Hydrogeochemical Signatures of Different Aquifer Layers in the Crystalline Basement of Oban Area ( SE Nigeria )

The shallow and deep aquifer horizons of the crystalline basement of Oban massif of (SE Nigeria) were studied during the dry and wet seasons. The criteria were ascertaining hydrogeochemistry relative to seasonal and spatial variations across the study region. The results obtained show that major elements such as Ca, Mg, Na and K were higher in the shallow aquifers than in the deep aquifers, during the study period. The major anions Cl, SO4, HCO3 and NO3 were higher in the deep aquifers compared to the shallow ones. Two water types were identified for both (shallow and deep) aquifers: Ca/Mg-HCO3 and Ca/Na-Cl/SO4. Most of the water parameters considered was within the international limits for drinking and domestic purposes. Assessment by use of Sodium Absorption Ratio (SAR), percent Sodium (% Na) and the Wilcox diagram reveal that the waters are suitable for irrigation purposes.


Introduction
Hydrogeochemical studies have over the years played an essential role in interpreting mineralogical composition of the sub-surface and inherent conditions in most geological settings.However, this approach has produced more comprehensive interpretation when applied together with other Earth science studies such as structural geology, photo-geology, remote sensing as well as deep geophysics.
In contrast to the large amount of work aimed at quantifying the permeability of unconsolidated and sedimentary rocks, little work has been reported on igneous and metamorphic rocks.Norton and Knapp (1977) stated that the fact that minerals tend to form along the walls of fractures and in veins demonstrates that fluid flow in sub-surface systems is controlled by fracture distribution.The major factor that controls or increases the permeability of igneous and metamorphic rocks is weathering.The combination of near surface weathering and fracturing will increase the permeability by two or four orders of magnitude (Davies andDe Weist, 1966 andDavis, 1969).In general these effects are apparent only within the first 20 m below the surface, but can extend down to as much as 100 m in tropical zones (Scheytt , 1997).Variations in groundwater geochemistry have also been shown to be related to the position of the aquifer units (Scheytt, 1997).
In basement terrains, there is a general belief that water exists in fractured rocks and overlying regolith and that such water is usually structurally controlled.
The study area (the Oban massif) forms part of the south-eastern basement complex of Nigeria, which lies between the Archaean cratons of west and central Africa.Recent and on going investigations within the study area notably Edet (1993), Edet et al., (1994), Edet et al., (1998), Okereke et al., (1998), Edet and Okereke (2005) Okereke et al. (1998), have led to the delineation of groundwater productive zones, quantitative and qualitative estimation of aquifer parameters within the massif.These have been expressed by interrelations between lineaments frequency and density of regolith development.These assessments have all been achieved based on remotely sensed data, aerial photos, deep geophysics, and geologic logs from exploration boreholes within the study area.This current research attempts to assess the hydrogeochemistry of the groundwaters relative to aquifer levels within the massif.

Description of Study Area
The Oban Massif lies between longitudes 8 º 00´E and 8 º 55 É and latitudes 5 º 00 Ń and 5 º 45´N covering an area of about 8,740 km 2 (Edet et al., 1998), Figure 1.This vast crystalline basement complex is characterised by isolated hills attaining a maximum height of about 1,200 m above sea level at locations on the eastern axis of the massif (Ayi, 1987).The topography exhibits sharp undulations with v-shaped valleys being typically forested at the highest peaks.The massif is well drained, controlled by weathered zones, fractured and jointed areas, coursing in two directions: southwards (seawards) and northwards to join the upper course of the Cross River in the Ikom depression.
The study area is characterized by a tropical climate with two distinct seasons: wet and dry.The wet season spans from May to October, while the dry extends from November to April.The general temperature trend for the study area is high with negligible diurnal and annual variations.The average monthly temperature in the area ranges from 29 to 34 º C. A mean annual rainfall of about 2,300 mm have been reported for the area, with annual mean daily relative humidity and evaporation of 86 % and 3.85 mm/day, respectively (CRBD, 2008).The regional run-off coefficient of the study area is in the order of 0.21-0.61and is due to topography and evaporation, (Petters et al., 1989).

