The hydrogeology of the Oman Mountains

Northern Oman is an arid area almost entirely dependent upon groundwater recharged by highly sporadic rainfall. Precipitation estimates are hampered by a lack of any reliable altitude-rainfall relationship. Below 700 m there is no statistically significant relationship. The isotopic composition of groundwater is strongly influenced by the rainfall amount (related to storm frequency), and not just by altitude/temperature. Storm events with long return periods are of disproportionate importance to recharge. Despite the huge volume of carbonate formations, holokarstic development is generally immature, and groundwater storage is greatest in alluvial piedmont surrounding main limestone massifs. Isotopes, chemistry and hydrologic measurements show that post-storm evaporative losses are very large. The origin of limestone springs and their chemical and physical anomalies are described. Structure rather than petrology controls groundwater flow in the limestones, hence regional differences in structural style produce contrasting hydrologic regimes between the various massifs. The Semail nappe mantle sequence is the only other hard-rock formation of groundwater significance. Though much less productive than the carbonates, these ultramafics display extraordinary chemical activity, yielding bicarbonate waters from the weathered zone, whilst more deeply circulating groundwaters produce hyperalkaline springs by low-temperature serpentinisation. Associated processes include solute reduction, hydrogen evolution, hydroxide and carbonate precipitation, hydroxide-basic rock reaction, salt enrichment and water-rock isotopic exchange. Throughout the interior catchments, groundwater mostly flows into narrow buried alluvial channels which are often constricted at hard-rock nodal points, thus facilitating very efficient interception and recovery by the "falaj" system. Traditional agriculture has evolved to cope with fluctuating groundwater supply but is sensitive to increased abstraction. On the Batinah plain, greatly increased coastal abstraction has locally induced moderate to severe salinisation. Existing process studies are insufficiently quantified to provide the resolution necessary to manage groundwater resources, especially in high-risk coastal areas.