Radiogenic 32P as a new dating tool to determine groundwater ages of 50–1000 years

• Radiogenic 32 P emits high-energy β - particles and is easily detected. • 32 Si and 32 P is in radioactive equilibrium in the range of 50–1000 a. • 32 P is enriched and extracted using Mg(OH) 2 co-precipitation method. • Radiogenic 32 P dating groundwater is feasible, convenient and reliable. Groundwater age has significant hydrogeological implication for groundwater renewal and solute transport. The period between 50 to 1000 years, marked by increased human activity, which is also a gap of groundwater dating. Cosmogenic 32 Si with a half-life of 144±11 a, is the only viable dating tool for this range but is seldom used due to complex pretreatment and time-consuming and expensive detection. Radiogenic 32 P, as a daughter isotope of 32 Si, decays by emitting high-energy β - particles and is easily detected. 32 Si and 32 P reaches radioactive equilibrium within 3 months, meaning they maintain consistent activity levels in groundwater aged between 50 and 1000 years. Therefore, 32 P can be used to date groundwater in the range of 50–1000 a. 32 P is enriched and extracted using Mg(OH) 2 co-precipitation method. The 32 P solution is then mixed with a solvent mixture containing Triton X-100 and Permafluor and detected using an ultra-low background liquid scintillation spectrometer. The counting time ranges from 2500 to 3000 min with an efficiency of 65 %, and the 32 P count peak surface is observed between channels 600 and 900. Comparing the 32 P ages with the 14 C and 32 Si ages of groundwater samples from the Jianghan Plain, Central China, reveals that 32 P ages and 14 C ages exhibit similar trends, and the 32 P ages and 32 Si ages are approximately consistent in the range of 50–1000 a. Therefore, radiogenic 32 P dating groundwater in the range of 50–1000 a is feasible, convenient and reliable.