Geochemical Characteristics and Quality of Groundwater Around Okemesi Fold Belt , South Western Nigeria

This study involves the determination of the hydrochemical character and quality of springs, shallow and deep wells around the Okemesi fold belt, south western Nigeria. This is with a view to elucidate their nature, type and evaluate portability, and suitability for agricultural and industrial purposes. The Okemesi fold belt is underlain by schistose rocks, mainly mica schist, quartz schist, and quartzite with minor amphibolites and gneisses. The major hydrochemical water type are Na – ( K ) – Cl – ( SO4 ) and Ca – (Mg) – ( Cl ) – SO4 water facies constituting about 52% and 47% respectively. The study showed that almost all of the physico-chemical parameters such as total dissolved solid TDS (18.75 790.50 mg/l), electrical conductivity EC (25 1054 μs/cm), pH (5.4 7.4), temperature (24.8 29.5°C), turbidity (2.2 40.5 N.T.U), total hardness (10 274 mg/l), total alkalinity (18 274 mg/l), dissolved oxygen (1.8 7 mg/l), chlorine demand (1.56 4.75 mg/l); and bacteriological analysis (5 80 MPN) results were within the World Health Organization (WHO) limits recommended for drinking water. However, some groundwater samples have Ni and K concentrations slightly above the recommended standard. These physico-chemical parameters, especially cations: Ca (1.6 72.8 mg/l), Mg (0 4.39 mg/l), Na (1.63 75.0 mg/l), K (0 108.3 mg/l) , Si (4.0 10 mg/l), Cu (0 0.391 mg/l), Zn (0 0.29 mg/l), Cd (0 0 mg/l), Pb (0 0 mg/l), Fe (0.1 0.1 mg/l), Ni (0 0.043 mg/l) and anions such as HCO3 (6.1 79.3 mg/l), Cl (5 109 mg/l), SO4 (38 76 mg/l), NO3 (0 0 mg/l) and PO4 (0.005 0.03 mg/l) seem to reflect the chemical nature of the underlying rock units suggesting dissolution, weathering and water – soil / rock interaction processes.


Introduction
As water percolates through the rocks, some ionic exchanges take place and invariably the water takes into solution in different concentrations some elemental composition of the rocks.(Abimbola et al., 2002) Therefore, the chemical property, and also bacteriological composition of groundwater is important amongst other criteria in the assessment of the quality of waters.While it is generally accepted that water bodies, rivers, lakes, dams and estuaries are continuously subject to dynamic state of change with respect to the geological age and geochemical characteristics (Adefemi & Awokunmi, 2010), the availability of good quality water derived from these sources is an indispensable feature for preventing diseases and improving quality of life (Oluduro & Adeyowe, 2007).It is now generally recognized that the quality of water is just as important as its quantity (Fan & Steinberg, 1996;Gbodo & Ogunyemi, 1999;Abimbola et al., 2002Abimbola et al., , 2008;;Tredoux et al., 2001;Adelana & Olasehinde, 2003;Adeyemi et al., 2003).Also apart from the fact that water quality is a function of physical, chemical and biological parameters, it is also subjective, as it depends on the particular intended use (Tijani, 1994).Therefore, water quality standards differ for various uses such as domestic (drinking), agriculture (irrigation) and industries.
The most significant geo hydraulic influence on groundwater chemistry arises from the source and circulation of groundwater itself (Amadi et al., 1989).Natural water contains some types of impurities whose nature and amount vary with source of water (Adeyemi et al., 2003).Metals for example, are introduced into aquatic system through several ways which include, weathering of rocks and leaching of soils, dissolution of aerosol particles from the atmosphere and from several human activities, including mining, processing and the use of metal based materials (Ipinmoroti, 1993;Adeyeye, 1994;Asaolu et al., 1997).
There has been some studies on rock composition, structure and association around Okemesi, southwestern Nigeria (Okunlola & Okoroafor, 2009;Ayodele & Odeyemi, 2010;) but little or none has been carried out on the quality of water in this region Hence the need for this study, which is aimed at elucidating the nature, origin and uses of the springs, shallow and deep well water in the study area..This involves the determination of the the physical and chemical properties together with bacteriological load in the groundwater.
The petrogenetic features of the schistose rocks around the Okemesi fold belt area of investigation, which is part of the Ife-Ilesha schist belt has been described as one of the most complex lithological and structural frameworks amongst the Nigeria's metasedimentary belts (Olobaniyi, 2003;Okunlola & Okoroafor, 2009).
Okemesi is located in areas of high relief, (Figure 1) as a result, runoff is high and infiltration rates are low.Hence, some of the hand dug wells and boreholes are concentrated at the center of the fold belt, while the rivers and the springs follow a dendritic pattern (Figure 1).The area of investigation lies completely within the 1: 50,000 sheet 253 (Ilesha NE) between latitude 7 0 49'-7 0 52' and longitude 4°54'-4°56', with the elevation ranging from 318 -484m above the sea level.Okemesi is the largest town in the area.Other settlements in this area include Ajindo, Koro and Ada.The area is a planned settlement in spite of its rugged relief (Figure 1) and well serviced by a network of roads and footpaths (Figure 2).A dendritic drainage system, upon which a trellis pattern has been super imposed mainly due to structural configuration characterize the area.Few large rivers and consequent streams that have their source from the nearby springs, together with deep and shallow wells present, are perennial, while small streams are seasonal with little or no meandering system (Figure 3).They flow westerly and a few northerly.The springs flow in a radial manner Geomorphologically, the area forms an oval topographic feature with enclosing schistose and the surrounding gneissic complex (Figure 4).The range is broken in places by rivers, like Osun and Ada which have cut steep valleys and run almost at right angles to the trend of the range.The climate is typically equatorial, hot, dry and wet with mean monthly temperature around 27°C (80°F).Cloudiness and heavy precipitation help to moderate the daily temperature.It experiences about seven months (April-November) of torrential rain and associated high runoff and erosion, while December to March are often characterized by dry conditions.

