Abstract EN:

This thesis comprises a study commissioned by the Council for Conservation of Environment and Water Resources to compliment ongoing exploration and evaluation of groundwater resources in Oman. Isotopic and geochemical methods are employed to determine groundwater mean circulation times and their recharge sources. Complementary investigations, carried out at the University of Paris-Sud, reconstruct the dominant late Pleistocene and Holocene paleoclimates’ in the region. Stable isotopes in rain and groundwater (¹⁸O, ²H), in dissolved carbonate, carbonate minerals and CO2 (¹³C, ¹⁸O) and in sulphate (¹⁸O, ³⁴S) are applied in identifying groundwater origins and recharge mechanisms. Mean groundwater circulation times are estimated through tritium (³H) and radiocarbon (¹⁴C) modelling. Chlorine-36 is used to provide estimates of rates of recharge. Groundwater flow systems within eighteen integral study areas are investigated individually, followed by a regional synthesis. In northern Oman, the most significant groundwater resources are shallow, associated with unconsolidated wadi and coastal alluvium, recharged by summer convective rainfall systems or in fractured bedrock aquifers, recharged at altitude by infrequent low pressure systems from the northwest. Tritium shows rapid circulation in less than 5 to 10 years. Deeper ground waters, in lower permeability aquifers, are considerably older (10 years). In southern Oman, deep confined ground waters in the interior (Najd) have mean residence times in excess of 10,000 years, probably recharged during the Holocene pluvial. The uppermost (phreatic) aquifers receive modern recharge from infrequent storms passing into the interior. Modern precipitation (monsoon) at altitude in the aquifer outcrop area is shown to recharge the coastal aquifers, with no evidence of flow to the Najd. Palaeoclimatic investigations are based on a new approach using travertines deposited by hyperalkaline ground waters (Ca +-OH-; pH > 11. 5). Variations in texture and stable isotope, signatures are shown to record the humid and arid climatic phases within the time scale of C dating (the past 33,000 years). Unusual isotope depletions (¹³CCaCO₃ = -33. 6 ‰ formed by atmospheric CO₂) associated with such travertine formation are investigated in a series of laboratory experiments, showing a combination of diffusion effects and reaction kinetics to be responsible.