Groundwater studies in parts of the Mamfe basin are sparse and the Mamfe area has the highest population density in the Mamfe basin. An in-depth study of groundwater rock interaction and groundwater quality is of vital importance. This same part of the basin is the economic centre and as such development of businesses in this area requires knowledge of the groundwater quality. Therefore, this study was undertaken to determine the input of the rock formations on the groundwater solute chemistry and groundwater domestic-agro-industrial quality using hydrogeochemical tools and physicochemical parameters: Ionic ratios, Gibbs diagrams, Piper diagrams, Durov diagrams and water quality indices. From physicochemical parameters, in the rainy season, pH ranged from, 4.3 - 8.6; EC, 3 - 1348 μS/cm; Temperature, 24.4℃ - 30.1℃  andTDS, 2.01 - 903.16 mg/L and in the dry season, pH ranged from 5.5 - 9.3; EC, 6 - 994 μS/cm; Temperature, 25℃ - 38.6℃ andTDS, 4.02 - 632.48 mg/L. Forty groundwater samples: 20 per season, wet and dry were analysed. The major ions fell below WHO acceptable limits for both seasons. The sequences of abundance of major ions were: Ca²⁺ > K⁺ > Mg²⁺ >  > Na⁺, Cl^- >  >  >  > NO₃ in wet season and Ca²⁺ > Mg²⁺ > K⁺ > Na⁺,  > Cl^- >  >  >  in dry season. Ion-exchange, simple dissolution and uncommon dissolution processes determined groundwater character. Groundwater ionic content was as a result of ion exchange from rock-weathering. Water types are: CaSO₄ and MgHCO₃ in both seasons. Hydrogeochemical facies are Ca-Mg-Cl-SO₄ and Ca-Mg-HCO₃. SAR for wet season 0.05 - 0.06 and dry season 0.00 - 0.05, %Na wet season 3.64 - 16.59 and dry season 1.22 - 10.97, KR wet season 0.01 - 0.02 and 0.00

Keywords

Geogenic Imprint, Hydrogeochemical Facies, Groundwater Quality, Mamfe, Cameroon

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1. Introduction

Water is an important natural resource essential for the existence of life and is basic human entity. Water resources are used for various purposes like drinking, agricultural, industrial, household, recreational, and environmental activities. Groundwater is one of the major sources of drinking water all over the world (Bear, 1979) . There has been tremendous increase in the demand for fresh water due to growth in population. Since groundwater is a renewable natural resource and a valuable component of the ecosystem, it is vulnerable to natural and human impacts. It is estimated that approximately one-third of the world’s population uses groundwater for drinking (Nickson et al., 2005) . In most parts of the Mamfe basin, groundwater is the major source of water supply for drinking and agricultural purposes. Few studies on groundwater quality have been carried out in this area, thus the groundwater might present a potential health hazard as such there was a need to determine the domestic-agro-industrial quality. Groundwater quality data give important clues to the geologic history, rock type and indications of groundwater recharge, discharge and storage (Walton, 1970) . Variations in groundwater quality in an area are a function of physical and chemical parameters that are greatly influenced by geological formations and anthropogenic properties (Kumar et al., 2011) . According to Babiker et al. (2007) , the chemistry of groundwater is not only related to the lithology of the area and the residence time the water is in contact with rock material, but also reflects inputs from the atmosphere, from soil and weathering as well as from pollutant sources such as mining, land clearance, saline intrusion, industrial and domestic wastes. Groundwater used for domestic and irrigation purposes can vary greatly in quality depending upon type and quantity of dissolved salts. It contains a wide variety of dissolved inorganic chemical constituents in various concentrations, resulting from chemical and biochemical interactions between water and the geological materials. Dissolved salts should be present in irrigation water in relatively small but significant amounts. They originate from dissolution or weathering of the rocks and soil, including dissolution of lime, gypsum and other slowly dissolved soil minerals. Research on groundwater studies in parts of the Mamfe basin is sparse and Mamfe area has the highest population density in the basin. An in-depth study of groundwater rock interaction and groundwater quality is of vital importance. This same part of the basin is the economic centre and as such development of business in the study area requires knowledge on groundwater quality. Therefore, the present study was carried out to determine the contribution of the formations to groundwater chemistry, groundwater quality and its suitability for drinking and agricultural uses in parts of the Mamfe basin.

