Hydrogeochemical process of ions in groundwater of communities along Southern-axis Atlantic Ocean in Bayelsa, Nigeria

ABSTRACT The hydrogeochemical process of groundwater from hand-dug well within communities located along the Southern axis of the Atlantic Ocean was examined based on its major ions concentration. Fifteen (15) water samples were collected across the study area and analyzed using the American Public Health Association (APHA) laboratory standard. With the aid of ArcGIS 10.7, the spatial distribution of physiochemical parameters and major ions (Ca2+, Mg2+, K+, Na+, HCO−3, SO42-, Cl− and PO43-) were determined. The cations and anions of the groundwater trended as Ca2+ > Mg2+ > K+ > Na+ and HCO−3> SO42- > Cl− > PO43-, respectively, while all values are within World Health Organization (WHO) allowable limit except Ca2+. The positive correlation among major ions illustrated similarities in the sources and distributions based on principal component analysis, while the cluster analysis established the complexity of the hydrogeochemical processes. Piper and Durov plot aided the geochemical process identification where the groundwater of the area showed the dominance of Ca2+-Mg2+-HCOˉ3, Na+- HCOˉ3 and Ca2±Mg2+- SO42-/Cl−; hence, the hydrogeochemical process was influenced by water–rock interaction of possible carbonate weathering characterized by the earth’s alkaline water and weak acids.


Introduction
Groundwater remains a vital renewable natural resource that supports organisms, including humans, for their industrial, agricultural and domestic activities (Abdelshafy, Saber, Abdelhaleem, Abdelrazek, & Seleem, 2019).The suitability of groundwater for a series of activities is influenced by natural (Jalali, 2007;Subramani, Rajmohan, & Elango, 2010) and human activities (Afolabi et al., 2022).The activities include geological structure, geochemical process, and leachate of domestic and industrial waste, contributing to the obtainable groundwater quality in an environment.Groundwater chemistry is based on various hydrogeochemical processes that the groundwater undergoes over space and time (Narany et al., 2014).
Bayelsa state is found within the Niger Delta region of Nigeria.According to (Abam, 1999), the region is enriched with surface and sub-surface water, crisscrossing many communities and maintaining their environmental stability.The study area consists of alluvial deposits and an extensive, low-lying, typical deltaic plain with essentially flat topography, which, in conjunction with the high annual rainfall, is responsible for the deplorable drainage conditions and the overall development of marshes and back swamps.There are several perennial streams, oxbow lakes and rivers in the area, e.g.Kolo Creek, Epie Creek, Yenagoa and Nun River.They all form a network which empties to the Atlantic Ocean through Nun River Estuary.The research area has exhibited substandard groundwater quality due to elevated levels of specific groundwater parameters and the intrusion of saltwater or brackish water into the freshwater aquifers (Nwankwoala & Peterside, 2019).According to (Nwankwoala & Daniel, 2016), the static water level in the area exhibits a range of 0-2 m during the rainy season and 1-3 m during the dry season.The water permeates the porous sands of the Benin Formation, effectively replenishing the aquifers (Nwankwoala & Udom, 2011).Groundwater in the region primarily occurs under water table circumstances, as documented by (Short & Stauble, 1967) and (Udom & Amah, 2006).
Several studies have adopted the use of major ion chemistry of groundwater to study the hydrogeochemical processes.Previous studies made use of physiochemical properties and rock interaction (Abdelshafy, Saber, Abdelhaleem, Abdelrazek, & Seleem, 2019); graphical representation in the form of a Piper diagram (Hussien & Faiyad, 2016;Nwankwoala & Udom, 2011;Peterside, Hart, & Nwankwoala, 2022); geostatistical and geochemical approach (Islam, Mostafa, & Cinti, 2022;Kumar & James, 2016;Narany et al., 2014), hydrogeochemical characteristics and health risk assessment (Davraz & Batur, 2021).Recently, studies have combined three or more techniques in their study of hydrogeochemical processes (Xu, Gong, Tao, & Liu, 2022;Zhi, Cao, Zhang, Li, & Ren, 2021).However, some of the related studies carried out in Nigeria focused on the influence of human activities and its interaction with subsurface water, and there is a limited study that focuses on the possible influence of natural phenomena such as seawater infiltration on shallow (hand-dug well) groundwater of a particular environment such as communities nearby the Atlantic Ocean.Therefore, the study aims to: (1) examine the spatial distribution of major ions in the shallow groundwater across the study area; (2) investigate the sources of the major ions in the shallow groundwater; and (3) identify the hydrogeochemical process.

