Hydro-geochemical evaluation of groundwater quality in Akoko North West local government area of Ondo State , Nigeria

A sudden geometric increase in population of Akoko North West Local Government Area of Ondo State has led to an increase in demand for water and harnessing of subsurface water reserve. A total of twenty six water samples obtained from both boreholes and hand-dug wells were analyzed for their physico-chemical characteristics with the aim of assessing their quality, usability and also to determine the level of their contamination in the local government which is dominated by granite gneisses, charnockites, and augen gneisses as the main rock types. The following physico-chemical properties were analyzed for in the samples collected: electrical conductivity (EC), pH, total alkalinity (TA), total dissolved solids (TDS), total suspended solids (TSS), acid neutralizing capacity (ANC) with major cations (Na + , Mg 2+ , Ca 2+ ), anions (PO4 3, HCO3 , SO4 3) and several heavy metals (Zn 2+ , Fe 2+ , As + , Ni + , Pb 2+ , Cd + , Cu 2+ ). The following results were obtained from the analysis: Zn 2+ (0.109 – 0.437 ppm), Fe 2+ (0.216 – 0.726 ppm), As + (0.01 – 0.03 ppm), Ni + (0.036 – 0.074), Pb 2+ (0.003 – 0.010), Cd + (0.002 – 0.007), Cu 2+ (0.086 – 0.241) with appreciable abundances of PO4 3, Cd + , Fe 2+ and Ni + some exceeding while some others are below the WHO acceptable level for drinking water, thereby making it a serious source of concern as it indicates varying levels of faecal contamination (from leaky septic tanks, interception with pit latrines) and heavy metal liberation into the water consumed by the residents. It was also noticed that the water was fresh based on the analysis of the Total Dissolved Solids and the Pb 2+ concentrations almost exceeded the standard permissible limit. Generally, it was concluded that the overall quality of the ground water is averagely good physico-chemically.


INTRODUCTION
Water is a very important component of the earthly environment.Throughout the history of man, water has always been sustaining life and serving communities.The importance of the quality of available water cannot, however, be over-emphasised.As far as Nigeria is concerned, there is abundant of surface and groundwater resources, particularly in the South-Western region which is entirely within the tropical rainforest zone (Obatoyinbo and Oyedotun, 2011).Rijswijk (1981) estimated the groundwater resources at 0 -50m depth in Nigeria to be 6 x 10 6 km 3 (6 x 10 6 m 3 ).However, from the eight mega regional aquifers in Nigeria which Akujieze et al., (2003) estimated, the deposit of the total groundwater yields additional groundwater resources of 7.2 times Rijswijk's figure, the total of which is estimated to be 50 million trillion l/year (Akujieze et al., 2003).Earlier, Hanidu (1990) has estimated the surface water resources in Nigeria to be 224 trillion l/year.Hence, with the available surface water resources of 224 trillion litre per year (l/year) and about 50 million trillion l/year as groundwater resources, there is an assured water abundance in Nigeria.Any shortfall in meeting the rising population needs is principally due to harnessing, distribution, delivery and quality (Hanidu, 1990;Akujieze et al., 2003).
Through the International Drinking Water Supply and Sanitation Decade (1980 -1990), the NPC (2006) recommended and proposed water for all by the year 2000.For this to be achieved, Nigeria launched a National Borehole Programme, which included 760 boreholes with only 228 (30%) being productive (Akujieze et al., 2003).The failure of this project may be due to poor knowledge of groundwater disposition in Nigeria, the bureaucratic nature of government projects or the attitude of new political government officials to discard their predecessors initiated projects, resulting in those ventures to be white-elephant projects.With the view and understanding that groundwater is much more of high quality than surface water (which are exposed to all sorts of pollution), individuals have been embarking on digging wells in their neighbourhood and apartments to meet their growing water needs for their multifarious purposes.
However, the quality of groundwater depends upon several factors such as lithology and conditions prevailing within formation, quantum of water available in the aquifer and its rate of circulation.Apart from these factors, the activities of microorganisms, temperature and pressure are also responsible for the chemical characteristic of groundwater (Ramanathan, 2004).Therefore, groundwater is not entirely pure water because it usually contains dissolved mineral ions (Okagbue, 1988).The type and concentration of these dissolved minerals can affect the usefulness of groundwater for different purposes (Boyle, 1988).If certain mineral constituent are present in excessive amounts, some type of treatment may be necessary to either change or remove the dissolved mineral before the water can be used for the intended purpose.The major cations found in groundwater include calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ), potassium (K + ) and anions such as bicarbonate (HCO 3 -), sulphates (SO 4 2-), chloride (Cl -) with non-ionic constituents like oxides, phenols, synthetic detergents, dissolved gases e.g.oxygen (O 2 ) and carbon dioxide (CO 2 ) (Tijani, 1994).These constituents result in the good quality of groundwater when they are present in optimum concentrations.
A sudden geometric increase in the population of Akoko North West Local Government of Ondo State, Nigeria which has reached 213,792 (UN, 2006) has led to an increase in the demand for water and harnessing of subsurface water reserve.This paper investigates the possible infiltration of dissolved mineral (either from weathered rock or anthropogenic sources) into subsurface water in the area (basement complex rocks), and comparing the mineral level with the set standard for quality water by the World Health Organization (WHO).The main objectives of this study are: to determine the suitability of the water in the study area for human consumption from WHO established standard, and to determine the level as well as source of contaminants where applicable.Physical tests and chemical tests for several parameters were also carried out using specific set of scientific experimental standard (Omotoyinbo and Okafor, 2008).

