Groundwater Age Dating using Basinal Radiogenic Helium Diffusion Profile

Saudi Arabia and the Gulf states have been classified as water-scarce countries by the United Nations, which urges the need for groundwater management and protection. Groundwater age is key to the protection and management of non-renewable groundwater resources. Helium diffusion within the fluids of sedimentary basins is a new and little explored groundwater dating tool. The current study constructs a helium diffusion profile from the crystalline basement to the surface for two basins in the Arabian Peninsula: The Jafurah Basin and the Northwestern Basin. These profiles were corrected using ⁴He concentrations (n=56) from fluids of different formations within the sedimentary succession of the two basins. All measured ⁴He concentrations were found to be of crustal origin, with (R/R_a) corrected values ranging between 0.005 and 0.078. The measured ⁴He groundwater concentrations were shown to be within 1σ uncertainty of the diffusion model results, indicating absence of advective groundwater transport and diffusive loss of ⁴He. It was found that major tectonic events at the Oligocene (33-21Ma) and the Miocene (19-8Ma) flushed ⁴He basement flux in the shallow groundwater aquifers, leaving room for in-situ production only. ⁴He produced in-situ was deemed sufficient to be used in groundwater age calculations beyond radiocarbon capabilities. Additionally, analysis of ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ²H, and radiocarbon proposed the presence of recent recharge. However, groundwater inheritance of paleo-seawater ⁸⁷Sr/⁸⁶Sr ratios indicated enhanced water-rock interaction (WRI), with little influence from modern-day seawater. This suggests mixing of recent recharge with the old groundwater depicted by ⁴He. Groundwater samples that had measurable ¹⁴C activity were also enriched in ⁴He, and such enrichment is too high to have accumulated over the residence time calculated by radiocarbon (2,817 – 30,100 yrs BP), especially in the absence of deep structural conduits e.g., faults and mega fractures. This challenges the conventional groundwater dating methods that presume one representative groundwater age. We conclude that groundwater age, especially that calculated from radiocarbon, represents a mean residence time of mixture of young and old endmembers, which proves ⁴He as a reliable chronometer for groundwater dating.