Study area

The Olbanita aquifer system is located in the lower Baringo basin of the Kenyan Rift (Figure 1). It is characterized by proximity to the equator and by its elevation which ranges from 1750 meters to 1880 meters above sea level. According to National Council for Population and Development,¹⁸ the demographic profile for Nakuru County (wherein the Olbanita aquifer system is located) in 2009 was 1 602 637 and is projected to increase to 2 400 367 in 2030 and 3 013 869 in 2050. The city of Nakuru, situated in the upper Lake Nakuru Basin in the south, relies heavily on the 8 wells drilled in the adjacent Lower Baringo basin for potable water supply. As noted by GOK,¹⁹ water availability problems in the greater Nakuru and Baringo basins had negatively impacted on resident communities and the regional gross domestic product. Within the city, where the abstracted water is used, high incidences of diarrhoeal infections become significant within the overall disease burden.²⁰

Figure 1.

Location of Olbanita aquifer in Kenya showing sampled boreholes (blue circled dots).

The hydrogeology of the area comprises fractured and weathered volcanic rocks and lacustrine sediments. The weathered tuffs sandwiched between the Samburu basalts and the Wasagess flows (phonolites and trachytes) of the Rumuruti group form the best aquifers in Olbanita area. The near N-S trending fault systems interrupting the aquifer system at 5 sites provide groundwater high porosity pathways. At a regional semi-arid climate, the open-faulted drainage, together with the semi-arid climate, renders groundwater in the area indispensable for human subsistence. The confined aquifer in the area recharges from the high altitude areas of Bahati (east of the area) and Menengai, where ecological conditions permit households to grow coffee, tomato, and maize as high-value enterprises.

Sampling and analysis methods

The sampling design in this study involved 6 out of 8 sampling sites constituted of boreholes in the Olbanita aquifer system located in the Kenya Rift (Figure 1). The sampling sites were located using a Garmin GPSmap60CSx model. At each sampling site, 4 replicate samples were taken monthly in 250-mL high-density plastic bottles which had already been precleaned by using concentrated nitric acid and drenched in deionized water. The samples were screened through Millipore membrane filters of pore size 0.2 micrometers to remove suspended solids. Thereafter, the water samples for sulfate analysis were precipitated using 0.2 M Zn(CH₃COO)₂ while samples for metal analysis were acidified in concentrated HCl or ${\mathrm{HCO}}_3^{-}$ 1 mL per 100 mL sample. In situ parameters (pH, electrical conductivity, and total dissolved solids) were measured using Hanna Instruments multiparameter meter model HI98194. Out of the 4 samples taken in plastic bottles, 2 replicate samples were stored in ice and taken to the laboratory for determination of geochemical parameters, namely Na⁺, K⁺, Ca²⁺, Cl⁻, F⁻, ${\mathrm{SO}}_4^{2-}$ , H₂S, Total CO₂, ${\mathrm{CO}}_3^{2-}$ , ${\mathrm{HCO}}_3^{-}$ , H₂CO₃, and ${\mathrm{NH}}_4^{+}$ . The concentrations of sulfate and chloride components in this study were quantified using ultraviolet-visible spectrophotometer model UV-1800 while the ions of Na, F, Ca, K, and NH₃ were quantified using an Ion-Selective Electrode (ISE) model ELIT 9801 as detailed by Victoria and the US Environmental Protection Agency (US EPA).²¹ Concentrations of nonionic components namely total carbonate carbon (TCC) and H₂S were quantified using titrimetric methods as also described in Victoria and the US EPA.²¹ Before ISE analysis, the acidified samples were re-digested using a strong alkali solution. TCC was further speciated into ${\mathrm{CO}}_3^{2-}$ , CO₂, ${\mathrm{H}}_2{\mathrm{CO}}_3^{-}$ , and ${\mathrm{HCO}}_3^{-}$ .

