Geochemical and isotopic evidence on the recharge and circulation of geothermal fluids in Tongzhou District, Beijing

Abstract

By employing chemical and isotopic tracers (2H, 18O, 3H, 14C and 87Sr/86Sr), we investigated the recharge-flow cycle characteristics of the Jixian karst geothermal system in Tongzhou District, Beijing. The temperature of geothermal water in Jixian system is in the ranges of 35-91?. The recharge source is
atmospheric precipitation in the northwestern or northern mountainous areas of Beijing, with an average recharge elevation of 1510 m. The age and temperature of geothermal water in the thermal reservoir show obvious tectonic control characteristics. In the western part of Tongzhou district, within the Daxingdian uplift tectonic unit, the water age increases from northwest (24 ka) to southeast (36 ka) with a transport rate of about 2 m/yr, and the thermal storage temperature increases from 57.4°C to 86.5°C. The Xiadian fracture in the southeast is a hydraulic and thermal conductivity fracture on which the geothermal water age decreases to 27,000 yr, while the thermal storage temperature increases to 107.8°C. Regionally, changes in strontium content and strontium isotope values can effectively trace the transport of geothermal fluids and the degree of water-rock interaction. Both strontium content and strontium isotope values in geothermal water increase along the direction of groundwater runoff, revealing the superimposed effects of two processes: dissolution of strontium in Jixian system carbonates and decay of 87Rb, the latter showing a significant time-accumulation effect, which is more evident in the southeastern part of the study area.