Quality and hydrochemical assessment of groundwater in geological transition zones: a case study from N.E. Nigeria

Sustainable management of groundwater resources in geological transition zones (GTZ) is essential due to their complex geology, increasing population, industrialization, and climate change. Groundwater quality monitoring and assessment represent a viable panacea to this problem. Therefore, there is a great need to investigate groundwater resources in terms of their chemistry and pollution to ascertain their quality and implement robust pollution abatement strategies. This study focused on the characterization of groundwater in a typical geological transition zone in northeastern Nigeria. Eighty-seven (87) groundwater samples were collected from dug wells and boreholes during the 2017 dry season. pH, conductivity, and total dissolved solids (TDS) were measured in situ using a multiparameter probe, while major cations and anions were measured using atomic absorption spectrometry and ion chromatography, respectively. Data were analyzed using descriptive statistics, principal component analysis (PCA), water quality index, and standard hydrochemical plots. TDS ranged between 95 and 1154 mg L−1 in basement terrains and between 49 and 1105 in sedimentary areas. pH ranged between 6.8 and 7.7 mg L−1 in basement terrains and between 5.0 and 6.5 in sedimentary areas, suggesting a moderately acidic to alkaline low mineralized groundwater. Calcium (2.6–128.0 mg L−1) was the dominant cation in the basement areas, suggesting silicate weathering/dissolution, while sodium (1.9–106.0 mg L−1) dominated the sedimentary zones due to base exchange reactions. The PCA analysis suggests that mineral dissolution (mostly silicate weathering) controls the hydrochemistry of the basement aquifers, while ion exchange and albite weathering, with some influence of anthropogenic factor, control the sedimentary aquifers. The water quality index revealed that the basement setting was predominated by poor to unsuitable groundwater, while the sedimentary terrain was characterized by potable groundwater. The dominant hydrochemical facie in the basement areas was Ca2+–(Mg2+)–HCO3− characteristic of recharge meteoric water. The Na+– (K+)–HCO3− facie characterized the sedimentary zones, indicative of cation exchange reactions, while the mixed water facie typifies the geological contact zones. The shallow nature of the basement groundwaters makes them more susceptible to geogenic and anthropogenic pollution compared to the sandstone aquifers. However, the basement aquifers have better irrigation indices (Kelly ratio and soluble sodium percent) as compared to the sandstone aquifers, which exhibit poor Kelly ratios (< 1) and soluble sodium percent (> 50) ratings. Results from the study clearly highlight the poor-unsuitable groundwater quality in parts of the studied GTZ and can be very instrumental to the policymakers in implementing sustainable water treatment strategies and cleaner production technologies in GTZ to forestall the incidence of water-related diseases.


Introduction
Availability and access to potable water are paramount in combating and averting numerous water-related diseases, especially in developing countries (Forstinus et al. 2016).
However, geogenic processes and anthropogenic factors due to increasing population, industrial activities, and urbanization have adversely impacted the quality of freshwater resources by aggravating water pollution . According to the World Economic Forum (2017), the nexus between food security and water security cannot be overstated as the water crisis remains a major threat to livelihood.
It is alarming that approximately one billion people worldwide lack access to potable drinking water and, consequently, approximately 2.2 billion people die yearly in third world countries like Nigeria as a result of water-related ailments (Olukanni et al. 2014;Ekere et al. 2019). Access to safe water represents a major struggle in Nigeria (Seiyaboh et al. 2017a, b), where approximately 66.3 million people (~ 33% of the population) do not have access to safe drinking water (Akinde et al. 2019), resulting in detrimental health problems (Allaire et al. 2018). Identifying the major sources of pollution as well as alternative sources of water and instituting the proper pollution mitigation approaches are needed (Eletta et al. 2020). Across Nigeria, groundwater represents the most reliable source of potable water due to its quality and availability, and the lack of infrastructures and the occurrence of maintenance problems typically related to surface water resources (Lapworth et al. 2012). However, groundwater in Nigeria is subject to multiple sources of pollution, including water-rock interaction (Talabi and Tijani 2013), landfill leachate (Gin et al. 2018), agricultural runoff (Edet et al. 2011), and domestic sewage (Ighalo et al. 2021). For example, in 2018 in the Bauchi state (N.E. Nigeria), a total number of 9405 cases of cholera with 35 deaths were reported due to contaminated water sources and poor sanitation facilities (Elimian et al. 2019).
During the past few decades, the implementation of waste management facilities in urban areas (Ajani 2008;Oloruntade et al. 2013) has partially attenuated the pollution from these sources. However, due to the limited coverage of these facilities across the country, groundwater contamination due to landfill leachate is still prevalent in rural areas (Uwadiegwu and Chukwu 2013). Chromium and other heavy metals (e.g., Cd, Pb, Zn, Ni, and Hg) have been reported in hand-dug wells in Akure and Lagos, S.W. Nigeria (Akinbile 2012; Olafisoye et al. 2013).
Geogenic processes represent an additional source of groundwater contamination (Ogunniyi et al. 2011;Emenike et al. 2017;Ighalo et al. 2021). The input from anthropogenic contaminants cannot be overemphasized and has resulted in a deplorable state of sanitation in many rural settings (Edet et al. 2011). While multiple studies linking the occurrence of geogenic metals in groundwater have been conducted in the southern part of the country (Ayantobo et al. 2014;Eyankware et al. 2019;Adewumi and Laniyan 2020;Emenike et al. 2020;Aladejana et al. 2020), there is a lack of studies investigating groundwater quality in the northern part of the country where cholera and other water-borne diseases are recurrent (Elimian et al. 2019;Ighalo et al. 2021). In addition to the lack of groundwater quality monitoring programs in the area, geological contact zones known for their hydrogeological complexity received little attention with respect to groundwater quality and hydrochemical assessment.
This study assessed the quality and hydrochemical characteristics of groundwater in the geological transition zones (GTZ) of Bauchi, in the semiarid zone in northeastern Nigeria. In this area, groundwater quality is faced with threats from geogenic and anthropogenic pollution (Jabbo et al. 2022), as well as the vulnerability of the groundwater to surface contamination due to the weak-moderate aquifer protective capacity in the area (Lawal et al. 2021b). Furthermore, the moderate-low recharge potential in the area is projected to decrease over time due to the urbanization, population increase, and climate change, which has necessitated the need for efficient groundwater resource protection and management (Lawal et al. 2021a). Previous studies have shown high levels of calcium, potassium, nitrate, hardness, and total dissolved solids in hand-dug wells and boreholes in residential areas in parts of Bauchi state due to landfill leachate (Gin et al. 2018). High amounts of sodium and chloride have been reported in parts of the industrial area in Bauchi (Dike et al. 2008). Jabbo et al. (2022) investigating the impacts of heavy metals on springs and wells using multivariate statistics and water quality index methods in the Yankari Game Reserve, N.E., and Bauchi State highlighted the need for groundwater quality monitoring, especially from the dug wells, to prevent possible water-related diseases. In addition, studies relating groundwater quality and hydrochemical evolution in the geological transition zones of Bauchi are limited. To the best of our knowledge, none of the available studies have evaluated the quality alongside the sources and hydrochemical processes governing groundwater chemistry in GTZ. Consequently, the present investigation attempts to fill this gap while trying to safeguard human lives and provide the necessary baseline data for policymakers to implement sustainable water treatment strategies and cleaner production technologies in GTZ.
The impact of anthropogenic and geogenic factors on hydrochemistry has been effectively assessed utilizing multivariate statistical analysis and traditional chemical approaches (Piper, Gibbs and Chadha diagrams, etc.), signifying the increasing application and popularity of statistical approaches in groundwater pollution studies (Liu et al. 2017;Bouteraa et al. 2019;Njuguna et al, 2020;Ismail et al. 2020;Ighalo and Adeniyi 2020). The objectives of this study, therefore, were to (1) assess the quality of groundwater in the geological transitional zones of Bauchi N. E, Nigeria, (2) evaluate the water quality indices, and (3) determine the hydrochemical facies as well as the main factors controlling the chemistry of the groundwater in such hydrogeologically complex zones. Such detailed but inexpensive assessment of groundwater quality across the entire state is needed in order to provide safe water to millions of people not only in NE Nigeria but across all developing countries.