To evaluate the appropriateness of geochemical data for factor analysis, Kaiser-Mayer-Olkin (KMO) measure of sampling adequacy and Barlett’s test of sphericity were performed.^22,23 The KMO index was found to be 0.6 indicating a moderate degree of correlation among the variables could be appropriately carried out. In addition, Barlett’s test of sphericity was significant (Bartlett χ² = .570, 66 df, P < .001), confirming the suitability of factor analysis on the data collected in this study. The results of the assessment depict a lower number of variables (factors) that may be used to elucidate the variability in the hydrochemical data. The correlation matrix was further inspected for correlation coefficients greater than 0.3.²⁴ Statistically significant correlations in physical-chemical data sets for each season were identified through the application of the Spearman’s rho for nonnormally distributed data. Most of the correlation coefficients are over 0.3 indicating that factor analysis may be utilized to provide significant reductions in data dimensionality. Helsel²⁵ and Rangeti et al²⁶ warn about the dangers of such simplified substitutions for nondetectable concentrations (by using numerical surrogates such as one-half the detection limit). Data were, therefore, automatically rescaled for HCA (because each predictor variable adopted a different scale of measurement), though exempted data sets for PCA. The laboratory geochemical data sets were utilized for PCA considering that the bottom-line correlation matrix (based on the Spearman rank) has the effect of standardizing the variable data.²⁷ Therefore, it is a more robust estimation technique that is less responsive to outliers compared with the widely used Pearson correlation matrix.²⁷

Groundwater quality data were further subjected to PCA and HCA. All statistical computations were executed using of MS Excel spreadsheet and SPSS software version 20.0. For the PCA matrix, orthogonalization of factors was based on the rotated varimax method (with significant eigenvalue loadings > ±0.5) and a derived scree plot (with the criterion of eigenvalues >1) was inspected for purposes of extracting varifactors.²⁸ For HCA, the software’s algorithm utilized Euclidean distances and “sum of squared errors” to minimize the criterion function.

General consideration of data sets

Based on the test of skewness which uses the arithmetic mean and standard deviation, the physical-chemical parameters such as total dissolved solids (TDS), pH, electrical conductivity (EC), and the ions such as Cl⁻, Na⁺, F⁻, Ca²⁺, ${\mathrm{SO}}_4^{2-}$ , ${\mathrm{H}}_2{\mathrm{CO}}_3^{-}$ , ${\mathrm{CO}}_3^{2-}$ , and H₂S are not considered to follow a normal frequency distribution²⁹ across the study area and between the sampling seasons. On the contrary, CO₂ and NH₄ depicted Gaussian distribution (ie, a significant departure from normality with a skew statistic >2) (Table 1). The binomial nature of the former is an indication of natural substandard waters characterized by high mineralization. Supersaturation of Olbanita groundwater may be linked to processes like the dissolution of halide, ion exchange, and weathering of sodium-rich plagioclases (usually giving rise to clay mineralogy). The high concentration of H₂S is an indication of very deep circulation attaining anoxic conditions accompanied by abundant bacterial activity.

Table 1.

Descriptive statistical data for all the parameters for the sampling period (n = 6, N = 42).

To offset the effects on mineral dissolution caused by dilution by meteoric waters from the analyses, seasonal data sets were analyzed independently. The initial step in the analysis was to account for the extent of mutual variability between individual pairs of water quality variables during the separate seasons. The inter-item correlation matrix of the measured parameters during the dry season and the wet season is provided in Tables 2 and 5, respectively.

Table 2.

Spearman rank correlation coefficients of the physicochemical parameters of groundwater for the dry season (November 2017–February 2018).

Generally, the rotated component matrix was found to contain both positive and negative loadings (Tables 4 and 7). The work of Liu et al³⁰ as enumerated in Mohapatra et al³¹ observed that eigenvalue loadings near ±1 designate a strong association between a variable and a principal component (PC); eigenvalues exceeding ±0.75 represent strong correlation, eigenvalues between ±0.5 and ±0.74 represent moderate correlation and those approaching 0 depict weak correlations. Each PC was attributed to a process owing to which the corresponding variables are probably linearly linked. The underlying processes occurring within the aquifer as construed from the consequent eigenvalue loadings are presented in Tables 4 and 7 where significant eigenvalue loadings are indicated by using the * mark.