Geologic and Hydrogeologic Setting
The Oban massif forms part of the huge spur of the western elongation of the Cameroon Mountains into the Cross River plains of south-eastern Nigeria (Ekwueme, 2003).Rahman et al. (1981) reported the rocks of the Oban massif to be dominated by: (1) locally migmatitic and shared gneissic rocks, paraschists, phyllites, metaconglomerates and quartzites, amphibolites and metadolerites, foliated pegmatites and aplites, pyroxenites, etc, (2) older synkinematic to late-kinematic intrusive series comprising different rock types such as meladiorites, granodiorites, adamellites to granites and weakly foliated to unfoliated pegmatites, aplites and quartz veins, (3) unmetamorphosed dolerites to microdioritic intrusives.These rock series exhibit variations across the sectors of the massif.
Based on a regional hydrogeological differentiation by Petters et al. (1989), the study area has been identified to belong to the basement complex province of the Cross River area of south-eastern Nigeria.Okereke et al. (1998) describe the Oban massif to be a three layer hydro-geolectric stratigraphic model composed of: -a top unsaturated clayey sand (lateritic); -middle gravelly sand and decomposed bedrock and; -fresh bedrock (fractured).
The occurrence of groundwater in the study area has been established to be controlled by structural discontinuities such as fractures, joints, fissures and regolith, (Petters et al., 1989;Edet 1993;Edet et al., 1994).Rates and levels of recharge to porous aquiferous media in the study area, suffer impedance due to the top lateritic cover characteristic of the area, Petters et al. (1989).This is attributed to the high clay contents of these lateritic top soils, hence their low permeability.
A low to moderate lineament density has been ascribed to the Oban massif (Edet et al., 1994).This reflects on the depth and extent of weathering profiles and consequently their groundwater potential.There is no general water table for the area, due to the variability of structural and geological controls.However groundwater generally occurs under water table conditions across the massif.Edet and Okereke, (2005) estimated the water bearing formations of the Oban massif to have a thickness of about 5-140 m.Okereke et al. (1998) employing Schlumberger electrical soundings and Werner profiling measurements, reported a 5 m average thickness for the dry overburden and aquifer components between 15-70 m.These layers overly highly resistive fresh bedrock.Groundwater sources from the basement are few, a scenario ascribed to the level of uncertainty and cost of groundwater exploration in the area.Aquifer parameters from the study area are reported in Table 1.Lithologic logs show the main rock types (e.g.gneisses, schists, granites, granodiorites, quartzites and amphibolites) revealing four to six layers across the massif.These layers represent different degrees of weathering processes.Representative sections (A-A' and B-B', Figure 1) across the area, based on drilled data, are presented in Figures 2 a and b.
Two basic aquifer units were identified and boreholes penetrated these layers at different levels across the massif.The aquifers are classified as shallow and deep aquifers relative to depth from surface with respect to the study area.The shallow aquifer boreholes were those with a maximum drilled depth of less than 15 m, hand dug and constituting about 60 % of the total number of sampled bores.The deep wells were those drilled at depths greater than 15 m, extending up to 30 m and beyond representing the remaining 40 % of sampled bores.
The deep aquifer boreholes were mechanically drilled and usually hand pump or motorized pump fitted.This scenario will hence entail the transfer of groundwater between these seemingly successive layers of different degrees of cross-cutting and depth of weathering.The fractures and lineaments act as conduits of groundwater from shallow aquifers to deeper levels.Data from drilled holes litho-sections were obtained for the borehole/deep wells as these were well documented by the government intervention agencies that were responsible for the drilling.Such information were poorly documented or lacking for the shallow hand dug wells, as they were drilled by the local population with no recourse for data documentation.
The drilled well in Oban town shows that the aquifer units are weathered gneissose units with depth between 5 and 10m.The aquifer layer at Abiati village spans between the second and third layers down the drilled section (3-9m) and is characterised by a granitic-gneissic lithology.At Aningeje village the aquifer layer is gneissic-granitic extending from 6 to15 m down profile.These are for locations on the eastern sector of the massif.
For the western sector, drilled holes at Iko Essai shows a granitic-gneissic unit extending from about 9 to 20 m being the aquifer layer.At Ibogo village the aquifer layer is a micaceous-schistose layer extending from about 4 to 35m down the profile.Lithologic log at Old Netim shows the aquifer layer to be granite-gneiss and biotite-granite-gneiss extending from 3 to 39 m.Ayaebam has a biotite-gneissic aquifer unit (between 6 and 32 m depth), while at Obung village the aquifer unit is pegmatitic-micaceous-gneissic with a depth extent of about 3 to 39m down hole.
Measured aquifer parameters are as presented in Table 2. Static water level was determined by the use of a water depth probe meter.Water levels for the open wells were determined for both dry and wet seasons.This was not possible for the boreholes as such information was only obtainable during borehole construction, prior to their sealing.Static water level varied across the massif through the seasons rising to surface at Camp IV, New Ndebiji and Igbofia in the wet season.The lowest depth of water was recorded at Oban at 6.20 m in the dry season.