2. Geologic Background of the Study Area

Mamfe and environs is situate between latitude 5.65 - 5.85N and longitude 9.25 - 9.55E Figure 1. Manyu Divisionis made up of four sub-divisions and occupied by four main ethnic groups: the Anyangs, Kenyangs, Akwayas, and Ejaghams. It has a population of over 200,000 inhabitants (Mamfe Council, 2014) . The area is made up of mostly farmers, business people and civil servants from almost every ethnic group in Cameroon and some from other countries. The climate in this division is hot and humid and consists of a rainy and a dry season modified by the deviation of the monsoon and the relief of Mount Cameroon (Ndip et al., 2018) . The vegetation is dominantly that of the equatorial rain forest, and the drainage system is principally that of the Cross River whose main source is found in Mount Bambouto. The sources of its main tributaries the Munaya and Badi Rivers are at Mount Rumpi and Nda Ali respectively (Ndip et al., 2018) .

The study area forms part of the Mamfe basin which lies along the Cameroon Volcanic Line. Sedimentation began in the Mamfe basin during the Albian (Dumort, 1968) and lithologies making-up the body of sediments are: basal conglomerates, conglomeratic sandstones, mudstones, shales, calcareous and carbonaceous rocks Figure 2 that are highly fractured (Lordon et al., 2017) . The Mamfe basin is a south-eastern trending Cretaceous rift basin that bifurcates off

the Benue trough and is linked with the West Central African Rift System (CWARS) thought to have formed during the Albian to Cenomanian (Eyong et al., 2013) as a result of basement rifting associated with the reactivation of E-W trending mylonite zones within the Pan-African basement (Dumort, 1968) . The basin narrows towards the east and widens towards the west across the Cameroon/Nigeria border into the Benue trough were Albianmarine deposits of Abakaliki Formation outcrops. The basin is fringed by reactivated, fault-bounded granite-gneissic rocks of the Pan-African Mobile Belt (550 ± 100 Ma) and are both intruded by volcanic rocks (Eyong et al., 2013) .

Mamfe Basin lies in a NW-SE trending trough with a length of 130 km and a width of 60 km and constitutes a small prolongation of the Benue trough (Nguimbous-Kouoh et al., 2012) . Ndougsa-Mbarga et al. (2007) described the Mamfe basin as the smallest of three side rifts associated with the Benue trough of west-central Africa. It extends from the lower Benue trough in Nigeria into Southwestern Cameroon where it narrows and terminates under the Cameroon volcanic line.

The Mamfe basin is bordered to the south by the Oban Massif granito-gneissic Precambrian Basement Complex which separates it from the Rio del Rey Basin and to the North by the Precambrian rocks of the Obudu Massif. To the West the Basin is open and continues as a part of Anambra basin of Nigeria and in the East and Northeast it narrows and terminates under the CVL.

3. Materials and Methods

3.1. Materials

The field materials and equipment used in the study are listed in Table 1.

3.2. Methods

A reconnaissance survey was carried out to identify wells, springs and streams in June 2016 as per ISO 5667-1 (2006) . Seasonal tests/measurements were carried out in September 2016 wet season and Dry season February 2017 respectively. 53 dug wells, were measured/tested in situ for: coordinates of wells, Surface elevation, Well water level, Dug wells depths well diameter, Electrical conductivity (EC), pH, Total dissolved solids (TDS) and Temperature (˚C). Forty (40) groundwater samples 20 in wet and dry seasons were collected in a high density polyethylene (HPDE) 500 ml bottles sealed and sent to the laboratory as per sampling protocols; ISO 5667-3 (2003) , ISO 5667-11 (2009) using the standard methods APHA (1995) to analyze for:

1) Major cations in mg/L: Ca²⁺, Mg²⁺, Na⁺, K⁺ and ${\text{NH}}_4^{+}$ .

2) Major anions in mg/L: ${\text{HCO}}_3^{-}$, Cl⁻, ${\text{SO}}_4^{2-}$, ${\text{HPO}}_4^{2-}$ and ${\text{NO}}_3^{-}$

Ionic ratio for indicative elements is a useful hydrogeochemical tool to identify source rock of ions and formation contribution to solute hydrogeochemistry Hounslow (1995) . These were used in this study.