Dry season water quality parameters

For the dry season data, the pairs pH-F, TDS-EC, TDS-CO₂, TDS-HCO₃, EC-CO₂, EC-HCO₃, Cl-SO₄, Ca-K, Ca-NH₄, F-NH₄, CO₂-HCO₃, as well as HCO₃-CO₃ showed strongly significant relationships. The pairs pH-Na, pH-Ca, pH-H₂CO₃, pH-NH₄, TDS-Na, EC-Na, K-Cl, Cl-CO₂, Cl-HCO₃, Na-F, Na-HCO₃, S0₄-CO₂, S0₄-HCO₃, Ca-F, NH₄-H₂CO₃, and HCO₃-H₂S depicted moderate correlations (Table 2).

Principal components extracted

Based on the eigenvalues > 1 criterion, 4 principal components explained variability in groundwater quality at the site (Figure 2). Based on the cumulative variance of the rotation sums of squared loadings of the dry season, the retained latent constructs account for 90.1% of the variance in the data set (Table 3). PC1 with the largest eigenvalue accounted for a maximum of the total variability (34.8%). PC 2 accounted for the total variation of 21%. The third PC explained 20.8% of the total variance, whereas final PC explained 13.6% of the remaining variation in the data. Observed eigenvalue decomposition corresponds to earlier observations by Hossain et al³² that after the first PC, the second PC explains the greatest of the residual variance and so forth.

Figure 2.

Cattel scree plot with eigenvalues > 1 criteria.

Table 3.

Total variance explained for the dry season data.

Table 4.

Extracted factor loadings of the measured parameters during the dry season which suited the provisions of orthogonal varimax rotation.

The first principal component, PC1 (the conductivity component), is associated with significantly high concentrations of electrolytic ions indicated by TDS, EC, Na⁺, Cl⁻, ${\mathrm{SO}}_4^{2-}$ , CO₂, and ${\mathrm{HCO}}_3^{-}$ . TDS depicted a strong correlation with EC (+0.99, α = 0.01) due to the extensive range in the solubility of/and mineral diversity within the aquifer system. Under sluggish flow during the dry season, groundwater can attain chemical saturation regarding TDS. Statistical analyses associate Na+ with the dissolution or chemical weathering of sodium-rich plagioclases (to produce clay minerals) or the dissolution of halide.¹² The moderate correlation observed between Na and F (+ 0.7) and Ca-F (–0.5) coupled with a relatively weak association between the pairs; Na-Cl (–0.4) supports partial derivation of electrolytic ions from weathering of Na-rich feldspathic rocks, dissolution of accessory mineral apatite as well as carbonate materials as opposed to dissolution of either halide rocks. The insoluble products of rock weathering such as Cl⁻ and ${\mathrm{SO}}_4^{2-}$ show a strong positive correlation (+0.84, α = 0.01) but cumulatively tend to inhibit the electrical conductivity of groundwater. Within the hydrogeological framework, the pattern and, therefore, the provenance of weathering can be accounted for by the roughly N-S fracture-fissure zones (Figure 1).

The second principal component, PC 2 (the salinity component), is depicted mainly by Cl⁻, ${\mathrm{SO}}_4^{2-}$ , Ca²⁺, and K⁺ ions in water. The anomalous distribution of Cl⁻, ${\mathrm{SO}}_4^{2-}$ , and Ca²⁺ is attributable to ion exchange mechanisms in saturated aquifer zones. The relatively strong association between the pairs Cl-SO₄ (+0.8), and the moderate association between the pairs Ca-F, K-Cl, S0₄-CO₂, Cl-HCO₃, and S0₄-HCO₃, indicate that aquifer water salinity is chiefly attributed to geologic derivation. In addition, agriculture is equally a major pollution source owing to the skewed distribution of Ca²⁺ which depicts strong correlations with K⁺ ions (+0.8) and ${\mathrm{NH}}_4^{-}$ (+0.8) at α = 0.01.

