The study area lies between 8.92N - 9.20N and 7.25E - 7.60E in Abuja Federal Capital Territory (FCT) as seen in Figure 1. It is bounded in the east by Nasarawa State, north by Kaduna State, west by Niger State and south by Kogi State.

The topography of Abuja is undulating with hills and inselbergs that rise northwestwards to a maximum of 1060 m above sea level. There are extensive plains found between hills in the study area. The Zuma rock stands out clearly on its own as the most conspicuous Inselberg at the boundary of the Abuja with Niger State. The lowest elevations are in the southwestern flood plains of the River Gurara, about 76 m above sea level. The rivers rise from the hills in the northeast and flow to the southwest. The area is drained by many rivers in and around Abuja including Rivers Gwagwalada and Usmanu while Rivers Wupa, Wosika and other smaller seasonal southerly-flowing streams form the tributaries and drain the study area. The drainage pattern generally varies from trellis to dendritic.

The major rivers join at Nyimbo village to form a tributary of River Niger in

the south. These rivers depend on rainfall for their recharge. As such, their stages are high in rainy season and decrease drastically during the dry season.

The area has its highest temperature of about 36˚C during the dry season, November to March. During the rainy season April to October, the temperature drops to a maximum of 24˚C. The annual rainfall ranges from 1100 mm to 1600 mm as discussed in [5]. Two types of vegetation occur; the forest predominantly of woody plants thorn bushes and trees in which grasses are virtually absent comprise mainly of secondary forest, which is continuously degraded for subsistence farming and habitation and the savanna herbs and shrubs, the study area being in Guinean Savanna Vegetation Zone of Nigeria.

There are seven subsurface aquiferous layers in the study area: 1) Topsoil, 2) Lateritic sand, 3) Clayey sand, 4) Weathered basement rock, 5) weathered/fractured basement rock, 6) Fresh/non-fractured basement rock and vii. Fresh fractured basement. There are numerous long and short fractures at depths ranging from 7.0 m to 36.00 m running through these layers whose structural trends are mainly in northeast-southwest direction corresponding to the major tectonic structural trend of the basement complex in Nigeria as discussed in [6].7]. The topsoil is aquiferous down the valleys (Hand-dug wells) and progressively unsaturated as we go uphill.

The geology of the study area has been described by many workers, including [8].9].10]. The study area is underlain by Precambrian rocks of the Nigerian Basement Complex which cover about 85% of the land surface and cretaceous sedimentary rocks belonging to the Bida Basin which cover the remaining 15%.

From field mapping, the major lithological units found in the study area are: Migmatite-gneiss; Biotite granites; Quartzites/Quartzite-schists; Amphibolite-Schists/Amphibolites as seen in Figure 2.

These Basement complex rocks are subdivided into:

1) The older metasediments

The migmatite gneiss complex is the commonest rock type in the Nigerian Basement complex. It comprises two main types of gneisses: the biotite gneiss and the banded gneiss. Very widespread, the biotitic gneisses are normally fine-grained with strong foliation caused by the parallel arrangement of alternating dark and light minerals. These banded gneisses show alternating light-colored and dark bands and exhibit intricate folding of their bands. The migmatite gneiss complex is the oldest basement rock, and is believed to be of

sedimentary origin but was later profoundly altered into metamorphic and granite conditions. The older metasediments were also among the earliest rocks to form on the Nigerian Basement Complex. Initially of sedimentary origin, with a more extensive distribution, the older metasediments underwent prolonged, repeated metamorphism; and now occur as quartzites (ancient sandstones), marble (ancient limestones), and other calcareous and relics of highly altered clayey sediments and igneous rocks.

2) The younger metasediments

Most parts of the Basement complex are underlain by belts of roughly north-south trending, slightly metamorphosed ancient Pre-cambrian sedimentary and volcanic rocks known as the younger metasediments with major rock types being ancient shaly rocks which are now referred to as quartz- biotite-muscovite schist. These change laterally into coarse-grained feldspar-bearing micaceous schists and schists in which graphite, phyllites and chlorite are common. Ferruginous quartzites also occur.

3) The older granites

The older granites vary extensively in composition. There is enormous variety in the granite composition of these rocks.

