mg) does not allow one to draw further inferences. Different from F1, the F2 factor is an indicator of a group of hydrogeochemical processes resulting from the CO2 pressure decrease in geothermal water during its ascent from the deep underground, including transformation of HCO3− to CO32−, precipitation of Ca2+ and Sr2+, and release of F− from some fluoride-bearing minerals of reservoir rocks. The plot of enthalpy vs. chloride, prepared on the basis of Na–K equilibrium temperatures, suggests that a parent geothermal liquid (PGL) with Cl− concentration of 185 mg/L (that of sample YYT-8) and enthalpy of 1020 J/g (corresponding to a temperature of 236–237 °C, i.e., somewhat higher than that of sample YYT-6) is present in the geothermal reservoir of the Yangyi area, below both the Qialagai valley and the Bujiemu valley, although the samples less affected by mixing and cooling (YYT-6 and YYT-7) come from the second site. The discharge of geothermal waters with high contents of toxic elements such as B, As and F into the Luolang River, the only drinking water source for local residents, has caused slight pollution of the river water. Great care should therefore be taken in the geothermal water resource management at Yangyi.