The third principal component, PC 3, is the alkalinity component indicated by pH, and Na⁺, F⁻, H₂CO₃, and ${\mathrm{NH}}_4^{-}$ . The pH, Na, and F have positive loadings, whereas H₂CO₃ and NH₃ have moderate negative loading on this PC. H₂CO₃ and NH₃ are slightly broken down (ie, at about 25°C) to release H⁺, ${\mathrm{HCO}}_3^{-}$ , ${\mathrm{CO}}_3^{2-}$ , and ${\mathrm{NH}}_4^{-}$ thereby reducing the pH. Reduced pH significantly increases the rates of weathering introducing more Na and F from geologic sources.

The fourth PC represents carbonate equilibrium with exogenic pH control. The external factor controlling pH is H₂S. Sulfide in borehole waters is probably due to inorganic and bacterial changes in the deep aquifer under low dissolved oxygen, optimum growth range in pH (between 5.5 and 8.5) and optimum temperature (between 24°C and 42°C). Besides the authigenic derivation, meteoric waters may also leach agricultural ${\mathrm{SO}}_4^{2-}$ which may also undergo reduction at depth to sulfides.³³ Unfortunately, the latter may effectively be ruled out due to a complete lack of strong correlation neither with sulfide nor with ammonia (Table 2). Deeper depths within the bedrock are readily flushed by low mobility, oxygen-deficient groundwater; effects that are conditioned by low permeability at depth. The sulfate-reducing bacteria, Desulfovibrio desulfuricans obtain energy via the interconversion between sulfates and sulfides within the larger sulfur cycle in the aquifer system.^34,35 Under these conditions, sulfates may be reduced to sulfides producing metallic sulfide which is again changed to H₂S under the action of H₂CO₃. The dissociation of H₂CO₃ yields the ${\mathrm{CO}}_3^{2-}$ and donates H⁺ which consequently reduces sulfates to sulfides.

Wet season water quality parameters

During the wet sampling season, the pairs pH-F, pH-H₂CO₃, pH-CO₃, TDS-EC, Na-F, F-H₂CO₃, F-CO₃, CO₂-H₂CO₃, and H₂CO₃-CO₃ showed strong positive correlations at α = 0.01 (Table 5). The pairs pH-Na, pH-NH₃, TDS-Cl, TDS-Na, TDS-F, EC-Cl, EC-Na, Cl-Na, K-Cl, Na-CO₃, SO₄-CO₂, SO₄-HCO₃, SO₄-H₂S, Ca-F, Ca-CO₂, and Ca-NH₄ depicted moderate correlations at α = 0.01.

Table 5.

Spearman rank correlation coefficients for physiochemical parameters of groundwater for the wet season (March-May 2018).

Principal components extracted

Based on the cumulative variance of the rotation sums of squared loadings of the wet season, the retained latent constructs account for 91.1% of the variability of the data set (Table 6).

Table 6.

Cumulative variance explained for the wet season data.

PC1 with the largest eigenvalue accounted for the maximum of the total variability (23.5%). PC 2 accounted for the total variation of 22.8% and corresponds in concept to the first PC. The third and fourth PCs explained 22.8% and 22% of the total variance, respectively. Cattel’s scree test plot is presented in Figure 2.

The first principal component, PC1 (the salinity component), is depicted mainly by K⁺, Ca²⁺, Cl⁻, ${\mathrm{SO}}_4^{2-}$ , H₂S, and ${\mathrm{NH}}_4^{-}$ ions in water. As the component explains the largest variance in the data, it can be inferred that the groundwater in the study area is mainly saline. As nearly all carbonate ions depict a negative correlation in explaining PC1 and the noncarbonate alkali exceeds 50% in all (except BH7) boreholes (Figure 3), it can be deduced that NaCl constitutes the primary salinity at Olbanita. Abnormal distribution of Ca²⁺ and ${\mathrm{SO}}_4^{2-}$ may be characteristically associated with the ion exchange mechanisms in saturated aquifer zones. At the wet season pH, for instance, the monovalent Na⁺ is depleted from groundwater as it substitutes for divalent Ca⁺² on exchangeable micro-pore surface water interfaces (Table 7). This inference is demonstrated by BH 7 where none of the cation or anion pairs exceed 50% setting forth a secondary salinity characterized by Ca-Mg-SO₄ and/or chloride mixed type waters (Figure 3). The ions ${\mathrm{SO}}_4^{2-}$ and H₂S depict a strong positive correlation (+0.8 at α = 0.01) indicating either geologic provenance, probably due to deep circulation of oxygen-saturated waters causing aerobic conditions (Table 5), or anthropogenic derivation, probably due to their simultaneous positive contribution to PC1 along with ammonia (Table 7). The moderate correlations within the pairs NH₃-Ca (+0.7) and NH₃-H₂CO₃ (+0.6) imply anthropogenic contributions to the pH controls on aquifer water salinity. During the wet season, decreases in H⁺ cause a reduction in the water concentrations of K⁺, Ca²⁺, and Cl⁻. Aquifer salinity, therefore, is chiefly a construct of water-rock interactions and to a lesser extent anthropogenic inputs.