Study area
The study was carried out at the communities within the Southern Ijaw Local Government Area of Bayelsa State, Nigeria, within the Longitude of 6º00'10"N and 6º25'15"N and Latitude of 4º40'07"E and 5º5'20" E (Figure 1).The geological formation of the region was described by (Short & Stauble, 1967) and (Kogbe, 1976) while (Abam & Nwankwoala, 2020) referred to the sedimentary formation to be a "complex regressive off-lap sequence of clastic sediments ranging in thickness from 9000 m − 12000 m.The region is arranged into the three-layered lithologic formation of Benin, Agbada and Akata formation, while the aquifers are fortified by permeable soil formation, which ensures accessibility to water at shallow aquifers due to the abundance of Benin Formation (Ngah & Nwankwoala, 2013;Nwankwoala & Omemu, 2019).The aquifers are arranged in stacks and easily recharged by precipitation accumulated in the region between 2540 and 5000 mm (Nwankwoala & Mzaga, 2017).The well in the region has a refilling rate of 20,000 l/hour and is typically higher with a borehole system (Amadi et al., 2012).
The anthropogenic activities of the study area are classified as either landward or seaward due to the region's unique geographical pattern that includes both terrestrial and aquatic ecosystems.The primary economic activity in the inland area is crop cultivation, while in the coastal region, fishing and aquaculture dominate (Ifeanyieze, Alkali, Okoye, & Ikehi, 2016).Economic activities can be categorized as either landbased or water-based and area has great potential for productive and economically beneficial land-based operations, especially for cultivating a wide range of crops (Ikehi, Ifeanyieze, Onu, Ejoifor, & Nwankwo, 2022).Additionally, many aquatic ecosystems in the area support economic activities that rely on water (Ifeanyieze, Alkali, Okoye, & Ikehi, 2016;Ikehi, Ifeanyieze, Onu, Ejoifor, & Nwankwo, 2022).

Sample collection
Groundwater from wells (hand-dug) was sourced from fifteen (15) positions across the study area.All the samples were collected during the rainy season (July 2020) using suitable techniques described by (Afolabi et al., 2022).The collected samples were taken to the laboratory, and the parameters were analyzed by Geospectra Engineering Services Limited, Port Harcourt using standard protocol for laboratory analysis.
To ascertain the quality of the outcome from various analyses, standard procedures and laboratory quality assurance were strictly followed while samples were analyzed in triplicates, and the mean was estimated for accuracy and precision.All analyses were subjected to a high-quality reagent from Geospectra Engineering Services Limited, and all instruments such as Pyrex glassware and containers for the analysis were washed adequately in deionized water, then soaked overnight with a 10% HNO 3 in 1% HCl solution and later washed with deionized water and desiccated.

Statistical analysis
The source and distribution of major ions of the groundwater were analyzed through statistical tools such as Pearson's Correlation Coefficient (PCC) analysis, Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) using the Statistical Package for the Social Sciences (SPSS) version 21 platform.The use of HCA has been previously adopted as clustering tools of various water quality parameters (Chegbeleh, Akurugu, & Yidana, 2020) and (Egbueri, 2020) while dendrograms show the quality of the clusters of the water sample parameters (Egbueri, 2020).PCC and PCA have been combined by previous studies as a tool for establishing possible relationship, extent of the relationship and sources identification among parameters (Afolabi et al., 2022;Afolabi et al., 2023).

