Renewal rate and age estimation of groundwater based on tritium in an unconfined aquifer in semi-arid continental climate area, Guide Basin, China.

Estimation of groundwater resources in plateau continental climate area is difficult due to the minimal groundwater recharge, high evaporation and frequent changes throughout the year. The renewal rate of aquifers can be calculated by constructing a model based on the tritium isotope data of atmospheric precipitation and groundwater. In this study, hydrogeochemical and isotopic methods were employed to study the hydrochemical composition and groundwater renewal processes in unconfined aquifers in Guide Basin. Results indicate that the phreatic water is mainly of the HCO₃-Ca-Na type, with chemical components originating mainly from the silicate weathering and carbonate dissolution in the aquifer. Isotopic analysis (δD and δ¹⁸O) shows that atmospheric precipitation is the main source of recharge for water bodies in the basin. Tritium content variations with depth indicate that groundwater circulation depth affects tritium decay processes. Groundwater age calculations based on tritium content in atmospheric precipitation and groundwater suggest that the hydrolyzed products of silicate gradually entered the groundwater with the increase of phreatic water runoff time. Contour maps of renewal rate and age of groundwater indicate that close hydraulic connection from river water brings the unconfined aquifer fine renewal capacity and a relatively younger age. The research findings not only establish a theoretical basis for groundwater resource management and sustainable development in Guide Basin, but also serve as a reference for the calculating the renewable capacity of groundwater resources in other basins.