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Application of environmental isotope to assess the renewability of groundwater of continental intercalaire aquifer of Sokoto Basin in Northwestern Nigeria
Groundwater for Sustainable Development 4 (2017) 35–41
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Research paper
Application of environmental isotope to assess the renewability of groundwater of continental intercalaire aquifer of Sokoto Basin in Northwestern Nigeria
MARK
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O.J. Ettea, , C.A. Okuofub, D.B. Adieb, S.B. Igborob, S.A. Alagbec, C.C. Ettehd a
Nigeria Atomic Energy Commission (NAEC), FCT, Abuja, Nigeria Department of Water Resources and Environmental Engineering, Ahmadu Bello University (ABU), Zaria, Nigeria c Department of Geology, Ahmadu Bello University, Zaria, Nigeria d Chemical Engineering Department, University of Aberdeen, Scotland, UK b
A R T I C L E I N F O
A BS T RAC T
Keywords: Water isotopes Tritium Meteoric water line D-excess Arid environment Nigeria
The southeast sector of Iullemmeden Basin is located in Nigeria and referred to as the Sokoto Basin. The aquifer system of Sokoto Basin is multilayered with Continental Intercalaire (CI) aquifer also known as Gundumi-Illo Formation at the bottom, overlain by the Rima Group, Sokoto Group and Continental Terminal (Gwandu Formation). This study is aimed at determining the validity of the statement that the Continental Intercalaire of the basin receives no recharge, through application of environmental isotopes of water molecules (3H, 2H, and 18 O) and characteristics deuterium excess (d-excess). The isotope result of oxygen-18 (δ18O) content ranges between −7.72‰ and 3.69‰, and deuterium (δ2H) content from −51‰ to 9.42‰. These results indicates presence of three categories of water, the waters depleted slightly in isotope signature, the moderately depleted waters and waters highly depleted in δ18O and δ2H. These results suggests that the waters depleted slightly in isotope signature may be evaporated waters, while moderately depleted values of 18O and 2H of water content from the Continental Intercalaire designates the modern waters and waters highly depleted in isotope signature may be regarded as paleowaters. The tritium (3H) value recorded falls within the range of 0.5–4 TU represents a mixture of pre-1952 and post-1952 water origin. It is thus concluded from the results that Continental Intercalaire aquifer of Sokoto Basin receives modern water as recharge.
1. Introduction Sokoto Sedimentary Basin is located in the south eastern part of the Iullemmeden aquifer system (Fig. 1). Groundwater is vital in Sokoto Basin where the amount of rainfall is limited to very few months of the year and surface water sources tend to dry up at peak dry season. The groundwater is accessed via dug wells. The total draft of water from dug wells in the Sokoto Sedimentary Basin is estimated to be less than 5 mgd, the individual draft per well is about 1000 gpd, (Anderson and Ogilbee, 1973). The dug wells are subject to seepage from nearby livestock and village wastes, and consequently the water is often contaminated. Sokoto Sedimentary Basin holds 34% of the population of Iullemmeden Basin of over 15million and the population is growing fast (OSS, 2008). Its groundwater resources are overexploited, considering the serious rainfall deficit from the 1980s to 2000 (GICRESAIT, 2012). Though, several irrigation programmes have been introduced in the
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Sokoto (Fadama) Basin. 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; Bell, 1961; Du Preeze and Barber, 1965; Anderson and Ogilbee, 1973; Kogbe, 1979; Oteze, 1989; Adelana et al., 2003; Alagbe, 2004; Okuofu, 2006; Sokona et al., 2008; AzTech, 2010; among many others) using mainly hydro-chemical approach. However, in Sokoto Sedimentary Basin, a greater part of Rima and Sokoto Formations are still uncovered using environmental isotope technique despite the limited application in Wurno irrigation scheme in Rima Formation. Environmental isotope hydrology is a relatively new field of investigation based on isotopic variations observed in natural waters. These isotopic characteristics have been established over a broad range and time scale. Isotopes of hydrogen and oxygen are ideal geochemical tracers of water because their concentrations are usually not subject to change by interaction with the aquifer material. They are currently referred to as the ‘DNA’ of water bodies because they can respond
Corresponding author. E-mail address: [email protected] (O.J. Ette).
http://dx.doi.org/10.1016/j.gsd.2016.11.003 Received 25 July 2016; Received in revised form 17 October 2016; Accepted 18 November 2016 Available online 25 November 2016 2352-801X/ © 2016 Elsevier B.V. All rights reserved.
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bottom to top into the late Jurassic to early Cretaceous Illo and Gundumi Formations (composed of sandstone and clay, belong to the Continental Intercalaire”), the Maestrichtian Rima Group (sub-divided into Taloka clayite, Dukamaje schist and Wurno soft sandstone Formations), the late Paleocene Sokoto Group (sub-divided into Dange pelite, Gamba and Kalambaina marine limestone which caps the Intercalary Continental of Sokoto Group) and the Eocene-Miocene Gwandu Formation (composed of sandstone and clayite formation, belong to Terminal Continental) (Fig. 4). Overlying all the formations is Dune and laterite hardpan formation of Quaternary period, yielding water to most dug wells in the area. The Sokoto Basin water resources can be divided into precipitation, surface water and groundwater. The precipitation in the area is concentrated within 3–5 months resulting in a periodic, short-lived but strong surface runoff (Adelana et al., 2003). The dry season lasts for about 7–8 months, the rainy season is short, with violent rain spells. The potential evapo-transpiration is very high, systematically higher than rainfall, except in August (AzTech, 2010). Rainfall is directly responsible for stream discharge in these areas; the groundwater contribution to stream flow is usually small. Due to the sporadic nature of rainfall and fast surface run-offs, infiltration into the groundwater system is drastically reduced. However, between Sabon Birni (about 70 km North East of Rabah village) and Sokoto town; the Rima River loses about 5.07×107 m3 of water to the ground every year (Oteze 1989; Adelana et al., 2003). This region displays a gentle relief ranging between 250 and 400 m above sea level where rainfall is irregular in the area with major interannual variability. The average rainfall is between 350 mm in the north in Kalmalo and 670 mm at the Sokoto airport. Prominent drainage systems are the Rivers Rima and Sokoto, joining close to Sokoto town, draining into River Niger and ultimately into the Atlantic Ocean. The headwaters of the Rivers Sokoto and Rima and their tributaries take off from the pre-Cretaceous crystalline basement terrain east of the basin and flows west and south. The River Niger gets about 46 million m3/ year from the CI and 79 million m3/year from the TC, for an annual total of 125 million m3. Its tributary Rima River provides about 20 million m3/year to the CI and receives about 12 million m3/year from the TC before joining the river Niger (OSS, 2008). Ground water in the Sokoto Basin is both confined as artesian water or unconfined beneath the water table, in most of the permeable members of the Cretaceous-Tertiary Sedimentary sequence. While confined water occurs downdip·and at depth in semi-consolidated sand or gravel of at least three important aquifers, in the Gundumi Formation, the Rima Group, and the Gwandu Formation. And unconfined ground water also occurs in the Quaternary alluvial. The climate is semi-arid or Sahelian.