Sources and distributions
The PCA coupled with the PCC, rotation component matrix for groundwater major ions and screen plot (Figure 3) were analyzed and presented accordingly.Among all the physicochemical and major ions parameters, the PCC analysis (Table 2) indicated that parameters such as EC, TDS, K + , and Cl − have a significant relationship and are positively correlated (where r ≥ 0.5, p = ≤ 0.05).Showing a similar trend was the relationship between Ca-Na ± and pH-HCO 3 − while pH-Cl − and Cl-HCO 3 − have a significant relationship and are negatively correlated (where r ≥ −0.5, p = ≤ 0.05).Among the carbonate components, pH indicated a weak positive correlation with Ca 2+ (r = 0.081) and negative correlation with Mg 2+ (r = −0.456).It has a significant role of low pH on the calco-carbonic equilibrium and potentiality of carbonate mineral/rock weathering (Islam, Mostafa, & Cinti, 2022) The weak negative correlation between Ca 2+ and Mg 2+ suggested that the ions may not necessarily be involved in several physicochemical reactions of the groundwater aquifer.All the major ions showed a weak and negative correlation among themselves, indicating a complex hydrogeochemical process, different hydrogeochemical composition and sources.
The PCA was carried out under varimax rotation, and four (4) principal components (PC-4) were noticed (Table 3).The analysis indicated a total variance of 77.35% with eigenvalue > 1 extracted by R-mode.From PC1, the variation among the parameters can only be explained by the 24.532% variance, while parameters such as Na + , Ca 2+ and K + are strongly correlated (0.835, 0.798 and 0.797 respectively) with eigenvalues > 1.The PC2 showed 22.078% of the variance, while a strong correlation of Cl − (0.858) and a strong negative correlation of HCO 3- (−0.740).PC3 revealed a strong correlation for SO 4 2-  (2007).NB: All parameters are in mgL −1 except EC (µS cm −1 ) and pH (no unit) in all reported Tables and Figures.R1 (Davraz & Batur, 2021), R2 (Utom, Odoh, & Egboka, 2013), R3 (Nwankwoala & Udom, 2011) and R4 (Islam, Mostafa, & Cinti, 2022).Mg 2+ at 16.422% variance; however, the first three (3) PC loading showed 63.033% of the total variance.According to (Islam, Mostafa, & Cinti, 2022), the values of PCA (with + or -signs) imply the possible impact and non-impact by the presence of extracted loads on an exact constituent.PCA revealed that a significant positive correlation among major ions illustrated similarities in the sources and distributions (Afolabi & Adesope, 2022) while the absence of such correlation between and among parameters implies a lack of mutual forms of source and distribution pattern (Ustaoğlu, Tepe, & Tas, 2020).
HCA was carried out for 15 samples using 11 parameters to discover the clusters of samples that showed similarities in their water chemistry (Figure 4).From the analysis, the major likes-variables maintained a specific cluster area and were closely connected with associated cluster(s), while clusters with fewer similarities were relatively connected to form a more extensive cluster (Chegbeleh, Akurugu, & Yidana, 2020).The hydrogeochemical parameters of samples showed five (5) and two (2) main cluster groups (cluster I and cluster II) based on a dendrogram.Cluster II showed a closed cluster distance between L8W and L11W with similar chemistries with L3W and L4W.This is reflected in all four (4) wells having TDS above allowable limits.The cluster arrangement of the wells further indicated the complexity of the hydrogeochemical processes within the groundwater, and each well showed similarities in their water chemistries.

