Review: Assessment of the aquifers in South Sudan with a focus on Lakes State

Abstract

The current state of knowledge on groundwater in South Sudan (a data-scarce country) is presented, based on extensive field surveys in Lakes State in the central part of the country, limited published literature, and unpublished consultancy reports. The Basement Complex and the unconsolidated sediments of the Umm Ruwaba Formation are the most extensive geological formations and are the most exploited for groundwater. The water-resource potential properties of the main Umm Ruwaba aquifer (the ‘second’ aquifer, mostly confined) in Lakes State are generally fair to good, with a shallow piezometric surface (<25 m deep), favourable transmissivity (median 8.4, mean 21.5 m2/day), and low salinity and nitrate content. However, some areas have poor groundwater potential due to deep piezometric levels (up to 80 m deep), low transmissivity (mean <5 m2/day) and/or brackish salinity. The estimated recharge is modest (1–8 mm/year) due to predominantly confined aquifer conditions. Aquifer recharge mainly occurs along the geological boundary with the Basement Complex and from the Bahr el Jebel River. Published literature shows saline groundwater and a groundwater trough in northeast South Sudan that was interpreted as being related to a buried saline lake. Additional data on groundwater levels indicate that the extent of the trough may be less than originally sketched. In the Basement Complex, the groundwater potential varies over short distances and depends on the thickness and sand content of the weathered top layer and/or the presence of fractures; the transmissivity is generally low (median for Lakes State is 4.2 m2/day).