The low-enthalpy geothermal system of Tianjin (average thermal gradient of 3 °C/100
m) is located in-and around the City of Tianjin (China), in the NE part of the
Hua-Bei plain, a tectonic depression affected by subsidence since the Neogene. In the
framework of a cooperation project between the Italian Ministry for Environment and
Territory (IMET) and the Ministry of Science and Technologies of the People's
Republic of China (MOST), major, trace and isotopic composition of 35 thermal water
and dissolved gases from both productive and reinjection wells in the Tinajin area
(North China) have been investigated to reconstruct the fluid circulation at depth.
The fluid exploited derives from two main aquifers. The shallower one (Tertiary and
Quaternary formations) is characterized by T < 80 °C, prevalent Na-HCO3 composition,
salinity <1,000 mg/L and high Na/K ratio (up to 300). The deeper one (pre-Tertiary
formations) has a temperature up to water boiling conditions, prevalent Na-Cl
composition, salinity >1,000 mg/L (up to 4,000 mg/L) and low Na/K ratio (<20). The
two aquifers are quite localized in their respective geological horizons, not showing
any really evident chemical mixing trend. The pH of both aquifers is
neutral-to-slightly basic, suggesting scarce contribution of acidic gas compounds
(such as H2S and CO2). Although several wells have CO2 as the main dissolved gas
species in solution, its concentration seems to be consistent with that expected,
according to the Henry’s law, by considering the increasing hydrostatic pressures. In
spite of the fact that the Tianjin geothermal field is located close to the ocean,
the thermal fluids do not show any significant contribution from seawater. This fact
suggests that the Na concentrations, relatively high with respect to the K contents,
as well as the prevalent Na-HCO3 composition of fluids hosted in the Tertiary, are
likely due, at least partly, the dissolution of continental evaporites that are
typically rich in Na-carbonate material. This interpretation is also indirectly
supported by the relatively low PCO2 values of the water phase, suggesting that the
typical Na-Ca exchange, commonly observed in sedimentary environments affected by a
high CO2 flux, can be excluded. On the basis of the 18O/16O and D/H isotopic ratios
the recharge waters of the two exploited aquifers seem to have a similar origin,
prevalently deriving from precipitations at elevations ranging roughly, 1200-1600.
According to the depth-temperature profiles, only few wells have a thermal gradient
up to 6 °C/100 m, whereas are either that of the average Earth crustal gradient (3.3
°C/100 m) or even lower. Therefore, the Tianjing geothermal field must be considered
as an excellent example of exploitation of a natural energy resource in an ordinary
thermal environment. However, excessive over-exploitation of the thermal aquifers
with possible depletion of the water resource, reinjection-induced scaling, plugging
phenomena and seawater intrusion have to be regarded as impending problems.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy