ABSTRACT
The Sokoto Basin,(the South-Eastern sectorof Iullemmeden basin), located in Nigeria, consists of an aquifer system which is bi-layered with cretaceous Continental Intercalaire (CI) sedimentary deposit at the bottom,(Sokoto Group, Rima Group, Gundumi-Illo Formation), by Continental Terminal (CT) Formation(Gwandu Formation) and surface water. The water of the basin is generally used for drinking, domestic, industrial and agricultural purposes. The water resources of the basin are being threatened by contamination, weak hydro-dynamic aquifer characteristics and sparse water quality data. Hydrochemical and isotope techniques have not really been applied to examine the water composition of the basin. In an attempt to bridge the gap in paucity of water quality data, hydro-chemical techniques and common environmental isotope signatures were applied to characterize the basin water composition. Eighty seven water samples consisting of eight surface water samples and seventy nine ground water samples were collected from the basin in July 2013 and April 2014; and analyzed for physico-chemical parameters (pH, Temp, EC, TDS, Ca, Mg, Na, K, F, B, Pb, Cr, Cd and Fe) and isotope constituents (�´18O, �´2H, 3H).Hydrochemistry of major ions (Ca+2, Na+, Mg+2, K+, HCO3-, SO4-2, Cl-)and common environmental isotopes (�´18O, �´2H, 3H) were used to categories the water resources of the basin into different water types. Heavy metals were subjected to further assessments and evaluation applying heavy metal index (MI) and heavy metal Degree of Contamination (CD) to find out the extent of metal contamination. The water quality for different uses was determined by assessment and evaluation of standard criteria.Three different water types were identified, calcium bicarbonate type (Ca-HCO3); calcium sulphate type (Ca-SO4); and sodium bicarbonate type (Na-HCO3,). The groundwater composition is largely controlled by dissolution of carbonate rocks, evaporite (gypsum) and cation-exchange of probably clay minerals. The isotope signature of the waters of the basin shows that the waters are made up of modern waters which are moderately depleted with �´18O content (-6.59%o to -3.20%o, mean= -4.49 Â�0.3%o) and �´2H content (-37.12%o to - 20.75%o, mean= -27.26%oÂ�1%o) compared with that ofpaleowaters(-7.72%o to -6.84%o, mean -6.87 Â�0.3%o) and�´2H content(-48.26%o to -46.96%o, mean=-47.13Â�1%o).This indicates that an appreciable fraction of modern waters recharge CT and CI aquifer system of the basin. Also, there is recharge from evaporated waters of mainly surface water origin enriched with stable isotope signature with delta values of �´18O (-3.52%o to -1.05%o, mean = -2.89%o) and�´2H of (-17.65%o to 6.75%o, mean= -9.79%o). The basin surface water is considerably more evaporated than the groundwater. The surface waters have generally low dissolved solid content and are due to the full exposure of the surface water to high atmospheric temperatures throughout the year.The unstable isotope (�´3H) signature confirms thatthe water composition of the basin consists of paleowater withextreme value of tritiumcontent from 0.0TU to 0.5TU andof young water from 0.5TU to 4.5TU. Determining the suitability of waters for drinking, domestic and industrial purposesthe mean concentrations of physico-chemical constituents in the water samples were compared to European Union(EU), United State Environmental Protection Agency (USEPA), World Health Organization(WHO), Bureau of Indian Standard (BIS) and Nigerian Industrial Standard (NIS).The results fall within permissible limits except for the heavy metals' concentrations in some wells located in urban areasthat exceed the permissible limits most probably due to anthropogenic inputs.The metal index values indicate that the water quality in the studied areas fall within ?low class' and ?medium class' according to the classification of heavy metal water quality using the heavy metal index. Sodium Adsorption Ratio (SAR), Percent Sodium (Na %), Magnesium hazard and Residual Sodium Carbonate (RSC) results revealed that most of the sampled waters are suitable for irrigation. Generally, this study characterized the aquifer system of the basin on the bases of physico-chemical and isotopic features and ascertained the contamination status of the aquifers of the basin. Besides, it was able to establish based on the findings that contrary to earlier claims and beliefs, a considerable amount of modern water enters the Continental Intercalaire aquifer and the waters of the aquifers are related. The suitability of the waters of the basin for different uses was also established and was found to be useful in diversified applications. However, it is recommended that climate variability and soil assessment of the basin be carried out for contamination index determination. It is further recommended that the quantity of modern water recharging the aquifer system of the basin and age of the waters should be determined.