Hydrogeochemical evolution of groundwater in the karstified aquifer system of the northeastern Al Hajar Mountains, Sultanate of Oman

Kim Hußmann1, Gösta Hoffmann1, Barbara Reichert1
1 Institut für Geowissenschaften, Rheinische Friedrich-Wilhelms-Universität Bonn

10.2 in Hydrogeologie arider und semiarider Gebiete

28.03.2020, 09:15-09:30, Händel-Saal

The northeastern Al Hajar Mountains in Oman are composed of paleogene carbonates which are underlaid by a complex of ophiolites. The carbonate formation is highly karstified as a result of humid periods during the Quaternary (Woods and Imes 1995). Therefore, a complex aquifer system with large caves and deeply carved Wadis has developed. The groundwater of the system is currently fundamental for drinking and hygiene purposes for the local population as well as for irrigation. Thus, an effective and sustainable water resource management is indispensable and accordingly, a sound understanding of the hydrogeological setting is required.

This study aims to characterize the hydrogeological setting visualized in a conceptual model. Hence, information about the aquifer structure and data to determine the evolution of groundwater are required. The mineralogical and geochemical composition of rocks in combination with the hydrochemistry of the water is used to investigate water-rock-interaction and to outline the hydrogeochemical evolution of groundwater. Thereby, the focus is especially on the interaction between groundwater and ophiolites.

During the first field investigations from February to March 2019, hydraulic conductivities were estimated according to the cubic law (e.g. Cook 2003). K-values in the karstified carbonates vary between 10‑2 to 10‑6 m/s while the ophiolite complex has k-values around 10-7 m/s. Thus, the carbonate formation is classified as an aquifer, whereas the underlaid ophiolite complex act as an aquitard. From both types of rocks 21 different samples were taken and analyzed. Depending on the stratigraphy the carbonate formation is composed of calcite or dolomite, while the ophiolites consist mainly of antigorite, clinochrysotile, and lizardite. For hydrochemical analyses 55 samples from groundwater and surface water were taken. Hydrochemical facies of the water were most of Ca-HCO3 and Mg-HCO3 type indicating calcite and dolomite dissolution. Moreover, Na-Cl facies near the coast suggest an influence of seawater. Calculated saturation indices (SI) reveal that hydrochemistry is also influenced by the underlaid ophiolite complex. Most SI values for chrysotile show an undersaturation indicating dissolution and therefore ophiolite weathering, while some positive SI values indicate precipitation of chrysotile.

Based on this snapshot sampling, a second field campaign in November and December 2019 will be performed to investigate seasonal variations in the hydrochemistry.

Cook, P.G. (2003): A guide to regional groundwater flow in fractured rock aquifers. Seaview Press, Henley Beach, South Australia.

Woods, W.W. & Imes, J.L. (1995): How wet is wet? Precipitation constraints on late Quaternary climate in the southern Arabian Peninsula. – Journal of Hydrology, 164: 263-268.

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