Occurrences and mechanisms of strontium-rich groundwater in Xinglong County, northern China: insight from hydrogeological and hydrogeochemical evidence

Xinglong County is an important groundwater recharge area for the Beijing-Tianjin-Hebei region, northern China, as well as a potential area to develop strontium (Sr)-rich mineral water. Combining the regional hydrogeological setting and hydrochemical investigation associated with five surface-water samples and 72 groundwater samples, the source and the spatial variation of Sr 2+ , the processes of water–rock interaction, and the Sr 2+ enrichment mechanism were elucidated. To reduce the uncertainty related to the potentially ambiguous interpretation of laboratory results, some leaching experiments were conducted. The results showed that the groundwater was mainly HCO 3 –Ca·Mg type, and the concentration of Sr 2+ ranged from 0.04 to 6.2 mg/L. Groundwater with elevated Sr 2+ concentration was mainly distributed in eastern and southern Xinglong County, where metamorphic rocks and intrusive rocks occur, while low Sr 2+ groundwater (<0.2 mg/L) was found in the aquifers comprised of pure carbonate rocks. Further analysis revealed that high Sr 2+ concentrations are not present in specific rocks but are closely related to minerals, mainly originating from the dissolution of celestite, gypsum, calcite and strontianite. In addition, the high level of Sr 2+ in groundwater is greatly determined by hydrogeological setting. The relatively impermeable intrusive rocks and overlying carbonate rocks provide sufficient water storage space, where Sr-rich groundwater was formed during the process of long-term water–rock interaction. The concept of groundwater storage space is proposed to explore the mechanism of elevated Sr 2+ concentration in groundwater from a new perspective. This study may provide support for the development and utilization of mineral water and water resources conservation.