Use of multiple age tracers to estimate groundwater residence times and long-term recharge rates in arid southern Oman

Multiple age tracers were measured to estimate groundwater residence times in the regional aquifer system underlying southwestern Oman. The three isotopic age tracers ¹⁴C, ⁴He, and ³⁶Cl were measured in waters collected from 20 wells along a line that extended roughly from the Dhofar Mountains near the Arabian Sea northward 400 km into the Empty Quarter of the Arabian Peninsula. The wells sampled were mostly open to the Umm Er Radhuma confined aquifer, although some were completed in the mostly unconfined Rus aquifer. The combined results from the three tracers indicate the age of the confined groundwater is <40k years in the recharge area in the Dhofar Mountains, >100 k years in the central section north of the mountains, and up to and >1 M years in the Empty Quarter. The ¹⁴C data were used to help calibrate the ⁴He and ³⁶Cl data. Additional measurements of noble gases made were consistent with the age interpretations. Mixing models suggest that long open boreholes north of the mountains compromise ¹⁴C-only interpretations there, in contrast to ⁴He and ³⁶Cl calculations that are less sensitive to borehole mixing. Thus only the latter two tracers from these more distal wells were considered reliable. In addition to the age tracers, ²H and ¹⁸O data suggest that seasonal monsoon and infrequent tropical cyclones are both substantial contributors to the recharge. The study highlights the advantages of using multiple chemical and isotopic data when estimating groundwater travel times and recharge rates, and differentiating recharge mechanisms.