Groundwater Evolution in an Arid Coastal Region of the Sultanate of Oman based on Geochemical and Isotopic Tracers

Abstract

In arid and semi-arid regions, conventional hydrological investigations often fail to adequately describe groundwater systems due to a large spatial and temporal variability of hydrological parameters. Alternatively, this study from the Sultanate of Oman utilizes a combination of geochemical and isotopic tracers (O, H, C, Sr) for the identification of aquifer units, recharge areas, groundwater flow paths and residence times in a coastal alluvial aquifer of the Eastern Batinah region in Northern Oman. The hydrochemical investigation of groundwater samples from more than 200 wells and springs clearly indicates that the main recharge areas for the coastal alluvium are the adjacent Oman Mountains, rising up to 3000 m. Groundwater that infiltrates in the high-altitude regions circulates rapidly through the karstified mountains, indicated by close to modern-day tritium activity values measured in springs and wells along the foothills of the mountains. In the piedmont areas, groundwater from the high-altitude regions is diverted by a less permeable ophiolite complex into two geochemically and isotopically distinct corridors (plumes) that stretch through two gaps in the ophiolite across the 50 km wide coastal plain to the Gulf of Oman. Within these plumes the chemical and isotopic signature from the high-altitude regions remains virtually unchanged horizontally as well as vertically to depths exceeding 300 m (deepest wells), suggesting that additional infiltration on the coastal plain is insignificant. Mixing calculations based on strontium isotopes indicate that infiltration on the coastal plain itself accounts for less than 10 % of the total groundwater recharge in the plume areas, which is consistent with the lack of tritium in these groundwater samples. The remaining 90 % of groundwater in the plumes originate in the Jabal Akhdar mountains. In contrast, direct infiltration and recharge on the coastal plain itself is the only source for groundwater in areas adjacent to the two plumes (downstream of ophiolite areas) and in these areas the alluvial aquifer is hydro-chemically layered. Groundwater samples from shallow parts of the aquifer (<50 m) contain tritium indicating modern recharge, whereas groundwater in the deeper aquifer (> 200 m) is of late Pleistocene origin.