GIS-Hydrogeochemical Model of the Yaoundé Fractured Rock Aquifer, Cameroon: Aquifer Setting, Seasonal Variations in Groundwater-Rock Interaction and Water Quality

This study of the gneiss-fractured-rock aquifer in Yaoundé capital of Cameroon determines: the aquifer setting-flow systems, the aquifer type, seasonal variations in rock-water interactions, evolution of the hydrogeochemical processes, physicochemical parameters and the suitability for domestic-agro-industrial use of the groundwater. Physicochemical field tests were carried out on 445 wells during four seasons for EC, pH, TDS, Temperature and static water level from July 2016 to May 2017. 90 well samples were analyzed 45 samples per season: wet/dry. 38 borewell logs were used together with structural data to determine the aquifer setting. The field physico-chemical and laboratory analysis data of well samples were mounted unto various GIS software platforms: Global mapper, AqQa, Aquachem, Rockworks, Logplot7, Surfer and ArcGIS, to get indices/parameters/figures, by use of Durov’s, Piper’s and Gibbs diagrams, Water quality index WQI, USSL ratio, Sodium Absorption ratio SAR, Percent sodium %Na, Kelly Ratio KR, Magnesium Absorption Ratio MAR, Total Hardness TH, Residual Sodium Carbonate RSC and Permeability Index PI that were determined. The process of groundwater ions acquisition is three-fold: by recharge through atmospheric precipitation, by ion exchange/simple dissolution between the rock-groundwater and by groundwater mixing in its flow path. Water types are Ca-HCO3, Mg-HCO3 and Mg-Cl while hydrogeochemical facies are Ca-Mg-HCO3 and Ca-Mg-Cl-SO4. Most water samples are fresh, potable and soft all seasons. The hydrogeologiHow to cite this paper: Akoachere, R. A. II, Yaya, O. O., Egbe, S. E., Eyong, T. A., Nji, B. N., & Tambe, D. B. (2019). GIS-Hydrogeochemical Model of the Yaoundé Fractured Rock Aquifer, Cameroon: Aquifer Setting, Seasonal Variations in Groundwater-Rock Interaction and Water Quality. Journal of Geoscience and Environment Protection, 7, 232-263. https://doi.org/10.4236/gep.2019.75018 Received: March 12, 2019 Accepted: May 27, 2019 Published: May 30, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access R. A. II Akoachere et al. DOI: 10.4236/gep.2019.75018 233 Journal of Geoscience and Environment Protection cal conceptual model is that of a three-layered single phreatic fractured-rock-aquifer while other researchers postulated a two-aquifer, phreatic and semi-confined, two-layered model.


