Geochemical and isotope constraints on the hydrogeology and geochemistry of the geothermal waters in the Shandong Peninsula, China

Utilization of low-temperature (< 150°C) geothermal waters plays an increasingly important role in the space heating and other direct uses in China, providing an alternative energy source to fossil fuels. In the Shandong Peninsula several local geothermal areas with low temperatures were discovered by drilling over 90 boreholes. The geothermal waters have surface temperatures ranging from 28 to 89°C, pH values between 6.8 and 8.5, and total dissolved solid contents between 150 and 10,000 mg/L. The mobile element systematics (B, Sr, and Cl) and isotope ratios (δ2H, δ18O) reveal that the geothermal waters are predominantly of meteoric origin with occasional minor seawater mixing and/or mineral salt leaching followed by modification through water–rock interactions and mixing with shallow non-thermal groundwaters. By applying multiple mineral-water equilibria geothermometry, higher subsurface water temperatures ranging from 70 to 160°C were obtained indicating conductive cooling and/or mixing with non-thermal groundwater as the geothermal waters ascends to the surface. Two types of geothermal waters were identified in the Shandong Peninsula. The type I geothermal waters are characterized by deep water circulation (∼2–5 km) associated with large and deep faults, low Cl concentrations (57.7–1781 mg/L), variable and often low 14C activity (6.94–82.6 pMC) and 3H activity (< 1–8.3 TU), and reasonably high reservoir temperatures (70–160°C). These results indicate mixing between reacted and “old” water components at depth and modern meteoric water sourced from the surface. The type II geothermal waters are associated with shallower fractures located away from the major granite intrusions. The geothermal waters have a narrow range of 3H activity (5.9–6.9) revealing a modern meteoric origin, whereas the 14C (54.26–131.78 pMC) activity ranges from modern to “old”. This is the result of input from multiple carbon sources rather than the input of only “old” water. The calculated reservoir temperatures (50–160°C) are generally lower than those of the type I geothermal waters and the water commonly display more elevated Cl concentrations (254–5316 mg/L) and a lower degree of water–rock interactions and rock leaching based on the B, Cl, and Sr systematics. Based on these findings, further prospects for geothermal exploration and exploitation may be feasible in the Shandong Peninsula in terms of deeper boreholes that may discharge higher temperature water.