4) The younger granites

Granitic alkaline ring complexes and the intruding volcanic rock.

Ninety-four (94) groundwater samples were collected from productive boreholes in the study area after a geological traverse field mapping exercise and borehole water field testing for physico-chemical parameters, following standard sampling protocols as discussed in [11]. Table 1 demonstrates geographical locations of tests sites in the study area.

Boreholes for tests and measurements were selected based on three criteria:

1) Availability of data, 2) Being functional and in use, 3) Not deeper than our water level indicator 50 m and Sonar bottom sounder 61 m. Groundwater samples were analyzed as discussed in [12].t Activation Laboratory (Actlabs), Canada. The following groundwater and borehole physical parameters were measured in-situ in the field using calibrated field instruments; Hanna HI 98127 (pH), HI 98304 (EC), HI 96304 (TDS), HI 9147 (DO), Groundwater temperature and electrical conductivity in boreholes was profiled real-time using Solinst levelogger for Static Water Level measurements. Geolocation and elevation measurements of boreholes were done using a Global Positioning System (GPS) Garmin 60CSx.

Water level measurements are useful in hydrogeology to:

1) Determine the ease of groundwater exploration, exploitation and distribution.

2) Determine groundwater flow direction, gradient and yields.

3) Determine aquifer parameters K, T, S and estimate recharge rates.

4) Determine short and long-term periodic changes in amount of water in storage.

5) Understand the mechanics of the aquifer’s matrix-fluid interaction.

6) Gain insight for well construction, development and efficient extraction.

Estimation of the Sustainable yield of a borehole.

The ratio of drawdown s to pumping rate Q is a constant for a well (if corrected for well losses). This constant only depends on the aquifer property transmissivity T and shall not exceed a maximum drawdown s_Available, the extrapolation of the measured pumping test drawdown can be used to determine the sustainable yield:

Q S u s t a i n a b l e = Q P u m p   T e s t S A v a i l a b l e ( t = t l o n g ) S P u m p T e s t ( t = t l o n g ) [13]

The available drawdown is for instance the position of the main water strike in the borehole. If the drawdown exceeds this position and a drastic decrease in the yield of the borehole occurs, it may dry up. The problem of extrapolating the drawdown measured during the pumping test from the time of the end of the pumping test (t) to a time (t_long) of around two to five years remains a compulsory requirement. This extrapolation is traditionally done by applying the Theis solution. A more sophisticated extrapolation of the pumping test drawdown beyond the time of the end of the measurement is obtained by using a Taylor series expansion based on the extrapolation of the measured drawdown curve including drawdown derivatives, and by accounting for boundaries as discussed in [13].

As discussed by [14].15].16].here are two Chloro-Alkaline Indices CAI₁ and CAI₂ for the interpretation of ion exchange between the groundwater and its surroundings during residence and/or travelling time in the aquifer.

The Chloro-Alkaline Indices are calculated from the following formula demonstrated in Table 2.

A positive value of CAI ₁ indicates there has been an exchange of sodium and potassium (Na⁺ + K⁺) in the groundwater with calcium and magnesium (Ca²⁺ + Mg²⁺) ions in weathered materials in the rocks by a type of base-exchange reaction. During this process, the host rocks are the primary sources of dissolved solids in the water. A negative value of CAI ₁ represents the absence of base-exchange reactions.

Ionic ratio for indicative elements is a useful hydrogeochemical tool to identify source rock of ions and formation contribution to solute hydrogeochemistry as discussed in [17]. These were used in this study.

Gibbs Diagram is a plot of Na⁺/(Na⁺ + K⁺ + Ca²⁺) and Cl⁻/(Cl + HCO 3 − ) as a function of TDS are widely employed to determine the sources of dissolved geochemical constituents. These plots reveal the relationships between water composition and the three main hydrogeochemical processes involved in ions acquisition; Atmospheric precipitation, rock weathering or evaporation crystallisation.