Introduction
Yaoundé city, Mfoundi division in the Centre Province is the capital city of Cameroon. The main activities carried out in the study area seat of all ministerial head offices are mostly the business of government, industrial activities and small scale agriculture. Yaoundé has a typical Classic equatorial Guinean climate which is tropically wet and dry with a regular and abundant precipitation (1600 mm per day), an average annual air temperature of approximately 24˚C and an evaporation of about 800 mm per year (Fouepe et al. 2011). This climate is characterized by four seasons: eight months of rainy season, from mid-August to mid-November and four months of dry season from mid-November to February. The climate also has a short rainy season from March-June and a short dry season from July-August.
Yaoundé's population is 3.066 million with a growth rate between 6% and 8% per annum (United States Central Intelligence Agency, 2018). This growth rate far exceeds the rate of development of infrastructure. As such, the government is left with a fire-brigade-reflex of pre-planned new-layouts and lots in response to housing; hoping other services will follow later. This results in chaotic urbanization which makes the development of water supply infrastructure problematic.
In Cameroon, pipe-borne water supply systems are presently catering for the needs of less than 50% of cities and towns. In Yaoundé, less than 30% of households have direct access to drinking water. This rate falls to less than 20% for households that have access to the drinking water network in sub-urban centers with <100,000 inhabitants and since the supply is erratic, most people will at some point use either wells, springs or rivers (Fouepe et al., 2011;Nola et al., 1998;Djeuda et al., 1999;Tanawa et al., 2002).
Dugwells are the most popular source of groundwater in villages and towns in Cameroon. Classifying the sources of groundwater by uses in order of importance in Yaoundé is as follows: dugwells 80% > springs 6% > borewells 4%. This is due to the fact that dugwells are easier to construct, need less complex technical skills, are cheaper and as such more cosmopolitan. Every house has one where possible. In Yaoundé where there are over 3000 dugwells, there are three dugwells in one household at Melen quarters, each belonging to one of three feuding brothers.
In Cameroon, sedimentary basins make up less than 20% of the land surface. As such, over 80% of the aquifers systems are fractured rock aquifers of igneous and metamorphic origin. There are however very few studies on the fractured rock aquifer systems in Cameroon.
The future development and expansion of Yaoundé as a capital city might depend on whether or not there is availability of water in quantity and in quality for use by the population. The Yaoundé fractured rock aquifer is therefore of great interest. Potable water is an existentially essential resource. There is a critical need for a complete understanding of the hydrogeology of the capital city Yaoundé. Also, the absence of proactive water management tools for planning an integrated hydrogeological approach to understanding the hydrogeology of Yaoundé begs our attention. The development of the full potential of this fractured rock aquifer depends on its systems/structural styles, an understanding of the rock/groundwater interaction, hydrogeochemical evolution, and the groundwater quality status for all uses and these must be part of the solution to satisfying the water needs of Yaoundé and other major fractured rock aquiferous formations in Cameroon.
Groundwater is a critical resource for people and ecosystems and tools are needed for efficient data management so that more technically sound and community-supported decisions may be made (Rossetto et al., 2018). This informed our decision to carry out this study to provide tools such as; chemical parameters and indices from analysis, interpretation of field data and the proposal of a good conceptual model of the aquiferous formations in Yaoundé. Although a comprehensive hydrogeological investigation has not been made heretofore of the whole city of Yaoundé, many researchers have carried out works on selected groundwater topics and in parts of Yaoundé. Notable amongst them are: Flow dynamics of lineaments in Yaoundé (Teikeu et al., 2016); mass balance of Nitrogen and Potassium in parts of Yaoundé (Kringel et al., 2016); assessment of groundwater quality for domestic and irrigation purposes in parts of Yaoundé (Tabue et al., 2012); use of geoelectric tools to explore the lithostratigraphy of the aquiferous formations in parts of Yaoundé (Teikeu et al., 2012); assessment of bacterial contamination in the Mfoundi Watershed (Kuitcha et al., 2010) and modeling of groundwater flow in Yaoundé IV district (Fouepe et al., 2011).
This study of the whole city of Yaoundé aimed at giving an in-depth into the hydrogeology of Yaoundé describes work on; the aquifer flow systems, hydrogeochemical processes in groundwater/rock interaction, a new conceptual model of the aquifer system(s), the seasonal variations in groundwater physico-chemical parameters and groundwater chemical quality all of which have not been dealt with in-depth by the other researchers; is situated between longitudes 11.42˚E to 11.56˚E and latitudes 3.78˚ to 3.92˚N with elevations of 550 -772 m. a.m.s.l (meters above mean sea level) covering an area of (12.41 Km × 17.12 Km) 212.46 Km 2 .