Fig. 1. The Iullemmeden Basin showing the location of the Sokoto Basin (Yellow) (Modified from Source: OSS, 2008).
sensitively to changes in the environment and trace these changes effectively (Hui et al., 2014). They cannot be controlled by man, but can be observed and interpreted to gain valuable regional and local information on the timescale of hydrological events, identify the origin of water, renewal potential of aquifer, turnover and transit time of water in the system which often may not be readily obtained by other techniques. The Continental Intercalaire aquifer is recharged principally on its outcrop area, by infiltration from precipitation and also by effluent seepage from streams in rainy season (Anderson and Ogilbee, 1973) however other previous research work in the area had it that the aquifer system consists of huge stock of non-renewable fossil water. According to Sokona et al. (2008), the Rima River is recharging part of the alluvial aquifer in the Sokoto Basin, while the portion of the paleowater related to the Continental Intercalaire (CI) receives no recharge. Hence, this study is mainly aimed at determining the veracity of the statement (that the groundwater of CI receives no recharge) and establishing whether or not there is a renewal potential of the Continental Intercalaire aquifer system of Sokoto Basin, underlying Sokoto, Kebbi, and part of Zamfara States in Nigeria, using environmental isotope signatures of deuterium (δ2H), oxygen-18 (δ18O) and tritium (3H). 2. Study area 2.1. Location, geography and morphology The Iullemeden Aquifer System (IAS) is located in the arid and sem-arid zone of three contiguous countries in Africa: Mali, Niger and Nigeria, (Fig. 1). This IAS is locally referred to as Sokoto Sedimentary Basin in Nigeria. The study area, Sokoto Sedimentary Basin covers three fourth of the States in the north western section of the Northern Region within Nigeria which make up Sokoto Basin. The States covered includes Sokoto, Kebbi and Zamfara States. The study area falls within latitude 8°35′0″N to 4°0′0″N and longitude 3°5′00″E to 8°30′00″E and covers a geographical areal extent of 60,000 km2 (Fig. 2).
3. Materials and methods Eighty seven water samples were collected from hand dugwells, tubewells, boreholes, and surface waters such as Rivers Rima, Sokoto, Zamfara, and reservoir like Goronyo Dam in two major sampling exercises. Garmin 72H model Global Positioning System (GPS) was used to take coordinates and altitude of each location. Water samples were collected in airtight High Density Polyethylene (HDPE) containers for further laboratory analyses. One 500 ml and one 60 ml HDPE containers were used for collection of water sample from each well for isotopic analyses. While the 500 ml HDPE container was utilized for stable isotope measurement and 60 ml HDPE container was used for collection of water sample for Tritium determination. The oxygen-18, deuterium, tritium analyses were carried out at the Center National des Sciences et Technologies Nuclear (CNESTEN), Rabat, Morocco. Tritium concentrations were measured with liquid scintillation counter and expressed in Tritium Units (TU) with a precision of ± 0.5. Deuterium and Oxygen-18 were measured by infrared Laser spectro-
2.2. Geology, hydrogeology, and climatic features The Precambrian basement occupied mainly the south-eastern segment of Sokoto River Basin; it was composed of crystalline rock (granite and related stones) and metamorphic rock (gneiss, schist, quartzite) that made up its Basement Complex (Fig. 3), which was part of the craton of the West African shield. Three major fault trends are prominent in the Sokoto basin (AzTech, 2010). The Sokoto Basin also consist of up to 2000 m of classic sequences that rests upon the Basement (Zboril, 1984). Moreover, in the Sokoto basin sequences of semi-consolidated gravels, sands, clay, some limestone and ironstone are found. According to Kogbe (1989), Adelana et al. (2003) and GICRESAIT (2012) the sedimentary sequences are sub-divided from 36
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Fig. 2. Location Map of Sokoto Basin (study area).
results.
scopy using a LGR/DLT-100. The results are expressed in per mil (‰) values relative to Vienna Standard Mean Ocean water (V-SMOM) with a reproducibility of ± 1‰ for deuterium (δ2H) and ± 0.3‰ for oxygen-18 (δ18O). Water sampling and isotope measurements were carried out in collaboration with Nigeria Institute of Hydrological Services Agency (NIHSA). Paleo-climate conditions were assessed from the isotopic composition of groundwater samples collected from respective aquifers using stable isotopes of δ18O, δ2H as well as radioactive isotope of 3H using Mazor (2004) guidelines. The Local meteoric water line (LMWL) were observed from the overall data from the study area using the least-square- fit linear regression (CelleJeonatan et al., 2001) between δ18O and δ2H. Statistica v 10′ and Aqua Chem v11 applications were used for further analysis of the laboratory
4. Results and discussion The results of recharge status of Continental Intercalaire (CI) of Sokoto Basin are presented and discussed based on descriptive statistical tools and selected isotope effects. 4.1. Descriptive statistical evidence The box and whisker multiple series plot of the isotopic composition for all the representative water qualities for Gundumi-Illo Formations covering Kebbi, Zamfara and Sokoto States of Sokoto 37
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Fig. 3. Geological Section of Sokoto Basin (Source: Kogbe, 1979).
Fig. 4. Geological Map of Sokoto Sedimentary Basin, North West of Nigeria, (Source: Adelana et al., 2003; AzTech, 2010; Oteze, 1991).
