AbstractInter-bedded limestone, dolostone and evaporite sequences are typical of shallow carbonate deposits in arid climates such as Qatar. Their chemical reactivity makes them prone to diagenetic alteration and dependent on groundwater chemistry and flow patterns at a range of scales.
Continuous core recovered from ~130m depth at three locations along a North-South transect on the crest of the Qatar Arch defined four hydrostratigraphic units: 1) crystalline calcite and dolomite of the Dammam Formation (upper aquifer), 2) Rus Formation, a carbonate and clay deposit (middle aquifer), 3) crystalline gypsum/anhydrite Rus Evaporite (aquiclude – absent in the north of the country) and 4) the dolomitic Upper Um Er Radhuma (UER) (lower aquifer).
Nested piezometers suggested a vertical hydraulic gradient governing flow within and between each aquifer system. In the south, an upward hydraulic gradient is due to a substantial thickness of crystalline Rus Evaporite confining the underlying saline UER. In the north, the absence of the Rus Evaporite means the different aquifers are in hydraulic continuity with a downward flow driven by meteoric recharge.
Groundwater analyses indicate differences between northern, central and southern Qatar within similar rock types in the same aquifers. In the south, concentrations of Ca2+, Mg2+ and SO42- in excess of predictions from linear mixing show sulfate enrichment. These southern waters were also depleted in Mg2+ relative to local seawater. As these waters ascend, they become less depleted in Mg+2 and less enriched in Ca2+ relative to SO42- enrichment. This vertical contrast suggests de-dolomitisation or recrystallization from less Ca-rich to more stoichiometric well-ordered (Ca=Mg) dolomites with Mg-rich clays. This trend is considered ongoing in centre and northern UER aquifers, although to a lesser degree.
Assessing these modern shallow groundwaters contribute to a better understanding of the deeper reservoirs, which have similar depositional and diagentic environments, and provide effective management of a crucial resource.
|Date of Award||23 Jan 2020|
|Supervisor||Fiona F Whitaker (Supervisor)|