Hydrology, Geology and Hydrogeology
Yaoundé city is drained by the Mfoundi River and its many tributaries. The stream and river network is dense and dendritic Figure 1. The Yaoundé series are a group of rocks that outcrop extensively around the Yaoundé city. They belongs to a regional scale mapped unit thrusted southwards unto to the Congo craton that resulted from the collision of the northern Cameroon basin and the Congo craton to the south. As a result of this collision, rocks of the Yaoundé series were squeezed and pushed over the Ntem and Nyong complexes by thrusting. Following this collision, the rocks of the northern Cameroon basement and those of the Congo carton were then fused together by the Yaoundé series to form a suture zone leading to the Yaoundé series having a high dip to the south. The Yaoundé series comprises of low to high garnet-bearing metapellites and orthogneisses metamorphosed under a medium to high pressure metamorphism reaching the granulite facies (T = 750˚C -800˚C and P = 100 -1200 Mpa). Meta-intrusive rocks include mafic to intermediate rocks; pyroclasites and serpentinized chromite and nickel, ferrous ultramafic rocks associated with gabbro, diorites and mafic dykes. The Yaoundé gneisses were probably derived from the metasediments of shales and greywacke deposited in an extensional environment related to the Congo craton (Nzenti et al., 1988). This basement consists of insoluble and impervious migmatites, gneisses and schists transpierced by faults and diaclases that give fissure permeability to the anisotropic and heterogeneous formations, which are highly weathered, producing predominantly well-drained Ferralitic soils. These weathered soils with relics of fractures from the pristine rocks serve as aquifers for shallow groundwater, while fractures and faults in deeper unaltered rock constitute the deeper aquifer. The hydrodynamic functioning of the weathered horizon-fresh rock system acts as a two layer aquifer components because the weathered zone contains the groundwater, which is drained by fractures in the rocks. The weathered horizon constitutes a shallow Journal of Geoscience and Environment Protection aquifers with thickness that varies from about 1 to 20 m, with a hydraulic conductivity from 10 −4 to 10 −6 m/s. Due to the undulating nature of the relief, the shallow aquifer gives rise to springs that oozes at the base of slopes forming spring lines and wetlands, which serve as discharge zones (springs and shallow wells) of water for domestic use and subsistence agriculture (Djeuda et al., 1999;Mvondo et al., 2007;Fouepe et al., 2011;Teikeu et al., 2016).

Methodology/Procedures and Data Used
An extensive field program of borewell; data acquisition, field hydrogeological measurement/tests, water quality sampling and laboratory analysis of collected water was conducted in Yaoundé as follows: The United States Geological Survey/National Aeronautics and Space Admin- well logs were used together with structural data to determine the aquifer setting.
The field acquired data were mounted on GIS platforms to create sample location, drainage and water level contour maps Figure 1. The city of Yaoundé was then divided into twenty zones and each zone covered by two field workers responsible for thirty 30 out of preselected 600 wells; 550 Dug wells and 50 Bore wells. Forty fieldworkers (Undergraduate geology students) were trained in April 2016 to use field equipment in Table 2, collect data, record data on data sheets and a reconnaissance survey to identify the pre-selected wells and self-introduction to private well owners for access/permissions. Four seasonal; single-day tests/measurements were carried out in June 2016 (Dry-wet season), September 2016 (Wet season), December 2016 (Wet-dry season) and March 2017 (Dry season) respectively. During each season, twenty commercial bikes (Bensikins) were hired for four hours each field day 8 am -12 noon simultaneously in all the twenty zones. Two field workers per bike measured/tested for the following in situ: Wells: Surface elevation, Well water level and Well depths. Groundwater: Electrical conductivity EC, pH, Total dissolved solids TDS and Temperature ˚C on 445 wells; 425 hand dug wells and 20 boreholes. 125 handdug wells and 30 boreholes with incomplete data were removed from the reconnaissance selected set of 600 wells due to: 1) Borehole access was impossible in the course of the study by private owners being absent on fixed data collection dates or access denied after initial acceptance.
2) Hand dug wells got dry in one or more season(s) in the course of the study.
3) Inconsistent data due to field workers' errors or high laboratory result ionic balance error (>10%).  Barcelona et al., 1985). All the collected pre-labeled sample-water-rinsed bottles were sent to Ac-  Table 3 to get indices/parameters and figures for interpretation.

Digital Elevation Model
Yaoundé has an undulating relief of hills and valleys. The land surface is covered by top soil in most areas though there are some areas where gneisses outcrop on hill tops and valleys Figure 3. Tectonic structures are hardly visible but for some joints and fissures.

Subsurface Structural Fissures
Subsurface structural fissures in the study area are mostly fissures and fractures     Table 4. The S 1 C 0 , S 1 C 1 , and S 1 C 2 make up the excellent, very good and good Classes respectively. For the Wet season 430 samples, Figure 7. Yaoundé groundwater pH values for four seasons; groundwater is alkaline in the rainy season; alkaline to slightly acidic in the wet/dry season; acidic in the dry season and acidic to slightly alkaline in the dry/wet season.