-2 Maximum
-10 Maximum
75th Percentile Median 25th Percentile
-4 -5
Minimum
-6 -7
-20
2H (%o)
18O (%o)
-3
75th Percentile Median 25th Percentile
-30
Minimum
-40
-50
-8
-60
KebbiZamfara Sokoto
KebbiZamfaraSokoto
Fig. 5. Box and Whisker Multiple Plots of Stable Isotope Signatures of CI of Sokoto Basin. (The light purple rectangular box between the two whiskers at the middle contains 50% of the data. The extreme upper hinge of the box indicates the 75th percentile of the data set, and the extreme lower hinge indicates the 25th percentile while the line in the box indicates the median value of the data. If the median line within the box is not equidistant from the hinges, then the data is skewed (Samir, 2011). The extremes of the vertical lines indicate the minimum and maximum data values).
signature are those of the Gundumi-Illo Formation located in Sokoto State and most depleted are also those from Sokoto State. The wide variation in isotope composition can be interpreted as a result of interaction between palaeowaters highly depleted in deuterium signature with modern waters comparatively enriched in deuterium content.
Basin are presented in Fig. 5. It is clear from Fig. 5 that the representative water samples of Gundumi-Illo Formation in Sokoto State exhibit the highest variability in terms of data distribution and the same fact remains true using the data of δ2H. Moreover, the most enriched groundwater in deuterium 38
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Fig. 6. Bivariate plot of δ18O (%o) and depth (m) of representative water samples of CI of Sokoto Basin.
Fig. 8. Relationship between δ18O and d-excess for CI aquifer of Sokoto Basin. (Abbrevation definition: SKG- Sokoto Group, RmG-Rima Group and GuF- GundumiIllo Formation).
Similar observations were also made with respect to Oxygen-18 isotope.
considering the result of (Onugba et al., 1990).
4.2. Altitude (depth) effect
4.4. Deuterium excess factor
The relationship between depths to water level (in meters) and δ O are presented in Fig. 6. The Fig. 6 shows a relationship in which two isotopically different waters (depleted and enriched) occur at different depths within the range of 147–275 m depth. This may suggests that oxygen-18 signature of rain that recharged each formation type is affected by altitude as also reported by Nti (2005). The CI of Sokoto Basin exhibits an altitude effect according to which, an interaction occurred between a fraction of young water and the supposed paleowater up to 275 m depth. This could also account for the observed differences in stable isotopes and confirms that the aquifer receives a considerable fraction of modern water recharging the aquifer. 18
The Deuterium excess (D-excess) value can be calculated from the equation formulated by Dansgaard (1964) as: d=δ2H-8δ18O. The Dexcess value can be greater than 10‰ it can range from 14‰ to 15‰ in tropical African region (Onugba et al., 1990). In Fig. 8, the relationships between the D-excess and 18O for the study area is presented. The range of d-excess value falls between +1.8‰ and +14.39‰. This falls within expected d-excess values of +14 and +15‰ for the Africa zone (Onugba et al., 1990). The wide range in d-excess values observed reflect interaction of recent recharge waters, enriched in δ18O values derived from rainfall of low mineralization, depleted in δ18O values (Maduabuchi et al., 2006: Hoefs, 2009; and Wassenaar, et al., 2011). In accordance with the findings of Samir (2011) the low ‘d-excess’ values (≤6‰) observed in the waters of the study area suggest that there is significant evaporation of rainwater leaving the residual groundwater with lower values of ‘d-excess’. The d-excess > 10‰ indicates that recharge is sourced from mixed oceanic and continental vapor (Samir, 2011). The d-excess constituent defined by Dansgaard (1964) as earlier described in this section, 1.4.4, enables us also to relate the isotopic composition of any water sample to the meteoric water line. As such, the distribution of values has meteorological significance (Onugba et al., 1990). The d-excess values generally deviates from 10‰ (Fig. 8). For central and east Africa region dexcess is 14‰ (Dansgaard, 1964). The high d-excess values indicate that the evaporative flux from continental waters is the major contributor to the total moisture balance of the air mass (Onugba et al., 1990). In this case, high d-excess values ( > 10‰) indicate that the evaporative flux from east Africa (or Sokoto Basin) contribute more significantly to precipitation that recharges the groundwater.
4.3. Evaporation effect Salinity increases as evaporation intensifies. Fig. 7 shows the scatter plot of oxygen-18 isotope values of groundwaters against salinity concentration measured in milligram per liter (mg/L). Although a difference in salinity in groundwater of each formation of various aquifer type exists, water clusters observed have closely related isotopic signatures present. However, this is not occurring in the whole system as can be seen in some parts of the Formations as shown in (Fig. 7), where groundwater from the respective aquifer type tends to show varying isotopic signature versus either a close or wide range of salinity variations. Samir (2011) suggests that such occurrence can be described as homogenous and heterogeneous interactions of water chemistry within the aquifer formation. In line with the above, the Continental Intercalair of Sokoto Basin is characterized by a mixture of the old and new waters. However, aquifer salinization cannot be explained by evaporation mechanisms only. Recent recharging water of low or high salinity could lead to a wide range in salinity
4.5. Conventional relationship between δ18O and δD The δ18O and δ2H composition of waters has long been known to vary in a systematic manner (Rozanski et al., 1993) such that the global meteoric water line (GMWL), is described by this regression line: δ2H=8δ18O +10 (Craig, 1961) where 8 is the intercept and 10 is the slope, also referred to as standard value of D-excess on the world scale and greater than 10‰ in tropical African region. Some of the groundwater samples tapping the Continental Intercalaire aquifer plot either on or above the GMWL while a considerable number plot slightly below the GMWL (Fig. 9). The deviation of some of the samples from the GMWL suggests that evaporation occurs prior to or during infiltration, or that recharge
Fig. 7. Salinity (mg/L) versus δ18O ‰ for CI of Sokoto Basin.
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5. Conclusions The present study examined the environmental isotope signature of groundwater in the Sokoto Basin to ascertain the renewability of the Continental Intercalaire Aquifer system of the basin. This was made possible only by using mainly isotope effects factors and characteristics d-excess to certify that the CI aquifer system of Sokoto Basin receives modern recharge water. Based on the discussions above these findings were reached:
• • •
Fig. 9. Meteoric Waterline plot of δ2H against δ18O in per mil (%0), (MAP denotes amount weighted annual precipitation). 50
•
Frequency (%)
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Acknowledgements 30
The authors would like to appreciate Nigeria Atomic Energy Commission for granting her study leave to enable her embark on this research being part of her Doctor postgraduate research project of Ahmadu Bello University, Zaria, Nigeria. We appreciate the Nigeria Institute for Hydrological Services Agency (NIHSA) for their support in the field sampling exercise and isotope measurement. The ideas (upon which this article was developed) of Samir Al-Gamal (2011) are well appreciated.