Description of the study location
The studied region lies in the southeastern part of Bauchi State (latitude: 10,000′N-10°30'′N, longitude: 10°00′E-10°30′E; Fig. 1). The region covers a total area of 3025 km 2 , cutting across four local government areas (Bauchi, Alkaleri, Kirfi, and Ganjuwa), includes over 200 villages, has peril-urban to rural settings, and farming, trading, cattle rearing, and fishery represents the most common occupations. During the 2017 dry season, when the study was conducted, the study area had a projected population of 665,800 based on the National Population Commission (NPC 2006).
The region is distinguished by hills (up to 706 m) in the northern and southern parts and by lowlands (up to 275 m) corresponding to stream channels. The area is drained by the Dindima River, Mansuri River, and Yashi River, which all tributaries of the Gongola River. The annual average rainfall in the area ranges between 700 mm in the north and 1300 mm in the south, while the monthly variation of relative humidity in the area show similitude with the precipitation pattern (unimodal), exhibiting low amounts (20-40%) between January to April and attains its maximum (80%) in August (NIMET 2014). The vegetation is characteristic of the Sudan Savannah type, with low yearly rainfall (< 1000 mm) and the extensive dry spells (6-9 months), which can only nourish limited trees and relatively shorter grasses (Acworth 1987).

Geology and hydrogeology
The detailed geological description of the area has been elucidated elsewhere (Dike 1993;Ferre and Caby 2007;Lawal et al. 2020). Briefly, the geological transition lies between the crystalline basement (Precambrian) of northern Nigeria and sedimentary units of Kerri-Kerri formation (Paleocene) (Fig. 1). The four major lithological units in the area are (i) migmatite gneiss, (ii) biotite hornblende granites, (iii) bauchite, and (iv) sandstone, with minor variations in the form of granite gneiss, biotite gneiss, and biotite granites and quartzites. The migmatite gneiss Fig. 1 Map of the study location and its geological components (modified after Lawal et al. 2020) belongs to a group of rocks known as the migmatite-gneissquartzite complex and represents close to 60% of the Nigerian basement rocks (Rahaman and Ocan 1978). This rock suite is arguably the oldest group of rocks in Nigeria, with geological ages between Pan-African to Eburnean (Dada 2006). The major assemblages of rocks in the complex comprise migmatite, paragneisses orthogneiss, ultrabasic, and basic metamorphosed rocks. These migmatites represent the primary rocks in the study area and are well exposed in the western parts. The biotite hornblende granites and the biotite muscovite granites are Neoproterozoic rounded to elongate intrusions emplaced within the migmatite during the Pan-African orogenic episode (Ferre and Caby 2007). The charnockitization associated with this orogenic event was responsible for the formation of the charnockites (Bauchites) in the area (Ferre and Caby 2007). Locally, these granites and charnokites occur as intrusive plutons within the migmatites in the western parts of the study area. The sandstones are the dominant rocks in the Kerri-Kerri Formation, the youngest sedimentary formation in the Upper Benue Basin (Dike 1993).
The Kerri-Kerri formation is a sedimentary assemblage defined by fining upward sequences, however, coarsening upward sequences have been reported (Adegoke et al. 1978;Dike 1993). The formation is Paleocene in age, occupies an area of 30,000 km 2 , and is found as far north as Chad Basin, where it underlies the Chad Formation (Dike 1993). The depositional environment of this formation has been described by many workers. Dike (1993) described it as an alluvial fluvial deposit, while Adegoke et al. (1978) recognized fluviatile, deltaic, marginal lacustrine, and transitional environments of deposition for this formation. Dike (1993) also reported the occurrence of an unconformity in the southern parts of the transition zones around Mainamaji, with characteristic basal conglomerates believed to be the products of weathering of the underlying Precambrian basement complex rocks. These basal conglomerates grade gravelly sandstones and conglomerates sands. Dike (1993) also mentioned the absence of coarsegrained facies at the basal part of the Kerri-Kerri Formation around Alkaleri in the central parts of the transition and parts of Mainamaji, suggesting that the contact zones were not always marked by the presence of basal conglomerates.
The hydrogeology of the southeastern part of Bauchi includes weathered and fractured layers forming the aquifers in the basement setting and sandstones constituting the aquifers in the sedimentary terrain (Lawal et al. 2020). The maximum groundwater resources in the basement complex occur along the main tectonic lines, which constitute the preferential flow paths for the groundwater (Edet and Okereke 1991;Edet et al. 1998;Anudu et al. 2014). Groundwater occurrence in the Basement Complex rocks of Bauchi is controlled by the nature of the weathered regoliths and the fractured basement rocks, or the sap rocks (Shemang and Jiba 2005). These aquifers are generally of limited capacity owing to the restricted permeability of the basement rocks. The depth of wells in the basement terrain varies between 13 to 61 m, while the water table and hydraulic head vary between 2 to 4 m and 390 to 570 m (above sea level), respectively (JICA 2009). The yields of boreholes, however, range between 9.6 and 405.6 m 3 /day (Shemang and Jiba 2005).
Hydrogeologically, the Kerr-Kerri Formation can be considered to have higher groundwater potential compared to the basement complex areas of the state due to their sedimentary nature and hence well-developed primary porosities (Akujieze et al. 2003;Eduvie 2006;Lawal et al. 2020). The Kerri-Kerri Formation represents one of the biggest water-bearing formations in Nigeria with an approximate yield of 1.3 to 10 L/s (Akujieze et al. 2003;Eduvie 2006). Lawal et al. (2020) identified three sand horizons in the Kerri-Kerri sub-basin and reported that the intermediate fine to coarse-grained sandstones constitute the saturated aquifer units. Wells tapping from these sandstone aquifers have a depth to the water table of 6 to 68 m, a well depth of 25 to 165 m, and a hydraulic gradient of 250 to 410 m (JICA 2009). The flow of groundwater in the basement portion of the transition is irregular compared to the sedimentary basins, where a more uniform flow occurs from North to South. Also, in terms of aquifer recharge, the area generally exhibits a moderate recharge potential (Lawal et al. 2021a, b). In terms of hydrochemistry and pollution sources, rock-water interaction has been reported as the most significant pollution source in the northeast and the distribution of chemicals in groundwater of sedimentary and basement complex settings have been documented to have similar concentrations, although these conclusions were made from limited data occasioned by the paucity of hydrochemistry in northeastern Nigeria (Edet et al. 2011;Ighalo et al. 2021).

