| Abstract |
The Tianjin low-to-medium temperature geothermal field is located in the North China Hua Bei Plain. Up to now from 1930, approximately 350 wells have been drilled into porous sandstone reservoirs (including 2 major productive zones) or karst/fractured basement rock reservoirs (including 3 major productive zones), to depths between 700 and 4041 m for their exploration, exploitation or reinjection. At present, about 300 of the total number of wells are in production and 52 are used for reinjection. A lot of water samples were analyzed in the past several decades, the discharge temperatures range from 40 to 102 C; the total dissolved solids (TDS) are between 453 and 8000 mg/l, and they also contain high contents of silica and boron. The TDS is generally controlled by the geological structures, faults, the hydrogeological properties of reserviors, lithologies, and the supply and circulating conditions. In general, the TDS increases gradually along the fluid flowing direction from NE to SW, and the water quality types vary from HCO3-Na, HCO3•ClNa，to Cl•HCO3－Na, Cl•SO4－Na, etc. The paper collected a large number of data of water examinations from wells of various reserviors in Tianjin for better understanding of the hydrochemical properties and their evolutive laws. |