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 hydrogeological 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.
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 Km2.
Hydrology, Geology and HydrogeologyYaoundé city is drained by the Mfoundi River and its many tributaries. The stream and river network is dense and dendritic
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 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).
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 Administration/Shuttle Radar Topography Mission’s USGS/NASA SRTM Digital Elevation Model DEM 60 m data for Yaoundé in decimal degrees and datum WGS84 (SRTM, 2018). This data was projected to the UTM coordinate system and clipped to the extent of the study area. Hydrogeological traverse field mapping was carried out in February - March 2016, 600 usable wells were identified and borehole drillers’ records, sandbox sketches, drill cuttings sketches, logs were acquired and converted to thirty-nine (39) lithologs for the study area using LogPlot7. Measurement of: longitudes, latitudes and elevations were carried out over the entire city. Ninety well samples were analyzed 45 samples per season; wet/dry for various sites in Yaoundé
| Location of Samples for chemical analysis | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Location | Coordinates | Location | Coordinates | Location | Coordinates | ||||||
| Name | Code | E | N | Name | Code | E | N | Name | Code | E | N |
| Nsam-1 | 1 | 11.6519 | 4.0669 | Nkolmesseng-1 | 16 | 11.7908 | 3.9269 | Madagascar-1 | 31 | 11.5878 | 3.8919 |
| Ekie | 2 | 11.5528 | 3.8272 | Essos | 17 | 11.5435 | 3.8727 | Madagascar-2 | 32 | 11.5056 | 4.0892 |
| Mvan | 3 | 11.5148 | 3.8174 | Nkolmesseng-3 | 18 | 11.7686 | 3.9511 | Madagascar-3 | 33 | 11.6450 | 4.0275 |
| Nsimeyong | 4 | 11.4914 | 3.8369 | Djongolo | 19 | 11.5227 | 3.8844 | Mballa-II | 34 | 11.5257 | 3.9011 |
| Ngoaekele | 5 | 11.5016 | 3.8649 | Nkolmesseng-5 | 20 | 11.7889 | 4.0986 | Madagascar-5 | 35 | 11.6019 | 4.1056 |
| Tamtam-1 | 6 | 11.5636 | 4.0283 | Etoug-ebe-1 | 21 | 11.6950 | 4.0019 | Mimbomann-1 | 36 | 11.6317 | 3.9811 |
| Mvogmbi | 7 | 11.5189 | 3.8422 | Etoug-ebe-2 | 22 | 11.7164 | 4.0467 | Mimbomann-2 | 37 | 11.7642 | 3.9781 |
| Mvogada | 8 | 11.5308 | 3.8624 | Bastos | 23 | 11.5122 | 3.8966 | Ngousso | 38 | 11.5831 | 3.9050 |
| Mvogbesi | 9 | 11.4822 | 3.8637 | Etoug-ebe-4 | 24 | 11.5650 | 3.8828 | Ekounou | 39 | 11.6897 | 4.0139 |
| EligEffa | 10 | 11.5021 | 3.8708 | Etoug-ebe-5 | 25 | 11.6936 | 4.0881 | Nkomo 1 | 40 | 11.7897 | 3.9533 |
| Simbock-1 | 11 | 11.6358 | 4.0172 | Damas-1 | 26 | 11.5942 | 3.9328 | Kondengui 1 | 41 | 11.6467 | 3.9556 |
| Citévert | 12 | 11.4940 | 3.8725 | Damas-2 | 27 | 11.5175 | 3.8183 | Kondengui 2 | 42 | 11.6789 | 3.9733 |
| Briqueterie | 13 | 11.5239 | 3.8851 | Damas-3 | 28 | 11.5281 | 3.9450 | Nkomo 2 | 43 | 11.7903 | 3.9558 |
| Simbock-4 | 14 | 11.5069 | 3.9753 | Oliga | 29 | 11.4929 | 3.8976 | Nkolndongo | 44 | 11.7489 | 3.9267 |
| Simbock-5 | 15 | 11.6181 | 3.9950 | Damas-5 | 30 | 11.7097 | 3.9311 | EkounouKoayos | 45 | 11.7550 | 4.0089 |
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
Forty fieldworkers (Undergraduate geology students) were trained in April 2016 to use field equipment in
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%).
