African Journal of Science and Technology (AJST) Science and Engineering Series Vol. 4, No. 2, pp. 80-89 ISOTOPE AND GEOCHEMICAL CHARACTERIZATION OF SURFACE AND SUBSURFACE WATERS IN THE SEMI-ARID SOKOTO BASIN, NIGERIA S.M.A. Adelana1, P.I. Olasehinde1 and P.Vrbka2 1Department of Geology & Mineral Sciences, University of Ilorin, P.M.B. 1515, Ilorin, Kwara State, Nigeria. 2Kaupstrasse 37, Germany (formerly Geology Institute, Technical University Darmstadt, Germany). ABSTRACT:- Stable isotopes and geochemical studies have been applied in the investigation of groundwater resources in Sokoto Basin, northwestern Nigeria. Generally, the characteristic hydrochemical classification in the study area is calcium-alkali-bicarbonate. Surface waters are characterized by alkali-calcium-bicarbonate while groundwater is of Ca-Mg-HCO3. The plot of δ18O versus δ2H shows that five isotopic groups can be distinguished. Group I-III is of groundwater origin while group IV and V represent surface water. 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). Keywords: stable isotopes, geochemical characterization, groundwater, Sokoto Basin. INTRODUCTION condition by one kilometre yearly into northern Nigeria threatens agriculture. It is in the light of this that the Northwestern Nigeria is a region with great potential for Federal government of Nigeria under a joint project with future large-scale economic development due to warm the International Atomic and Energy Agency, Vienna has temperatures and bountiful resources; including thermal planned a number of irrigation schemes in order to energy, farmlands and minerals. Water resources data in increase agricultural activities in the area to two planting this area, as far back as the 60s, are available in literature seasons, thereby boosting food production. (du Preez and Barber 1965, Ogilbee and Anderson 1965, FAO 1969, MWWR 1965-1970, FMWR 1999). In the Sokoto Basin, water resources can be divided into precipitation, surface water and groundwater. Prominent However, water, being the primary restrictive factor of surface waters are the rivers Rima and Sokoto, joining economic development and exploitation of land and other close to Sokoto town, draining into river Niger and resources, makes continuous evaluation of water ultimately into the Atlantic Ocean. Intensive, small scale resources important in a semi-arid region like the Sokoto and low-cost farming is practiced in the area, yet the Basin. Moreover, infrequent and short period of rainfall impacts of the irrigation activities and the application of (<60 raining days) in most part of the basin prevents the agro-chemicals on the quality on ground or surface water development of agriculture, restricting crop production are not yet fully known. The overall aim of this study is to to only one planting season per year. Because of the evaluate and characterize water resources in the basin in semi-arid nature of most parts of the terrain, this area terms of quality as well as quantity to aid in it’s planning suffered heavily during the last Sahelian drought (1972- and management to meet the demands of the growing 1974). Furthermore, the encroachment of desert population in this area. AJST, Vol. 4, No. 2: December, 2003 76 Isotope and Geochemical Characterization of Surface and Subsurface Waters in the Semi-arid Sokoto Basin, Nigeria. DESCRIPTION OF THE STUDY AREA season having temperature range of 37-40 °C with the highest temperature in April. Diurnal temperature varies Physiography from 21-27 °C and class A evaporation ranges from <2000mm/a to 2500mm/a in the extreme north The Iullemmeden Basin of West Africa covering an (UNESCO, 1978) with potential evapotranspiration far estimated area of 700,000km2(Kogbe, 1989) extends into exceeding precipitation (see Fig. 2). As rainfall is northwestern Nigeria where it is referred to as the “Sokoto concentrated within 3-5 months, a periodic, short-lived Basin”. but strong surface runoff exists; resulting in high stream discharge. Generally, the advancement of the desert The Sokoto Basin lies in the sub-Saharan Sudan belt of southwards that is accompanying natural climatic West Africa in zone of Savanna-type vegetation generally fluctuations both make the whole of the savanna belt classified as semi-arid. It lies between latitudes 10 ° and subject to aridity (Fagoyinbo, 1990). 