Geophysical Investigation of Groundwater Quality and Salinity Distribution in the Sokoto-Rima Basin, North West Nigeria

The Sokoto-Rima Basin in North West Nigeria contains extensive sedimentary aquifer systems, but spatial variation in groundwater salinity associated with lithological heterogeneity and evaporite dissolution within the Rima Group creates significant management challenges for water supply authorities in Sokoto and Kebbi states. This professional study maps groundwater salinity distribution and identifies zones of fresh, brackish, and saline groundwater in the Sokoto-Rima Basin using time-domain electromagnetic sounding integrated with hydrochemical data. TDEM soundings were acquired at 60 stations within an 8,000-square-kilometre survey area, with loop sizes optimised for investigation depths of up to 250 metres. Inverted one-dimensional resistivity models were quality-controlled by comparison with existing borehole completion logs. A resistivity threshold derived from formation factor analysis, calibrated against groundwater total dissolved solids measurements in existing boreholes, was applied to the resistivity models to classify aquifer zones into fresh (TDS less than 500 milligrams per litre), brackish (500 to 1,500 milligrams per litre), and saline (greater than 1,500 milligrams per litre) categories. Fresh groundwater zones are predominantly associated with the Gwandu Formation sandstones at intermediate depths of 40 to 100 metres, while saline zones occur in the deeper Dange Formation in the northern basin. The salinity map provides a critical spatial framework for optimising borehole siting and design to access fresh groundwater and avoid saline zones in future water supply development. Keywords: groundwater salinity, TDEM, Sokoto-Rima Basin, hydrochemistry, aquifer management