| Equipment/Softwares | Specifications | Functions |
|---|---|---|
| Bike | Commercial bikes (Bensikin) | To transport fieldworkers to wells |
| GPS | GARMIN GPSMAP 60CSx | To measure longitude, latitude and elevation of wells |
| EC Meter | HANNA HI 98304/HI98303 | To measure Electrical Conductivity of water. |
| pH Meter | HANNA HI 98127/HI98107 | To measure pH of water. |
| Water level indicator | Solinst Model 102M | To indicate static water levels of water in wells |
| Measuring Tape | Weighted measuring tape | Measurement of well diameter and depth. |
| Digital Thermometer | Extech 39240 (−50˚C to 200˚C) | To measure temperature of water |
| TD S meter | Hanna HI 96301 with ATC | To measure Total dissolved solids in water |
| Water sampler | Gallenkampf 1000 ml | To collect well water sample from well |
| Sample bottles | Polystyrene 1000 ml | To hold sample for onward transmission to laboratory |
| ArcGIS | Version 10.1 | GIS Drawing sampling/Tests location maps |
| Global Mapper | Version 11 | GIS Geolocation of wells |
| Surfer Golden Software | Version 12 | GIS plotting contours for spatial distribution |
| AqQA/Aquachem | Version 15 | For the analysis/interpretation of water chemistry results |
| Rockworks | Version 16 | For the analysis/interpretation of water chemistry results |
| Logplot | Version 7 | For production of lithologs from borehole logs/data |
8900 groundwater physico-chemical field tests; 445 wells *5*4 seasons for EC, pH, TDS, Temperature, Depth to water carried out during these four seasons. Ninety groundwater samples were collected. Two selected wells per zone were sampled for two seasons; wet and dry; one out of every fifteen tested wells. Two samples for chemical analysis were collected at each site. One sample was filtered using Sartorius polycarbonate filtering apparatus and a 0.45 µm cellulose acetate filter membrane, acidified with Merck ultra-pure nitric acid to pH < 2 in situ in sample-rinsed high density linear polyethylene (HPDE) 50 ml bottles. The other sample was unfiltered into 50 ml bottles with sealed lids as per sampling protocols (ISO 5667-3 2003; ISO 5667-1 2006; ISO 5667-11 2009; Barcelona et al., 1985). All the collected pre-labeled sample-water-rinsed bottles were sent to Actlabs Canada to analyze for (APHA, 1995):
1) Major cations in mg/L: Ca2+, Mg2+, Na+, K+ and NH 4 + .
2) Major anions in mg/L: HCO 3 − , Cl−, SO 4 2 − , HPO 4 2 − and NO 3 − .
3) Trace elements; results not evaluated here.