Groundwater sampling and analysis
Eighty-seven (87) water samples were collected from different rock units distributed across the study area to evaluate the effect of lithology on groundwater composition. Prior to the collection of each sample, water was allowed to flow for 10-15 min to avoid possible collection of stagnant water. Field parameters, including electrical conductivity (EC), temperature, total dissolved solids (TDS), and pH were measured using a multiparameter probe (HANNA Instruments). Split water samples were obtained in duplicates, collected in 100 ml polyethylene bottles during the dry season in November 2017, stored at 4 °C, and analyzed at the Nebraska Water Center's Water Sciences Laboratory (WSL) for alkalinity, anions, and cations. Alkalinity was measured by acid titration (APHA 2017), which involved the use of a standard acid to titrate the sample to set endpoints, pH = 4.5 represents total alkalinity, while pH = 8.3 stands for phenolphthalein alkalinity. Major anions were measured with an ion chromatograph (ICS-90) (Dionex, Bannockburn, IL, USA) with a detection limit of 0.1 mgL −1 . Major cations (calcium, magnesium, sodium, and potassium) were measured in acid-preserved (3-4 drops of concentrated nitric acid using a Perki-nElmer (PerkinElmer Inc., Waltham, MA, USA) AA400 Atomic Absorption Spectrometer with a detection limit of 0.01 mgL −1 . Quality control and quality assurance samples, including laboratory reagent blanks (LRB), laboratory fortified blanks (LFB), laboratory fortified matrix (LFM), and laboratory duplicate samples (LD1 and LD2) were implemented throughout the study, and the corresponding charge balance errors were calculated to ascertain the reliability of the results (± 10%, Ramesh et al. 2021).
In addition, to water quality analysis, depth and water level were determined with a calibrated water level meter. The type of well and well protection, major land use, and distance from pollution sources were also documented.

Data evaluation and analysis
Hydrochemistry data were analyzed using descriptive statistics, bivariate plots, multivariate principal component analysis, and hydrochemical plots (e.g., piper, ternary, Gibbs, Chadha, etc.). The water quality index (WQI) and irrigation water quality indices were also determined and evaluated. Descriptive statistics were used to summarize the hydrochemical data, while box and whisker and bivariate plots were employed to compare the relative proportions of the chemical species in the samples. Hydrochemical plots provided information on the processes that control groundwater chemistry and evolution. WQI was used to evaluate the influence of the dissolution of rocks and human-related activities on water quality. The evaluation of the WQI entailed the assignment of weight to the physicochemical parameters based on the relative impact of the parameters on the general quality of water. The principal component analysis or PCA, was utilized as a quantitative method to decrease the dimensionality of the hydrochemical data by evaluating the association between the variables of the samples and as a measure of the correlation between the groundwater constituents. Geostatistical analysis was done to assess the spatial distribution of WQI and the irrigation parameters. The main statistical packages deployed for the analyses are the Aquachem version 2014.2 (Waterloo Hydrogeologic, Waterloo, ON, Canada), Origin Pro 8 (OriginLab Corporation, Northampton, MA, USA), and the ArcGIS software (ESRI Headquarters, Redlands, CA, USA).

Well inventory and physico-chemical parameters
The elevation of the investigated wells ranged between 308.0 (sandstone) and 572.0 m (basement terrain) with an average of 460.0 ± 79.7 m (Table 1). Similarly, the hydraulic head ranged between 292.0 (sandstone) and 561.4 m (basement terrains), with an average of 444.7 ± 75.9 m, suggesting that groundwater flows from the basement terrain located in the western region to the sandstones located in the eastern region. The water column in the study region ranged between 0.5 and 12.6 m (9.87 ± 9.4 m), while the well depths ranged between 3.0 and 53.0 m in the basement setting and between 8.0 and 114.0 m in the sedimentary terrain (Table 1). The basement aquifers consist of weathered overburden materials with limited spatial extent and thicknesses (Wright 1992;Eduvie 2006). The sandstone aquifers are characterized by large spatial extent and thicknesses, in addition to well-developed primary porosity (Eduvie 2006). Therefore, the shallow aquifers in the basement terrain are more susceptible to local anthropogenic contamination, especially when the well heads are not adequately protected from unhygienic conditions and agricultural runoff.
Among the different water quality parameters investigated, pH varies between 6.4 and 7.7 (7.2 ± 0.3) in the weathered basement aquifer and 5.0 to 8.5 (6.0 ± 0.7) in the sandstone aquifers, indicating a moderately acidic to slightly alkaline groundwater system (Table 1). Among the different aquifer settings, pH ranged between 6.4 and 7.7 (7.2 ± 0.3), 6.8 and 7.5 (7.1 ± 0.3), 6.4 and 7.7 (7.2 ± 0.4), and 5.0 and 8.5 (6.1 ± 0.7) in the migmatite, granite, charnockite, and the sandstone, respectively. In the sandstones, EC and TDS ranged from 10.0 to 1520.0 μS cm −1 (276.1 ± 327.0 μS cm −1 ) and from 15.0 to 1105.0 mg L −1 (205.6 ± 235 mg L −1 ), respectively. In the basement setting, EC and TDS ranged from 120.0 to 2040.0 μS cm −1 (706.0 ± 387.8 μS cm −1 ) and from 95.0 to 1558.0 mg L −1 (512.0 ± 288.6 mg L −1 ), respectively (Table 1, Fig. 2). These results suggest low mineralized groundwater with limited migratory history within the sedimentary aquifers. Values of EC and TDS exceeding the permissible limits, EC-1000 μS cm −1 and TDS-500 mg L −1 (NSDQ 2007;WHO 2011), are predominant in the shallow dug wells in the basement setting, which are often located in the vicinity of waste drainages and dumpsites in villages or wells sited close to latrines in residences, suggesting a possible contribution from anthropogenic sources. Based on the above premise, it is evident that well depth, well head protection, and anthropogenic and geogenic processes have a profound effect on the groundwater quality in the area. In terms of hardness, 55% of the samples in the area, comprising largely of those from the sandstone aquifers (89%) and the bauchite terrain (58%), displayed hardness values below the permissible limit of 150 mg L −1 (NSDQW 2007), while the remaining 45%, primarily from migmatite and biotite hornblende granite zone (75% each), showed hardness values greater than the permissible limit.