Groundwater Content of Major Ions mg/L Table 5
The occurrence of ions in groundwater in Yaoundé in decreasing order of magnitude: for cations; Ca 2+ > K + > Mg 2+ > Na + > 4 NH + and anions; HPO − . The evaluation of ions is based on recommended guideline values (WHO, 2017) ( Table 6).     3) Calcium ionic concentration ranges from Wet season 4.24 -112.8 and Dry season 1.38 -74.45. Acceptable limit of calcium in drinking water is 75 mg/L. 200 mg/L in case of no other alternative source. Calcium ion is necessary for proper mineralization of bones and bone strength. Deficiency in intake of calcium leads to eventual demineralization of bones for complementing the inade-quate amounts of calcium in the body. 4) Magnesium ionic concentrations of Yaoundé groundwater range from: 1.01 -32.9 in the Wet season and 0 -14.16 in the Dry season. Magnesium Acceptable limit in drinking water is 30 mg/L (100 mg/L in case of no other alternative source). Magnesium helps in maintaining normal nerve and muscle function, a healthy immune system and helps bones remain strong. It also helps in regulation of regulate blood glucose levels and aid in the production of energy and protein. Deficiency of magnesium in the human diet might lead to anxiety, fatigue or anorexia. 5) Ammonium Soil/rock-water interactions can result to weathering and enrichment of the groundwater with ammonium ions, since groundwater quality is a function of the chemical composition of the soil/rock through which it passes. However rel-atively little of the nitrate found in natural waters is of mineral origin; most coming from organic and inorganic sources, the former including waste discharges and the latter comprising chiefly oxidation of ammonia from artificial agricultural fertilizer run-off. However, bacterial oxidation and fixing of nitrogen by plants can both produce nitrates. Interest is centered on nitrate concentrations for various reasons. Most importantly, high nitrate levels in waters to be used for drinking will render them hazardous to infants as they induce the "blue baby" syndrome (methaemoglobinaemia). The nitrate itself is not a direct toxicant but is a health hazard because of its conversion to nitrite which reacts with blood hemoglobin to cause methaemoglobinaemia.
The Ca, Mg, Na, K, HCO 3 , Cl and SO 4 ionic concentrations show considerable seasonal variations. This is similar to results in the basement complex in Uganda imputing the variations to geology, climatic and hydrologic conditions (Nyende et al., 2014).

Rock-Groundwater Interaction in Yaoundé
To have an insight into the interaction between rock matrix and the groundwater in Yaoundé, a Gibb's diagram was used (Gibbs, 1970). Rock/mineral weathering, reverse/inverse ion-exchange, dissociation, and anthropogenic inputs are major solute acquisition mechanisms for the acquisition of major ions content in groundwater with respect to; atmospheric-precipitation, rock-weathering and evaporation-crystallization. This gives the controlling geochemical processes for the groundwater in Yaoundé.
Gibbs plot for Yaoundé Figure 12 and Table 7   weathering of the aquifer matrix is the primary dominant process in the acquisition of ions while atmospheric precipitation is the secondary process controlling the hydrogeochemistry in Yaoundé.

Hydrogeochemical Character of Yaoundé Groundwater
Durov diagram is a composite plot consisting of two ternary diagrams where the milliequivalent percentages of cations are plotted perpendicularly against those of anions (Durov, 1948); the sides of the triangles form a central rectangular binary plot of total cation vs. total anion concentrations. These are divided into nine Classes which give the hydrogeochemical processes determining the character of the water types in the aquiferous formation (Lloyd & Heathcoat, 1985).  There are no Classes 5 -9 in the rainy season and no Classes 7 -9 in the dry season samples in groundwater from Yaoundé Figure 13. 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, recharging 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 recharging groundwater and the pre-existing groundwater in the formation, piston flow. The absence of samples showing Na + and Cl − as dominant cation/anion, no Classes 5 -9 in the rainy season and no Classes 7 -9 in the dry season, indicates that the groundwater in Yaoundé is not related to reverse or inverse ion exchange of NaCl waters or any end-point down gradient waters.