Geological Setting:
Geologically, the study area is around the Okemesi fold belt of Ife -Ilesha schist belt within the Pre -Cambrian to Late Proterozoic Basement Complex rocks of southwestern Nigeria which in turn is part of the Pan -African mobile belt lying east of West African Craton (Jones & Hockey, 1964;Odeyemi, 1976;Rahaman, 1976).The major rock units in the area of investigation are ampibolite, banded gneiss, mica schist, quartz schist and quartzite (Okunlola & Okoroafor, 2009) (Figure 4).The details of the geological setting are contained in the aforementioned reference.
The basement rocks are commonly considered as poor aquifers because of their crystalline nature which leads to low porosity and permeability.However, appreciable porosity and permeability are developed through fracturing and weathering of the rocks (Davis & Deweist, 1966).This makes an otherwise barren rock to function as groundwater aquifer (Abimbola et al., 2002).The rocks in the area of investigation have undergone different tectonic episodes and have as a result responded differently to tectonic deformation that has affected the terrain, hence the numbers and sizes of lineaments (fractures) (Ayodele & Odeyemi, 2010) giving rise to the springs.

Materials and Methods
A total of twenty one representative samples of surface and groundwater were collected around Okemesi, Koro, Ajindo and Ada.Comprising 2 springs, 10 hand dug wells, 5 boreholes and 4 rivers which constitute the major water supply for the inhabitant.Sampling was done before the onset of heavy rain to minimize dilution effect.At each location, an aliquot of water to be sampled was used to rinse the plastic container to avoid contamination before collecting the water into 2 well drained separate clean new set of plastic containers or bottles, one for the determination of anions while the other set of samples were for cations and these were acidified immediately with two drops of HNO 3 in order to prevent loss of metals, bacterial and fungal growth.The first set was also stored in the refrigerator before bacteriological analysis.
The electrical conductivity, pH and temperature as a sensitive physico-chemical parameters were determined directly in the field using digital Hack pH meter while the location coordinates and elevation were determined by Ground Positioning System (GPS).Turbidity was determined by hack turbidity meter.Total hardness, total alkalinity, chlorine demand, dissolved oxygen, NO 3 2-and SO 4 2-Ca 2+ , Mg 2+ , Fe 3+ , HCO 3 and silica were determined by titrimetry method at the Central Laboratory, Water Corporation of Oyo State, Nigeria.The laboratory method used for the detection and enumeration of total coliform count is the multiple tube method.
Cations such as K + and Na + together with the trace elements such as Cu, Zn, Cd, Pb, and Ni were analyzed using the Atomic Absorption Spectrometer (Bulk scientific 210/211 VGP AAS), 220GF Graphite Furnace and 220AS Autosampler at the Agronomy laboratory, University of Ibadan, Oyo state, Nigeria while PO 3 2-was analyzed by Ascorbic Acid method before determining its content in the solution by Spectrophotometer 70 at 882mμ.The detection limits of the metals are 0.001mg/l while for K + and Na + is 0.01mg/l