Physicochemical Characteristics
The geochemical nature of groundwater may depend on the depth of flow paths reflecting effects of the weathered profile and cavity control at different horizons within sub-surface.
Aquifer units within the massif occur basically at two levels: within 15 m from the surface (shallow aquifers) and beyond 15m down hole (deep aquifers).The shallow aquifers were mainly exploited by hand dug open wells, while the deep ones were mechanically drilled, and down hole pumps have been set up.The groundwaters exhibit both physical and chemical variations, from the shallow to the deep aquifers, with recognizable influences of the sampling season.Results of chemical analysis of sampled groundwaters are presented in Tables 3(a) and (b).Assessment of quality of data was accomplished by calculating the ion balance error using equation (1), Hounslow (1995): cations anions Error of ion balance cations anions An error within limits of + or -5% is tolerable.Approximately all water quality data for this study were within this range.A statistical summary of measured parameters are shown in Tables 4(a) and (b).

Deep Aquifers
Data from dry season sampling revealed groundwaters from deep aquifers/boreholes to exhibit: pH values between 5.08 and 6.34, temperature ranging from 26 to 32 º C, while electrical conductivity (EC) values are between 34.6 and 326 µS/cm and Total Dissolved Solids (TDS) from 22.18 to 208.97 ppm.In the wet season, pH values ranged from 6.27 to 8.65, temperature from 26 to 30.4 º C, EC from 60 to 430 µS/cm, while TDS values were between 40 to 570 ppm.

Shallow Aquifers
Groundwater samples collected from the shallow wells during the dry season show pH values ranging from 5.12 -6.86, temperatures between 26 to 28 o C, EC values in the range of 24.8 to 622 µS/cm and TDS values between 15.90 and 398.72 ppm.In the wet season, pH values ranged from 5.94 to 7.69, temperature was between 27 and 30 o C, while EC values ranged from 80 to 580 µS/cm and TDS values were in the range 60 to 890 ppm.Chemical analysis of groundwater samples from the dry season had Ca² + ranging from 9.00 to 135.20 mg/L, Mg 2+ from 0.45 to 5.59, Na + from 1.16 to 4.55 mg/L and K + ranged from 0.54 to 5.92 mg/L.Anion analysis for these samples revealed Cl -from 1.02 -87.47 mg/L, HCO 3 -ranged from 18.30 to 122.30 mg/L, SO 4 2-from 1.28 to 117.90 mg/L and NO 3 -from 0.009 to 45.89 mg/L.
In the wet season, Ca 2+ ranged from 6.04 to 97.14 mg/L Mg 2+ from 0.32 to 3.42 mg/L, Na + from 2.45 to 7.00 mg/L and K + ranged from 0.99 to 45.89 mg/L.Anions showed concentrations values ranging from 22.14 to 308.70 mg/L for Cl -, from 12.14 to 384.30 mg/L for HCO 3 -, from 13.64 to 471.70 mg/L for SO 4 2-and from 0.23 to 11.72 mg/L for NO 3 -.

Discussion
For both shallow and deep wells the difference in the physical parameters and average temperatures are insignificant.The difference in pH values is also negligible.pH values close to 7 or higher are characteristic of deep wells and groundwaters of artesian origin (Langmuir, 1997).
Electrical conductivity (EC) and total dissolved solids (TDS) were higher for the shallow wells than the deep wells.The shallow wells are characterized by higher degree of regolith development and abundance of free ions in waters.This can be attributed to equilibrium between the water and soluble rock type (Hem, 1986).
Assessment of results from chemical analyses shows that concentration levels of the major elements are higher for the shallow wells than for the deep wells.These variations in the concentration levels could be attributed to high weathering levels and development of regolith as considered for shallow wells.Clay content decrease down hole due to weathering decreasing and the more open joints or fractures.With greater depth however, the joints are closed yielding less groundwater.Maturity of waters relative to resident times and history of travel along interconnected pore spaces also determine concentration levels of these mobile chemical species in shallow aquifers (Sears and Langmuir, 1982).
Concentration levels of anions are higher in the deep wells than in the shallow wells, with exception of NO 3 which is higher for the shallow wells.Nitrate concentrations can be used as an indicator of anthropogenic pollution.Levels of about 8.5 mg/L are considered to represent low level contamination (Hallberg, 1989).This could be interpreted for the groundwaters from shallow wells based on their proximity to surface anthropogenic inputs.Probable source of nitrate would be the use of fertilizers in farming and dumping of human and animal waste in the environment.
The noticeable anion content variation trend between shallow and deep waters, could be attributed to higher in-situ concentrations of chemical species in the waters of the deeper aquifers with longer resident times of water-rock interaction.