Groundwater Types Occurring in Yaoundé
In saturated formations; aquifers, groundwater occurs in varied chemical modes associated ionic complexes that differ in their chemical composition as a function of; lithology, ionic content, solution kinetics and flow patterns called hydrogeochemical facies (Todd, 1980). Hydrogeochemical facies, water types and the geochemical evolution of the ionic content of groundwater is elucidated by plotting water sample's laboratory analysis results on Piper's trilinear diagram (Piper, 1944). In Figure 14, (Durov, 1948;Lloyd & Heathcoat, 1985). No samples plotted on field II and field III (Langguth, 1996 that has not moved a long distance from its source (Hounslow, 1995). No samples plotted on Field II and Field III. The high contribution of alkaline earth elements 77.8% in the rainy season and 64.4% in the dry season may be due to direct ion-exchange processes which enrich groundwater with alkaline earth elements. The dominance of Ca-Mg-HCO 3 hydrogeochemical facies in this area could be due to dissolution of gases and minerals, particularly CO 2 and CO 2 -related compounds from the atmosphere dissolved in precipitation and during groundwater infiltration through the vadose zone.

2) Piper-Langguth groundwater types
The diamond field of piper diagram has further been divided into seven fields Classifying water types and designated with alphabets from A to G (Langguth, 1996). Using this Classification, the water from the study area is distinguished into the A, B, and C categories ( Table 9). The D, E, F, and G water types are absent. In the rainy season; Category A, 32 samples, 64.4%; characterized by normal Table 9. Seasonal variations in groundwater types from Piper's trilinear diagram, Yaoundé (Langguth, 1996). SO − or Cl − in the dry season. This groundwater has a modern age and is being rapidly recharged by precipitation.

Water Quality for Drinking Purposes
In urban and semi urban slums and some rural areas in Cameroon, groundwater sources in the form of dugwells or borewells are the only source for drinking water. In the present study, to ascertain whether or not the groundwater in use by the population of the capital city in Cameroon meets the chemical water quality guidelines, the following water quality parameters are determined (WHO, 2017).

1) Water Quality Index WQI
The WHO permissible values of ions present in the groundwater have been used to calculate WQI values (Pradhan et al., 1998;Asadi et al., 2007) Figure 15.

2) Total Hardness
During the rainy season total hardness (TH) ranges between 1.00 to 17.00 mg/L with an average of 8.36 mg/L, and during dry season, it ranges between 0.50 to 11.00 mg/L with an average of 6.82 mg/L. All the 45 (100%) groundwater samples for both seasons are (<50 mg/L) soft. The total hardness (TH) of water is a measure of mainly calcium carbonate and magnesium carbonate dissolved in The general acceptance level of hardness is 300 mg/L, although WHO has set an allowable limit of 600 mg/L ( Figure 16). Water hardness has no known adverse effects; however, some evidence indicates its role in heart disease.
Hard water is unsuitable for domestic use and it is a measure of the Ca 2+ and Mg 2+ content expressed in equivalent of calcium carbonate (WHO, 2017).

Water Quality for Agro-Industrial Perposes
To assess the overall irrigational water quality of the samples collected, six com-    Figure 19. Wilcox plot of Na% vs. EC for groundwater in Yaoundé; in the rainy season 42 samples, 93.3 % is in Class I "Excellent to good" and in Class II; 3 samples, 6.7% "Good to Permissible" while in the dry season 44 samples, 97.8% Class I "Excellent to good" and 01 sample, 2.2% in Class II "Good to Permissible" for irrigation. Journal of Geoscience and Environment Protection on internal drainage patterns in soil as release of calcium and magnesium ions are facilitated due to absorption of sodium by clay particles.

3) Permeability Index (PI)
The soil permeability of an area eventually decreases due to continuous irrigational practices and is defined based on quantity of bicarbonate, sodium, calcium and magnesium in water an empirical index termed, "Permeability Index" (Doneen, 1964). Permeability index in Yaoundé Figure

5) Magnesium Adsorption Ratio (MAR)
MAR categorizes water into two broad Classes: water having MAR < 50 considered suitable for irrigation and MAR > 50 considered unsuitable (Paliwal, 1972 (Hem, 1985). During equilibrium more Mg 2+ in groundwater adversely affects the soil quality rendering it alkaline which result in decrease of crop yield (Giggenbach, 1988).