Results and Discussion
The data for physical, chemical and bacteriological parameters of both groundwater and surface water in the study area are presented in the Tables 1, 2, 3 and 4 respectively.The mean concentration of the cations is in the order Ca 2+ > K + > Na + >Mg 2+ and the mean concentration of anions is in the order SO 4 2-> Cl -> HCO 3 > Silica > PO 4 3-> NO 3 2-while the trace metals mean concentration is in the order of Fe > Cu > Zn> Ni > Cd > Pb and that of coliform mean concentration is 24ppm.Although all the analyzed parameters have concentration values less than the World Health Organization (WHO, 2004) recommended limits, K + in locations WL6 (16.20mg/l),WL13 (25.40mg/l),BL17 (23.6mg/l),BL19 (108.3mg/l),WL20 (48.2mg/l),WL23 (86.6mg/l), and WL25 (46.6mg/l) were observed to have the concentration value above WHO standard recommended limit (Table 5).These are all groundwater.Generally the concentration of calcium is much higher in the deep wells than the surface waters.for instance, sample BL17, BL19,WL20, and WL25 have calcium concentration of 72.8mg/l, 36.8mg/l,64.0mg/l, and 48.0mg/l respectively while RL1, RL3, SPL9 and SPL14 have calcium concentration 4.8 mg/l, 9.6 mg/l, 4.0 mg/l, and 1.6 mg/l respectively.Above 50,000 -In this study, the highest concentration of Ca 2+ is found in borehole location BL17 at Iro, Okemesi having value of 72.8 mg/l.This may be attributed to dissolution of the water with the weathered calc-plagioclase feldspar, amphibole and pyroxene of the aquiferous zone by the groundwater.However, Mg 2+ concentration in all the location is very low with mean value of 0.84 mg/l when compared to WHO highest desirable level of 39 mg/l for drinking water and could be attributed to generally lower dissolution of magnesium in water.Its values range from 0.146 mg/l to 2.245 mg/l in most places except in location RL5 at river Koro where the concentration is 4.390 mg/l.This river also has the highest concentration of Ca 2+ of 28.8 mg/l for surface water.This may have incorporated the elemental component of the amphibolite in this area that the river passes through (Table 7).Generally, the concentration of K + and Na + varies for surface and groundwater.The highest K + and Na + concentration value of 108.3 mg/l and 75 mg/l respectively is found in borehole BL19 at Odofin while the lowest values of 0.00 and 1.63 for the two cations are found respectively at Ikanwo hills at location SPL9.This can be attributed to difference in the rock types and elevations of these two locations.While Odofin rock is banded gneiss with elevation of 414m, Ikanwo hills is quartzitic in composition with elevation of 484m above the sea level (Table 7).Hence, the borehole must have incorporated some minerals such as albite and orthoclase in the gneiss.The chloride concentration seems to have relationship with sodium and potassium in the area of investigation.This is because high concentration of chloride ion is found in area of high K + and Na + .For example, sample BL19 with abnormal Cl -, K + and Na + concentration values of 109 mg/l, 108.3 mg/l, and 75 mg/l, may have important geological control.
The source of HCO 3 with mean concentration of 23.53 mg/l can be attributed to dissolution of carbon dioxide of the air and organic matter.The lower concentration or absence of SO 4 2-, PO 4 3-, NO 3 2-, Fe, Cu, Zn, Cd and Pb in all the locations could be attributed to absent of industrial discharges, sewage discharge, landfill leachates and ore bodies.However, Ni concentration value is slightly above the maximum recommended value by WHO in 3 wells in location WL20, WL15 and WL4.Elevated trace metals concentrations in soils and shallow groundwater systems have been attributed to anthropogenic sources through agricultural practices (Abimbola et al., 1999: Mapanda et al., 2005;Tijani et al., 2009;Tijani, 2010) the possibility of geogenic contribution is high considering the presence of a basic rock unit associated with the gneisses in the area.The relatively higher electrical conductivity with a range of 50μs/cm to1054μs/cm in the groundwater compared to a range of 25 μs/cm to 535μs/cm in the surface water may also be due to increased water rock reaction and dissolution of solid components of the bed rock.However majority of the values are still within those for unpolluted waters Statistical correlation using product moment coefficient and scattered plot indicates positive correlation between some pairs of parameters (Tables 8 & 9) and.Correlation studies show that calcium has positive correlation with cations: Mg (r = 0.584), Na (r = 0.633), K (r = 0.481) and some heavy metals: Zn (0.342) and Ni (0.505) indicating common source but show negative correlation with Cu (r = -0.099)suggesting different source.Also, potassium correlates positively   Ca -(Mg) -(Cl) -SO 4 water facies constitute about 47.62% of the total water sample and is also referred to as normal alkaline earth with predominance of sulphate.This water suggests mixing process attributed to its interaction by dilution with weathered rock (Figure 5).It may also be due to anthropogenic input from inhabitant in the study area .Na -( k ) -( SO 4 ) -Cl water facies constitute about 52.38% of the total surface and groundwater samples and as a result is predominant water type in the study area.This refers to as alkaline water with high concentration of sodium and potassium.
This water type is common in groundwater in the area and likely to have originated from the dissolution of weathered gneissic rocks and schist, that is geogenic in origin.4), the water within the study area can be characterized as Ca -(Mg) -Na -(K) -(SO 4 ) -Cl water type because it constitutes about 99% of the total water types in the area.This is similar to the Piper characterization and is due to high percentage of chloride and sulphide ions in the water body, which is higher when compared to other ions within the study area.measure of the sodium hazard for irrigation is the SAR which is used to express reactions with the soil (Sadashivaiah et al., 2008).The classification of water samples from the study area with respect to SAR using Wilcox model is represented in Table 11.The SAR value of all the samples are found to be less than 10, and are classified as excellent for irrigation except in sample BL19 where SAR and electrical conductivity values are 12.22 and 1054 μs/cm respectively.This sample is within the permissible class and medium salinity hazard level.In general, all the water samples analysed are good for agricultural and irrigation purposes.However, other conditions necessary for plant growth such as amount of water, good soil type and favourable climate have to be met Also most of the samples (67%) have total hardness within 0 -60mg/l,implying they are soft while while the rest are moderately hard (Table 12)