Cation and anion chemistry
Among the major cation investigated, calcium (Ca 2+ ) and magnesium (Mg 2+ ) ranged from 0.2 to 216.2 mg L −1 (38.2 ± 41.7 mg L −1 ) and from 0.1 to 57. 7 mg L −1 (14.4 ± 15.3 mg L −1 ), respectively (Table 1, Fig. S1), and fall within desirable limits for drinking water purposes (NSDQW2007; WHO 2011). The occurrence of calcium and magnesium in water depends on the weathering and dissolution of calcium carbonates or silicate minerals in basement complex rocks (Fisher and Mullican 1997). Similarly, sodium (Na + ) ranged between 1.59 and 130.0 mg L −1 (27.6 ± 27.9 mg L −1 ), significantly below the permissible limit of 200 mg L −1 (NSDQW 2007;WHO 2011). In contrast, potassium ranged from 0.2 to 134.9 mg L −1 (9.23 ± 21.6 mg L −1 ), with fifteen samples having concentration greater than permissible limit (12 mg L −1 , WHO 2011). While sodium is commonly derived from halite dissolution, weathering of silicates and agricultural activities, potassium is released by conversion of feldspars to clay minerals (Fisher and Mullican 1997;Ali and Ali 2018). The study region, besides being a farming area, is dominated by crystalline rocks rich in feldspars and other silicate minerals (e.g., biotite and hornblende), known to be unstable and susceptible to weathering.
Among the major anions investigated, chloride ranged between 0.4 mg L −1 , in the basement terrain, and 318 mg L −1 , in the sandstones. The permissible limit for Cl − in groundwater ranges between 100 mg L −1 (NSDQW 2007) and 250 (WHO 2011). Only 8% (7 locations: 14, 15, 33, 55, 64, 66, and 84) of the groundwater samples displayed chloride values exceeding the NSDQW standard. These samples were primarily collected from wells near waste drainages and dumpsites in the villages and, to a limited extent, from wells sited within residences. Sulfate ranged between below the analytical detection limit and 117 mg L −1 (13 ± 20 mg L −1 ) and it was consistently below the permissible limits of 250 mg L −1 (WHO 2011). However, X samples exceeded the Nigerian permissible limit of 100 mg L −1 (NSDQW 2007). Nitrate (NO 3 − ) ranged from below the analytical detection limit to 719 mg L −1 (107 ± 136 mg L −1 ). Fifty-three percent (53%) of the water samples revealed nitrate levels exceeding the permissible limit (45 mg L −1 , WHO 2011; 50 mg L −1 , NSDQW 2007). According to the WHO (2011), nitrate concentrations greater than 50 mg L −1 are clear indications of shallow groundwater contamination. High levels of nitrate were detected in samples collected from shallow wells (Fig. 2). Deeper wells (e.g., 20-40 m and greater than 40 m), mainly located in the Kerri-Kerri Formation, showed low values and variability. Field observations suggested that the occurrence of high concentrations of nitrate was related to inputs from domestic waste/sewage, open defecation, and ponding of wastewater in the vicinity of the well (< 5 m), as well as agro-pastoral activities.

Lithology-based hydrochemical characterization
The lithology-based summary of the measured chemical parameters at the study location is given in Table S1. The biotite hornblende granite zone showed the highest average concentration of calcium (Ca 2+ ), 81 mg L −1 , followed by migmatite, 53 mg L −1 , and bauchite, 39 mg L −1 . Sedimentary terrain recorded a low concentration of calcium (average: 13.57 mg L −1 ). Magnesium (Mg 2+ ) displayed average concentrations of 26 mg L −1 , 23 mg L −1 , 9 mg L −1 , and 6 mg L −1 in the biotite hornblende granite, migmatite, bauchite, and the sandstone aquifers, respectively. The low levels of the alkali earth metals in groundwater from the sandstone aquifers were linked to the low amounts of unstable silicate minerals in the sandstones compared to the basement's terrain (Fisher and Mullican 1997). Na + averaged 43 mg L −1 (migmatite bedrocks), 36 mg L −1 (biotite hornblende granites), and 26 mg L −1 (bauchite) and 13 mg L −1 in the sandstone aquifers, in agreement with the general decrease in TDS observed from the basement complex areas to the sedimentary zones. Potassium (K + ), which is generally less abundant than sodium in nature, averaged 4 mg L −1 , 8 mg L −1 , 16 mg L −1 , and 12 mg L −1 in migmatite, biotite hornblende granite, bauchite and sandstone aquifers, respectively, suggesting substantial amounts of orthoclase feldspar in bauchites and sandstones (arkosic in nature). The K + in the orthoclase may replace divalent calcium from the calcic plagioclase, thereby releasing K + in water through cation exchange reactions (Morales et al. 2016). The overall assessment of the cations in this study revealed that there was a general depletion of alkali earth metals (Ca 2+ and Mg 2+ ) and overall enrichment of the alkalis (Na + and K + ) moving from the basement aquifers to the sedimentary aquifers due to a possible cation exchange reaction (Eq. 1). A monovalent K + may replace divalent cations from feldspars, as exemplified by the reaction of orthoclase (Na, K) AlSi 3 O 8 ) to produce anorthite (CaAl 2 Si 2 O 8 ) releasing Na + and K + The comparative abundance of the cations in the basement setting was such that Ca 2+ > Na + > Mg 2+ > K + in the groundwater from migmatite and biotite hornblende setting while in the bauchite the order was Ca 2+ > Na + > K + > Mg 2+ . In a typical basement setting, the amount of alkaline earth metals in the groundwater were expected to considerably surpass the alkali metals, which is the scenario in basement terrain in the research area, and this is in tandem with studies by Mapoma et al. (2017). In the sandstone aquifers, however, a different trend, Na + > Ca 2+ > K + > Mg 2+ , was observed, revealing enrichment of the alkalis and depletion of the alkali earth metals suggestive of the cation exchange reaction.
As revealed by Table S1, HCO 3 − was the prevalent anion in the area with the migmatite showing the highest average concentration of 206 mg L −1 , followed by the biotite hornblende aquifers at 197 mg L −1 , and the bauchite aquifers at 121 mg L −1 . The concentration of bicarbonates in the sandstones averaged 41 mg L −1 . Nitrate averaged 71 mg L −1 in the sandstone aquifers and 215 mg L −1 , 115 and 81 mg L −1 in migmatite, biotite hornblende and bauchite aquifers, respectively. Groundwater with high levels of HCO 3 − similar to those observed in the basement aquifers derived their solutes from silicate weathering (Fisher and Mullican 1997). The concentration of the strong acids (Cl − and SO 4 2− ) was dominant in the basement settings as compared to the sedimentary terrain (Fig. S1). The concentration of Cl − -ranged between 79 mg L −1 in the biotite hornblende granite to 17 mg L −1 in the sandstone aquifers, with the migmatite and bauchite averaging 34 and 36 mg L −1 , respectively. It is important to note that SO 4 2− showed the same increasing order as the Cl − with respect to the different bedrock. The biotite hornblende granite revealed an average SO 4 2− concentration of 30.1 mg L −1 , while migmatite, bauchite, and the sandstones averaged 18 mg L −1 , 9 mg L −1 , and 4 mg L −1 , respectively. The order of abundance of major anions in the wells was HCO 3 − > Cl − > SO 4 2− . The hydrochemical profile of the southeastern part of Bauchi has revealed that the dominant cation and anion in the basement setting are calcium and bicarbonate, respectively, and this is attributable to CO 2 -charged meteoric recharge in basement rocks (Lloyd and Heathcoat 1985;Fisher and Mullican 1997). Although, the possibility of having calcite and dolomite dissolution in the area was very low owing to the generally low range of pH in the area (pH: 5 -8.5) and coupled with the fact that the average TDS was generally lower 600 mg L −1 in accordance with Zhang et al. (2007). The sedimentary aquifers, on the other hand, revealed Na + and bicarbonates as the dominant ions indicating a possible case of CO 2 -charged meteoric water and ion-exchanged water. Furthermore, the influence of anthropogenic factors in this study was suggested by the general abundance of nitrate in the groundwater in the area (Hallouche et al. 2017). Based on the above discourse, it can be summarized that the groundwater in the area is influenced by both natural/geogenic factors, possibly precipitation and silicate weathering (as reflected by the dominance of HCO 3 − ) and anthropogenic factors, as indicated by the high content of nitrate in some of the studied groundwater samples (Fig. S1).