For a better understanding of the rock-water interactions, evolution of the hydrogeochemistry, water quality evaluation for human consumption, comparing the water types and interpretation of seasonal variations in hydrochemical processes and suitability for agro-industrial use; Durov’s diagram, Piper’s diagram, Gibbs diagram, USSL ratio, Sodium Absorption ratio SAR, Percent sodium %Na, Kelly Ratio KR, Magnesium Absorption Ratio MAR, Total Hardness TH, Residual Sodium Carbonate RSC, Permeability Index PI and the field physico-chemical and laboratory analysis data were mounted unto various software platforms
| Indices | Formula | Reference |
|---|---|---|
| Percentage Sodium, Na% | % Na = Na + + K + Na + + K + + Ca 2 + + Mg 2 + × 100 | Wilcox (1955) |
| Kelly Ratio, KR | KR = Na + Ca 2 + + Mg 2 + | Kelley (1940) |
| Magnesium Absorption Ratio, MAR | MAR = ( Mg 2 + Mg 2 + + Ca 2 + ) ⋅ 100 | Szaboles & Barab (1964) |
| Total Hardness, HT | TH (CaCO3) mg/L = 2.5 Ca2+ + 4.1 Mg2+ | Todd (1980) |
| Residual Sodium Carbonate, RSC | RSC = ( CO 3 + HCO 3 − ( Ca + Mg ) ) | Eaton (1950) and Raghunath (1987) |
| Sodium Absorption Ratio, SAR | SAR = Na Ca + Mg 2 | Richards (1954) |
| Permeability Index, PI | PI = ( ( Na + K ) + HCO 3 ) ∗ 100 Ca + Mg + Na + K | Doneen (1962) |
| Water Quality Index, WQI | WQI = ∑ i = 1 n W i q i [ ∑ i = 1 n W i ] − 1 | Sisodia & Moundiotiya (2006) |
The aquiferous formations in Yaoundé are made up from top to bottom,; top soil 0.50 - 9.00 m, clayey Sand 1.00 - 25.00 m, Silty mottled clay containing preserved fractures 10.00 - 42.10 m, Ferralitic Laterite 1.60 - 37.30 m, fractured weathered gneiss and unweathered fractured gneiss and migmatites of the Yaoundé series 1.50 - 38.00 m in thickness, up to a total depth of 84 m
Fractured basement and weathered layers of crystalline rocks form two major types of aquifers important for water supply in hard rock areas. These weathering profiles generally develop under both stable geodynamic conditions where weathering rate far exceeds erosion rate with important amounts of precipitation and high temperatures (Wyns et al., 2004).
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
Subsurface structural fissures in the study area are mostly fissures and fractures
1) Depth-to-water values (m) of groundwater in wells in Yaoundé range from: 0.40 - 22.30 in the Wet season; 0.64 - 22.42 in the Wet/dry season; 1.31 - 21.56 in the Dry season to 1.95 - 23.30 in the Dry/wet season
2) Borewells have depths of 17 m - 83 m while Dugwells range in depth from 80 cm at the valleys to 19 m at higher altitudes. Most shallow dugwells < 5 m at elevations above 660 m dried up completely in the Dry/wet season 4.
3) Surface elevation and Groundwater contours
The surface elevation and groundwater contours are mostly concentric around the hills and are a direct replica of surface morphology
the groundwater levels are similar to surface elevation in profile typical of phreatic aquifers.
1) pH values of Yaoundé groundwater range from: 4.0 - 8.8 in the Wet season to 2.6 - 12.0 in the Dry/wet season
2) Total Dissolved Solids (TDS) (mg/L) values of Yaoundé groundwater range from: 4 - 650 in the wet to 0.01 - 800 in the dry season
3) Temperature T ˚C values of Yaoundé groundwater range from: 21.0 - 27.5 in the wet season to 19.5 - 27.5 in the dry season
4) Electrical conductivity EC µS/cm values of Yaoundé groundwater range from 10 - 1300 in the wet season, 90 - 1690 in the dry/wet season
96.7%; Wet/dry 424 samples, 95.3%; Dry season 433 samples 97.3% and Dry/wet season 431 samples, 96.9% are thus considered as good for residual salinity hazard based on the electrical conductivity