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10
0 0
1
2
3
4
Tritium (T.U.)
References
Fig. 10. Tritium Concentration in groundwater samples of the in the Sokoto Basin.
represents an interaction of isotopically enriched and depleted waters (Yidana, 2013). Fig. 9 also reveals that, the majority of groundwater samples taken from Gundumi-Illo Formation, plot slightly below the global meteoric water line which can be associated with marine air mass coming from the Atlantic Ocean that eventually leads to a low slope ( < 10) and low intercept ( < 8) in the local meteoric water line, this may suggest that infiltrating rainwater were enriched in heavy isotopes prior to recharge and during the process of infiltration and percolation through the unsaturated zone to the saturated zone (Yidana, 2013). The decrease in slope value relative to standard value (10) may suggest that infiltrating rainwater were enriched in heavy isotopes prior to recharge (Carol et al., 2009; Carucci et al., 2012; Yidana, 2013). As such, this may imply that the groundwaters of CI aquifer are being recharged by modern water such that there is a mixture of young and old waters.
Adelana, S.M.A., Olasehinde, P.I., Vrbka, P., 2003. Isotope and geochemical characterization of surface and subsurface waters in the semi-arid sokoto basin, Nigeria. Afr. J. Sci. Technol. Sci. Eng. Ser. 4 (2), 80–89. Alagbe, S.A., 2004. Hydrogeology of the Kalambaina Limestone Aquifer, Sokoto Basin, Northwestern Nigeria (Unpublished Ph.D. Thesis). Ahmadu Bello University, Zaria, Nigeria. Anderson, H.R., Ogilbee, W., 1973. Aquifers in the Sokoto Basin. Geological Survey Water Supply 1757-L, pp. 79. AzTech, Environmental Impact Assessment of Zauro Polder Irrigation project. Kebbi State Nigeria. Unpubl. Report submitted to FMWR, Nigeria, 2010 Bell, J.P., 1961. The Sokoto limestone investigation, a supplementary report on the Kalambaina area: Nigeria Geol Survey Open-file Report, 1184, 10. Carol, E., Eduardo Kruse, E., Mas-Pla, J., 2009. Hydrochemical and isotopical evidence of ground water salinization processes on the coastal plain of Samborombón Bay, Argentina. J. Hydrol. 36, 335–345 〈http://www.elsevier.com/locate/jhydrol〉. Carucci, V., MarcoP., Ramon, A., 2012. Interaction between shallow and deep aquifers in the Tivoli Plain (Central Italy) enhanced by groundwater extraction: a multi-isotope approach and geochemical modeling. Appl. Geochem. 27, pp. 266–280. Available online at: 〈www.elsevier.com/locate/apgeochem〉. Celle-Jeonatan, H., Zouar, K., Travi, Y., Daoued, A., 2001. Characterization isotopique des pluies en Tunisie. Essai de typologie dans la région de Sfax. C.R. Acad. Sci. Paris Sci. Terre Planètes 333, 625–631. Craig, H., 1961. Isotopic variation in meteoric waters. Science 133 (3465), 1702–1703. Dansgaard, W., 1964. Stable isotopes in precipitation. Tellus 16, 436–467. Du Preez, J.W., Barber, W., 1965. The distribution and chemical quality of groundwater in Northern Nigeria. Geol. Surv. Niger. Bull. 36, 38–45. Falconer, J.D., 1911. The Geology and Geography of Northern Nigeria. MacMillan, London. GICRESAIT (2012). Integrated and Concerted Water Resources Management of the Aquifer Systems of Iullemmeden, Taoudéni/Tanezrouft and the Niger River Hoefs, J., 2009. Stable Isotope Geochemistry 6th ed., 36–87 , Available on〈http://www. springer.com〉. Hui, Q., Jianhua, W., Yahong, Z., Peiyue, L., 2014. Stable oxygen and hydrogen isotopes as indicators of lake water recharge and evaporation in the lakes of the Yinchuan Plain. Hydrol. Process. 28, 3554–3562. http://dx.doi.org/10.1002/hyp.9915. Jones, B., 1948. The sedimentary rocks of Sokoto Province. Niger. Geol. Surv. Bull. 180, 75. Kogbe, C.A., 1979. Geology of the Southeastern (Sokoto) sector of the Iullemmeden Basin 2. Bull, Ahmadu Bello University, Zaria, Nigeria, 1420. Kogbe, C.A., 1989. Cretaceous and Tertiary of the Iullemmeden Basin in Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Rock View Publ. Co. Jos, 377–421. Maduabuchi, C., Faye, S., Maloszewski, P., 2006. Isotope evidence of paleorecharge and paleoclimate in the deep confined aquifers of the Chad Basin, NE Nigeria. Sci. Total Environ. 370, 467–479.
4.6. Evaluation using tritium (3H) Cosmic ray neutrons interact in the upper atmosphere with nitrogen, producing 15N, which is radioactive and disintegrates into common carbon (12C) and tritium: 14
The stable and unstable isotope data of the water resources of Sokoto Basin have been determined. Continental Intercalaire aquifer system receives modern water as recharge contrary to previous claims. The study classified waters of the Basin into three different water types- Paleowater, (more depleted); Modern water (depleted) and Most enriched water (Evaporated waters) The waters of the various aquifers of Sokoto Basin are interacting.
N+n→15N→12C+3H
The tritium atoms are oxidized to water and become mixed with precipitation, and so enter the groundwater. Presented in Fig. 10 is a histogram of tritium concentration measured. The Fig. 10 obviously shows that 44% of the waters are of pre-1952 era interacting with waters of pre-1952 and post-1952 era in accordance with Mazor (2004) semi-quantitative dating interpretation.
