1. Introduction

The central part of Ekiti-State which is predominantly basement terrain is characterized with occurrence of numerous surface water bodies. Groundwater occurrence in the area is erratic and wells failures have been on the increase. These factors prompt the inhabitants to harness the surface water for drinking and domestic uses including agricultural activities. These surface water bodies are prone to impacts from anthropogenic activities apart from geogenic solute inputs from dissolved minerals. Major ion composition of surface water bodies are controlled by the interaction of precipitation with surficial geological and biological materials ^[1]. Therefore, knowledge of dissolved salts in such water bodies is a pre-requisite for making any decision on their proposed or potential use for any specific industrial, irrigational or domestic purposes.

A lot of studies abound in literature on hydrochemistry of surface waters. Such works include ^{[2, 3, 4, 5, 6]}. In most of these studies, water-rock interaction contributed significantly to hydrochemical evolution of the water bodies. ^[7] reported that about 40% or even more disease outbreaks are waterborne in nature. In the present study, the peasant farmers that constitute about 80% of the workforce of the study area rely mostly on surface water [replenish through annual rainfall] availability for yearly maintenance of farming activities paying little or no attention to the quality status of the water bodies which are prone to anthropogenic contamination.

This study aimed at evaluating the hydrogeochemistry of surface water in the central part of Ekiti-State in terms of domestic and irrigational quality assessments as well as to reveal the factors controlling the distribution and concentration of elements in the water bodies.

2. Study Area

The study area is within the basement complex terrain of southwestern Nigeria. It lies within latitudes 7° 30' to 7° 45' N and longitudes 5° 00' to 5° 30' E [Figure 1]. The area is a hummocky environment with average elevation of 250m above sea level ^[8]. It is drained by many rivers/streams flowing dendritically towards the major rivers [Ose and Ogbese].The area has tropical climate characterized by high humidity [60 – 80%] and mean annual rainfall (1500mm) ^[8]. Two prominent seasons occur in the area with a long rainy season of March to November and short dry season commencing towards ending of November and terminating in early March. The effects of climate change are becoming apparent with exact periods of rainfall activities becoming unpredictable.

Geologically, the area is an Archean to early Proterozoic terrain underlain mainly by Precambrian rock units comprising of migmatite–gneiss, quartzite, granite and charnockite. [Figure 1]. The rocks in many places are covered with sediments resulting from the weathering of the basement rocks. These weathered regoliths along with fractured saprolite constitute the major aquifers in the study area.

3. Materials and Methods

Sampling exercise was carried out in January 2013. The samples’ locations were noted and recorded in the field using Magellan eXplorist Hand held GPS. Forty rivers/streams samples were picked in the study area [Figure 1]. The sampling operation was conducted in a manner that allowed four samples per designated river or stream with a view to obtain an average value per river. At each location, water sample was put into polyethylene bottles [in triplicate] that have been previously properly rinsed with distilled water and air dried in the laboratory [to avoid contamination]. The water samples were collected as far as possible from the edges of the water bodies and as deep as possible from the flow paths of the rivers. Water samples for cations determination were acidified with concentrated nitric acid to prevent reactions with the container. All samples were stored in an ice packed large rubber container and transported to the Laboratory where they were kept in refrigerator at 4ºC before analysis. The first two sets of samples were used for cations and anions determinations respectively while the third set was for bacteriological analyses. In-situ parameters including water temperature, pH and electrical conductivity [EC] were measured immediately samples were collected using a multiparameter portable meter [model Testr-35]. Chemical and bacteriological analyses were carried out at the Federal Ministry of Water Resources Laboratoy, Akure in Ondo State, Nigeria. Anions and cations were analysed using colorimetric and atomic absorption spectrometry methods respectively.

The set of samples for bacteriological analysis were subjected to total bacteria count and coliform count. Nutrient agar medium was used to obtain plate count of living bacteria [viable cell count]. The procedure involved mixing 1 ml of water sample with liquefied agar at 40 °C in a Petri dish. The agar sets to a jelly, thus fixing the bacteria cell in position. The plate was then incubated under appropriate condition [24 hours at 37 ºC for bacteria organism from animal or man]. At the end of the incubation, the individual bacteria would have produced colonies visible to the naked eyes and the number of colonies was assumed to be a function of the viable cells in the original sample. Coliform count was achieved using a lactose medium inoculated with serial dilution of the sample. The appearance of acid and gas after 24 hours at 37 °C was taken as positive indication of the presence of coliform bacteria; results were expressed as number of colonies per 100 ml.