1.1.The Study Area
The study area (Akoko North West) is one of the eighteen Local Governments that made up Ondo State in the South-Western Region of Nigeria.It falls within latitudes 7°30′ and 7°35′N and longitudes 5°43′ and 5°49′E.The area is accessible by roads and footpaths and it occupies about 0.83km 2 in aerial extent (Figure 1).Topographically, the area is characterised by a relatively rugged, undulating, topography with outcrops of charnockites, migmatite gneiss with other gneissic rocks as highlands which range between 600 and 1500feet above sea level.It is situated within the Precambrian Basement Complex with the outcrops which are predominantly gneiss and migmatite (Figure 2).With this type of rock types, their ability to serve as aquifers compared to sedimentary aquifers is very minimal with limited small amount of groundwater storage within the crystalline basement.The failure rate of the new boreholes in the study area is as high as 80% (Edet et al., 1998), which is as a result of the geological configurations of the study area.The groundwater availability is, however, limited to fracture zones and areas of deep weathering.The topography of Ikare (the capital of the Local Government) is high with undulating relief of about 200m to more than 1500m high.There are knolls, ridges and flat lying exposures observed in the area.The western part is characterized by a conical hill reliefoutcrops of batholiths is scattered in the southern part of the area towards Ikare and Okeagbe.Eastward, the area is featured by lowland outcrops and sparsely characterized with gneissic ridges.The major road runs from southwest to northwest.
The climate can be said to be subequatorial with two peaks of rainfall.The first peak comes up between April and July while the second peak comes up between late August and late October.These two peaks are marked by heavy rainfall and the mean annual rainfall is 1500 -2000 mm with a relative humidity of about 75 -95%.Since the climate is subequatorial, temperature could sometimes be severe.The mean annual temperature is 23 -26°C (Duze and Ojo, 1982).The area under study is situated in the deciduous rain forest area within south-western Nigeria.It has evergreen vegetation and urban settlement.The vegetation of this area reflects the rainforest and Guinea Savannah's vegetation, which is characterized by different plants and trees which may reach a height of 5 m and even more.They consist of light forests, shrubs and scattered cultivation.
There are areas where rocks are covered by vegetation which is also an indication of the porosity of the rocks and function of the grain size.Trees and plants like timber, oil palm, kolanut, rubber, cocoa and citrus are very prominent in these areas.A high forest zone is found in the north while the southern part is mostly Sub-Savannah due to the farming activities in the area, which had actually reduced the thickly vegetated area.

MATERIAL AND METHODS
Groundwater samples were collected from the hand-held boreholes, Solar powered boreholes and hand-dug wells at various locations and different depths in the study area.The details of the sampling site and depths are given in Table 1 below.The samples were then taken to the International Institute for Tropical Agriculture (IITA), Ibadan, Oyo State of Nigeria for the analysis of all the parameters (physico-chemical parameters, cations, anions and the heavy metals), after which relevant statistical packages were also used to analyse the data and their level of significance was determined.The chemical character of any water determines its quality and utilization.The quality is a function of the physical, chemical and biological parameters and could be subjective since it depends on a particular intended use (Tijani, 1994).Hence, there are different water quality standards and are generally based on two main criteria.These include the presence of objectionable taste, odour and colour plus the availability of substances with adverse physiological health effects (Tijani, 1994).
The quality of water, whether it is used for drinking, irrigation or recreational purposes is significant for health in both developing and developed countries worldwide.Water quality can have a major impact on health, either through outbreaks of waterborne diseases or by contributing to the background rates of the diseases.Accordingly, countries develop water quality standards to protect public health (Ramanathan, 2004).
The WHO has developed a series of normative guidelines for assessment of health risk and hazards through water.These principal guidelines are intended to assist countries in establishing effective national or regional strategies and standards.The physical parameters such as pH, EC, TDS etc and chemical parameters such as Ca 2+ , Mg 2+ , Na + , Fe 2+ , Cl -, SO 4 2with other heavy metal concentrations should be within the WHO acceptable standards.Therefore, the findings from the analyses are compared with WHO set standard to evaluate the groundwater quality in the local government (the study area).The World Health Organization (WHO, 1984) recommended a standard for drinking water quality.Table 2 below gives the example of these recommendations.

