Groundwater in Sudan: An improved understanding of wadi-directed recharge. (December 2018)

• Record Type: Journal Article
• Authors: Lanzoni, M.; Darling, W.G.; Edmunds, W.M.
• Is Part Of: Applied geochemistry. Volume 99(2018)
• Page Start: 55
• Page End: 64
• Publication Date: 2018-12
• Subjects: Wadi recharge -- Sudan -- Environmental isotopes -- Palaeowater -- Nubian sandstone aquifer; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2018.10.020
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: Like many dryland regions around the world, the Butana Region of Sudan is almost entirely dependent on groundwater, and its main source of recharge comes from the highly variable 150–200 mm/yr rain that arrives mostly during a three-month rainy season. These rains cause ephemeral wadis to flow and flood, and provide a critical component of shallow aquifer replenishment. This study uses water table fluctuations along a 20 km wadi reach and chemical and stable isotopic tracers in 27 shallow (<40 m) and 6 deep (70–170 m) wells to show differences between mean recharge in the alluvial aquifer, estimated as 38–105 mm/yr, and the basal Nubian sandstone aquifer unit at 13–36 mm/yr. This study suggests while groundwater from the Nubian sandstone is predominantly palaeowater of −8 to −6‰ δ 18 O formed during a wetter, early-Holocene climate, modern recharge does reach the Nubian sandstone aquifer unit in areas <0.3 km from the wadi. An upstream dam may also focus this wadi-directed recharge. In the 1980s, conjunctive use of major and trace geochemistry, isotope, and physical hydrology in the study of wadi recharge was only beginning to gain traction as a sub-discipline, and the study of Edmunds et al. (1987) in the Abu Delaig area paved the way for hydrogeochemical studies in unmonitored dryland catchments around the world. Thirty years on from that study, the local population has grown tenfold, water extraction has increased to accommodate mining, and an upstream dam has been built. This study, conducted in a 400 km 2 subset of the original Lower Atbara Basin study, answers a previously unanswered question about wadi recharge to the deeper, Nubian sandstone aquifer unit and offers a unique three-decade perspective on groundwater development in a rural, arid environment expanding beyond its margin of groundwater sustainability. Highlights: Focused, wadi recharge penetrates the underlying, Nubian sandstone aquifer locally in the Abu Delaig catchment. D/δ 18 O shows modern recharge in the Nubian sandstone <0.3 km from the wadi, while palaeowater is suggested >5 km away. Water elevation suggests the alluvial aquifer flows northeast along the wadi, while the Nubian unit flows north/northwest.