Hydrogeochemical process
The hydrogeochemical process of a place/region is based on the precipitation, evaporation and rockwater interaction process, which eventually influences the groundwater chemistry (Kumar & James, 2016).A combined graphical representation plotting of the Piper diagram (Figure 5) and Durov diagram (Figure 6) was adopted in describing the relative enrichment of cations and anions and the geochemical process of the groundwater (Piper, 1944).The  Piper diagram is a very useful tool in bringing out the groundwater chemical relationship in more concise terms (Walton, 1970).From the analysis, three different hydrogeochemical facies were identified.The Ca 2+ -Mg 2+ -HCOˉ3 type of water dominates among the facies, followed by Na + -HCOˉ3 and lastly by Ca2±Mg 2+ -SO 4 2-/Cl − .The high concentration of HCOˉ3 is an indication of chemical weathering (Yusuf, 2020).A similar outcome was reported for a freshwater aquifer along a coastal belt (Yusuf, Abiye, Ibrahim, & Abubakar, 2021).The Ca 2+ -HCOˉ3 water type, therefore, is thought to be generally derived from the carbonate dissolution such as calcite, which contributes Ca 2+ and HCOˉ3 to the groundwater (Afolabi, Ogundana, & Ogundipe, 2022;Yusuf, Abiye, Ibrahim, & Abubakar, 2021).Furthermore, the Na + -HCOˉ3 and Ca 2+ -Mg 2+ -HCOˉ3 types are typically dominated by the earth's alkaline water and weak acids, as observed in the present study (Karanth, 1987).The concentration of Ca 2+ and Mg 2+ relatively indicated the total cation dominance of about 75%, and therefore, the study proposed a possible carbonate weathering that influences the groundwater quality of the area.The outcome was validated by the R 2 value of both Ca 2+ and Mg 2+ at 0.798 and 0.681, respectively.For the Durov plot, which defines the hydrogeochemical processes, anions and cations are projected on a square shape subdivided into nine facies based on (Lloyd & Heathcote's, 1985) water classification scheme.The water type showed a dominance of Ca 2+ and HCOˉ3 (in Fields 1 and 2), Na + and HCOˉ3 (Field 3), Ca 2+ and SO 4 2-(Field 4), Field 5 showed no dominance of either anion or cation, Na + and SO 4 2-(Field 6) while Na + and Cl − were dominant in Field 7-9.This outcome further established the complexity of the hydrogeochemical processes from the cluster analysis, which also indicated that some wells indicated similarities in their water chemistry.

Conclusion
This study investigates the hydrogeochemical processes of major ions in groundwater aquifers close to the Atlantic Ocean's coastal belt, with the aim of comprehending the geochemical dynamics inside and around these aquifers.Based on the average values, the physiochemical parameters demonstrate conformity to the standards set by the WHO and NIS, indicating that all parameters fall within the acceptable range, with the exception of Ca 2+ .However, it should be noted that EC and TDS levels exceed the prescribed limit in some wells.The cations and anions of the groundwater trended as Ca 2+ > Mg 2+ > K + > Na + and HCO − 3 > SO 4 2-> Cl − > PO 4 3-, respectively.The Piper diagram revealed that the groundwater in the region had a prevalence of Ca 2+ -Mg 2+ -HCOˉ3, Na + -HCOˉ3, and Ca 2+ -Mg 2+ -SO 4 2-/Cl − as dominant ion compositions.Conversely, the Durov plot suggested that the cations Ca 2+ and Na + were dominant, while the anions HCOˉ3 and SO 4 2-exhibited dominance.The primary finding of this study indicates that natural processes substantially impact groundwater chemistry, as evidenced by the observed water-rock interaction.Specifically, carbonate dissolution and ion exchange were identified as the prominent geochemical processes governing groundwater chemistry in the investigated area.However, it is important to note that this study did not observe any potential anthropogenic influences on groundwater chemistry.
In summary, primary determinants influencing the hydrogeochemical processes of groundwater in the communities located along the Southern-Axis Atlantic Ocean in Bayelsa, Nigeria, have been inferred.Hence, it is advisable to ensure that anthropogenic activities in the region are conducted ecologically friendly to mitigate pollution.Additionally, it is essential to continuously monitor the aquifer to assess pollution trends across the basin, both temporally and spatially.

Figure 1 .
Figure 1.Overview of the Southern Ijaw local government area in Bayelsa state.

Figure 2b .
Figure 2b.Spatial variation of the physicochemical and major ions Concentrations.

Figure 3 .
Figure 3. Scree plot of eigenvalues after PCA and rotated component matrix of groundwater Ion.

Figure 4 .
Figure 4. Dendrogram of hierarchical cluster analysis for sampled groundwater.

Table 1 .
Physiochemical and major ion parameters.

Table 3 .
Rotation component matrix for major ions.