New insight into groundwater 4He ages based on Ne isotopic equilibrium in Jianghan Plain, Central China

Abstract

The change of hydrostatic pressure caused by the fluctuation of groundwater table in the aquifer will lead to the partial dissolution of excess ⁴He gas, resulting in the isotope imbalance of ³He/⁴He-⁴He. The dissolved Ne in groundwater is mainly derived from the atmosphere, and its isotopic composition can correct the isotopic imbalance of ³He/⁴He-⁴He. We collected thirty-eight groundwater samples from the second aquifer of the Jianghan Plain, and the isotopic concentrations and ratios of He and Ne were measured. ²¹Ne/²²Ne-²⁰Ne/²²Ne illustration is proposed to estimate the shares of atmospheric and mantle components. The ²¹Ne content and isotopic ratios of atmospheric and mantle components are used to estimate a calculated Ne content. The difference between the calculated Ne content and the measured Ne content (∆Ne) is used to evaluate the percentage of error estimated“excess air”. The accumulation of crustal ⁴He is corrected with the measured ⁴He content and the percentage of error estimated “excess air”. We found the maximum percentage of error estimated “excess air” was 7.57 % occurring in the groundwater samples from the second aquifer of Jianghan Plain, and the disequilibrium of ³He/⁴He-⁴He led to overestimation of the share of mantle He. The percentage of mantle He in total dissolved components is reassessed and range from 0.03 % to 0.74 %, indicating the mantle component is minor. The reassessed ⁴He ages (1.79 ka to 21.90 ka) were uniformly older than those estimated by traditional method which only use the measured ³He/⁴He ratio to distinguish the crust ⁴He (1.28 ka to 18.74 ka). ⁴He age is significantly underestimated up to 47.05 %.