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Rozanski, K., Araguas-Araguas, L., Gonfiantini, R., 1993. Isotopic patterns in modern global precipitation. In: Swart, P.K., Lohmann, K.L., McKenzie, J., Savin, S. (Eds.), Climate Change in Continental Isotopic Records. American Geophysical Union, 1–37. Samir, A., 2011. An assessment of recharge possibility to North-Western Sahara Aquifer System (NWSAS) using environmental isotopes. J. Hydrol. 398 (2011), 184–190 , Available from〈http://www.elsevier.com/locate/jhydrol〉. Sokona, Y., Al-Gamal, S., Dodo, A., 2008. Isotopic properties of rainfall and groundwater resources in the Iullemeden Basin, West Africa. J. Int. Assoc. Environ. Hydrol. 1 (16), 1–7 , Available from〈http://www.hydroweb.com〉. Wassenaar, L.I., Athanasopoulos, P., Hendry, M.J., 2011. Isotope hydology of precipitation, surface and groundwaters in the Okanagan valley, Britian Columbia, Canada. J. Hydrol. 411, 37–48 , Available online:〈http://www.elsevier.com/locate/ jhydrol〉. Yidana, S.M., 2013. The stable isotope characteristics of groundwater in the Voltaian Basin – an evaluation of the role of Meteoric recharge in the Basin. J. Hydrogeol. Hydrol. Eng.. http://dx.doi.org/10.4172/2325-9647.1000106. Zboril, L., 1984. Sketch Map of Oil-Bearing Structures. Geofyzika, Brno, Czechoslovakia, 94.
Mazor, E., 2004. Chemical and Isotopic Groundwater Hydrology. (3rd ed). Available from: 〈http://www.dekker.com〉. Nti, 2005. Hydrochemtcal and Isotopic Characterization of Groundwater in the Buem, Voltaian and Togo Formations of the Volta Region, Ghana (Published Master's Thesis). University of Ghana legon. Okuofu, C.A., 2006. Analysis of Transboundry Risks, Impacts in the Iullemeden (IAS). Paper Presented at the Transboundary Diagnostic Analysis Workshop for the Iullemeden Aquifer System (IAS), Abuja. Onugba, A., Blavoux, B., Dray, M., 1990. The environmental isotopes in monthly precipitation at Kano (Nigeria) from 1961–1973. In: Proceedings of the 1st Biennial National Hydrology Symposium, Maiduguri, UNESCO, pp. 67–88. OSS, 2008. Iullemeden aquifer system (MALI, NIGER, NIGERIA), Concerted management ofshared water resources of a Sahelian transboundary aquifer Tunis (2). Synthesis. Rep. Project, OSS, Tunis. Oteze, G.E., 1991. Potability of Groundwater from the Rima Group Aquifers in the Sokoto Basin, Nigeria. Journal of Min. Geol. 27 (1), 17–23. Oteze, G.E., 1989a. The hydrogeology of northwestern Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Rock View Pub. Co., Jos, 455–472. Raeburn, C., Tattam, C.M., 1930. A preliminary note on the sedimentary rocks of Sokoto Province. Niger. Geol. Surv. Bull. 13, 57–60.
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Groundwater for Sustainable Development journal homepage: www.elsevier.com/locate/gsd
Research paper
Application of environmental isotope to assess the renewability of groundwater of continental intercalaire aquifer of Sokoto Basin in Northwestern Nigeria
MARK
⁎
O.J. Ettea, , C.A. Okuofub, D.B. Adieb, S.B. Igborob, S.A. Alagbec, C.C. Ettehd a
Nigeria Atomic Energy Commission (NAEC), FCT, Abuja, Nigeria Department of Water Resources and Environmental Engineering, Ahmadu Bello University (ABU), Zaria, Nigeria c Department of Geology, Ahmadu Bello University, Zaria, Nigeria d Chemical Engineering Department, University of Aberdeen, Scotland, UK b
A R T I C L E I N F O
A BS T RAC T
Keywords: Water isotopes Tritium Meteoric water line D-excess Arid environment Nigeria
The southeast sector of Iullemmeden Basin is located in Nigeria and referred to as the Sokoto Basin. The aquifer system of Sokoto Basin is multilayered with Continental Intercalaire (CI) aquifer also known as Gundumi-Illo Formation at the bottom, overlain by the Rima Group, Sokoto Group and Continental Terminal (Gwandu Formation). This study is aimed at determining the validity of the statement that the Continental Intercalaire of the basin receives no recharge, through application of environmental isotopes of water molecules (3H, 2H, and 18 O) and characteristics deuterium excess (d-excess). The isotope result of oxygen-18 (δ18O) content ranges between −7.72‰ and 3.69‰, and deuterium (δ2H) content from −51‰ to 9.42‰. These results indicates presence of three categories of water, the waters depleted slightly in isotope signature, the moderately depleted waters and waters highly depleted in δ18O and δ2H. These results suggests that the waters depleted slightly in isotope signature may be evaporated waters, while moderately depleted values of 18O and 2H of water content from the Continental Intercalaire designates the modern waters and waters highly depleted in isotope signature may be regarded as paleowaters. The tritium (3H) value recorded falls within the range of 0.5–4 TU represents a mixture of pre-1952 and post-1952 water origin. It is thus concluded from the results that Continental Intercalaire aquifer of Sokoto Basin receives modern water as recharge.
1. Introduction Sokoto Sedimentary Basin is located in the south eastern part of the Iullemmeden aquifer system (Fig. 1). Groundwater is vital in Sokoto Basin where the amount of rainfall is limited to very few months of the year and surface water sources tend to dry up at peak dry season. The groundwater is accessed via dug wells. The total draft of water from dug wells in the Sokoto Sedimentary Basin is estimated to be less than 5 mgd, the individual draft per well is about 1000 gpd, (Anderson and Ogilbee, 1973). The dug wells are subject to seepage from nearby livestock and village wastes, and consequently the water is often contaminated. Sokoto Sedimentary Basin holds 34% of the population of Iullemmeden Basin of over 15million and the population is growing fast (OSS, 2008). Its groundwater resources are overexploited, considering the serious rainfall deficit from the 1980s to 2000 (GICRESAIT, 2012). Though, several irrigation programmes have been introduced in the
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Sokoto (Fadama) Basin. 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; Bell, 1961; Du Preeze and Barber, 1965; Anderson and Ogilbee, 1973; Kogbe, 1979; Oteze, 1989; Adelana et al., 2003; Alagbe, 2004; Okuofu, 2006; Sokona et al., 2008; AzTech, 2010; among many others) using mainly hydro-chemical approach. However, in Sokoto Sedimentary Basin, a greater part of Rima and Sokoto Formations are still uncovered using environmental isotope technique despite the limited application in Wurno irrigation scheme in Rima Formation. Environmental isotope hydrology is a relatively new field of investigation based on isotopic variations observed in natural waters. These isotopic characteristics have been established over a broad range and time scale. Isotopes of hydrogen and oxygen are ideal geochemical tracers of water because their concentrations are usually not subject to change by interaction with the aquifer material. They are currently referred to as the ‘DNA’ of water bodies because they can respond
Corresponding author. E-mail address: [email protected] (O.J. Ette).
http://dx.doi.org/10.1016/j.gsd.2016.11.003 Received 25 July 2016; Received in revised form 17 October 2016; Accepted 18 November 2016 Available online 25 November 2016 2352-801X/ © 2016 Elsevier B.V. All rights reserved.