Chemical data were subjected to statistical evaluation using SPSS 17 while Chloro-Alkaline Indices [CAI-1 and CAI-2] were calculated [expressed as meq/l] using equation 1 and 2 to find out the ion exchange processes responsible for the concentration of ions in groundwater ^{[9, 10]}.

(1)

(2)

Irrigation quality was assessed using estimated irrigation quality indices including salinity hazard, Sodium Absorption Ratio [SAR], Magnesium Absorption Ratio [MAR], Kelly’s Ratio [KR] and Permeability Index [PI].

4. Results and Discussion

Table 1 and Table 2 present the physical and chemical parameters of surface water in the study area respectively while the statistical summary of the physico-chemical data is as presented in Table 3. The average temperature of the water was closer to the mean atmospheric temperature of 28°C, suggesting present day climatic influence and recharge. The pH of the water samples ranged between 8.3 and 9.6 signifying alkaline water. The EC and TDS values of the surface water bodies were low with range values of 84 – 205[av. 142.10] μS/cm and 63 – 153.75[av.106.58] mg/L respectively suggesting low-mineralised freshwater. Similarly, major ion concentrations were low [Table 2] and the order of ionic concentrations in [mg/L] for the cations is Na⁺>Ca²⁺>Mg²⁺>K⁺ and for the anions, Cl^- >SO₄^2-> HCO₃^->NO₃^- [Figure 2]. All ionic concentrations are within approved ^[11] standard for drinking water indicating chemically potable water. The dissolve ionic species in the surface water are the resultant product of weathering of rock forming minerals with minor contribution from atmospheric precipitation and anthropogenic activities ^[12]. The contribution of atmospheric sources to the dissolved salts in the water bodies can be assessed by considering the rain water chemistry or by taking the ratios of elements to Cl [13, 14]^{[13, 14]}. The average Na/Cl and K/Cl ratios of the surface water in the area come out to be 0.71 and 0.30 respectively which are lower than the marine aerosol [Na/Cl = 0.85] and higher than that for [K/Cl = 0.0176] ^[15]. This suggests that the ionic contribution from the atmospheric precipitation is limited in the area. The ionic contribution in the area is from rock water interaction. This observation was further confirmed by ^[16] diagram [Figure 3]. The Gibb’s diagrams represent the plot of the ratios of Na⁺/[Na⁺+Ca²⁺] and Cl^-/[Cl^-+HCO₃^-] against TDS. The diagrams are widely utilized to assess the functional source of dissolved chemical constituents, such as precipitation dominance, rock dominance and evaporation dominance ^[16]. In this study, Gibb’s diagram plot of analytical data of the water samples cluster in the regions of rock and precipitation dominance indicating chemical weathering piloted by precipitation as the major factor controlling the chemistry of surface water.

The lack/weak correlations among most ionic parameters [Table 4] are indicative that varied hydrochemical processes account for the ionic concentrations in solution. However, the likely source of Cl^- is atmospheric inputs given the positive correlation with Na⁺ [r = 0.92] and surface anthropogenic activities. A plot of Na⁺ vs Cl^- [Figure 4] indicates most of water samples lie below the aquiline, implying that evaporation may be the cause of addition of Cl^-, due to more salt dissolution from soil as the water interacts with the rocks/soil in the area. Na⁺ concentration is also being reduced by ion exchange. Hence Na⁺ and Cl^- do not increase simultaneously. A Na⁺/Cl^- ratio approximately equal to one ^[1] is usually attributed to halite dissolution whereas the ratio greater than one ^[1] indicates that sodium is released from silicate weathering ^[13]. In this study majority of the sample have Na⁺/Cl^- ratio close to one ^[1] indicating that halite dissolution arising from precipitation interaction with the soil is responsible for the Cl^- in the water bodies of the study area.

In the study area, the value of the chloro-alkaline indices; CAI-1 and CAI-2 vary from -0.38 to 0.23 and -0.11 to 0.17 respectively [Figure 5]. Whenever, the indices are positive, it is indicating ion exchange of Na⁺ or K⁺ in the surface water with the Ca²⁺ or Mg²⁺ in the aquifer weathered material that infers chloro-alkaline equilibrium ^[9]. However, in reverse exchange, indices are negative indicating exchange of Ca²⁺ and Mg²⁺ in the water bodies with Na⁺ or K⁺ of the rocks signifying chloro-alkaline disequilibrium. There is no systematic variation in the values of indices. So the ion exchange reactions seem to occur in both the negative and positive directions depending on the rivers flow path and the mixing of water process. The observed chloro-alkaline process indicated that the cation-anion exchange process is one of the important geochemical processes that control the water chemistry of the area.

