ABSTRACT
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.
CHAPTER ONE
1.0 INTRODUCTION
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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