Figure 3.

Major composition in groundwater during the wet season.

Table 7.

The factor loadings of the measured parameters during the wet season which suited the provisions of orthogonal varimax rotation.

The second principal component, PC 2, depicts carbonate equilibrium with external pH control. The external factors controlling pH are ${\mathrm{SO}}_4^{2-}$ and H₂S, which are strongly correlated (+ 0.8, α = 0.01). Sulfate and sulfide transformations in borehole waters are invariably mediated by bacterial changes under anaerobic conditions. D desulfuricans may also produce H₂S under conditions of the measured pH range (optimum growth range in pH is between 5.5 and 8.5) and optimum temperature between 24°C and 42°C. Under these conditions, sulfides (in the form of metal sulfide) undergo oxidation to sulfates which are again converted to H₂S under the action of H₂CO₃. The dissociation of H₂CO₃ yields the ions ${\mathrm{CO}}_3^{2-}$ and H⁺ which causes the reduction of sulfates to sulfides. Due to protracted mobility (diminutive residence period) of oxygen-saturated water in the wet season, reduced dissociation of H₂CO₃ yields less ${\mathrm{CO}}_3^{2-}$ and H⁺ which consequently causes oxidation of sulfides to sulfates. BH 7 depicts the mixing effects probably conditioned by such pH shifts resulting from ${\mathrm{SO}}_4^{2-}$ and H₂S variability (Figure 3).

The third principal component, PC 3, is the alkalinity component as indicated by pH, ${\mathrm{CO}}_3^{2-}$ , ${\mathrm{H}}_2{\mathrm{CO}}_3^{-}$ , F⁻ and ${\mathrm{NH}}_4^{-}$ . The pH, ${\mathrm{CO}}_3^{2-}$ , and F have positive loading, whereas ${\mathrm{H}}_2{\mathrm{CO}}_3^{-}$ and ${\mathrm{NH}}_4^{-}$ have negative loading on this PC. The PC explains the dissolution of fluoride through microbial activity releasing ammonia and thereby decreasing the pH. In addition, weak acids such as H₂CO₃ ionize sequentially releasing ${\mathrm{CO}}_3^{2-}$ , which again raises the pH of water.

The fourth PC is the electrolytic conductivity component which is associated with significantly high concentrations of electrolytic ions indicated by TDS, EC, Na⁺, and F⁻. Statistical analyses (eg, Pazand⁹) associate F⁻ with weathering of the fluoro-apatite and silicate mineralogy, whereas Moghimi¹² linked Na⁺ with the dissolution or weathering of sodium-rich plagioclases (clay mineralogy) or the dissolution of halide. The strong correlation observed between Na⁺ and F⁻ (+ 0.9) and the weak association between Na⁺ and Cl⁻ (–0.6) supports weathering plagioclase feldspars as the chief source as opposed to the dissolution of halide. Na and F ions are the intrinsic constructs responsible for electrolytic conduction, as supported by their strong correlations with TDS. Within the hydrogeological framework, the pattern and, therefore, the provenance of weathering can be accounted for by the roughly N-S fracture-fissure zones. The component accounts for the lowest variability because of the reduced residence time of groundwater during the wet season exerting substantial reduction in TDS.