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bottom to top into the late Jurassic to early Cretaceous Illo and Gundumi Formations (composed of sandstone and clay, belong to the Continental Intercalaire”), the Maestrichtian Rima Group (sub-divided into Taloka clayite, Dukamaje schist and Wurno soft sandstone Formations), the late Paleocene Sokoto Group (sub-divided into Dange pelite, Gamba and Kalambaina marine limestone which caps the Intercalary Continental of Sokoto Group) and the Eocene-Miocene Gwandu Formation (composed of sandstone and clayite formation, belong to Terminal Continental) (Fig. 4). Overlying all the formations is Dune and laterite hardpan formation of Quaternary period, yielding water to most dug wells in the area. The Sokoto Basin water resources can be divided into precipitation, surface water and groundwater. The precipitation in the area is concentrated within 3–5 months resulting in a periodic, short-lived but strong surface runoff (Adelana et al., 2003). The dry season lasts for about 7–8 months, the rainy season is short, with violent rain spells. The potential evapo-transpiration is very high, systematically higher than rainfall, except in August (AzTech, 2010). Rainfall is directly responsible for stream discharge in these areas; the groundwater contribution to stream flow is usually small. Due to the sporadic nature of rainfall and fast surface run-offs, infiltration into the groundwater system is drastically reduced. However, between Sabon Birni (about 70 km North East of Rabah village) and Sokoto town; the Rima River loses about 5.07×107 m3 of water to the ground every year (Oteze 1989; Adelana et al., 2003). This region displays a gentle relief ranging between 250 and 400 m above sea level where rainfall is irregular in the area with major interannual variability. The average rainfall is between 350 mm in the north in Kalmalo and 670 mm at the Sokoto airport. Prominent drainage systems are the Rivers Rima and Sokoto, joining close to Sokoto town, draining into River Niger and ultimately into the Atlantic Ocean. The headwaters of the Rivers Sokoto and Rima and their tributaries take off from the pre-Cretaceous crystalline basement terrain east of the basin and flows west and south. The River Niger gets about 46 million m3/ year from the CI and 79 million m3/year from the TC, for an annual total of 125 million m3. Its tributary Rima River provides about 20 million m3/year to the CI and receives about 12 million m3/year from the TC before joining the river Niger (OSS, 2008). Ground water in the Sokoto Basin is both confined as artesian water or unconfined beneath the water table, in most of the permeable members of the Cretaceous-Tertiary Sedimentary sequence. While confined water occurs downdip·and at depth in semi-consolidated sand or gravel of at least three important aquifers, in the Gundumi Formation, the Rima Group, and the Gwandu Formation. And unconfined ground water also occurs in the Quaternary alluvial. The climate is semi-arid or Sahelian.
Fig. 1. The Iullemmeden Basin showing the location of the Sokoto Basin (Yellow) (Modified from Source: OSS, 2008).
sensitively to changes in the environment and trace these changes effectively (Hui et al., 2014). They cannot be controlled by man, but can be observed and interpreted to gain valuable regional and local information on the timescale of hydrological events, identify the origin of water, renewal potential of aquifer, turnover and transit time of water in the system which often may not be readily obtained by other techniques. The Continental Intercalaire aquifer is recharged principally on its outcrop area, by infiltration from precipitation and also by effluent seepage from streams in rainy season (Anderson and Ogilbee, 1973) however other previous research work in the area had it that the aquifer system consists of huge stock of non-renewable fossil water. According to Sokona et al. (2008), the Rima River is recharging part of the alluvial aquifer in the Sokoto Basin, while the portion of the paleowater related to the Continental Intercalaire (CI) receives no recharge. Hence, this study is mainly aimed at determining the veracity of the statement (that the groundwater of CI receives no recharge) and establishing whether or not there is a renewal potential of the Continental Intercalaire aquifer system of Sokoto Basin, underlying Sokoto, Kebbi, and part of Zamfara States in Nigeria, using environmental isotope signatures of deuterium (δ2H), oxygen-18 (δ18O) and tritium (3H). 2. Study area 2.1. Location, geography and morphology The Iullemeden Aquifer System (IAS) is located in the arid and sem-arid zone of three contiguous countries in Africa: Mali, Niger and Nigeria, (Fig. 1). This IAS is locally referred to as Sokoto Sedimentary Basin in Nigeria. The study area, Sokoto Sedimentary Basin covers three fourth of the States in the north western section of the Northern Region within Nigeria which make up Sokoto Basin. The States covered includes Sokoto, Kebbi and Zamfara States. The study area falls within latitude 8°35′0″N to 4°0′0″N and longitude 3°5′00″E to 8°30′00″E and covers a geographical areal extent of 60,000 km2 (Fig. 2).
3. Materials and methods Eighty seven water samples were collected from hand dugwells, tubewells, boreholes, and surface waters such as Rivers Rima, Sokoto, Zamfara, and reservoir like Goronyo Dam in two major sampling exercises. Garmin 72H model Global Positioning System (GPS) was used to take coordinates and altitude of each location. Water samples were collected in airtight High Density Polyethylene (HDPE) containers for further laboratory analyses. One 500 ml and one 60 ml HDPE containers were used for collection of water sample from each well for isotopic analyses. While the 500 ml HDPE container was utilized for stable isotope measurement and 60 ml HDPE container was used for collection of water sample for Tritium determination. The oxygen-18, deuterium, tritium analyses were carried out at the Center National des Sciences et Technologies Nuclear (CNESTEN), Rabat, Morocco. Tritium concentrations were measured with liquid scintillation counter and expressed in Tritium Units (TU) with a precision of ± 0.5. Deuterium and Oxygen-18 were measured by infrared Laser spectro-
2.2. Geology, hydrogeology, and climatic features The Precambrian basement occupied mainly the south-eastern segment of Sokoto River Basin; it was composed of crystalline rock (granite and related stones) and metamorphic rock (gneiss, schist, quartzite) that made up its Basement Complex (Fig. 3), which was part of the craton of the West African shield. Three major fault trends are prominent in the Sokoto basin (AzTech, 2010). The Sokoto Basin also consist of up to 2000 m of classic sequences that rests upon the Basement (Zboril, 1984). Moreover, in the Sokoto basin sequences of semi-consolidated gravels, sands, clay, some limestone and ironstone are found. According to Kogbe (1989), Adelana et al. (2003) and GICRESAIT (2012) the sedimentary sequences are sub-divided from 36
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Fig. 2. Location Map of Sokoto Basin (study area).
