[en] The Sokoto hydrological basin in Northwestern Nigeria belongs to a larger geographic unit, the Iullemmeden Basin in West Africa covering an estimated area of 700,000 Km². It is a semi-arid region with mean annual rainfall (over a period of 90 years) ranging from 350mm at Kalmalo in the extreme north and 670mm (at Sokoto). The potential evapotranspiration at Sokoto is calculated to about 2,500mm/a, thus exceeding precipitation by as much as a factor of 5. The aim of this study is to characterize water resources in the basin and investigate the risk of pollution. This is to aid the planning and management of water resources in the semi-arid areas of northwestern Nigeria to meet the demands of the growing population. In order to achieve the aim of the present study more than 190 samples were taken from dugwells and tubewells, boreholes, lake as well as rivers Rima and Sokoto in four major sampling campaigns in the Federal Ministry of Water Resources (Nigeria) under the joint RAF/08/22 project with the International Atomic and Energy Agency, Vienna (Austria) and three fieldwork in the University of Ilorin (Nigeria) in collaboration with Technical University, Darmstadt (Germany). As a reference to groundwater 10 rainwater samples were collected from 3 different stations (Goronyo, Wurno and Sokoto) for isotope analyses. Physical parameters like temperature, pH, electrical conductivity (EC), dissolved oxygen and alkalinity were measured in the field using potable meters and 'AquaMerck' titration kits. Main chemical components and the environmental isotopes H-2, H-3, C-13, C-14 and O-18 have been analyzed. Stable isotopes ratios are expressed as delta in per mil relative to VSMOW (Vienna Standard Mean Ocean Water). The isotope precision of measurement based on VSMOW is ±0.15 per mille for ¹⁸O and ±1 per mille for ²H. Results of field analysis of the water samples have shown a pH range of 5.5 - 8.3; electrical conductivity of 45 -1, 155 μS/cm and total dissolved solids (TDS) between 16 to 1,063 mg/l. The dominant anion is bicarbonate although places of significant concentration of sulphate were recorded depending on where the sample was taken. It is generally difficult to find any significant criterion in the chemical solutions of groundwater for distinction of the different aquifer types. However, five groups of water samples were formed to plot the equivalent concentration of the major ions and ionic combinations. These groups, with their distinguishing chemical composition, are as follows: (I) deep groundwater from boreholes (HCO₃>SO₄>Cl), (II) shallow groundwater mainly from dug wells outside the irrigation area (SO₄>HCO₃>Cl), (III) shallower groundwater from tube wells and some dugwells around the Wurno Irrigation Scheme, (Cl>HCO₃>SO₄), (IV) Lake and Carnal waters (HCO₃>Cl>SO₄), (V) Rivers Rima and Sokoto (HCO₃>SO₄>Cl). The study of stable isotope contents (¹⁸O, ²H) also identified the five groups. Group I-III is of groundwater origin while group IV and V represent surface water. It is obvious that, between the deeper groundwater (group I) and the overlying water bodies, no transition zones are known, as they can be clearly separated. For the shallower groundwater, however, admixtures of groups II, III, with groups IV and V are obvious. Group II and III stem mainly from direct recharge by precipitation. However, due to lack of sufficient information on the position of the screens the depth penetration of recharging water is not known. Except few samples of shallow and surface water which defined a local evaporation line of slope 5.1, no signification evaporation was found to occur during infiltration; most of the points plot close to the Global Meteoric Water Line (GMWL) and the weighted mean of precipitation in Kano. A combination of the hydrochemical and isotope data (¹⁴C, ¹³C and ³H) reveals the Sokoto basin aquifers generally contains good quality groundwater of Holocene age (100 to 10,000 years Bp)