Gibbs Diagram is a plot of Na⁺/(Na⁺ + ${\text{HCO}}_3^{-}$ Ca²⁺) and Cl⁻/(Cl + ${\text{HCO}}_3^{-}$ ) as a function of TDS are widely employed to determine the sources of dissolved geochemical constituents. These plots reveal the relationships between water composition and the three main hydrogeochemical processes involved in ions acquisition; Atmospheric precipitation, rock weathering or evaporation crystallisation.

Pipers Diagram is a graphical representation of the chemistry of water sample on three fields; the cation ternary field with Ca, Mg and Na + Kapices, the anion ternary field with HCO₃, SO₄ and Cl⁻ apices. These two fields are projected onto a third diamond field. The diamond field is a matrix transformation of the graph of the anions [sulfate + chloride]/Ʃ anions and cations [Na + K]/Ʃ cations. This plot is a useful hydrogeochemical tool to compare water samples, determine water type and hydrogeochemical facies Langguth (1966) . This has been used here for these purposes.

Durov diagram is a composite plot consisting of two ternary diagrams where the milliequivalent percentages of cations are plotted perpendicularly against those of anions; the sides of the triangles form a central rectangular binary plot of total cation vs. total anion concentrations. These are divided into nine classes by Lloyd and Heathcoat (1985) which give the hydrogeochemical processes determining the character of the water types in the aquiferous formation Langguth (1966) .

WQI was calculated by adopting Weighted Arithmetical Index method considering thirteen water quality parameters (pH, EC, TDS, total alkalinity, total hardness, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, ${\text{SO}}_4^{2-}$, ${\text{NO}}_3^{-}$, ${\text{NH}}_4^{+}$ ) in order to assess the degree of groundwater contamination and suitability Table 2.

For Agro-industrial suitability the following parameters were used; sodium adsorption ratio SAR, permeability index PI, Magnesium adsorption ratio MAR , percent sodium %Na, Kelly’s ratio KR and Residual sodium carbonate RSC and Wilcox diagram Table 2.

The following sofwares; Surfer 12, Global mapper 11 and AqQA 1.5 AGIS 10.3 were used for data presentation, interpretation and analysis.

4. Results and Interpretation

4.1. Physicochemical Parameters

The physicochemical parameters groundwater in Mamfe: Temperature, pH, EC

and TDS for 53 wells were evaluated as shown in Table 3. From Table 3 all physicochemical parameters vary with seasons indicating seasonal influence phreatic aquifer.

Water Level Fluctuations

Depth-to statues water values (m) of groundwater in Mamfe ranged from: 0.5 - 9.5 in the Wet season and 0.5 - 14.5 in the dry season Figure 3.

Groundwater flow direction

Groundwater flows towards the Northwestern part of the study area during the wet season and dry season but during the dry season some water flows towards Bachuo-Akagbe Figure 4.

Temperature

Temperature values ˚C of Mamfe groundwater ranged from: 24.4 - 30.1 wet season 25˚C - 38.6˚C Figure 5.

pH

The pH value of most of the groundwater samples in the study area ranged from 4.3 - 8.6 in the wet season and 5.5 - 9.3 in the dry season Figure 6. This

clearly shows that the groundwater in the study area is slightly acidic to alkaline in both seasons.

Electrical conductivity (EC)

The EC ranges from 3 - 1348 µS/cm during the wet season and 6 - 944 µS/cm during the dry season Figure 7.

The high electrical conductivity is due to high solute concentration in water.

Total dissolved solids (TDS)

The total dissolved solids range from 2.0 - 903.16 mg/L in the wet season and 4.02 - 632.48 mg/L in the dry season Figure 8.

4.2. Chemical Parameters of Groundwater

The results of the chemical analysis varied in both seasons Table 4(a) and Table 4(b) (Figure 9 & Figure 10).

Mechanism controlling water chemistry

Ionic ratios of groundwater in Mamfe

18 ionic ratios in groundwater were used to deduce formation inputs in parts of the Mamfe basin, as follows Tables 5(a)-(c).

12 of the 18 66.7% ionic ratios calculated gave indices indicating weathering of geologic formations in the Mamfe basin as a source of solute concentration in the groundwater while nitrate ratio indicates no anthropogenic contribution and sulfate indices indicates no oxidation of sulfides. Ca is sourced from gypsum while Na is sourced from halite-albite and ion exchange. Mg is contributed by dolomite dissolution, calcite precipitation or saltwater. There is no plagioclase weathering. These high indices values are found in the following localities Lalla, Okoyong, Bachuo-Ntai, Bachuo-Akagbe and Egbekaw.

Gibbs diagrams of groundwater in Mamfe

The Gibbs diagrams were used. In the wet season 17 samples 85% plot in the rock-weathering dominance field and 3 samples 85% plot in the atmospheric precipitation dominance field. In the dry season 16 samples 80% plot in the rock-weathering dominance field and 4 samples 20% plot in atmospheric dominance field Figure 11. This indicates the mechanism contributing solute to groundwater in Mamfe is rock-weathering 80% - 85% followed by atmospheric precipitation 15% - 20% in the dry and wet season respectively (Table 6).

Groundwater types

The diamond field of piper diagram after Piper (1944) has further been divided into seven fields classifying water types and designated with alphabets from A to G, Langguth (1966) . Using this classification, the water from the study area is distinguished into the A, B, and C categories. The D, E, F, and G water types are absent. In the rainy season; Category A, 3 samples 15%; characterized by normal earth alkaline water with prevailing bicarbonate. Category B, 3 samples 15% are characterized by normal earth alkaline water with prevailing sulfate or chloride and Category C, 14 samples 70% Figure 12 are characterized by alkaline earth water with increased portions of alkalis with prevailing bicarbonate. In the dry season; Category A, 14 samples 70%, Category B, 5 samples 25% Category C, 1 sample 5%. The dominant water types are Category C 70%; Normal earth alkaline water; prevailing ${\text{SO}}_4^{2-}$ or Cl⁻ in the wet season and Category A 70%; Normal earth alkaline water; prevailing ${\text{NO}}_3^{-}$ in the dry season. The water types in Mamfe groundwater are Ca-SO₄ and Mg-HCO₃ for both seasons Table 7.

Piper’s Hydrogeochemical facies

From the Piper’s diagrams, field I: Ca-Mg-Cl⁻-SO₄ hydrogeochemical facies has 15 samples 75% in the rainy and 2 samples 10% in the dry season demonstrating the dominance of alkaline earths over alkali Ca + Mg > Na + K and strong acidic anions over weak acidic anions. Field IV, Ca-Mg-HCO₃ hydrogeochemical facies has 5 samples 25% in the rainy and 18 samples 90% in the dry season Table 8 signifying the dominance of alkaline earths over alkali and weak acidic anions over strong acidic anions. This facies is characteristic of freshly recharged groundwater that has equilibrated with CO₂ and soluble carbonate minerals under an open system conditions in the vadose zone typical of shallow groundwater flow systems in crystalline phreatic aquifers.

No samples plotted on field II and field III. The dominance of Ca-Mg-HCO₃ hydrogeochemical facies in this area could be due to dissolution of gases and minerals, particularly CO₂ and CO₂-related compounds from the atmosphere

dissolved in precipitation and during groundwater infiltration through the vadose zone.

Durov diagram

Based on the classification of Lloyd and Heathcoat (1985) : Six classes of processes occur in the rainy season; Class 1 recharging waters: 10 samples 50%; Class 2 ion exchange water: 5 samples 15%; Class 3 ion exchange water: 1 samples 5%; Class 5 simple dissolution or mixing: 1 samples 5%; Class 6 probable mixing or uncommon dissolution influences: 1 sample respectively 5% and Class 7 2 samples respectively 10%; Cl and Na dominant is frequently encountered, Otherwise the water may result from reverse ion exchange of Na-Cl waters Table 9. Five classes of processes occur in the dry season: Class 2 ion exchange water: 1 sample 5%; Class 3 ion exchange water: 3 samples 15%; Class 5 simple dissolution or mixing: 1 sample 5%; Class 6 probable mixing or uncommon dissolution influences: 11 samples 55% respectively Figure 13. There are no Classes 4 and 9 in the rainy season and no Classes 1, 7 and 9 in the dry season samples in groundwater from Mamfe. In the rainy season, fresh recently recharging water exchanges ions with the matrix of the formation, while simple dissolution or mixing also goes on between the recently recharging and the existing groundwater in the formation. In the dry season, recharged groundwater having spent more time in the formation continues to exchange ions to a lesser extent with the matrix of the formation while increasingly; simple dissolution or mixing also goes on between the recently recharged groundwater and the pre-existing groundwater in the formation.