14 ° N and longitudes 3 ° and 7 °E and covering an area of about 65,000 square kilometres (Fig.1). The area is GEOLOGICAL AND HYDROGEOLOGICAL bounded on the north and west by Niger Republic and on SETTING the southwest by Benin Republic. Although the Sokoto Basin of Nigeria appears extensive in area extent, it only Basement rocks dominate about 50% of Nigeria’s surface represents about one-tenth of the entire Iullemmeden area while Cretaceous and Cenozoic sediments cover the Basin of West Africa (Greigert 1961). The basin broadly other 50%. In the southeastern sector of the covers an area underlain predominantly by crystalline rocks Iullemmeden Basin, i.e. Sokoto Basin, up to 2000 meters to the east and sedimentary terrain to the northwestern of clastic sequences rests upon the Basement (Zboril half. Most of the samples discussed in this publication 1984). Moreover, in the Sokoto basin sequences of semi- are within the Sokoto sedimentary Basin in the consolidated gravels, sands, clay, some limestone and northwestern corner of northern Nigeria (see fig.1). ironstone are found. According to Kogbe (1989) the sedimentary sequences are sub-divided from bottom to Geomorphologically, the area is mainly undulating, only top into the late Jurassic to early Cretaceous Illo & with depressions caused by wadis and watercourses (rivers Gundumi Formations (“Continental Intercalaire”), the Rima and Sokoto). The elevation of the ground is between Maestrichtian Rima Group (sub-divided into Taloka, 250 and 350 metres above sea level. Resistant crusts of Dukamaje and Wurno Formations), the late Paleocene laterites and ironstones characteristically cap the hills in Sokoto Group (sub-divided into Dange, Kalambaina and this area. The River Sokoto-River Rima system represents Gamba Formations) and the Eocene-Miocene Gwandu the principal drainage network in this region. Other rivers Formation (fig.3). Overlying all the formations is a like Zamfara, Gulbin Ka, and Gagere are the main laterite cap usually up to 12 metres thick in some tributaries. localities yielding water to most dugwells in the area. Climate The analysis of pumping tests data carried out in the shallow aquifer yielded transmissivities in the range of The Sokoto Basin, in northwestern Nigeria, is a semi-arid 200 to 5000 m2/d and storage coefficients of 10-2 to region marked by distinct weather conditions-the wet and 10-5 indicating semi-unconfined to confined conditions dry seasons. Within the Sokoto basin there is considerable (Bassey et al., 1999). Based on these, the hydraulic variation in rainfall distribution with mean annual rainfall conductivity varies between 10-4 to >10-3m/sec. The yield (over a period of 90 years) ranging from 350 mm at of tubewells up to 20meters depth is generally >2 L/ Kurdula in the extreme northwest and 670 mm (at Sokoto sec. The fluctuation of the water table in the fadama area Airport). The rainy season, which usually starts from May is about 2-3 meters throughout the year. The water table or June of each year, lasts till September or early October is lowest in June and highest in September at the end of depending on the rainfall pattern for that year. The average the raining season. As precipitation in the area is annual rainfall during this season (from 1916-2000) at concentrated within 3-5 months a periodic, short-lived Sokoto is presented as figure 2, usually with July and but strong surface runoff results. August as the months of heavy downpours. Between November and March is the dry harmattan period, which The headwaters of the rivers Sokoto and Rima as well as is completely without rain, but dust-laden and cold winds their tributaries take off from the pre-Cretaceous blow in from the northwest. The departure of harmattan crystalline basement terrain east of the basin and flows and the onset of rain are usually marked by a hot sunny west and south. Stream discharge has been measured on 81 AJST, Vol. 4, No.2: December, 2003 S.M.A. ADELANA Kalmalo Lake NIGER A1/7dw NIGER Kurdula A1/6dw F1/10bh F1/11hp Tangaza F1/9dw Bale A1/5dw A1/8hp F1/13rr F1/5rr F1/14bh Bacaka A1/10hp+11hp A1/9dw R i m a R Samples W. area A1/12hp A1/4dw F1/7bh F1/1- 24 A1/3r F1/8dw 13°N A2/11bh A2/8hp A3/6p A2/10hp A2/9bh A1/2dw A2/15hp Sokoto A2/16dw Yeldu A2/13hp A2/12dw A2/14bh A2/17dw A1/1hp A2/22art A2/26dw A2/18bh A2/21dw S o k o t o R . Birnin Kebbi A2/19dw A2/23bh A2/25bh A2/20dw A2/7bh A2/24bh A2/6hp Talata A2/5bh Gw andu Ma f ar a A2/1dw +2dw 12° A 2/3hp+4hp Zamfara R. Gus au Niger A3/3sr Gulbin Ka River A3/2sr A3/1nr 11° River G H A N A A3/4nr A3/5nr 10° 3.5° 4° 5° 6° E ILULLEMMEDEN BASIN 50 0 50Km NIGER BENIN Lake Chad NIGERIA N 1000 0 1000 Km Figure 1: Map of study area with sampling locations and sample numbers (Below-Map of Africa showing the Iullemmeden Basin of West Africa in dark shade) AJST, Vol. 4, No. 2: December, 2003 82 Isotope and Geochemical Characterization of Surface and Subsurface Waters in the Semi-arid Sokoto Basin, Nigeria. 400 350 300 250 h 200 mm/mont 150 100 50 0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Precipitation Pot.