Principal component analysis (PCA)
The hydrochemical data obtained from the analysis of the eighty-seven (87) groundwater samples collected across the study area were subjected to a principal component analysis to enhance the data for better evaluation and understanding (Eslami et al. 2019;Jabbo et al. 2022). PCA yielded 3 significant principal components (factor loadings) based on Eigen value > 1 and their high cumulative variance of 87.0%, as shown in Table 2. The first factor accounts for 63.8% of the total variance, and it is characterized by high loadings of EC, TDS, Cl − , NO 3 − , SO 4 2− , Ca 2+ , Mg 2+ , HCO 3 − , and Na + . The Na + , Ca 2+ , Mg 2+ , and SO 4 2− may reflect the contributions of other hydrochemical processes (ion exchange, gypsum dissolution, and silicate. The high loadings of EC, TDS, Ca 2+ , Mg 2+ , HCO 3 − , and Na + in component 1 reveals the contribution of hydrochemical processes such as ion exchange, silicate weathering or carbonate dissolution and gypsum dissolution, while the strong loadings of Cl − , NO 3 − , and SO 4 2− suggest a possible influence of agricultural fertilizers and human and animal sewage confirming presence of significant anthropogenic activities in the area (Edet et al. 2011).
The second factor accounting for 15.4% of the total variance, revealed high loadings of pH and HCO 3 − , implying that the presence of different carbonate species in water is controlled by the pH variability. The third factor, accounting for 8.6% of the total variance, revealed high loadings of K + due to the release of potassium ions as a result of the conversion of feldspars to clay minerals (Fisher and Mullican 1997;Ali and Ali 2018).

Drinking water quality index (WQI)
WQI is a dependable and effective technique for evaluating, interpreting, and reporting water quality information (Asadi et al. 2007). In line with Vasanthavigar et al. (2010), suitable ranks were allocated to the physical and chemical parameters based on their perceived contamination potential (Table 3), whereas the relative weight Wi for every parameter was calculated using the equation provided by Krishna et al. (2015). The WQI of groundwater from the sandstone setting ranged between 2 and 114.0 (24.1 ± 61.8), while in the basement setting the WQI ranged between 8.0 and 115.0 (58.3 ± 75.3) (Fig. 3a, b). However, four groundwater samples from the sedimentary setting and six from the basement bedrocks revealed anomalously high WQI values of between 140 and 370 and between 182 and 374, respectively, attributable to contamination from nitrate, chloride, and sulfate due to human sewage (as suggested by the prevalence of open defecation in the study area), salts from fertilizers and other pesticide utilized by farmers, and leachates from dumpsites in these areas (Fig. S1). In line with WQI ratings developed by Mapoma et al. (2017), the sedimentary bedrocks generally have very good drinking WQI (< 25) as compared to the basement complex bedrocks which generally revealed poor WQI (55-70). The quality of groundwater in the study area was affected by the depth of the wells as well as inadequate well protection and poor sanitation in the vicinity of the wells (Fig. 3b). The wells in the Kerri-Kerri settings with depths of between 15 and 114 m generally revealed good and very good ratings (25 > WQI < 40), while the basement settings, dominated by shallow wells with depths between 3 and 20 m, are characterized by predominantly poor WQI (55 > WQI > 70). The above scenario can be attributed to contamination of the shallow basement wells by nitrate derived from human sewage, chloride, sulfate from fertilizers, and other dissolved chemicals form open dumpsites. These contaminants easily find their way into shallow dug wells (characteristic of the basement setting) where well heads are predominantly uncovered and unprotected. Thirteen samples from the biotite hornblende granite and migmatite settings revealed exceptionally unsuitable WQI (> 100) due to the occurrence of latrines, dumpsites, and farmlands where the effects of fertilizers, and open defecation are particularly high in the proximity of the wells. In the investigated region, the influence of anthropogenic factors on groundwater declined with increasing well depth owing to the fact that the longer times required for the dissolved nitrate to travel from the surface to the well allows for attenuation or denitrification of the nitrate before the contaminate the wells. Therefore, deeper wells, mostly located in the sedimentary terrain, were less susceptible to surface contamination than the shallower wells dominating the basement areas. However, there were a few exceptions where unsuitable water type occurs in very deep wells (KK5, KK6, KK29, and KK35). This can be related to the dissolution of gypsum in deeper parts of the sedimentary rocks, possibly from the lacustrine units of the Kerri-Kerri Formation (sandstone aquifers), inadequate well head protection from unhygienic human activities, and poor sanitary conditions around the wells. From the spatial distribution map of the WQI of the research area (Fig. 3b), it is evident that the sandstone aquifers have the best drinking WQI, followed by the bauchite, migmatite, and biotite hornblende granite settings, respectively. In conclusion, the WQI result indicated that 33.3% of the entire samples are unsuitable for drinking (WQI > 70), while 31% belongs to the very good category (WQI < 25). 17.1% are classified as good (WQI > 25 < 40) and 10.3 and 8.1% fall under moderate (WQI > 40 < 55) and poor water quality (WQI > 55 < 70) respectively.