Pipers Diagram is a graphical representation of the chemistry of water sample on three fields; the cation ternary field with Ca, Mg and Na + K apices, the anion ternary field with HCO₃, SO₄ and Cl⁻ apices. These two fields are projected onto a third diamond field. The diamond field is a matrix transformation of the graph of the anions [sulphate + chloride]/Σ anions and cations [Na + K]/Σ cations. This plot is a useful hydrogeochemical tool to compare water samples, determine water type and hydrogeochemical facies as discussed in [18]. This has been used here for these purposes.

Durov diagram is a composite plot consisting of two ternary diagrams where the milliequivalent percentages of cations are plotted perpendicularly against those of anions; the sides of the triangles form a central rectangular binary plot of total cation vs. total anion concentrations. These are divided into nine classes by [19].hich give the hydrogeochemical processes determining the character of the water types in the aquiferous formation as discussed in [18].

WQI was calculated by adopting Weighted Arithmetical Index method considering thirteen water quality parameters (pH, EC, TDS, total alkalinity, total hardness, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO 4 2 − , NO 3 − , NH 4 + ) in order to assess the degree of groundwater contamination and suitability as demonstrated in Table 2.

Data from the geological traverse field mapping, field tests, field measurements and laboratory analysis were placed on MS Excel spreadsheets, and then mounted unto various GIS and software platforms, Rockworks14, Surfer V12, Grapher, AQqa and Enviroinsite where they were vigorously queried as demonstrated in Table 3.

The physicochemical parameters of groundwater in Abuja: Temperature, pH, EC and TDS for 94 boreholes were evaluated and the basic statistics as demonstrated in Table 4.

Borehole depths ranged from 19.5 m to 34.5 m, as seen in Figure 3. Borehole depth measurement is one of the first health checks for a borehole. It tells us

how well the borehole was constructed, developed, correctness of the borehole initial data and its state during exploitation. Silt and sand may have settled at the bottom of a well, well construction diagrams may be unavailable or inaccurate. The thickness of weathered basement is 15 m - 35 m with an average of 26 m.

In the study area, water levels for sampled wells ranged from 3 to over 12 m a.m.s.l. as seen in Figure 4.

The importance of water level measurements in aquiferous formations is one of the most understated, under-emphasized and most taken-for-granted

parameters in hydrogeological investigations. How shallow or deep the groundwater level in an area is tells us a whole lot about the ease with which groundwater exploration, exploitation and distribution can be carried out in an area. Also, it tells us about the vulnerability of the aquifer to pollution and the surface/subsurface interaction of surface/groundwater. Measurements of water levels in wells provide the most fundamental indicator of the status of water resources and are critical to meaningful evaluations of the quantity and quality of ground water and its interaction with surface water. Water-level measurements from wells are the principal source of information about the hydrologic stresses acting on aquifers and how these stresses affect groundwater recharge, storage, and discharge. The water levels are relatively shallow and as such prone to pollution.

Profiled borehole groundwater temperatures were close to air temperatures, ranged from 26.5˚C to 36˚C and had no significant vertical variations, as seen in Figure 6. Profiling groundwater temperatures with depth in boreholes can be used to detect fracture positions, different aquiferous formations and systems through which the well has been drilled and also as a tracer to determine groundwater velocity as discussed in [21]. [22].sed shallow subsurface temperatures to detect areas of increased flow to wells from riverbed infiltration, and to pinpoint zones of relatively better permeability in glacial outwash as discussed in [23].howed that variations in the smoother groundwater regional isotherm maps represented locations where differing land-use practices or groundwater movement disturbed the normal formational thermal system.

pondus Hydrogenium (pH): Weight of hydrogen ion in a solution; is a number

(0 - 14) equal to the negative logarithm of the hydrogen ionic concentration. The value of pH of a water sample is recognized as an index of classifying groundwater as acidic < 5.5, slightly acidic 5.5 - 6.5, neutral 6.5 - 7.5, slightly alkaline 7.5 - 8, moderately alkaline 8 - 9 and alkaline > 9.

The groundwater pH in the study area ranges from acidic (<5.5); Saprock with leached horizon-A as a result of precipitation and weathering, Neutral (6.5 - 7.5), humid climatic conditions having neutral environment with Ca and Mg as dominant ions; to slightly alkaline (7.5 - 8.0) with measurable carbonate in the groundwater causing silica to be soluble and mobile as seen in Figure 7.