results.
scopy using a LGR/DLT-100. The results are expressed in per mil (‰) values relative to Vienna Standard Mean Ocean water (V-SMOM) with a reproducibility of ± 1‰ for deuterium (δ2H) and ± 0.3‰ for oxygen-18 (δ18O). Water sampling and isotope measurements were carried out in collaboration with Nigeria Institute of Hydrological Services Agency (NIHSA). Paleo-climate conditions were assessed from the isotopic composition of groundwater samples collected from respective aquifers using stable isotopes of δ18O, δ2H as well as radioactive isotope of 3H using Mazor (2004) guidelines. The Local meteoric water line (LMWL) were observed from the overall data from the study area using the least-square- fit linear regression (CelleJeonatan et al., 2001) between δ18O and δ2H. Statistica v 10′ and Aqua Chem v11 applications were used for further analysis of the laboratory
4. Results and discussion The results of recharge status of Continental Intercalaire (CI) of Sokoto Basin are presented and discussed based on descriptive statistical tools and selected isotope effects. 4.1. Descriptive statistical evidence The box and whisker multiple series plot of the isotopic composition for all the representative water qualities for Gundumi-Illo Formations covering Kebbi, Zamfara and Sokoto States of Sokoto 37
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Fig. 3. Geological Section of Sokoto Basin (Source: Kogbe, 1979).
Fig. 4. Geological Map of Sokoto Sedimentary Basin, North West of Nigeria, (Source: Adelana et al., 2003; AzTech, 2010; Oteze, 1991).
-2 Maximum
-10 Maximum
75th Percentile Median 25th Percentile
-4 -5
Minimum
-6 -7
-20
2H (%o)
18O (%o)
-3
75th Percentile Median 25th Percentile
-30
Minimum
-40
-50
-8
-60
KebbiZamfara Sokoto
KebbiZamfaraSokoto
Fig. 5. Box and Whisker Multiple Plots of Stable Isotope Signatures of CI of Sokoto Basin. (The light purple rectangular box between the two whiskers at the middle contains 50% of the data. The extreme upper hinge of the box indicates the 75th percentile of the data set, and the extreme lower hinge indicates the 25th percentile while the line in the box indicates the median value of the data. If the median line within the box is not equidistant from the hinges, then the data is skewed (Samir, 2011). The extremes of the vertical lines indicate the minimum and maximum data values).
signature are those of the Gundumi-Illo Formation located in Sokoto State and most depleted are also those from Sokoto State. The wide variation in isotope composition can be interpreted as a result of interaction between palaeowaters highly depleted in deuterium signature with modern waters comparatively enriched in deuterium content.
Basin are presented in Fig. 5. It is clear from Fig. 5 that the representative water samples of Gundumi-Illo Formation in Sokoto State exhibit the highest variability in terms of data distribution and the same fact remains true using the data of δ2H. Moreover, the most enriched groundwater in deuterium 38
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Fig. 6. Bivariate plot of δ18O (%o) and depth (m) of representative water samples of CI of Sokoto Basin.
Fig. 8. Relationship between δ18O and d-excess for CI aquifer of Sokoto Basin. (Abbrevation definition: SKG- Sokoto Group, RmG-Rima Group and GuF- GundumiIllo Formation).
Similar observations were also made with respect to Oxygen-18 isotope.
considering the result of (Onugba et al., 1990).
4.2. Altitude (depth) effect
4.4. Deuterium excess factor
The relationship between depths to water level (in meters) and δ O are presented in Fig. 6. The Fig. 6 shows a relationship in which two isotopically different waters (depleted and enriched) occur at different depths within the range of 147–275 m depth. This may suggests that oxygen-18 signature of rain that recharged each formation type is affected by altitude as also reported by Nti (2005). The CI of Sokoto Basin exhibits an altitude effect according to which, an interaction occurred between a fraction of young water and the supposed paleowater up to 275 m depth. This could also account for the observed differences in stable isotopes and confirms that the aquifer receives a considerable fraction of modern water recharging the aquifer. 18
The Deuterium excess (D-excess) value can be calculated from the equation formulated by Dansgaard (1964) as: d=δ2H-8δ18O. The Dexcess value can be greater than 10‰ it can range from 14‰ to 15‰ in tropical African region (Onugba et al., 1990). In Fig. 8, the relationships between the D-excess and 18O for the study area is presented. The range of d-excess value falls between +1.8‰ and +14.39‰. This falls within expected d-excess values of +14 and +15‰ for the Africa zone (Onugba et al., 1990). The wide range in d-excess values observed reflect interaction of recent recharge waters, enriched in δ18O values derived from rainfall of low mineralization, depleted in δ18O values (Maduabuchi et al., 2006: Hoefs, 2009; and Wassenaar, et al., 2011). In accordance with the findings of Samir (2011) the low ‘d-excess’ values (≤6‰) observed in the waters of the study area suggest that there is significant evaporation of rainwater leaving the residual groundwater with lower values of ‘d-excess’. The d-excess > 10‰ indicates that recharge is sourced from mixed oceanic and continental vapor (Samir, 2011). The d-excess constituent defined by Dansgaard (1964) as earlier described in this section, 1.4.4, enables us also to relate the isotopic composition of any water sample to the meteoric water line. As such, the distribution of values has meteorological significance (Onugba et al., 1990). The d-excess values generally deviates from 10‰ (Fig. 8). For central and east Africa region dexcess is 14‰ (Dansgaard, 1964). The high d-excess values indicate that the evaporative flux from continental waters is the major contributor to the total moisture balance of the air mass (Onugba et al., 1990). In this case, high d-excess values ( > 10‰) indicate that the evaporative flux from east Africa (or Sokoto Basin) contribute more significantly to precipitation that recharges the groundwater.