The occurrence of ions in groundwater in Yaoundé in decreasing order of magnitude: for cations; Ca2+ > K+ > Mg2+ > Na+ > NH 4 + and anions; HCO 3 − > Cl− > SO 4 2 − > NO 3 − > HPO 4 2 − . The evaluation of ions is based on recommended guideline values (WHO, 2017) (
| Season | Wet | Wetdry | Dry | Drywet | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Parameter | Min | Max | Av | Min | Max | Av | Min | Max | Av | Min | Max | Av |
| Depth to water (m) | 0.40 | 22.30 | 1.22 | 0.64 | 22.42 | 1.86 | 1.31 | 21.56 | 2.41 | 1.95 | 23.30 | 4.22 |
| pH | 4.0 | 8.8 | 7.8 | 3.4 | 9.0 | 7.5 | 6.2 | 9.0 | 6.0 | 2.6 | 12 | 5.6 |
| Electrical conductivity (µS/cm) | 10 | 1300 | 246 | 20 | 1700 | 348 | 50 | 1050 | 452 | 90 | 1690 | 276 |
| Temperature (˚C) | 19.0 | 27.5 | 24.8 | 19.0 | 27.5 | 25.3 | 21.0 | 28.0 | 23.6 | 19.5 | 27.5 | 25.0 |
| Total dissolved solids (mg/l) | 0.0 | 850 | 186 | 0.0 | 800.0 | 200 | 0.0 | 650.0 | 250 | 0.0 | 850.0 | 150 |
| Hazard Class | EC µS/cm | Quality | Season1 No % | Season2 No % | Season3 No % | Season4 No % | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| C0 | 0 - 100 | Excellent | 141 | 31.7 | 118 | 26.5 | 112 | 25.2 | 112 | 25.2 |
| C1 | 101 - 250 | Very Good | 149 | 33.5 | 145 | 32.6 | 156 | 35.1 | 154 | 34.6 |
| C2 | 251 - 750 | Good | 140 | 31.5 | 161 | 36.2 | 165 | 37.0 | 165 | 37.1 |
| C3 | 751 - 2250 | Doubtful | 15 | 3.4 | 21 | 4.7 | 11 | 2.5 | 13 | 29.2 |
| C4 | >2250 | Unsuitable | 0 | 0 | 0 | 0 | 1 | 0.2 | 1 | 0.2 |
1) Sodium ionic concentration values of Yaoundé groundwater range from: 0.05 - 3.31 in Wet season to 0.05 - 3.42 in the Dry season. Sodium regulates blood pressure levels in the human body and increased levels of sodium in blood leads to rise in blood pressure.
| Ions | Wet Season | Dry season | ||
|---|---|---|---|---|
| min | Max | min | max | |
| Na+ | 0.05 | 3.31 | 0.05 | 3.42 |
| K+ | 0.55 | 41.69 | 0.38 | 32.02 |
| Ca2+ | 4.24 | 112.8 | 1.38 | 74.45 |
| Mg2+ | 1.01 | 32.9 | 0 | 14.16 |
| NH 4 + | 0.01 | 0.16 | 0 | 30.05 |
| HCO 3 − | 2.44 | 368.76 | 4.88 | 287.45 |
| NO 3 − | 0 | 4.32 | 0.07 | 64.86 |
| SO 4 2 − | 0.57 | 14.37 | 0.36 | 25.94 |
| Cl− | 1 | 184 | 0.32 | 89.2 |
| HPO 4 2 − | 0 | 0.19 | 0 | 0 |
| Hardness | 1.74 | 18.21 | 0.34 | 11.02 |
2) Potassium ionic concentration values of Yaoundé groundwater range from: 0.55 - 41.69 in the Wet season to 0.38 - 32.02 in the Dry season. Potassium Controls body balance and maintains normal growth of the human body. Deficiency of potassium might lead to weakness of muscles and rise in blood pressure. No standard limits have been provided by the WHO Standards for level of sodium and potassium in drinking water.
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 inadequate 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 NH 4 + ionic concentrations of Yaoundé groundwater range from: 1.01 - 32.9 in the Wet season and 0 - 14.16 in the Dry season. These values are above allowable limits in some areas in both the wet and dry seasons. Ammonium is generally present in natural waters, though in very small amounts, as a result of microbiological activity which causes the reduction of nitrogen-containing compounds. When present in levels above 0.1 mg/L N, sewage or industrial contamination may be indicated. From the viewpoint of human health the significance of ammonium is marked because it indicates the possibility of sewage pollution and the consequent possible presence of pathogenic micro-organisms.
6) Bicarbonate HCO 3 − ionic concentrations of Yaoundé groundwater range from: 2.44 - 368.76 in the Wet season and 4.88 - 287.45 in the Dry season. No standard limits have been provided by the WHO Standards for level of carbonate and bicarbonate in drinking water.
