| There is huge geothermal resource in China, including high and low to medium temperature geothermal resource. Compared to high temperature geothermal resource, the low to medium geothermal resource is rich in reserve, wide in distribution and cost-effective for exploration. In recent years, more and more attentions have been paid on low to medium geothermal resource as the demand for cleaner energy keeps increasing. To study on the water-rock interaction, the chemical components and its evolution of thermal water, is significance for analyzing the genesis and utilization of geothermal system.Since1958, there are four stages for the development of geothermal exploration in Guangdong, including the investigation of hot spring, the exploration of remarkable thermal fields, the underlying geothermal fields and commercial geothermal exploration. Books of "The hydrogeological characteristics and exploration methods of thermal water in Guangdong" and "The map of hot mineral water and its instruction in Guangdong" are published in1981and1983, which systematically summarized the formation and distribution of thermal water, the characteristic of reservoir, and its exploration methods. However, little research is carried on the hydrogeochemical evolution of chemical components and the genesis of geothermal systems in Guangdong, especially in the coastal area.In this paper, water samples, including cold groundwater, hot spring, river water, rainy water and seawater, were collected to analyze the hydrogeochemical characteristics of the low to medium geothermal system. The occurrence of thermal water was studied based on regional geology, tectonics, hydrogeology and geothermal geology. The chemical and isotope characteristics, the calculated temperature of geothermal reservoir, the genesis of saline water, were analyzed and discussed by using the chemistry and isotope analysis results. A typical geothermal system, the Xinzhou geothermal field, was selected to discuss the recharge area, water-rock interaction, mixing processes of sea water and hot water. Finally, a conceptual model for the Xinzhou geothermal field was given. 1. Hydrogeochemistry of thermal water in the coastal areas of GuangdongOne hundred and forty-seven water samples and eight rock samples were collected to determine the major, minor and trace elements, and the types of dissolved gas in the study area. The results are as follows:(1) There are four hydrochemical types, named Na-HCO3, Na-Cl and Na-Ca-Cl, Ca-SO4, and the mixing type, in the coastal area of Guangdong. In addition, from the coastal area to the inland, the water changes from Cl to HCO3for hydrochemical type, and there is Ca-SO4type in the area of Yangchun sity.(2) The concentrations of Ca2+、Mg2+、Na+、K+、 HCO3-、SO42-、Cl-and EC increase from inland to coastal area, indicating that the impact of sea water on the chemical components of thermal water.(3) Nitrogen (N2) and oxygen (O2) are the major dissolved gas in water samples, and the percentage values of Nitrogen are78%to93%, and85%to94%for cold groundwater and thermal water, respectively. The results of dissolved gas indicate that the thermal water is meteoric origin. The contents of hydrogen (H2) in thermal water are used to determine the high and low temperature geothermal system of geothermal fields. However, the percentage of H2in thermal water is almost zero, which means that there are not high temperature geothermal systems in Guangdong.2. Thermal water-rock interactionThe hydrochemical processes of major ions in thermal water were analyzed by the relationship between the chemical constituents and temperature, and ion ratios, the results are shown as follows:(1) The cold groundwater in the west coastal area of Guangdong is so-called "immature water", while the thermal water was fell into the area between "mature water" and "immature water". It indicated that the ion exchange between water and rock is still under equilibrium. The groundwater collected in the east coastal area of Guangdong is also plotted into the area between "mature water" and "immature water", moreover, it near to the "immature water" area. Compare thermal water of the east coastal area with that of the west coastal area, there is more extensive water-rock interaction in the west coastal area, or other "mature water" mixed into thermal water.(2) From the plot of TDS and Cl-/(Cl-+HCO3-),(Ca2++Mg2+) and (SO42-+HCO3-),(Mg2+/Ca2+) and (Na+/Ca2+),(Na++K+) and Cl-, it can be suggested that the dissolved of silicate, carbonate and sulfate minerals influence the chemical component of thermal water. In addition, there is a mixture between sea water and thermal water.