4.3. Evaporation effect Salinity increases as evaporation intensifies. Fig. 7 shows the scatter plot of oxygen-18 isotope values of groundwaters against salinity concentration measured in milligram per liter (mg/L). Although a difference in salinity in groundwater of each formation of various aquifer type exists, water clusters observed have closely related isotopic signatures present. However, this is not occurring in the whole system as can be seen in some parts of the Formations as shown in (Fig. 7), where groundwater from the respective aquifer type tends to show varying isotopic signature versus either a close or wide range of salinity variations. Samir (2011) suggests that such occurrence can be described as homogenous and heterogeneous interactions of water chemistry within the aquifer formation. In line with the above, the Continental Intercalair of Sokoto Basin is characterized by a mixture of the old and new waters. However, aquifer salinization cannot be explained by evaporation mechanisms only. Recent recharging water of low or high salinity could lead to a wide range in salinity
4.5. Conventional relationship between δ18O and δD The δ18O and δ2H composition of waters has long been known to vary in a systematic manner (Rozanski et al., 1993) such that the global meteoric water line (GMWL), is described by this regression line: δ2H=8δ18O +10 (Craig, 1961) where 8 is the intercept and 10 is the slope, also referred to as standard value of D-excess on the world scale and greater than 10‰ in tropical African region. Some of the groundwater samples tapping the Continental Intercalaire aquifer plot either on or above the GMWL while a considerable number plot slightly below the GMWL (Fig. 9). The deviation of some of the samples from the GMWL suggests that evaporation occurs prior to or during infiltration, or that recharge
Fig. 7. Salinity (mg/L) versus δ18O ‰ for CI of Sokoto Basin.
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5. Conclusions The present study examined the environmental isotope signature of groundwater in the Sokoto Basin to ascertain the renewability of the Continental Intercalaire Aquifer system of the basin. This was made possible only by using mainly isotope effects factors and characteristics d-excess to certify that the CI aquifer system of Sokoto Basin receives modern recharge water. Based on the discussions above these findings were reached:
• • •
Fig. 9. Meteoric Waterline plot of δ2H against δ18O in per mil (%0), (MAP denotes amount weighted annual precipitation). 50
•
Frequency (%)
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Acknowledgements 30
The authors would like to appreciate Nigeria Atomic Energy Commission for granting her study leave to enable her embark on this research being part of her Doctor postgraduate research project of Ahmadu Bello University, Zaria, Nigeria. We appreciate the Nigeria Institute for Hydrological Services Agency (NIHSA) for their support in the field sampling exercise and isotope measurement. The ideas (upon which this article was developed) of Samir Al-Gamal (2011) are well appreciated.
20
10
0 0
1
2
3
4
Tritium (T.U.)
References
Fig. 10. Tritium Concentration in groundwater samples of the in the Sokoto Basin.
represents an interaction of isotopically enriched and depleted waters (Yidana, 2013). Fig. 9 also reveals that, the majority of groundwater samples taken from Gundumi-Illo Formation, plot slightly below the global meteoric water line which can be associated with marine air mass coming from the Atlantic Ocean that eventually leads to a low slope ( < 10) and low intercept ( < 8) in the local meteoric water line, this may suggest that infiltrating rainwater were enriched in heavy isotopes prior to recharge and during the process of infiltration and percolation through the unsaturated zone to the saturated zone (Yidana, 2013). The decrease in slope value relative to standard value (10) may suggest that infiltrating rainwater were enriched in heavy isotopes prior to recharge (Carol et al., 2009; Carucci et al., 2012; Yidana, 2013). As such, this may imply that the groundwaters of CI aquifer are being recharged by modern water such that there is a mixture of young and old waters.
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4.6. Evaluation using tritium (3H) Cosmic ray neutrons interact in the upper atmosphere with nitrogen, producing 15N, which is radioactive and disintegrates into common carbon (12C) and tritium: 14
The stable and unstable isotope data of the water resources of Sokoto Basin have been determined. Continental Intercalaire aquifer system receives modern water as recharge contrary to previous claims. The study classified waters of the Basin into three different water types- Paleowater, (more depleted); Modern water (depleted) and Most enriched water (Evaporated waters) The waters of the various aquifers of Sokoto Basin are interacting.
N+n→15N→12C+3H
The tritium atoms are oxidized to water and become mixed with precipitation, and so enter the groundwater. Presented in Fig. 10 is a histogram of tritium concentration measured. The Fig. 10 obviously shows that 44% of the waters are of pre-1952 era interacting with waters of pre-1952 and post-1952 era in accordance with Mazor (2004) semi-quantitative dating interpretation.
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Mazor, E., 2004. Chemical and Isotopic Groundwater Hydrology. (3rd ed). Available from: 〈http://www.dekker.com〉. Nti, 2005. Hydrochemtcal and Isotopic Characterization of Groundwater in the Buem, Voltaian and Togo Formations of the Volta Region, Ghana (Published Master's Thesis). University of Ghana legon. Okuofu, C.A., 2006. Analysis of Transboundry Risks, Impacts in the Iullemeden (IAS). Paper Presented at the Transboundary Diagnostic Analysis Workshop for the Iullemeden Aquifer System (IAS), Abuja. Onugba, A., Blavoux, B., Dray, M., 1990. The environmental isotopes in monthly precipitation at Kano (Nigeria) from 1961–1973. In: Proceedings of the 1st Biennial National Hydrology Symposium, Maiduguri, UNESCO, pp. 67–88. OSS, 2008. Iullemeden aquifer system (MALI, NIGER, NIGERIA), Concerted management ofshared water resources of a Sahelian transboundary aquifer Tunis (2). Synthesis. Rep. Project, OSS, Tunis. Oteze, G.E., 1991. Potability of Groundwater from the Rima Group Aquifers in the Sokoto Basin, Nigeria. Journal of Min. Geol. 27 (1), 17–23. Oteze, G.E., 1989a. The hydrogeology of northwestern Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Rock View Pub. Co., Jos, 455–472. Raeburn, C., Tattam, C.M., 1930. A preliminary note on the sedimentary rocks of Sokoto Province. Niger. Geol. Surv. Bull. 13, 57–60.
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