Electrical conductivity is used for indicating the total concentration of ions in a groundwater sample. It is closely related to the sum of cations (or anions) as determined chemically and usually correlates closely with the total dissolve solids. Since temperature affects chemical conductivity, it is customary to express it at 25˚C. The electrical conductivity ranges from 13.4 to 1634 with a mean of 265.21

as seen in Figure 8. (0 - 250) low salinity, (250 - 750) moderately saline water (750 - 2250) High saline waters, >2250 high concentration Na⁺, HCO₃ and CO 3 2 − ions characterized by high pH.

TDS values of groundwater in Abuja vary between (<500 mg/L) fresh water to (500 - 2000 mg/L) slightly brackish water. This indicates the presence of some organics and silica in colloidal suspension. Total dissolved solid (TDS) is a measure of the combined content of all inorganic and organic substances contained in groundwater in molecular, ionized or micro-granular (colloidal sol) suspended form, used as an indicator test for the general quality of the water. Elevated total dissolved solids may suggest that toxic metals may be present at an elevated level. Very lower TDS concentration (<100 mg/L) may be corrosive and corrosive waters may leak toxic metals such as copper and lead from the household plumbing. This also means that trace metals could be present at levels that may pose a health risk as seen in Figure 9.

Groundwater migrates through aquiferous formations over different space and time scales. During this process there are water/formation and water/formation-water interactions; dilution by percolating rainwater, inflow/outflow of fresher groundwater and (in shallow phreatic formations) effects of deep evapotranspiration and weathering. The result is groundwater mixing. Here absolute values of parameters and/or ionic concentrations can only give marginal groundwater interpretation indices. The use of parameter and ionic ratios is the most apt tool to enable a good interpretation of the hydrogeology of aquiferous formations.

The relation between the different major ions has been elucidated through the determination of the ionic ratios as demonstrated in Table 5.

The expression of the ionic relationships in terms of mathematical ratio is quite helpful for establishing the chemical similarities among water, representing a single geologic terrain or single aquifer (Na⁺/Cl⁻). The collected samples have ratio ranging from 0 to 195.26, and the increase is due to the impact of marine salts on the groundwater composition, dissolution, ion exchange process and also may attributed to the addition of sodium salts of terrestrial origin to the water.

Eighteen (18) ionic ratios in groundwater were used to deduce formation inputs in Abuja, as demonstrated in Table 5. Twelve (12) of the 18 ionic ratios calculated gave indices indicating weathering of geologic formations in Abuja as a source of solute concentration in the groundwater while nitrate ratio indicates no anthropogenic contribution and sulfate indices indicates no oxidation of sulfides. Ca is sourced from gypsum while Na is sourced from halite-albite and ion exchange. Mg is contributed by dolomite dissolution, calcite precipitation or saltwater and Seawater or brine or evaporates as discussed in [17].

To know the groundwater chemistry and the relationship of the chemical components of the groundwater to their respective aquifers; chemistry of the rock types, the chemistry of precipitated water, and the rate of evaporation, according to [24].iagram in which the ratio of dominant anions and cations are plotted against the value of TDS was done. The chemical data of the collected groundwater samples are plotted in the Gibbs diagram as seen in Figure 12. Ninety-five (95) percent of the groundwater chemistry is a result of chemical weathering of rock-forming minerals through the dissolution of the host rock. Four percent of the groundwater chemistry results from atmospheric precipitation. One percent of the groundwater chemistry results from evaporation and recrystallization of minerals in the aquifer Table 6.

The concept of hydrogeochemical facies was developed in order to understand, identify and classify water composition in different classes as discussed in [17].28]. Facies are recognizable distinct zones that possess cation and anion concentration categories belonging to any genetically related system with. According to [29].iper diagram is one of many trilinear diagrams used to infer hydrogeochemical facies. It consists of three fields in a large triangle; two smaller triangular fields, one for plotting cations and another for plotting anions, are projected unto the third larger diamond shaped field, from which inference is drawn on the basis of hydrogeochemical facies concept. This tri-linear diagram is useful in bringing out chemical relationships among the groundwater samples in more definite terms rather than with any other possible plotting methods. A piper diagram was generated for the study area using the analytical data obtained from the hydrogeochemical analysis of the ninety-four groundwater samples as seen in Figure 13. This diagram revealed the dissimilarities and analogies in the different class types of water in the study area, which are identified and demonstrated in Table 7.