Water quality for irrigation purposes
Irrigation quality was assessed using salinity hazard, sodium absorption ratio (SAR), residual sodium carbonate (RSC), soluble sodium percentage (SSP), permeability index (PI), Kelly ratio (KR), and magnesium hazard (MH). The mathematical formulation and a summary of the irrigation quality parameters of the study area are presented in Table S2 and Table 4, respectively. EC highly impact crops' yield. For example, low values (EC < 250 μs cm −1 ) provide favorable agricultural conditions while high values (EC > 750 μs cm −1 ) provide poor agricultural conditions (Richards 1954). Therefore, sedimentary aquifers dominate the excellent to good category with 91.7%, followed by the bauchite aquifers with 83.3%, as compared to 59.3 and 58.3 in migmatite and biotite hornblende granites, respectively. The variation in salinity hazard within the different bedrock settings is attributable to weathering of rocks/dissolution of silicate minerals, and anthropogenic activities prevailing in the different regions.
With respect to salinity and sodium hazard (Fig. S2), most of the Kerri-Kerri groundwater revealed predominantly C1S1 (low salinity with low sodium) and C2S1 (medium salinity with low sodium), while the bauchite settings mostly C2S1 (medium salinity with low sodium) and may be applied to soils with moderate leaching. The migmatite and biotite hornblende bedrocks, however, revealed medium (C2S1) to high (C3S1) salinity water, which can only be utilized for irrigation purposes on well-drained soils with moderate leaching potential.
SSP, also known as % Na, is a measure of the dissolved sodium content of the water. The basement aquifers revealed good SSP values (< 50%) suggesting excellent irrigation water quality; however, the Kerri-Kerri aquifers have a greater percentage (> 40%) of unsuitable irrigation waters based on SSP and this can be ascribed to cation exchange process which is more prevalent in the sedimentary settings. In terms of Kelly ratio, nearly all the samples Table 3 WQI parameters and weight assignment Weights, wi, weight of each parameter, and relative weights, and Wi, were adapted after Vasanthavigar et al. (2010) and Krishna et al. (2015), respectively. Anions, cations, total hardness (TH), TDS as mg/L, and EC as μs/cm from the basement bedrocks of the area are good for irrigation, while close to 50% of the Kerri-Kerri groundwaters are of unsuitable quality. The migmatite and the biotite hornblende granites recorded the highest percentages of samples with poor permeability index values, 33.3 and 50%, respectively, and this is attributable to the generally high levels of Ca 2+ and Mg 2+ . Overall, the groundwater from the basement setting would be best suited for irrigation or agricultural purposes. The sandstone aquifers are likely to be less suitable for irrigation use due to their high percentages of poor KR (< 1) and SSP (> 50) ratings, which is attributable to sodium enrichment occasioned by cation exchange reactions in the sandstone aquifers.

Hydrochemical facies
Hydrochemical facies are determined by the flow patterns, aquifer's geology, and solution kinetics. In the present study, piper trilinear and stiff diagrams were employed to evaluate the different hydrochemical facies. The Piper plot (Fig. 4) represents an efficient instrument for presenting hydrochemical information and characterizing hydrochemical facies.   (Fig. 4). Furthermore, weak acids exceed strong acids in 81.2% of the groundwater samples (CO 3 2− + HCO 3 − > SO 4 2− + Cl − ). Based on the Piper trilinear plot, the predominant water type in the studied aquifers was temporary hard water (50.6%), followed by the mixed water with no abundant cation and anion (28.2%) and by alkali carbonate water (HCO 3 − CO 3 2− , and Na + -K + ; 14.1%).
The groundwater facies show a general dominance of the Ca 2+ -Mg 2+ -HCO 3 − facie in the basement aquifers as compared to the sedimentary aquifers and this could be attributable to recharging meteoric water which dissolves mobile Ca 2+ and Mg 2+ from rock surfaces. Among the migmatite aquifers, twenty samples (74%) fall under the Ca 2+ -Mg 2+ -HCO 3 − water type, while reduced occurrences were observed among biotite hornblende granite (67%), bauchite (50%) and sandstone aquifers (27%). The plots also demonstrated the dominance of the alkali earth over the alkalis in the basement aquifers with the biotite hornblende granite aquifers having 100% dominance, migmatite (81.5%), bauchite (58.3%), and Kerri-Kerri (44.1%). The Piper plot showed a reverse trend in the distribution of the Na + -K + -HCO 3 − facie which is predominant (32.4%) in the Kerri-Kerri aquifers compared to the bauchite (8.3%) confirming the earlier inference suggesting that mineralization of the Kerri-Kerri groundwater can be attributed to cation exchange reactions with the associated lacustrine clays of the sedimentary units (Adegoke et al. 1978).
The Stiff plot provides a visual picture of the water chemistry by displaying distinct polygons typical of the hydrochemical facie of groundwater (Uliana and Sharp 2001). Representative samples of the stiff diagrams for the corresponding bedrock types were plotted and superimposed to  . 4 Combined piper trilinear plots of the samples from the area the geology map of the area to provide a spatial distribution of the hydrochemical facies of the aquifers combined with the degree of mineralization (Fig. 5). Groundwater from the basement settings is more mineralized than groundwater in the sedimentary settings (Fig. 5). This is in agreement with the TDS results, > 500 mg L −1 in the basement and ~ 205 mg L −1 in the sedimentary settings. Overall, three major hydrochemical facies were identified in this study: (1) calcium bicarbonate facies, dominating the basement settings (northwestern and southwestern zones), are a product of carbon dioxide charged meteoric recharge and silicate weathering and dissolution of rocks, (2) sodium bicarbonate water type dominating the sandstone aquifers, and (3) mixed group which occur the immediate zones of the geological contact and represents products of mixing of the aforementioned water types.