Hydrochemical Facies
Observations across the massif, shows that shallow groundwaters are diluted waters, slightly acidic with mean pH values of 6.02, and belong mainly to the Ca/Mg-HCO 3 /SO 4 facies, and in a lesser extent to the Ca/Mg-SO 4 /HCO 3 water type.As the depth of aquifer increases the shallow groundwaters evolve to higher mean pH values of 7.17, become slightly alkaline and Ca/Na/Mg-HCO 3 /SO 4 type waters.The prevailing geochemical signatures of the groundwaters are Ca-HCO 3 /SO 4 .This classification was also proposed by Langmuir (1997), and reflects the processes involved in chemical weathering of silicates and the common occurrence of calcium carbonate.
The apparently dominant Ca/Mg-HCO 3 water type is defined as the normal alkaline group of water.Amadi (1987) describes this type of water as typical of Nigerian basement terrain with limited mixing, perhaps reflecting a primary stage of evolution of its groundwater system.A similar water type has been reported for the western basement complex of Nigeria by Elueze et al., (2004) and Tijani (1994).The chemical composition of this water type is ascribed to the dissolution of silicate minerals in the bedrock and aluminosilicates in the weathered regolith, Tijani (1994).In these waters, Ca 2+ +Mg 2+ concentrations are higher than HCO 3 concentrations.Such water type is believed to show permanent hardness with no bicarbonate hazard for irrigation (Naik et al., 2001).

Irrigation Suitability
The major characteristics of water that determine its suitability for irrigation purposes are: (i) total concentration of soluble salts (ii) relative proportion of sodium to other principal cations and (iii) bicarbonate concentration in relation to the concentration of Ca 2+ + Mg 2+ .These have been called as salinity hazard (Wilcox, 1995), sodium hazard and bicarbonate hazard.
The water suitability for irrigation purposes is better adjudged by its sodium hazard potential.This issue is being called sodium absorption ratio (SAR) and expresses the reactions with the soil.The SAR can be computed as follows: where all ionic concentrations are expressed in milliequivalents per litre (meq/L).
The classification of groundwaters from the Oban massif with respect to SAR is presented in Table 5.The waters from the shallow and deep aquifers from both sampling seasons are found to be less than 10 (limit for excellent water classification), thus they are classified as excellent for irrigation.
Deep aquifer waters have sodium contents between of 2.40 and 29.50 % in the wet season and in the range of 2.64 to 25.66 % in the dry season.The sodium contents in waters from shallow aquifers range from 3.60 to 27.53 % and from 2.53 to 19.12 % in the wet and dry seasons, respectively.These results show that the waters from both aquifer types are within excellent (< 20 % Na) to good (20 -40 % Na) category.
All the water types sampled in both seasons plot within the excellent to good field of the Wilcox diagram (Figure 3).

Conclusions
The waters from the shallow and deep aquifers are slightly acidic in the dry season and evolve to a slightly alkaline nature in the wet season.Calcium and magnesium are the dominant cations which are generally higher in the shallow aquifers.Anions such as Cl -, SO 4 2-, HCO 3 -and NO 3 -are higher in the deep wells for both sampling seasons.Two types of hydrogeochemical facies were identified in the area: Ca/Mg-HCO 3 and Ca/Na-Cl/SO 4 .The water chemistry is influenced by rainfall, weathering processes (mainly silicate weathering) and water mixing.Deductions from SAR and % Na calculations as well as the Wilcox diagram, suggest that the waters can be used for irrigation purposes.

Figure 1 .
Figure1.Schematic geological map of the study area (Oban massif): inserted is the map of Nigeria (modified fromEkwueme, 2003)

Table 1 .
Representative aquifer parameters across the Oban massif

Table 3 (
a). Results of representative analysis of sampled groundwaters (dry season)

Table 4 (
a). Descriptive statistics of parameters as measured in the dry season

Table 4 (
b). Descriptive statistics of parameters as measured in the wet season

Table 5 .
Sodium hazard and percent sodium classification of groundwaters from the Oban massif