Hydrogeochemical Features of the Thermal Groundwater

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Answer 1

Sampling of the thermal groundw ater w as conducted in the central part and on the northeastern margin of the city of Tianjin. Chemical and isotopic constituents of 5 representative samples are presented in Table 18. 1. The three w ater samples collected in the central city 15 years before are also listed for comparison. Chemical compositions of w ater samples taken from the same aquifer are observed to be almost identical. It is also found that except the free CO₂， constituents of the thermal groundw ater in the same aquifer changed very little in the 15 years period. The chemical compositions of the thermal groundw ater in the bedrock aquifers in the city of Tianjin are believed to almost not change w ith time w ithin a certain period of time.

Table 18. 1 Chemical and isotopic compositions of the thermal groundwater in the bedrock aquifers in Tianjin ( mg / L)

Note that O， and Pt stand for Ordovician，Cambrian and Meso-to-Neo Proterozoic aquifers，respectively.

Thermal groundw ater in the Meso-to-Neo Proterozoic aquifer and the Cambrian aquifer have similar hydrogeochemical features. The w ater in these tw o aquifers have betw een 1. 64 g / L and 1. 98 g / L TDS. Cl and SO4are the dominant anions and constitute more than 43% and 25% of the anion milliequivalents. Na is the major cation and accounts for more than 80% of the milliequivalents of the cations. Water type of the tw o aquifers are dominated by Cl·SO₄-Na type. Cl·SO₄·HCO₃-Na and Cl·HCO₃·SO₄-Na types of water are also encountered. The composition of the thermal w ater in the Ordovician aquifer is dominated by SO₄and Ca. More than 62% of the anion milliequivalents are SO₄，34% are Cl. More than 50% of the cation milliequivalents are Na，and 35% are Ca. The w ater in this aquifer contains TDS of 4. 45 g / L to 4. 57 g / L and has high concentrations of SO₄of about 2. 1 g / L，w hich are about 2. 5 times and 6 times greater than those of the w aters occurring in the Meso-to-Neo Proterozoic aquifer and the Cambrian aquifer，respectively. High TDS and enrichment in SO₄make the w ater in the Ordovician aquifer differ from those in the Meso-to-Neo Proterozoic aquifer and the Cambrian aquifer. The thermal w ater w ith relatively low temperature is of SO₄·Cl － Na·Ca type. Other constituents，such as Sr，Li，Br，NH4and Al are commonly detected in the w aters in the three aquifers. The concentrations of H₂BO₃and H₂SiO₃，in particular，are relatively abundant in the thermal groundw ater. The value of pH of the w aters is around 7 and does not show any systematic change.

The contents of SiO2and fluoride，ranging from 30 mg / L to 71 mg / L and from 4 mg / L to 10. 4 mg / L，respectively，are relatively high in the thermal groundw ater. The locations w ith the highest values correspond to the uplifts of the bedrocks，making them useful geochemical makers for determining locations of heat flow anomalies. There are rough correlations betw een the contents of SiO2and fluoride and the temperatures of the hot w ater，indicating that the contents of SiO₂and fluoride increase w ith temperature in the range 30 － 100℃. The content of SiO₂increases at a rate of 12 mg / L to 15 mg / L w hen the temperature of the w ater increases at a rate of 10℃. Fluoride concentration increases at a rate of 2. 3 mg / L to 2. 5 mg / L w hen the temperature of the w ater increases at a rate of 10℃.