Abstract
The North-Western Sahara aquifer system (NWSAS) forms an important transboundary groundwater resource whose properties remain to be fully understood across its whole extent. For example, groundwater flow in the Cretaceous Continental Intercalaire (CI) unit of the NWSAS is well-characterised in the northern part of its range around the Algeria/Tunisia/Libya borders and in the Great Eastern Erg sub-basin immediately to the south. To the southwest, however, the CI of the Great Western Erg sub-basin has been much less studied. The present paper reports hydrogeochemical data from a wellfield in central Algeria which will contribute to a better understanding of this sector of the NWSAS in terms of the age and origin of groundwater within it.Groundwater pumped from five deep boreholes in the CI aquifer overlying the Krechba gas field has been studied using a variety of environmental tracers including hydrochemistry, environmental isotopes, and reactive and noble gases (the latter being reported for the first time for this sub-basin). All the waters were dilute (SEC 460–600 μS/cm), contained detectable O2 (6.3–7.5 mg/L), showed evidence of evaporation (relative enrichment in δ18O), gave late-Pleistocene 14C model ages (13.5–19.3 ka), and yielded lower than present-day noble gas recharge temperatures (14.3–17.6 °C). Various lines of evidence suggest that these waters are the product of mixing between water recharged direct to the CI and leakage from the Neogene–Quaternary Erg aquifer. The results support the long-held concept of regional flow from a palaeo-recharge area to the northwest. Finally, while the Krechba gas field (Carboniferous) has been since 2004 the site of a pilot carbon capture and storage (CCS) project, the data revealed no evidence for leakage of fluids (gas or brines) into the overlying CI aquifer at the time of sampling (October 2014).
Highlights
The North-Western Sahara aquifer system (NWSAS) is one of the largest in the world and includes two main aquifers, the Continental Intercalaire (CI) and the overlying Complexe Terminal (CT), shared between Algeria, Tunisia and Libya
It remains to be fully characterised in some areas, the CI in the southwest of the region, where it underlies a part of the Sahara Desert known as the Great Western Erg (Fig. 1), or GWE
The existence of a wellfield at Krechba has provided a window into groundwater conditions in the transboundary Continental Intercalaire (CI) aquifer in an area of central Algeria with otherwise sparse data
Summary
The North-Western Sahara aquifer system (NWSAS) is one of the largest in the world and includes two main aquifers, the Continental Intercalaire (CI) and the overlying Complexe Terminal (CT), shared between Algeria, Tunisia and Libya. While the combination of the Tademait and the Mzab topographically separates the two Erg (dune) areas, for the purposes of this study the groundwater divide will be taken as the subbasin boundary, which marks the southwestern limit of the CT as a significant aquifer. On this basis, the CI and CT beneath the GEE discharge to the northeast, while the CI of the GWE discharges to the southwest (Fig. 1). The CI is confined in this area by Cenomanian mudstones (Fig. 2b); piezometric measurements suggest that flow across the wellfield is from NE to SW (Fig. 2a), but the estimated 1:3000 the gradient is extremely low so the precise direction remains uncertain
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