Mechanisms for hydrochemical evolution
The source of solutes and mechanisms responsible for groundwater hydrochemistry in the investigated aquifers was investigated using the distribution and compositional relationship of the major dissolved species in the groundwater. The Gibbs plot (Fig. 6) revealed that the groundwater system is primarily affected by rock dominance, suggesting that the hydrochemistry of the various groundwater in this study were mainly influenced by the rock dominance (the intermediate zone), which further confirms the silicate weathering/ dissolution origin made in earlier sections. The examination of the stoichiometric relations between cations and anions has been used to investigate the mechanism or combination of processes responsible for the evolution of the groundwater chemistry in aquifers (Fisher and Mullican 1997;Mapoma et al. 2017). Consequently, a hydrochemical diagram was developed following Chadha (1999) and used to further investigate the mechanism responsible for the evolution of the hydrochemistry in the research region. Among the four possible mechanisms identified, recharging water type was the dominant mechanism controlling the hydrochemical evolution of the investigated aquifers. In particular, the main mechanism controlling groundwater chemistry in the basement aquifers was the recharging water/ mineral dissolution, as 75%, 74%, and 50% of the samples from the biotite hornblende granite, migmatite, and bauchite, respectively, revealed recharging water and rock weathering as the chief source of solute in the groundwaters. Conversely, in the sedimentary aquifers (Kerri-Kerri), only 22% of the samples revealed the recharging water mechanism as the source of dissolved species in the aquifer units. Ion exchange reactions were more peculiar to the Kerri-Kerri aquifers of the area as 58% of the samples from these sedimentary aquifers covering mostly the eastern half of the studied area, revealed evolution from base ion reactions as compared to 33.3%, 11.1%, and 0% in the bauchite, migmatite, and the biotite hornblende granite aquifers, respectively. The chloroalkaline indices have been a vital tool used by hydrochemists to detect the mechanism base ion exchange reactions which control groundwater chemistry (Zhu et al. 2007). Negative values of chloro-alkaline indices imply the occurrence of ion exchange reactions. The dominance of negative values (89%) of the chloro-alkaline indices further confirms the role of base ion exchange reactions as a major mechanism controlling the hydrochemical development in sedimentary aquifers.
While the Gibbs and Chadha plots provided useful information on the mechanism for the hydrochemical evolution of groundwater in the studied aquifers, only a little information has been provided on the nature and lithology of the rock species involved in the hydrochemical reactions in the study area (Fig. 7). To overcome this limitation, cross plots of the major ions were employed to confirm the processes controlling the groundwater chemistry with more emphasis on the reacting masses following Hallouche et al. (2017). Figure 8 (alkaline earth ions (Ca 2+ + Mg 2+ ) vs. HCO 3 − ) shows that the majority of samples do not scatter along a 1:1 line, indicating that these solutes are not contributed by the dissolution of carbonates but rather by other sources, possibly silicate weathering/weathering of amphiboles and pyroxene minerals from the basement rocks (Hallouche et al. 2017). A ratio of Ca 2+ + Mg 2+ /HCO 3 − lower than 1 suggests recent meteoric recharge (Nazzal et al. 2014). Most of the studied samples, with the exception of a few locations (6,8,32,33,34,31,55, and 58 from migmatite aquifers and 15, 62, and 65 within biotite hornblende granite aquifers), satisfied the above condition, thus showing great input of fresh groundwater recharge in the investigated area (Fig. 8a). The cross plot of Na + against Cl − (Fig. 8b) revealed that most of the samples were > 1 or fell away from 1:1 line suggesting input from silicate weathering (Al-Amry 2008). Halim et al. (2010) and Yidana et al. (2010) also surmised that halite dissolution is not the primary cause of high Na + in groundwater when the samples disperse considerably away from the halite dissolution line (1:1) in Na + vs Cl − plots (Fig. 8b). However, when the samples scatter significantly away from the halite dissolution line (1:1), the causative mechanism could be ascribed to base ion exchange reactions, and to some extent, weathering of silicates, which is the case in parts of the charnorckite and the sedimentary areas of the study area.
Similarly, ratios of Ca 2+ /Mg 2+ > 2 indicates silicate weathering and this corroborates the aforementioned ionic relationships (Ca 2+ + Mg 2+ /HCO 3 − and Na + vs Cl − ) because the mean values of the Ca 2+ /Mg 2+ for all the rocks are greater than 2, except for the migmatite aquifers, which is 1.52, but when the whole basement is considered, the ratio is greater than 2, indicating a silicate weathering origin for the solutes in the aquifers under study and this is in tandem with study conducted by Ayadi et al. (2018). Furthermore, the plot of Ca 2+ + Mg 2+ vs. cation (total) (Fig. 8c) revealed a growing contribution of Na and K as TDS increases as most of the samples falls below the 1:1 line. The dominance of Na + in sandstone aquifers is a weathering index which indicates that the ions result dissolution of salts (from connate water or cementing material) and this in agreement with the work of Rahman et al. (2011). The normalized Mg 2+ vs. Ca 2+ (Fig. 9a) suggests silicate weathering as the primary source of chemicals in the studied groundwater (Appelo and Postma 2005), as most plots disperse in the silicate weathering region, with 13 samples (having representation in all the aquifer types) approaching carbonate dissolution. Similarly, a plot of normalized HCO 3 vs. Ca 2+ (Fig. 9b) shows that elevated Ca 2+ in the system is mostly from silicate weathering and possibly carbonate dissolution (Mukherjee et al. 2009;Halim et al. 2010). Most groundwater samples fell in the proximity of the silicate dissolution end member, with a few points plotting close to the carbonate and evaporite dissolution. The normalized plot Mg 2+ vs. Ca 2+ (Fig. 9a) suggests silicate weathering as the primary source of solutes in the groundwater, as most plots disperse in the silicate weathering region, with 13 samples (having representation in all the aquifer types) approaching carbonate dissolution. Similarly, a plot of normalized HCO 3 − vs. Ca 2+ (Fig. 9b) also revealed that a high amount of Ca 2+ in the groundwater is derived from lithogenic processes (Mukherjee et al. 2009;Halim et al. 2010).

