The Sokoto Basin,(the South-Eastern sector of 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 (δ18O, δ2H, 3H).Hydrochemistry of major ions (Ca+2, Na+, Mg+2, K+, HCO3-, SO4-2, Cl-)and common environmental isotopes (δ18O, δ2H, 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 δ18O content (-6.59%o to -3.20%o, mean= -4.49 ±0.3%o) and δ2H content (-37.12%o to – 20.75%o, mean= -27.26%o±1%o) compared with that of paleowaters(-7.72%o to -6.84%o, mean -6.87 ±0.3%o) andδ2H content(-48.26%o to -46.96%o, mean=-47.13±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 δ18O (-3.52%o to -1.05%o, mean = -2.89%o) andδ2H 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 (δ3H) signature confirms that the water composition of the basin consists of paleowater with extreme value of tritium content from 0.0TU to 0.5TU and of young water from 0.5TU to 4.5TU. Determining the suitability of waters for drinking, domestic and industrial purposes the 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 areas that 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.

1.1       Background of Study

In 1931, the phrase ‗Iullemmeden Basin‘ was earlier defined by Radieras the Sedimentary Basin which extends from Mali and the Western boundary of the Republic of Niger through northern Benin Republic and northern Nigeria(Kogbe,1979) where it is referred to as ‗Sokoto Basin‘, into eastern Niger (Fig. 1.1).
It is located in the arid and semi-arid area of West Africa with the population of approximately 15million, which is projected to riseto 28 million in 2025 (GEF, 2003).The Iullemmeden Aquifer System (IAS) properties according to (GEF, 2003) is such that the depth

ranges between 1500 to 2000mbsl (with aquifer section below 250mbsl are not usually considered to take active part in modern hydrological cycle), the water level vary from 0 to 80m and transmissivity fall between 10-3 to 10-4m2/sec. The recharge zones of the sedimentary deposit for IAS is in the northeast (in Niger) and northwest (in the Taoudeni basin in Mali) ridges, and in the south with input from the Rima River (or Goulbi de Maradi), and the Drainage zones are mainly the Niger River and the Rima River, one of its affluent (GEF, 2003; OSS, 2008 and AzTech, 2010).The aquifer system covers an area of 525,000km2, with 31,000km2 in Mali, 434,000km2 in Niger and 60,000km2 in Nigeria, where it is also known as Sokoto Basin. Sokoto Basin which covers Sokoto, Kebbi, Zamfara and parts of Kastina States in Nigeria is the area for this study.It stretches between latitudes 10o and 14o North and longitudes 3o and 7o east, as shown in the base map of the study location(Appendix, I).

The Basin lies in the sub-Saharan Sudan belt of West Africa in the area of savanna-type vegetation categorized as semi-arid. The Basin broadly covers an area underlain predominantly by crystalline rocks to the east and sedimentary terrain to the northwestern half according to Anderson and Ogilbee (1973). Similar to Iullemmeden aquifer system, Sokoto Basin is multi-layered including a cretaceous Continental Intercalaire (CI) sedimentary deposit at the bottom (Gundumi-Ilo Formation)overlain by three layers of Continental Terminal (CT) Formations (Gwandu Formation, Sokoto Group and Rima Group), as shown in (Fig. 1.2). The Sokoto Group consist of three distinct sedimentary formations which includes: Gamba, Kalambaina and Dange Formations and Rima Group also consist of distinct Formations which include: Wurno, Dukamaje and Taloka Formations.

Geomorphologically, the area is undulating with depressions due to Wadis and tributaries, with laterites and iron stones that characteristically cover the hills of the basin, (AzTech, 2010). The Sokoto and Rima Rivers are the principal drainage network, (Kogbe, 1979). The headwaters of the Sokoto and Rima rivers as well as their tributaries rise in pre-Cretaceous crystalline rock terrain east of the Sokoto Basin and flow west and south across the outcrop zones of the Gundumi-Ilo Formation, the Rima and Sokoto Groups, and the Gwandu Formation, (Anderson and Ogilbee, 1973). There are other tributaries to the main drainage networks which include Shella River, Bunsuru River, Gagere River, Kware River, among many others. Rainfall in this area percolates directly into the sandy soil or flows in minor tributaries where it infiltrates or evaporates and may eventually dry out during the peak of dry season. The major urban settlements in the area are: Sokoto, Birnin Kebbi, Gusau and Gwandu. There
are low-lying flood plains (Fadamas) which are used during the dry season for irrigated agriculture. The characteristic agricultural products are grains and vegetables. Dams like the Goronyo, Bakolori, Zobe, Shagari and Jibiya dams, (Okuofu, 2006) are identified reservoirs in the region.Traditionally, water supply in many rural settlements in the Sokoto Basin is based on rainwater, surface water and ground water.

The degradation of water quality has been identified as one of the major challenges of the basin by previous researchers. There is sparse data about water quality and existing data on the hydrodynamic aquifer characteristic from this region is weak (Okuofu, 2006).The Intercalary Continental, have little outcrop, are not easily renewed, some authors report that its non renewable and are thus threatened to mining exploitation. Presently, these water resources face the consequences of over exploitation such as decline of the piezometric level, degradation of water quality and the effects of climate change and variability (OSS, 2008).As such, this study focused on gap bridge in limited data on water quality in the basin.Groundwater, unless contaminated, is generally of better quality than surface water and in an effort to provide the population with safe, adequate, and sustainable water supply, the significant groundwater resources, have increasingly been exploited and contaminated.The recorded water quality issues in the basin are: high ground water salinity, high fluorine contents, pollution from agricultural sources, and infiltration of toxic chemicals and organic polluting waste from agro-industries. The contaminants may be due to mineral exploration for precious metals and petroleum in the region GEF(2003) and OSS(2008).This will have indirect impact on other riparian countries (transboundary aquifer). Therefore, need for sustainable water resources management of the Sokoto Basin through application of environmental Isotope in combination with hydrochemical technique to study the dynamics of water composition in the basin is vital. Numerous studies have been carried out in this Basin, including studies on geological
formations of the basin to water resources integration of the basin (Falconer, 1911; Raeburn and Tattam, 1930; Jones, 1948; Radier, 1957; Bell, 1961; Du–Preeze et al., 1965; Anderson and Ogilbee, 1973; Kogbe, 1979; Oteze, 1989a; Adelana et al, 2003; Alagbe, 2004; Okuofu, 2006; Sokonaet al., 2008; AzTech, 2010; Al-Gamal, 2011 and among many others) using mainly hydro-chemical approach.In this respect, environmental isotope in complementary with hydrochemical approach were applied in conducting this study. Isotope and related techniques are unique and effective tools for obtaining hydrologic information for a broad range of water resource management issues; timescale of hydrological events, identify the origin of water, renewal potential of an aquifer e.t.c.Therefore, in the present study, a joint application of hydro-chemistry and common environmental isotopes were applied to study the dynamics of the water composition of the aquifer in the basin in order to bridge the gap in sparse water quality data which will aid in making sound water planning decision for sustainable development.

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