A Study Of Hydrochemistry And Isotopes In Thermal Groundwater In The Urban Geothermal Fileld, Beijing

The Beijing urban geothermal field is a famous geothermal field in China. It hasabundant low-temperature geothermal resources and gains extensive applications. Itsproduction history spans more than 30 years and shows better social, economical andenvironmental benefits. The geothermal system consists basically of three reservoirs, theTieling Group (Jxt), the Wumishan Group (Jxw) and the Hongshuizhuang Group (Jxh) ofthe Jixian (Jx) system.The geochemical characteristics of thermal fluid are results of geological functionduring the long period of geothermal formation. This kind of function will have betterapplication to geochemical characters of thermal fluid. The geochemical methods adoptedin this paper are used to interpret the chemical and isotopic characteristics of thermalwaters in the Urban geothermal field. According to the geochemical characters of thermalwater, we can ascertain the cause of formation in the Urban geothermal field and tospeculate the potential of geothermal development.According to the Cl-SO4-HCO3 triangular diagram and the Piper diagram, the mainchemical type of geothermal water is HCO3?SO4-Na?Ca. It can educed that the age ofthermal water in the urban field is about 5 to 48 a based on the messure of Tritium, it showsthat the geothermal water stay for a long time in the ground and it may come from reservoir.The relationship between stable isotopes of D and δ18O indicates the meteoric origin ofthermal water in the Urban field. To determine the scale trend of thermal water, we haveadopted index method and WATCH program. Most of the thermal water in the Urban fieldhave the potential of scaling. Because of the relatively low temperature of the thermalwater, the thermal fluid does not seem to be in equilibrium with any hydrothermalminerals in the reservoir, except for chalcedony and quartz. Several kinds ofgeothermometers are used to predict the subsurface temperatures in the Urban geothermalfield. The subsurface temperature is in the range of 51.4-103.8℃ on the base of calculationon the chalcedony geothermometer. All the geothermometer calculations indicate that onecan expect the highest temperature in the centre of the sedimentary basin. Therefore, themain potential for further development of the geothermal resources is in that region. Asfor mixing, the binary plot, Schoellor diagram and silica-enthalpy mixing model are used,indicating that the fluids in the Urban geothermal field may have been mixed with coldgroundwater.Over-exploitation of geothermal resources makes the water level decline for 50 m inthe Urban field from the year 1971 to the end of 2000. The chemical composition of thewater has changed very little during the period of 1984-2000 on the data of dynamicmonitor of geothermal water in the Urban field. It indicates that the geothermal resources inthe Urban field are abundant and the thermal water has steady supply.