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Title (Primary) Evaluation of groundwater recharge in Najd aquifers using hydraulics, hydrochemical and isotope evidences
Author Al-Mushaikhi, K.S.A.
Source Titel PhD Dissertation
Year 2011
Department HDG
Volume 9/2011
Page To 176
Language englisch
UFZ inventory Leipzig, Bibliothek, Reportsammlung, 00450645, 11-0702 F/E
Abstract

This study presents a multi-indicator approach about the Najd region in south-west Oman; a significant area for groundwater resources and a future of agricultural development. The Najd area mainly consists of marine limestone of Tertiary age and hydrogeologicaly classified to four aquifers A to D following the order from groundlevel, downwards. Aquifer A is located in the Rus formation, the upper deposits, whereas the aquifers from B to D are restricted with the Umm Er Radhuma formation (UER).

On the north side of the recharge area at the Jabal chain, only aquifer D exists, whereas the other aquifers are dry due to geological structural influences. This dominance leads us to believe that aquifer D is the main aquifer in Najd. This aquifer has the potential to collect recharge from the Jabal chain and the nearby north side. This occurs particularly during cyclone events and in monsoon season. The aquifer conducts the water through faults and fissures to upper aquifers further north.

Results were detected from monitoring the level of water in boreholes during flood period occurrences. It is evident that these observations support the idea that cyclone events are a significant source of recharge to Najd groundwater. Thus, the estimated time for recharge to reach the groundwater was calculated between a range from 7 days to 6 months depending on the cyclone intensity and longevity, the aquifer characteristics and distances between the recharge occurrence locations and the monitored boreholes. In comparison to water levels are declining in central Najd in all aquifers A to D on the range between 0.12m to 2.05m annually. However, the highest decreases are found at Helat Ar Rakah in aquifer C, with a depressive affect extending more than 30 km.

In general, groundwater mineralization in all aquifers increases in a northeastward direction. With exception of the Jabal chain, Najd groundwater does not meet with the Omani standard for human drinking water. However, the majority of groundwater in all aquifers is classified as suitable for agricultural purposes, ie. Irrigation, according to SAR and EC concentrations, follow the order of D>C>A>B.

The stable water isotopes reveal that 2H and 18O are depleted in all aquifers within the flow direction. They reflect the isotopic composition of cyclone events as well as in fossil groundwater. However, enrichment in 2H and 18O is found in or at least close to the Jabal chain and two other locations further north: at Thumrait (MOD-15) and at the Poultry Farm (332/014) at Hanfeet. This enrichment indicates recent groundwater along the recharge direction as one of the findings in this study. Tritium was detected in the Jabal chain, at Shsir in aquifer A and in DEP-9 in aquifer B reflecting sub-modern groundwater recharge. These arguments conclude that groundwater hydrochemistry and water stable isotopes are mainly controlled by geological structures and that aquifer properties contain a certain quality and quantity of recharge. As a result, heterogeneity of aquifers was observed.

14C data and model age calculations show that the groundwater age in Najd aquifers varies between modern and up to 22,000 years. However, majority of groundwater belongs to the Holocene period (<10,000 yr). With exception of the Jabal chain several boreholes from different aquifers were indicated as modern or containing a young groundwater age, such as Helat Ar Rakah, Shsir, the South of Qitbit and east Al Mazyunah. The data analyses of 14C, 13CDIC and 18O confirm three sources of the origin of groundwater: monsoon, recent and paleo-cyclones.

The noble gases temperature (NGT) reveals paleo-climate conditions, keeping in mind the recharge to groundwater in the past, which occurred under colder climate conditions compared with recent times. Additionally, the groundwater temperature increases towards the northeast which supports the hypothesis of humid or colder climates near the Jabal in the past than further north in the interiors of Najd. The Ne/He and 3He/4He data presents an atmospheric helium source in aquifers A, B and D as an evidence of recent recharge, however, radiogenic helium in aquifer C points to a paleo-recharge.

Persistent UFZ Identifier https://www.ufz.de/index.php?en=20939&ufzPublicationIdentifier=11350
Al-Mushaikhi, K.S.A. (2011):
Evaluation of groundwater recharge in Najd aquifers using hydraulics, hydrochemical and isotope evidences
Dissertation, Friedrich-Schiller Universität Jena
PhD Dissertation 9/2011
Helmholtz-Zentrum für Umweltforschung - UFZ, Leipzig, 176 pp.