4.3. Water Quality

Domestic Water Quality

Ionic content of water in the study area was used to evaluate groundwater suitability for domestic use: The recommended values are of the WHO (2017) guidelines. The quality standards for drinking water have been specified by the World Health Organization (WHO) 2017 . The suitability of groundwater in the study area based on WQI and total hardness H_T are discussed below.

Water quality index (WQI)

The WHO (2017) permissible values of ions present in the groundwater have been used to calculate WQI values, Asadi et al. (2007) . Water Quality Index WQI considered the most effective tool to convey the water quality information in the simplest form to the public (Babaei, 2011). The WQI values range from 0 - 70.79 in the wet season and 13.2 - 276.6 in the dry. 80% of the water samples in the wet season can be considered suitable for domestic and other utilitarian purposes as they belong to excellent to good water quality classes Table 10. Remaining 20% of the samples are said to be unfit for consumption as they belong to poor class Figure 14 whereas 20% of the water samples in the dry season can be considered suitable for domestic and other utilitarian purposes as they belong to excellent to good water quality classes. Remaining 80% of the samples are said to be unfit for consumption as they belong to poor, very poor and unsuitable classes Table 10.

Total hardness (TH)

The total hardness of groundwater samples range from 20.63 - 244.87 mg/L in the wet season and 86.66 - 338.47 mg/L in the dry season Figure 15. 80% of groundwater in the study area can be classified as soft, 10% fall into the moderately hard category, and just 10% in the wet season is hard water that may be a potential health risk factor whereas in the dry season 0% of groundwaters in the study area can be classified as soft, 65% fall into the moderately hard category, 30% is hard water and just 5% is very hard Table 11.

4.4. Water Quality for Irrigational Use

The important parameters which determine the irrigation water quality of the study area are discussed below;

Sodium percent

Sodium percent values range from 3.64 - 16.59 in wet season and 1.22 - 10.97 dry season. Sodium along with carbonate forms alkaline soil; while sodium with chloride forms saline soil; both of these are not suitable for the growth of plants (Pandian & Shankar 2007) . The quality classifications of irrigation water based on the values of sodium percentage as proposed by Wilcox (1955) suggest that the groundwater of the study area is good to permissible category both in the wet and dry season Figure 16 indicating the water is suitable for irrigation (Figure 17).

Sodium adsorption ratio

SAR values range from 0.005 - 0.075 in rainy season and 0.01 - 0.14 during dry season. Based on the SAR values, all samples have low sodium hazard and on plotting over the USSL Salinity diagram (USSL 1954) Figure 18. All the 20 groundwater samples fall in the S₁-C_0-3 classes Table 12 for both rainy season and dry season considered suitable for irrigation (Figure 19).

Permeability index

The PI values range 0.89 - 68.63 in the wet season and 18.75 - 73.35 in the dry season Figure 20. The classification of irrigation waters has been attempted on the basis of permeability Index, as suggested by Doneen (1962) . The groundwater samples of the study area fall in class-I and II as per Doneen chart, the groundwater samples of the study area are of good quality for irrigation. The increased percentage of groundwater samples under class–I is due to dilution and subsequently the lower values of permeability index (Figure 21).

Magnesium Adsorption Ratio

Magnesium Ratio adsorption values range from 13.3 - 67.88 in the wet season and 27.02 - 77.01 in the dry season Figure 22. Magnesium Ratio adsorption less than 50% it is considered as suitable for irrigation purpose. In the study area, 90% of the samples are suitable for irrigation during the wet season whereas 25% of the samples are suitable for irrigation during the dry season.

