Geochemical behaviors of boron and its isotopes in aqueous environment of the Yangbajing and Yangyi geothermal fields, Tibet, China

• The δ11B values of geothermal water from Yangbajing and Yangyi were evaluated.
• Reservoir host rocks and magma degassing are sources of boron in geothermal water.
• The possible processes responsible for boron isotope fractionation were delineated.
• Boron and its isotopes were used to trace aqueous environment contamination.

Fifty-two samples including geothermal well water, hot spring water, cold groundwater and river water were collected from the Yangbajing and Yangyi geothermal fields to determine their boron concentrations and δ11B values. The results show that the concentrations of boron in the geothermal waters range from 38.5 to 165.4 mg/L, much higher than those of cold groundwaters and river waters (0.3–0.7 and 0.1–3.9 mg/L respectively). The δ11B values are between − 11.4 and − 12.3‰ for the Yangbajing geothermal waters and between − 5.0 and − 9.7‰ for the Yangyi geothermal waters, while those of the cold groundwaters are from − 2.6 to − 4.8‰. The δ11B values and B/Cl ratios indicate that boron in geothermal water is mainly leached from reservoir host rocks, whereas magma degassing is possibly another source of boron. Although the dissolution of tourmaline-rich granitic rocks is the primary factor controlling the boron concentrations and δ11B values of the Yangbajing and Yangyi geothermal waters, other secondary processes, e.g. CO2 degassing as well as subsequent pH increase and precipitation of carbonate minerals, also affect the boron isotope fractionation occurring in the reservoirs. In addition, the geothermal exploitation for electricity generation at Yangbajing had a significant impact on geothermal water chemistry, which may have relations to boron isotopic fractionation as well. For the Zangbo River flowing through the Yangbajing field, the boron isotopic geochemistry of river waters was significantly influenced by geothermal wastewater near its drainage exit. However, the recharge of the tributaries of the Zangbo River and the desorption of boron from river-bed sediments are major processes affecting the δ11B values of river waters at the far downstream of the drainage exit. The results of this study validate that boron and its isotopes can be used to trace the origin of geothermal water and to identify contamination induced by geothermal wastewater drainage.