Hydrogeochemical and isotopic evolution of water in the Complexe Terminal aquifer in the Algerian Sahara

• Published in: Hydrogeology journal Vol. 11; no. 4; pp. 483 - 495
• Main Authors: Guendouz, A., Moulla, A. S., Edmunds, W. M., Zouari, K., Shand, P., Mamou, A.
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01-08-2003
• Subjects: Aquifers; Deuterium; Evolution; Geology; Groundwater; Holocene; Isotopes; Mineralization; Pleistocene; Pliocene; Recharge; Redox reactions; Sedimentary basins; Strontium; Trace elements; Algeria
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s10040-003-0263-7

The hydrogeochemical and isotopic evolution of groundwaters in the Mio-Pliocene sands of the Complexe Terminal (CT) aquifer in central Algeria are described. The CT aquifer is located in the large sedimentary basin of the Great Oriental Erg. Down-gradient groundwater evolution is considered along the main representative aquifer cross section (south-north), from the southern recharge area (Tinrhert Plateau and Great Oriental Erg) over about 700 km. Groundwater mineralisation increases along the flow line, from 1.5 to 8 g l^sup -1^, primarily as a result of dissolution of evaporite minerals, as shown by Br/Cl and strontium isotope ratios. Trends in both major and trace elements demonstrate a progressive evolution along the flow path. Redox reactions are important and the persistence of oxidising conditions favours the increase in some trace elements (e.g. Cr) and also NO^sub 3^^sup -^, which reaches concentrations of 16.8 mg l^sup -1^ NO^sub 3^-N. The range in ^sup 14^C, 0-8.4 pmc in the deeper groundwaters, corresponds with late Pleistocene recharge, although there then follows a hiatus in the data with no results in the range 10-20 pmc, interpreted as a gap in recharge coincident with hyper-arid but cool conditions across the Sahara; groundwater in the range 24.7-38.9 pmc signifies a distinct period of Holocene recharge. All δ^sup 18^O compositions are enriched relative to deuterium and are considered to be derived by evaporative enrichment from a parent rainfall around -11[per thousand] δ^sup 18^O, signifying cooler conditions in the late Pleistocene and possibly heavy monsoon rains during the Holocene.[PUBLICATION ABSTRACT]