Residual Sodium Carbonate

The RSC values −4.59 to −0.33 in the wet season and −5.13 - 0.3 mg/L in the dry season Figure 23. RSC values < 1.25 mg/L are considered as safe for irrigation while those from 1.25 mg/L to 2.5 mg/L are marginally suitable for irrigation. If RSC values are >2.5 the groundwater is unsuitable for irrigation (Eaton, 1950; Richards, 1954) . All the RSC values are <1.25 in the study area thus rendering the water suitable for irrigation in both seasons.

Kelly’s Ratio (KR)

KR < 1 is considered suitable for irrigation and KR > 1 is unsuitable. During rainy season, KR values vary between 0.01 - 0.02 and during the dry season the values vary between 0.00 - 0.02 Figure 24. All groundwater samples in Mamfe are suitable for irrigation for both seasons.

5. Discussion

Temperature, pH, EC and TDS vary with seasons as such the groundwater is in hydraulic connectivity with the atmosphere indicative of a phreatic aquifer. All major ions fell below WHO (2017) acceptable limits for both seasons. From ionic ratios there are additional sources of SO₄, silicate weathering possibly of the sandstones, conglomerates and other rocks in this area. Weathering of Na-feldspar or other Na-silicates and Ca-carbonate dissolution or Ca-silicate weathering. Cation-exchange of the silicate rocks with the groundwater. Ironic ratio values for nitrate and sulfate are very low as such there are no anthropogenic contribution and no oxidation of sulphides. Solutes from weathering reactions and inputs of dissolved species in precipitation get into the aquifer indicating a recharge zone. From Gibbs diagram there is the dominance of rock-weathering and atmospheric precipitation dominance. From Durov diagrams the processes involve are ion exchange, dissolution and mixing. From the Piper’s diagrams, the dominant hydrogeochemical facies are Ca-Mg-Cl-SO₄ and Ca-Mg-HCO₃ for both seasons. This facies is characteristic of freshly recharged groundwater that has equilibrated with CO₂ and soluble carbonate minerals under an open system conditions in the vadose zone typical of shallow groundwater flow systems in crystalline phreatic aquifers.

From the above groundwater synthesis, WQI values indicate that 80% of the groundwater in the wet season can be considered suitable for domestic use, 20% are classified to be unfit for consumption whereas in the dry season 20% are suitable for domestic purposes and 80% are unfit for consumption as they belong to poor, very poor and unsuitable classes. Therefore the water in Mamfe is more suitable for domestic purposes in the wet season than in the dry season similar to work done in Kumba, by Akoachere et al. (2018) .

The values of SAR, PI, %Na, KR and WQI, RSC and Wilcox diagram indicate that most groundwater in the study area are suitable for irrigation purposes but of MAR with values higher in Mile-1, Kumba road, Egbekaw, and Bachuo-Ntai rendering the water unfit for irrigation.

Since there exist little hydrogeological or hydrological work in this part of the basin, there is little to compare the results here-in, this being the first study that sheds light into these unexplored aspects of the basin.

6. Conclusion

The geogenic input to groundwater is the weathering of rocks possibly of the granites, gneisses, sandstones, conglomerates, shales and other rocks in this area.

Groundwater ionic content was as a result of ion exchange from rock-weathering of the aquiferous formations in the area, dissolution and recharge from precipitation.

Water types are: CaSO₄ and MgHCO₃ in both seasons. Precipitation recharge, ion-exchange and simple dissolution are the processes determining groundwater character.

Hydrogeochemical facies are Ca-Mg-Cl-SO₄ and Ca-Mg-HCO₃.

All major ions concentrations are below WHO acceptable guidelines for both seasons.

Water quality indices: SAR, PI, %Na, KR and WQI, RSC and Wilcox diagram indicate that water in the study area is irrigation suitability assessment but of MAR were the values that are higher in Mile-1, Kumba road, Egbekaw, and Bachuo-Ntai rendering the water unfit for irrigation.

There is a need for more studies in order to determine the aquifer extent: lateral and vertical, aquifer boundaries, aquifer hydraulic parameters (permeability, transmissivity and storage capacity), the biological and organic water quality, necessary tools for a good management of this important resource in the aquiferous formations in Mamfe. These more detailed studies will throw more light on the groundwater capacity and residence time in the different aquiferous formations in the Mamfe Basin.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest

The authors declare no conflicts of interest.

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