Mechanism of rock/mineral weathering and the impact on groundwater quality in the transition zone
Chemical reactions alter rocks and minerals exposed to the earth's crust. These reactions, which can be very complex, take place between rocks/minerals and atmospheric substances like water, carbon dioxide and oxygen, giving rise to new minerals or dissolved minerals, thereby impacting the quality of the groundwater system in an area. As rain drops through the atmosphere, it combines with carbon dioxide to form weak acid (carbonic acid) (Eq. 2).
Following the formation of the weak carbonic acid, it dissociates into hydrogen and bicarbonate ions. The carbonic acid becomes more acidic as it dissolves more CO 2 in the  The common rock-forming minerals in the basement rocks are quartz (SiO 2 ), muscovite (K Al (Si 3 Al) O 10 (OH, F)), albite (NaAlSi 3 O 8 ), orthoclase KAlSi 3 O 8 ), biotite (K(Mg, Fe) 2-3 Al 1-2 Si 2-3 O 10 (OH,F) 2 ), hornblende (Ca, Na) 2-3 (Mg, Fe, Al) 5 (Si, Al) 8 O 22 (OH,F) 2 ), and fayalite (Fe 2 SiO 4 ). The degree to which the mineral phases react with the groundwater depends on the availability of protons (H + ), the contact time, and the surface area per unit volume of water (Hem (3) CaAl 2 Si 2 O 8 + H 2 CO 3 + 1∕2 O 2 → Al 2 Si 2 O 5 (OH) 4 + Ca 2+ + HCO 2− 3 (4) . The groundwater in the basement bedrocks of the study area comprises hard and very hard water, which could be attributed to the dissolution of silicate minerals from rocks as elucidated in the above equations (Fig. 10). The weathering of anorthite (calcium-rich feldspars) and hornblende is responsible for the abundance Ca 2+ in the basement groundwaters (Ganyaglo et al. 2010;Ali and Ali 2018).
Conversely, the soft water species are predominant in the Kerri-Kerri formation, and this can also be attributed to the fact that the sedimentary rocks are majorly sandstones, which are rich in silica and generally low in calcium and magnesium-rich minerals. The Na + in the sedimentary groundwater is attributable to the weathering of albite and halite in the rocks in the area or as a result of anthropogenic processes relating to by-products of agricultural activities in the area (Sultana 2009;Khan et al. 2014;Mostafa et al. 2017 Fig. 10 Representation of geogenic processes in the study area: a basement setting, b sedimentary settings, and c transition zone groundwaters is an indication of ion exchange reactions (Morales et al. 2016). The necessary exchangers may be provided for by the presence of clay minerals such as kaolinite or lacustrine shales, which have been observed in the study area (Fig. 10) and presented in Eq. 5, where X represents clay: In the transitional zones, however, the interaction between the groundwater from the different aquifers was observed as these zones were typified by mixed water facies (Ca-Na-Mg HCO 3 , Na-Mg-Ca HCO 3 , and Na-Ca-Cl-HCO 3 ), which are products of groundwater mixing and common along a groundwater flow paths (Eslami et al. 2019). As shown in the groundwater facie map of the area, the northern parts of the transition where the groundwater flows from (Lawal et al. 2021b), revealed predominance of the sodium bicarbonate water and the sodium chloride facie, suggesting more influence of the sedimentary lithogenic processes (albite and halite weathering). The middle zone of the transition is typified by Mg-HCO 3 , CaCl, NaCl, and mixed facies, while the southern parts of the transition revealed a combination of the sodium bicarbonate and calcium bicarbonate groundwater, suggesting a mix of basement and sedimentary geogenic processes (Fig. 10). This part of the transition has been reported to be characterized by basal conglomerates believed to be the products of weathering of the underlying Precambrian basement complex rocks (Dike 1993).
In terms of the quality of the groundwater in the geological transition zone under study, close to 40% of the samples from the vicinity of the contact zone revealed unsuitable water quality, while the remaining samples indicated moderate to very good water quality. The former were found to have high TDS (274-908 mg/L), EC (380-1520 μs/cm), and nitrate contents (150-619 mg/L), attributable to rock water interaction and anthropogenic processes like agricultural run-off, leachates from human and animal waste, as well as leachates from polluted ponds, which are common practices in the study area (Fig. S1). The above assertion corroborates studies by Edet et al. 2011;Talabi and Tijani 2013;Jeelani et al. 2014;Hallouche et al. 2017;Eslami et al. 2019;Saha et al. 2019;Ighalo et al. 2021;Jabbo et al. 2022.

Conclusion
The assessment of the hydrochemical information from the research region unraveled the groundwater system's quality and hydrochemistry. Hydrochemical data revealed the presence of minimally mineralized fresh water (15 ≤ TDS ≤ 1588 mg L −1 ). Furthermore, the results showed that (5) 2Na + + Ca∕Mg − X 2 ↔ Ca 2+ + 2Na − X groundwater is slightly acidic to alkaline (5.0 ≤ pH ≤ 8.5), and 55% of samples can be classified as soft to moderate (TH ≤ 150 mg L −1 ). The soft water species predominate the sandstone aquifers while the hard and very hard water types are prevalent in the migmatite and biotite hornblende granite area. Elevated nitrate (> 50 mg L −1 ) and EC values (> 1000 mg L −1 ) observed among some of the basement aquifers can be related to the occurrence of shallow wells and anthropogenic contamination. The PCA analysis suggests that the physicochemical parameters controlling the mineralization of the basement aquifers in the area are Mg 2+ , TDS, T.H, SO 4 2− , Ca 2+ , pH, and HCO 3 − and this reflects the mineral dissolution (mostly silicate weathering). In contrast, the sandstone aquifers are partly defined by EC, Na + , Cl − , NO 3 − , and K + , which is an indication of possible cation exchange and some influence of anthropogenic factors.
The water quality index revealed generally poor to unsuitable drinking water quality in the basement areas and very good to moderate in the sedimentary areas. Sixty (60%) of the groundwater from the vicinity of the contact zone revealed moderate to very good water quality, while 40% of the samples indicated unsuitable water quality, attributable to rock water interaction and anthropogenic processes like agricultural runoff, leachates from human and animal waste, as well as leachates from polluted ponds, which are common practices in the study area. In terms of agricultural uses, most of the calculated indices were within the prescribed limits for irrigation purposes in the basement areas. The sandstone aquifers, however, are likely to be less suitable for irrigation use due to their poor Kelly ratio and soluble sodium percent ratings.
The study revealed 3 major hydrochemical facies (calcium bicarbonate, sodium bicarbonate, and mixed water types). The mixed water facie with no abundant cation and anion represents the product of the mixing of the different water types, suggesting a heterogeneous and complex hydrogeological system in the studied geological transitional zones. Recharging water/weathering of silicates and cation exchange (in the sedimentary basin) represent the major mechanisms for hydrochemical evolution in the area. A detailed but low-cost preliminary characterization of potential sources of water and pollution sources across the entire state is crucial in order to provide safe water to millions of people, not only in NE Nigeria but across all developing countries.
As a form of recommendation, future hydrochemical research in the area should include refining the basin conceptual model in terms of groundwater age and residence time distributions and analysis of geochemical data from rocks so as to examine the influence of the mineral assemblages of the different rock types on the hydrochemistry of the studied area and the mixing dynamics.