Tengchong is located in Yuannan province, and the Tengchong volcanic district, located in southern China near the border with Burma (Myanmar), was active during five periods ranging from the early Pliocene to the Holocene. The Tengchong district is the site of active geothermal fields, the largest and highest temperature of which is the Rehai geothermal field. More than 20 hydrothermal eruptions have occurred at the Rehai geothermal field since 1993.
Tengchong is one of China's three major geothermal areas, known as "Land Abounding in Geothermal Resources". There are plenty of springs of hot water and hot vapour, called "Hot Sea". There are quite a number of hot fields as well.
The area is well known for its geothermal sites and over 80 hot springs are recorded, but the difficult terrain, long distances and lack of east-west routes, restricted the number of locations that could be studied in the time available.
Rehai lies 10.5 kilometres south of Tengchong and is approached through a steep narrow valley. It is national scenic geological park, famous for the number of geothermal manifestations and has been significantly developed as a tourist area.
Rehai (Hot Sea), and its neighboring area, is different from the other known geothermal areas in the world, since it lacks typical acidic extrusive rocks such as rhyodacite, rhyolite and tuffs. Consequently, the major hydrothermal fluids have an alkaline pH.
The geochemistry of hydrothermal fluids in Rehai is as follows:
1. The major composition of hydrothermal gases is of CO2-H2S-CH4-N2-O2. Minor amounts of H2 have been also detected from the central manifestation of Rehai. Such a composition is broadly similar to those from the majority of volcanic geothermal regions of the world.
2. HCO3-Na type is the most common species of thermal spring water. Cl-HCO3-Na or HCO3-Cl-Na type water issues from high temperature hydrothermal areas close to the young volcanoes. The acidic SO4-Na type also accompanies these spring waters.
3. The estimated average underground temperatures for Rehai are more than 150°C. The hydrochemistry model of Rehai geothermal field suggests that the temperature of the parent hot water from the heated zone may be no less than 270°C.
Langpu is a small village south west of Tengchong on the eastern side of a broad river valley. The ground rises steeply to the east and this is where the thermal sites are to be found. Several decades ago Langpu was developed as a thermal spa but the project was never completed and the site is overgrown, the buildings derelict and abandoned. Although a few natural springs were found, the site has suffered from considerable human intervention and it is likely that the main thermal source lies beneath the ruined buildings.