6) Kelly's Ratio (KR)
This is ratio is of sodium ion concentration over calcium and magnesium ion concentrations (Kelly, 1963). KR < 1 is considered suitable for irrigation and R. A. II Akoachere et al.

Aquifer Setting
From field observations, borehole lithostratigraphic sections and by comparing various case studies in similar terrain in gneiss-type rocks and in some parts of Yaoundé have indicated that topography controls groundwater flow and that base flow to rivers is an important factor moderating groundwater movement in Yaoundé as shown here by the groundwater contours for four seasons (Teikeu et al., 2016;Wyns et al., 2004;Clark, 1985;Acworth, 1987;Obiefuna & Sheriff, 2011;Fouépé Takounjou et al., 2012). Lithologies from their VES curves corroborated the lithologs of boreholes from this area (Teikeu et al., 2012). Some researchers have concluded with a dual aquifer conceptual model for fractured rock gneiss aquifers (Teikeu et al., 2012;Wyns et al., 2004). Although weathered gneiss terrains are layered, aquifer models suitable for sedimentary terrains of layered formations cannot apply here where the fissures and fractures cut across the layering in the formations' regolith that still retain relics of the continuous gneiss fractures and fissures intact in saprolite, weathered basement and fresh fractured basement. Similar weathering and erosional processes induce similar geological structures and that at catchment scale for water resources applications, the fresh basement is considered as impermeable; of very low storativity and that in highly foliated rocks (i.e., gneisses or schists) the orientation of the fissures is controlled by the rock structure (Dewandel et al., 2006). Also, fissured layers assume storage functions in composite aquifers with the fresh basement being permeable only locally, where tectonic fractures are present (Wyns et al., 2004). This depends to a large extent on the density of fractures (the number of fissures/fractures per unit area). At high density shear zones as in Yaoundé; these rocks could store huge reserves of groundwater in the fractures and fissures. As a consequence of the hydrogeochemical analysis above, a modified hydrogeological conceptual model of weathered migmatite/gneiss-type aquifer in Yaoundé is proposed (Figure 25).  takes place with limited connection to the regional subsystem which could supply dugwells throughout the year and low discharge borewells in the rainy season.
3) The fracture/fissure interconnected network infrastructure which also serves as the major conduit/channel for storage and flow of groundwater; the Regional subsystem. Here groundwater is stored (channel storage) and flows throughout the region through interconnected fracture/fissure networks that can supply large discharge borewells. The Yaoundé groundwater samples in this fractured rock aquifer belong to the bicarbonate group with significant amounts of magnesium and calcium cations. The Sodium and Potassium content can be attributed to the presence of clays in the geology resulting in the release of potassium and sodium ions from the argillaceous materials by ion-exchange processes between the formation waters and clay minerals.
The overall chemical qualities of ground water with respect to drinking standards are good in all seasons with low TDS and are soft.
Quantitative chemical analysis results reflect the dominant anions to be calcium and Magnesium and the dominant anions bicarbonate and chloride characteristic of migmatite gneiss basement in Africa.
The pH, electrical conductivity, total dissolved solids and temperature values of water samples are below guideline acceptable limits and variable with seasons indicative of phreatic aquifers (WHO, 2017).
RSC, SAR, SSP, MAR, PI and KR indicating the suitability of groundwater samples for irrigation is Excellent-to-Good in almost all cases. By these indices the groundwater quality of Yaoundé has been assessed for its agro-industrial suitability, found to neither cause salinity hazards nor have an adverse effect on the soil properties and is thus suitable for irrigational purposes.

Conclusion
The process of ions acquisition of Yaoundé groundwater is three-fold: by recharge through atmospheric precipitation, by ion exchange/simple dissolution between the rock-groundwater and by groundwater mixing in its flow path.
The groundwater in Yaoundé is of CaHCO 3 , MgHCO 3 and MgCl − freshwater types low in ionic content that vary with seasons and soft, typical of phreatic aquifers in basement aquifers. There is a need for more detailed studies to determine aquifer characteristics: permeability, transmissivity and storativity, vertical and lateral regional extent of aquifer boundaries in this aquifer, groundwater pollution potentials for biological, organic and trace metals to enable a complete characterization of these aquiferous formations.