The dry to wet seasonal shift in variability from electrolytic conduction to low alkali salinity, respectively, is probably due to differential seasonal rates of weathering, flow and dilution processes in the aquifer. Significant correlations (at α = 0.01) in the pairs Cl-SO₄, Ca-K, Ca-NH₃, F-NH₃, CO₂-HCO₃, HCO₃-CO₃, Na-F, and F-H₂CO₃ indicate that the water is alkaline to mildly acidic which are a manifestation of authigenic and to a lesser scale anthropogenic imprints. Key groundwater evolutionary trends suggest that silicate, carbonate, and/or accessory mineral apatite dissolution, as well as ion exchange at sorption sites with the clay-water interface, are the central sources of variability in the groundwater chemistry of the aquifer. Protracted mobility of oxygen-saturated water in the wet season, probably conditioned seasonal by ${\mathrm{SO}}_4^{2-}$ and H₂S variability, though being an exogenic factor, restrained the pH constraint and hence carbonate equilibrium mixing reactions.

Spatial variability between sampling sites

The results of hierarchical clustering procedures were discrete clusters presented graphically in the form of a dendrogram by an averaging algorithm (Figure 4).

Figure 4.

Dendrogram for clustering of groundwater sampling sites using the Euclidean distance metric (Ward’s method).

Based on rescaled Euclidean distances and the “sums of squared errors,” 2 main borehole clusters are conspicuous in the area. {Where to “prune” the tree (eg, using the continuous bold lines) is a vital factor in interpreting the results of the analysis. The within-cluster medium depict translational invariance in sample composition. Alternate shading was thus introduced to facilitate review}. The first cluster (forming the left-hand group) consists of the western zone cases (boreholes 2, 3, 4, 5 and 6), whereas the second cluster (forming the right-hand group) consists of the eastern zone borehole cases (boreholes 1, 7, and 7A). The former boreholes were deciphered to be hydraulically connected by a major inferred north west – south east fault which corresponds to a calibrated transmissivity-formation resistivity model for the aquifer. Intrinsic permeability was empirically elevated along major fracture traces, consequently increasing yields of the affected boreholes. In addition, some wet season samples obtained from boreholes 4 (sample Nos. 27 and 33), 5 (sample Nos. 28, 34 and 40), and 6 (sample Nos. 29, 35 and 41) formed a mini-cluster within the former main group. The samples represent the effects of a high permeability fault/fracture structure which accentuates deep circulation of oxygen-saturated waters from recent precipitation events coupled with dilution within the corresponding season. We suggested low residence times of groundwater in the zone. Worth noting was the strong indication of low-carbonate alkali to mildly acidic mixed waters in the western zone at BH 6, which transits to become Ca-Mg sulfate and/or chloride water types of at BH 7 in the eastern periphery in the wet season.

The latter cluster contains boreholes located in the eastern compartment of the aquifer. Elevated values of Cl⁻, ${\mathrm{SO}}_4^{2-}$ , and exceptionally low values of EC, TDS, pH, Na⁺, T-CO₂, and HCO⁻₃ were recorded in these groundwater boreholes. However, values of EC, TDS, Ca²⁺, ${\mathrm{SO}}_4^{2-}$ , and K⁺ depicted an upward trend, whereas those of Na⁺, T-CO₂, ${\mathrm{CO}}_3^{2-}$ , and ${\mathrm{HCO}}_3^{-}$ showed a downward trend from the dry season toward the wet season for these boreholes. By contrast, boreholes in the zone are not hydraulically connected via major fault structures. We, therefore, suggested that pore-level adsorption/desorption processes in which vast quantities of monovalent ions such as Na⁺ and K⁺ are removed from the groundwater in exchange for divalent ions control the observed variability in groundwater hydrochemistry in this zone. Lack of aquifer-scale hydraulic networks and the presence of clay micropores signify extended groundwater residence time favoring ion exchange reactions in the zone. Borehole 8 drilled in this zone dried-up after its completion because a clay layer was inadvertently targeted for production.