Aflaj-aquifer interconnectivity in northern Oman, revealed from geochemical and isotopic composition of aflaj waters

The aflaj system in Oman is an ancient irrigation network that delivers water supply to the local population, using gravity transport from the aflaj's water sources. Ensuring the sustainability of these water sources is crucial, and a thorough understanding of recharge and hydrodynamic relationships among different aflaj is necessary for future planning and development. Major chemical and isotope compositions, including 2H, 18O, and Sr, are used in this study to identify the sources of water in different aflaj systems and explore connectivity among the various hydrologic systems. The investigation covers a 38,325-km2 area in northern Oman, focusing on springs (ainy aflaj), groundwater collection channels (daoodi aflaj), and surface-water collectors (ghaili aflaj). The primary aquifers in the region include Hajar Super Group (HSG), Ophiolite, Tertiary, Alluvium, and Hawasina. The chemistry of aflaj waters is mainly controlled by evaporation and weathering processes. The 2H vs 18O data display a slope of 5.04, indicating the influence of evaporation. Analyzing the 2H vs 18O data reveals two groups of springs: one group is recharged from the HSG and Hawasina aquifers, while the other is recharged from HSG and Ophiolite. Daoodi aflaj, on the other hand, are primarily recharged from HSG and Ophiolite, with some contribution from Hawasina. The Mg/Ca ratio exhibits a connection between waters from Hawasina and both HSG and Ophiolite, indicating their hydraulic connectivity. This study suggests that all aflaj receive water contributions from at least two aquifer units, with HSG being the primary source of recharge before connecting with other aquifers.