Hydrogeochemical and isotopic characteristics of groundwater in Xinchang preselected site and their implications.

The safety disposal of high-level radioactive waste (HLW) has become an important issue for nuclear energy and environmental protection. Water chemistry and environmental isotope are accepted as feasible ways to trace groundwater circulation; it can effectively reveal the conversion relationship between different groundwater of the disposal site. Geochemical and isotopic tracers were used to constrain origins and chemical evolution of groundwater in the arid fissure system of the Xinchang preselected site for high-level radioactive waste geological disposal in China. Groundwater level, water temperature, and water chemistry information at different depths were obtained by multi-layer groundwater monitoring. The results show that the chemical and isotopic composition of groundwater in this fissure system is mainly controlled by evaporation, the water chemistry type of the shallow groundwater is mainly Na-Cl-SO4 or Na-Ca-Cl-SO4, and the deep groundwater is mainly Na-Cl-SO4. Based on the values of monitoring data in deep borehole, the fluctuation of groundwater level is less than 0.40 m with weak hydrodynamic condition, and the geothermal gradient is 1.91 °C/100 m. The isotope analyses indicate that the groundwater in the system recharged by local atmospheric precipitation, and the deep groundwater recharged capacity of the site is weak and with no deep cycle.