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Hydrochemical and isotopic approach to dynamic recharge of a dolomite aquifer in South Africa

Xiao, Liang, Xu, Yongxin, Talma, A. S.
Hydrogeology journal 2019 v.27 no.3 pp. 945-964
aquifers, calcium, carbon, chlorides, deuterium, dolomite, groundwater, groundwater recharge, hydrodynamics, magnesium, models, oxygen, radionuclides, rain, sodium, springs (water), stable isotopes, time series analysis, tritium, South Africa
The dolomite aquifer is the largest water source in northern South Africa. The flow dynamics of the dolomite aquifer is investigated by using analyses of the hydro-chemical parameters and isotopes (³H, δ²H, δ¹⁸O, δ¹³C-DIC and ¹⁴C-DIC) of spring samples. Recharge areas of the dolomite aquifer are confirmed through interpretation of the hydro-geochemical types of the spring samples. The important role of rainfall in groundwater recharge is suggested by low Na⁺ and Cl⁻ concentrations and by the δ²H and δ¹⁸O values of the spring samples. Groundwater mean residence time (MRT) and its temporal and spatial distributions within the young dolomite spring system can be analyzed using an improved lumped-parameter model based on the time series of ¹⁴C-DIC, initial ¹⁴C activities and δ¹³C-DIC values of the spring samples collected during the 1970s and 2000s. The results show that the spring samples have about 50–80% of the initial ¹⁴C activities and the MRTs of the dolomite spring system range from ≤10–51 years. At five spring sites, the temporal distributions of groundwater MRTs are identified to be significantly influenced by the variability of the local rainfall. At the Kuruman sites only, an increasing trend of the groundwater MRTs and the evolution of the [Ca²⁺]/[Mg²⁺] ratio along the flow direction indicate an important role of deep groundwater inflow to the spring flow. The results provide basic scientific information required for sustainable management of the dolomite aquifer.