No abstract

[1] Oxygen and hydrogen isotope analyses of rainfall samples collected on the eastern Batinah coastal plain of northern Oman between 1995 and 1998 indicate two different principal water vapor sources for precipitation in the area: a northern, Mediterranean source and a southern, Indian Ocean source. As a result, two new local meteoric water lines were defined for the study area. Isotopic analyses of groundwater samples from over 200 springs and wells indicate that the main source of water to the Batinah coastal alluvial aquifer is high-altitude rainfall from the adjacent Jabal Akhdar Mountains, originating from a combination of northern and southern moisture sources. The groundwater recharged at high-altitude forms two plumes of water which is depleted in the heavy isotopes 18 O and 2 H and stretches from the mountains across the coastal plain to the sea, thereby retaining a chemical homogeneity horizontally and vertically down to a depth exceeding 300 m. In contrast, in areas adjacent to these two plumes the alluvial aquifer is geochemically stratified. Near the coast, saline intrusion results in abrupt changes in chloride concentrations and isotope values.

Concentrations of atmospheric noble gases (neon, argon, krypton, and xenon) dissolved in groundwaters from northern Oman indicate that the average ground temperature during the Late Pleistocene (15,000 to 24,000 years before present) was 6.5 degrees +/- 0.6 degrees C lower than that of today. Stable oxygen and hydrogen isotopic groundwater data show that the origin of atmospheric water vapor changed from a primarily southern, Indian Ocean source during the Late Pleistocene to a dominantly northern, Mediterranean source today. The reduced northern water vapor source is consistent with a drier Last Glacial Maximum through much of northern Africa and Arabia.

Warm springs emanating from deep-reaching faults in orogenic belts with high topography and orographic precipitation attest to circulation of meteoric water through crystalline bedrock. The depth to which this circulation occurs is unclear, yet it is important for the cooling history of exhuming orogens, for the exploitation potential of orogenic geothermal systems, and for the seismicity of regional faults. The orogenic geothermal system at Grimsel Pass, Swiss Alps, is manifested by warm springs with a clear isotopic fingerprint of high-altitude meteoric recharge. Their water composition and their occurrence within a 3 Ma fossil upflow zone render them particularly favorable for estimating the temperature along the deep flow path via geochemical modeling. Because the background geotherm has remained stable at 25 °C/km and other heat sources are unavailable, the penetration depth can be derived from the deep-water temperature. We thus estimated the base of the Grimsel system to be at 230-250 °C and 9-10 km depth. We propose that deep temperatures in such systems, particularly those with normal background geotherms (<30 °C/km), have been systematically underestimated. Consequently, far more enthalpy may be accessible for geothermal energy exploitation around the upflow zones than previously thought. Further, the prevalence of recent earthquake foci at ≤10 km beneath Grimsel suggests that meteoric water is involved in the seismicity of the host faults. Our results therefore call for reappraisal of the heat budget and the role of meteoric water in seismogenesis in uplifting orogens.

A regional hydrogeochemical model was developed to evaluate the geochemical evolution of different groundwaters in an alluvial aquifer system in the Interior of Oman. In combination with environmental isotopes the model is able to extract qualitative and quantitative information about recharge, groundwater flow paths and hydraulic connections between different aquifers. The main source of water to the alluvial aquifer along the flow paths of Wadi Abyadh and Wadi M'uaydin in the piedmont is groundwater from the high-altitude areas of the Jabal Akhdar and local infiltration along the wadi channels. In contrast, the piedmont alluvial aquifer along Wadi Halfayn is primarily replenished by lateral recharge from the ophiolite foothills to the east besides smaller contributions from the Jabal Akhdar and local infiltration. Further down gradient in the Southern Alluvial Plain aquifer a significant source of recharge is direct infiltration of rain and surface runoff, originating from a moisture source that approaches Oman from the south. The model shows that the main geochemical evolution of the alluvial groundwaters occurs along the flow path from the piedmont to the Southern Alluvial Plain, where dedolomitization is responsible for the observed changes in the chemical and carbon isotope composition in these waters.