Hardness is caused by compounds of calcium and magnesium and by a variety of other metals. General guidelines for classification of waters are: 0 to 60 mg/L as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard ^[17]. Surface water hardness in the study area ranged between 52 and 100 with 20% in the soft water category while the remaining 80% were in the moderately hard class. There is no health risk attached to the hardness depicted by the surface water as only water in hard and very hard categories might lead to pre-natal mortality, cardio-vascular diseases ^[18].

Figure 1. Location and Geologic map indicating sampling locations [Modified after 8]

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The surface water samples were plotted on the Piper trilinear which permits the cations and anions compositions of the samples to be represented on a single graph in which major groupings or trends in the data can be discerned visually ^[19]. Water types are often used in the characterization of waters as a diagnostic tool ^[20]. Piper trilinear diagram [Figure 6] for the study area shows that mixed Ca²⁺-Mg²⁺-Cl^- representing 70% of the surface water as dominant with Na⁺-Cl^- and Ca²⁺-Cl^- representing 20% and 10% respectively as minor water types. In addition to the Piper trilinear diagram an overall characterization of hydrochemical facies of the surface water was carried out by using ^[21] plot where the rectangular field was divided into eight sub-fields, each of which represents a water type and hardness domain

[Figure 7]. Majority of water samples in this study fall in domain six ^[6] which signifies water with permanent hardness i.e. alkaline earths and strong acidic anions exceeded both alkali metals and weak acidic anions. Water in this category represents Ca²⁺-Mg²⁺-Cl water with minor facies represented by Na⁺-Cl^- and Ca²⁺-Cl^- as indicated by the Piper diagram ^[22].

5. Bacteriological Analysis

Results of the bacteriological analysis of the water samples are presented in Table 2. All sampled water tested positive to bacteria counts, necessitating treatment of water before consumption. The Total Bacterial Count [TBC] ranged from 16 – 43 [av. 30.50] cfu/100ml while the E-Coli are from 4 – 11 [av. 7.6] cfu/100ml. The total bacterial counts for all the water samples were within the limit of 1.0X10² cfu/ml which is the standard limit of heterotrophic count for drinking water ^[23]. However, the total coliform count for all samples were higher than the EPA maximum contamination level [MCL] for coliform bacteria in drinking water of zero total coliform per 100ml of water ^[24]. The high coliform count obtained in the samples may be an indication that the water sources were faecally contaminated ^[25]. None of the water samples complied with EPA standard for coliform in water. The primary sources of these bacteria in water are animal and human wastes introduced into surface runoff, pasture and other land areas where animal wastes were deposited. Seepage or discharge from septic tanks, sewage treatment facilities and natural soil/plant bacteria may also contribute to bacterial contamination of water ^[23].

Table 1. Results of average values of physical parameters of surface water in the study area [no of samples = 4]

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Table 2. Mean values of chemical/bacteriological parameters of surface water in the study area (no of samples = 4)

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Table 3. Summary statistics of measured parameters of surface water in the study area

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Table 4. Correlations of measured parameters in the study area

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Figure 2. Schoeller diagram indicating ionic concentrations of surface water in the study area

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6. Irrigation Quality Assessment

The control of salt and alkali in the soil is as important as the supplying of irrigation water for the development and maintenance of successful irrigation projects ^[26]. In this study, salinity, sodium absorption ratio [SAR], residual sodium bicarbonate [RSBC], and permeability index [PI] were important irrigation quality parameters employed for determining the suitability of the surface water for agricultural uses.

Electrical conductivity is a good measure of salinity hazard to crops as it reflects the TDS in water bodies. The EC value of surface water of the study area ranged from 84 to 205 μS/cm with an average value of 142.10μS/cm, which signified ‘excellent to good’ irrigation water ^[27]. In terms of the ‘degree of restriction on use’, EC value of < 700 μS/cm refers the water to ‘none’; 700-3000 μS/cm ‘slight to moderate’ and above 3000 μS/cm ‘severe’ ^[28].