The diamond field of Piper’s diagram has seven classes A-G classifying water types and designated with alphabets from A to G as seen in Figure 13. Water from Abuja falls into A, B, D, E and G categories Table 8 and there is no category F. Category A 9 samples has 9.6% of samples; which indicates bicarbonate as prevailing ion of groundwater. Category B; 18 samples; 19.14% are characterized by normal earth alkaline water with prevailing bicarbonate. Category C; 15 samples, 15.95% are characterized by alkaline earth water with increased portions of alkalis with prevailing bicarbonate. Category D; 19 samples, 31.91%; are characterized by earth alkaline water with prevailing HCO 3 − ; Category E; 30 samples, 20.21%; are characterized by earth alkaline water with added portions of alkalis with prevailing chloride and Category G; 3 samples, 3.19%; are characterized by alkaline water with prevailing sulfate or chloride. Groundwater in Abuja is made up of 2 major water types; MgHCO₃ and CaHCO₃ as demonstrated in Table 6.

From the Piper’s diagram Figure 13, groundwater in Abuja is characterized by three hydrogeochemical facies. Field (I): Ca-Mg-Cl-SO₄ hydrogeochemical facies, 51 samples 54.25%, this facies are characteristic of groundwater some distance along its flow paths Field (II) Na-K-Cl-SO₄ hydrogeochemical facies has 40 samples, 3.19% and Field (IV) Ca-Mg-HCO₃ hydrogeochemical facie has 40 samples, 42.56%, this facies is characteristic of freshly recharged groundwater that has equilibrated with CO₂ and soluble carbonate minerals under an open system conditions in the vadose zone typical of shallow groundwater flow systems in crystalline phreatic aquifers. No samples plotted on Field III. The high contribution of alkaline earth elements in all seasons is due to direct ion-exchange processes which enriched groundwater with alkaline earth elements.

For better understanding the hydrochemistry and comparing the water types,

Durov Diagram was plotted as seen in Figure 14. The 9 fields of the Durov diagram on which the samples fall is given in below.

According to [30].urov diagram is a composite plot consisting of two ternary diagrams where the milliequivalent percentages of cations are plotted perpendicularly against those of anions; the sides of the triangles form a central rectangular binary plot of total cation vs. total anion concentrations. These are divided into nine Classes which give the hydrogeochemical processes determining the character of the water types in the aquiferous formation as discussed in [19]. Based on this Classification Table 7; seven Classes occur in the study area; Class 1 recharging waters: 3 samples, 3.19%; Class 2 ion exchange water: 1 sample, 1.06%; Class 3 ion exchange water: 2 samples, 2.13%, Class 4 simple dissolution: 19 samples, 20.21%; Class 5 simple dissolution or mixing: 60 samples, 63.83%; Class 6 probable mixing or uncommon dissolution influences: 6 samples, 6.38%; Class 7 Cl and Na dominant is frequently encountered: 3 samples, 3.19% respectively as demonstrated in Table 8. There are no Classes 8 and 9 in the groundwater from Abuja FCT.

Dominance of Ca and Mg in the groundwater samples collected from the high topography suggested an inverse ion exchange process. During this process Ca from the aquifer matrix will be exchanged by Na from the groundwater. However, in the lower topographic region water is dominated by the Na and Cl ions, which is represented by the discharge zone. Sluggish flow in these relatively flat regions enables sufficient rock-water interactions.

Using [31].uideline values of ions present in the groundwater, WQI values were determined according to [32]. WQI values ranged from −220 - 180. Groundwater in Abuja is excellent-unsuitable for domestic purpose base on WQI as demonstrated in Table 9 and seen in Figure 15.

Total hardness values ranged from: 0 - 519.12 as seen in Figure 16. This is indicative that groundwater in Abuja is of soft, hard moderately hard and hard categories as demonstrated in Table 10.