RESULTS
The analysis of the findings, which is centred on laboratory test of the water samples collected from both (mechanical and solar powered) boreholes and hand-dug wells and are termed to be ideal for human consumption, are compared with the WHO standards for drinking waters.
The analysis is presented by plotting the concentration of the analytical values of each element and the WHO standard in mgL -1 (ppm) with the twentysix sample locations so as to have an overview of the concentration of each element and its probable effect on the quality of the water.The results of the analysis are presented in Tables 3 -4.The derived results from the analysis are then plotted in comparison with the WHO stand values, the bar chart of which are presented in Figures 4 -6.The elements analyzed are Ca, Mg, Na, As, Fe, Zn, Cu, Cd, Pb and Ni.

DISCUSSION
The characteristics of each element varies from one sample point to the other and this indicates the difference in the quality of the water sample collected from that particular point.The WHO acceptable values are then compared with the various values to determine and compare the suitability and effect of continual consumption of such water.
The Figures 4, 5 and 6 above indicate the comparison of the analysed values for the groundwater parameters in Akoko North West Local Government of Ondo State with the WHO established standard.For cadmium concentrations, the analyzed values of the samples taken at Arigidi-Akoko are the only ones within the WHO permissible range (0.003 mgL -1 ) Figure 4(i), in other areas, the difference are less than the acceptable WHO standard concentration (0.003 mgL -1 ), and are still assumed to be okay for domestic use except for two peaks at locations 9 and 11 in Arigidi.On the other hand, the analyzed values of the samples taken at Ikare and the Ministry area are far above the permissible concentration.It can therefore be concluded that the concentration of cadmium in these areas are anomalous and as such geological.
For the lead concentrations (Figure 4(ii)), the analyzed values fall within the permissible range for lead concentration based on WHO standard concentration (0.01 mgL -1 ).It can therefore be concluded that though the concentration of the various sample locations have a concentration between the range, the lead intake of the residents within the study area is lesser than the acceptable which is what it ought to be since lead intake is hazardous to the health of man.Nevertheless care should be taken at the three locations (I 8 , I 11 and M 6 ) that have the

CONCLUSION
It can be concluded that the sampled groundwater are in most cases not suitable for human consumption judging by the various concentrations of the physicochemical parameters analyzed from the samples.Because of the total reliance of the entire population in the study area on groundwater for their various uses, there is the need to carry out a research on the link between quality of water intake and the health conditions of the populace in the study area.It was also observed that some of the heavy metals, anions and cations, with the physical parameters are much higher than the WHO acceptable standard for drinking water.Although there were also cases where in some of the analyzed samples, the range and the concentration were within the WHO acceptable standard for drinking water.Most of the high concentrations in the analyzed groundwater are generally from anthropogenic sources and not natural or geological.
The following are hereby recommended as possible solutions to the groundwater quality problems in the study area: i. that waters in such areas where the physic-chemical constituents are lower than the acceptable standards of the WHO are unsafe for human consumption and domestic use, and therefore should be treated to reduce or ameliorate the problems; ii. that further work should be carried out to ascertain the specific anthropogenic contaminants in the analyzed waters so as to determine the level of contamination and be able to proffer appropriate remediation; iii. that in areas where the cationic and anionic concentrations are far below the WHO acceptable levels, supplements should be advised to the residents of such areas or such be distributed so as to make up for the minimal intake recommended for domestic use by the World Health Organization; iv. that further investigation of the entire area is necessary to determine the specific causes for the chemistry observed and to establish hypotheses on reasons for some of the abnormalities; v. for the research approach, a larger number of samples should be taken to allow the optimization of a more routine study in the area; and, vi. the startling statistics from this research work should, therefore, serve as a wake-up call for urgent remedial action from both the Local and State Governments, if the residents in these areas are to continually live healthily.They should be properly orientated on the need not to dump untreated/raw sewage into flowing surface waters and the environments so as to reduce the increasing rate of fecal contamination of both the surface and groundwater bodies from where majority of domestic water supply come from.

Figure 1 .
Figure 1.Map of Ondo State Showing the study area.Inset: Map of Nigeria showing Ondo State.

Figure 2 .
Figure 2. Geologic Map of Ondo State showing Geology and the location of the study area.Source: Geological Survey Nigeria, Ondo State.

Figure 3 .
Figure 3. Example of (a) Hand Held Borehole, (b) Solar Powered Borehole and (c) Hand-dug well in the study area.

Figure 4 .
Figure 4. Graph of physico-chemical analytical values of groundwater samples with WHO Acceptable Value.(i) Cadmium (ii) Lead (iii) Arsenic (iv) Nickel (v) Iron and (vi) Cooper.

Figure 6 .
Figure 6.Graph of physico-chemical analytical values of groundwater samples with WHO Acceptable Value (i) Bicarbonate (ii) pH (iii) Electrical Conductivity (EC) and (iv) Total Dissolved Solids.

Table 1 .
Summary of the derivations from the GPS readings from sample collection and other readings.

Table 2 .
Summary of Physical and Chemical Characteristics and WHO (1984)standards for drinking water.

Table 3 .
The concentrations of Anion, Cation and heavy metal from the water samples at the study area.

Table 4 .
The concentrations of physical parameters from the water samples at the study area TDS (mgL -1 ); TSS (mgL -1 ).