Figure 3. Overall processes regulating geochemistry of surface water in the study area

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Figure 4. Na versus Cl plot

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Figure 5. Chloro-Alkaline Indices of water samples from the study area

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It is easily deduced that the surface water from the study area in terms of EC values, was suitable for irrigation purpose as it fell under category ‘none’ ^[28]. High Na⁺ concentration in water can lead to displacement of exchangeable cations Ca²⁺ and Mg²⁺ from the clay minerals of the soil and such soil becomes impermeable leading to low fertility and cultivation ability ^[29]. SAR is an important parameter for determining the suitability of groundwater for irrigation because it is a measure of alkali/sodium hazard to crops ^[30]. Salinity and Toxicity problems of irrigation water are attributed to SAR ^[31]. It is defined by ^[32], that sodium – rich water may deteriorate the physical structure of the soil [pore Clogging]. The SAR was calculated by the following equation given by ^[33] as:

(3)

where, all the ions are expressed in meq/L. The values of SAR of the collected surface water samples ranged from 0.34 to 1.46 with an average value of 0.74 [Table 5]. Salinity classification was done using a quality diagram [Figure 8] given by the U. S. Salinity Laboratory ^[33]. The diagram has 16 classes, with reference to SAR as an index of sodium hazard and EC as an index of salinity hazard .

By plotting the obtained results in the diagram [Figure 5], all the sampled surface water was categorized into “C₁-S₁” an indication that such water can be safely used for irrigation purposes ^[33].

The RSBC was estimated according to ^[34] using;

(4)

Where, the concentrations of the constituents are expressed in meq/L. According to the US Department of Agriculture, water having more than 2.5 epm of RSBC is not suitable for irrigation purposes. All surface water samples of the study area had RSBC less than 2.5epm indicating that the water bodies are safe for irrigation purposes.

Kelly’s ratio entails measuring Na⁺ against Ca²⁺ and Mg^{2+ [35]} and was calculated using:

(5)

where, all the ionic concentrations are expressed in meq/L. The KR values computed for the surface water in the study area ranged from 0.25 to 1.26meq/L with mean of 0.57meq/L. Ninety percent [90%] of the surface water have values less than one and as such suitable for irrigational uses.

Generally, calcium and magnesium maintain a state of equilibrium in most waters. More magnesium in water will adversely affect crop yields as the soils become more saline ^[36]. The values of the magnesium adsorption ratio of surface water in this present study ranged from 24.08 to 76.40% with 50% of the water samples within the acceptable limit of 50% ^[37]. Fifty percent of the water was therefore considered unsuitable for irrigation since their magnesium adsorption ratio were higher than 50% and as such produce harmful effect to soil.

Table 5. Irrigation indices of water samples from the study area

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The permeability of soil is influenced by sodium, calcium, and magnesium and bicarbonate contents in the soil as well as by long-term use of irrigation water ^[38]. ^[39] evolved a criterion for evaluating the suitability of water for irrigation based on PI. PI is calculated using the following equation:

(6)

where, all ions are expressed in meq/l. The PI values computed for the area ranged from 45.76 to 80.24% with mean of 61.79%. Table 5 indicated that 20% of the samples fell under class II while 80% fell under class III of the Doneen chart. According to ^[40], waters can be classified into class I, II and III based on PI values. Class I and class II waters are categorized as good for irrigation with 75% or more maximum permeability while class III waters are unsuitable with 25% of maximum permeability. Majority of water in the study area fell in good irrigation class.

Figure 6. Piper Diagram [modified after 41]

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Figure 7. Hydrochemical Facies of surface water in the study area [after 21]

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Figure 8. Salinity classification of irrigation water [33]

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7. Conclusion

This study has assessed the hydrochemistry, quality and suitability of surface waters in the central part of Ekiti state, southwestern, Nigeria. The physico-chemical parameters of the surface waters fell within the ^[11] Guidelines for drinking water and depict that the waters are chemically potable. Evaluation of the results showed that the waters are fresh (63≤TDS≤154 mg/ L), soft to moderately hard 52≤TH≤100 mg/ L) and alkaline (8.3≤pH≤9.6) with low residence time and low water-rock interaction. All the surface water tested positive to total bacterial count and e-coli and need to be dis-infected before consumption. Mixed Ca²⁺-Mg²⁺-Cl^- was the dominant water type in the study area while Na⁺-Cl^- and Ca²⁺-Cl^- were the minority water facies. Irrigation assessment using SAR, RSC, PI and KR indicated that the surface waters from the area were within excellent to good irrigation water class with low salinity hazard and are suitable for most crops on most soils. The Gibbs plot depicted that the chemistry of waters were modified by chemical weathering piloted by precipitation as the major factor controlling the chemistry of the surface waters.

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