Conclusions
This study showed that most of the hydrochemical parameters in the groundwater around Okemesi are within the WHO limits for drinking water, posing no health threat to consumers.However, the levels of potassium, especially in some groundwater samples especially those of WL6 (16.20mg/l),WL13 (25.4 mg/l), BL17 (108.3 mg/l), WL20 (48.2 mg/l), WL23 (86.6 mg/l) and WL25 (46.6 mg/l), and, possibly, all the other areas such as WL4 (0.043mg/l), WL15 (0.033) and WL20 (0.033) where nickel levels exceeded the critical value of 0.02mg/l, should be closely monitored.Geological processes such as weathering and dilution of the water with rocks in this area have great impact on the quality and characteristics of the groundwater.Also a shallow well at WL 2 from Oke-Onire is found to have total coli form count of 80MPN, suggestive of heavy impact of human activity and is due to unnatural agents such as leakage of wastewaters and soak away.
Two major hydrochemical water facies were identified.These are Ca -(Mg) -(Cl) -SO 4 and Na -(K) -Cl -(SO 4 ) water types.Forty eight per cent of the water sources are mixed water type having either HCO 3 , SO 4 2-or Cl -ions as the main anions predominating (Table 11).
Generally, the groundwater quality in this area is appropriate for drinking and agricultural purposes, except in few locations highlighted.

Figure 1 .
Figure 1.Map of sampling locations of surface and sub surface waters around Okemesi range of schistose outcrops of Okemesi fold belt.Specifically, rivers like Oruro, Eleyinmi and Koro have utilized fractures in the area and cut across the divide.

Figure 4 .
Figure 4. Scholler diagram showing chemical ions in the study area

Table 1 .
The result of physical characteristics of surface water and groundwater around Okemesi SAR-sodium absorption ratio TA-toltal alkalinity

Table 2 .
The result of chemical characterstics (cations and trace metals) of surface water and groundwater around okemesi

Table 3 .
The result of chemical characterstics (anions) of surface water and groundwater around okemesi The summary of the various parameters, their mean values as compared to the values ofWHO (2004)standards is shown in table 5, while the result of the bacteriological analysis as compared with World Health Organization (W.H.O) 2004 Bacteriological Standard for Drinking Water is in Table6.

Table 5 .
Summary of the physical and chemical parameters of waters around okemesi compared withWHO (2004)standard

Table 7 .
The statistical summary of average elemental composition of water sample control by rock type around

Table 8 .
Correlation coefficients for the physico-chemical parameters around Okemesi

Table 9 .
Correlations between some of the hydrochemical parameters around Okemesi

Table 10 .
Characterization of groundwater and surface water around Okemesi on the basis of Piper tri-linear diagram