Environmental impacts of geothermal waters with extremely high boron concentrations: Insight from a case study in Tibet, China

Abstract

Boron is a potentially toxic element in the environment and commonly enriched in high-temperature geothermal waters. In this study, seven typical boron-rich geothermal areas in Tibet (China) were selected as the study areas for comprehensively investigating the geochemistry of geothermal boron and its environmental impact. The geothermal waters were classified into four types: acid low-boron waters, acid high-boron waters, neutral/alkaline low-boron waters, and neutral/alkaline high-boron waters. The enrichment of boron in the neutral/alkaline high-boron geothermal waters is attributed to the input of magmatic fluids, whereas the host rock leaching is the primary boron source for the neutral/alkaline low-boron geothermal waters. The acid low-boron waters are essentially locally-perched groundwaters heated by geothermal vapors separated from flashed deep geothermal fluids, and thereby their boron comes mainly from the limited leaching of the host rocks of the shallow aquifers. As for the acid high-boron waters, they could be formed via a mixing between acid low-boron waters and neutral/alkaline geothermal waters, with the latter as the primary boron source. The boron species in these geothermal waters comprise mainly H3BO3, B(OH)4−, polyborate ions and fluoroborate complexes, all of which are affected by the pH, temperature, and F and B concentrations of the waters. When the geothermal waters flow out of the hot spring vents, the rapid changes in unstable parameters, like pH and water temperature, will lead to the conversion of B(OH)4− and polyborate ions to H3BO3. The discharge of the boron-rich geothermal waters has contaminated the surface waters and sediments in and around the investigated geothermal areas. In turn, the boron sorbed onto/into the hot spring sediments is liable to be released either in short-term if it is originally exchangeable boron or in long-term if carbonates-, Fe-Mn oxides-, or organic matters/sulfides-combined boron. This study enhances the understanding of geothermal boron speciation and its environmental impact, and provides insights for rational exploitation of hydrothermal resource without inducing geothermal boron pollution.