(3) The contents of F-in thermal water are controlled by hydrochemical types and pH values. The concentrations of F in HCO3-type are relatively higher than that of in SO42-and Cl-types. In addition, the contents of F-in alkaline water are high.3. Isotope geochemistry in thermal water (1) Hydrogen and oxygen isotope①The cold groundwater, thermal water, and surface water samples are all plot along the global meteoric water line, indicating those water samples are recharged by precipitation.②The values of8I8O and δD in cold groundwater, thermal water, river water are different, and scatted in the plot of δ18O and δD. It indicates that those water samples belong to different water system③There are linear relationship between δ18O vs Cl and EC, which suggest that cold groundwater and sea water are possible two end-members for thermal water. These results are consistent with that of the chemical analysis.(2) Carbon isotope①The corrected14C data of thermal water samples in the west and east coastal area of Guangdong range from933-11721a, and4937-8239a, respectively. According to the counter map, the values of14C increase from inland to coastal area, while the corrected14C data decrease. In addition, the counter map of14C enclosed in Yangjiang and Yangchun sity, indicating a local groundwater recharge and drainage system. It is consistent with the results; of chemical analysis.②The value of δ13C ranges from-17.8‰to1.7‰, with an average of-8.9‰, indicating a variety of source.(3) Strontium isotopeThe value of87Sr/86Sr ratio in thermal water is close to that of sand and shale, but lower to that of granitic rock, indicating water-rock interaction is the main processes for87Sr/86Sr ratio in thermal water. In fact, most of thermal water samples display on the boundary of granitic rock and sedimentary rock, then the components of wall rock can be dissolved into thermal water during water up flow forward to surface.(4) Boron isotopeFour distinct area can be distinguished for groundwater based on the value of B and δ11B: the first one is B>0.1mg/L and δ11B>10‰, most thermal water with TDS value higher than4g/L belongs to this type; the second one is B>0.1mg/L and δ11B<10‰, and most fissure water are of this type, and the value of δ11B ranges from-10to10‰, which is similar to the hot spring of Yellowstone Park; the third one is B<0.1mg/L and δ11B>10‰, and cold water is this type; the last is B<0.1mg/L and δ11B<10‰, some of fissure water, pore water samples belong to this type.②Groundwater samples of west coastal area scatted in the plot of δ11B vs Cl/B, and Li/B, and Sr/B, indicating the different groundwater systems. However, the thermal water samples, with TDS value higher than4g/L, are plotted into a sea water mixing line, indicating the mixing processes of sea water.4. The genesis of saline thermal water Water samples with TDS value between1g/L and13g/L are classified into saline thermal water, and the genesis of this type is discussed by using H、O and B isotopes, the results are as follows:①It is shown from the ratios of Ca/Cl、Mg/Cl、Na/Cl、K/Cl、HCO3/Cl、SO4/Cl and Cl/Br, and the plots of Ca vs Cl、Na vs Cl、Br vs Cl, that there is a mixing process between sea water and groundwater.②The initial values of δD and δ18O for one end-member are of-50.73‰and-7.25‰, respectively, based on the plot of δD and δ18O vs Cl-.③Another result of this end-member can be calculated from the intersection between the meteoric water line and the fitting line of δ18O and δD values, the value of δD and δ18O is-51.86‰and7.73‰, respectively.5. The calculated temperature of geothermal reservoir and the relative geothermal gradientThe geothermometer of silica and cation is used to calculate the geothermal reservoirs, the results are:①Compare to the water temperatures tested in situ, the calculated values of silica are more reasonable than that of K-Na-Ca geothermometer, and the estimated values of Na-K geothermometer is the highest among other results.②The value of relative geothermal gradient in coastal area is relatively higher than that of inland, with a range of70to85. All in all, there may not exist high temperature geothermal system in Guangdong according to the relative geothermal gradient.③The largest and lowest circulation depth of thermal water in the study area ranges from1013to5679m and566to3148m, respectively.6. Hydrogeochemistry of typical geothermal fieldThe Xinzhou geothermal field is selected as a typical area to analyze the evolution of hydrogeochemistry, the results are shown as follows:①From the ion ratios and hydrogen and oxygen isotope results, it can be seen that the thermal water collected in and around Xinzhou geothermal field are mixture of sea water and cold groundwater, and the mixing ratio of sea water are of9.8to11.4%for thermal water of Xinzhou geothermal field, and38%for Shenzao geothermal field.②One end-member of Xinzhou geothermal field is paleo-seawater, with low B contents and high δ11B values.③The conceptual model of Xinzhou geothermal field was given. |
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