7) Chloride Cl− ionic concentrations of Yaoundé groundwater range from: 1 - 184 in the Wet season and 0.32 - 89.2 in the Dry season. Acceptable limit of chloride in drinking water is 250 mg/L (1000 mg/L in case of no other alternative source) (WHO, 2017). Large amounts of chloride ions leads to bad taste in groundwater. Increase in chloride levels in our body might lead to increase in blood pressure levels and rise in body fluids.
8) Sulphate SO 4 2 − ionic concentrations of Yaoundé groundwater range from: 0.57 - 14.37 in the Wet season and 0.36 - 25.94 in the Dry season. The sulphate ion causes no particular harmful effects on soils or plants; however, it contributes in increasing the salinity in the soil solution. Acceptable limit of sulphate in drinking water is 200 mg/L. Excess sulphate consumption through water might lead to occurrence of diarrhoea in humans.
9) Phosphate HPO 4 2 − ionic concentrations of Yaoundé groundwater range from: 0 - 0.19 in the Wet season and 0 in the Dry season. The mean phosphate levels in all seasons for groundwater samples in Yaoundé are below the recommended maximum contaminant level. 1.0 mg/L for drinking water.
10) Nitrate NO 3 − ionic concentrations of Yaoundé groundwater range from: 0 - 4.32 in the Wet season and 0.07 - 64.86 in the Dry season. For the dry seasons the values are above the recommended value of 50 mg/L hazardous to infants. 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 relatively 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, HCO3, Cl and SO4 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).
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
| Gibbs Yaoundé | Wet season | Dry season | ||
|---|---|---|---|---|
| Ions | samples | % | Samples | % |
| Cations | ||||
| Atmospheric precipitation dominance | 7 | 16 | 8 | 18 |
| Rock-weathering dominance | 38 | 84 | 37 | 82 |
| Anions | ||||
| Atmospheric precipitation dominance | 9 | 20 | 11 | 25 |
| Rock-weathering dominance | 36 | 80 | 34 | 75 |
| Cation + Anion | ||||
| Atmospheric precipitation dominance | 16 | 17.8 | 19 | 21 |
| Rock-weathering dominance | 74 | 82.2 | 63 | 79 |
During the dry season, for cations; 67 ions, 34 samples 73.3% rock weathering dominance and 23 ions 11 samples, 26.7% evaporation-crystallization dominance. For anions, 33 samples 73.3% are of rock weathering dominance and 12 samples 26.7% are of the atmospheric precipitation dominance.
Considering cations + anions, in the rainy season 70 ions, 35 samples 77.8% of the samples fall in the rock weathering dominance and while 20 ions, 10 samples 22.2% fall under atmospheric precipitation dominance. No sample fell under evaporation-crystallization dominance in both seasons. This reveals that, the 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.
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). Based on this Classification
| SN | Durov’s Diagram Yaoundé | No. | % | ||
|---|---|---|---|---|---|
| Water type and hydrogeochemical process | Wet | Dry | Wet | Dry | |
| 1 | HCO 3 − and Ca+ dominant, indicates recharging waters in migmatites and gneiss. | 8 | 19 | 15.6 | 42.2 |
| 2 | This water type is dominated by Ca+ and HCO 3 − ions. Association with dolomite is presumed if Mg+ is significant. However, those samples in which Na+ is significant, an important ion exchange is presumed | 10 | 11 | 22.2 | 24.4 |
| 3 | HCO 3 − and Na+ are dominant, normally indicates ion exchanged water, although the generation of CO2 at depth can produce HCO 3 − where Na+ is dominant under certain circumstances | 28 | 11 | 62.2 | 24.4 |
| 4 | SO 4 2 − dominates, or anion discriminant and Ca+ dominant, Ca+ and SO 4 2 − dominant, frequently indicates recharge water in lava and gypsiferous deposits, otherwise mixed water or water exhibiting simple dissolution may be indicated. | 0 | 2 | 0 | 4.4 |
| 5 | No dominant anion or cation, indicates water exhibiting simple dissolution or mixing | 0 | 1 | 0 | 2.2 |
| 6 | SO 4 2 − dominant or anion discriminate and Na+ dominant; This water type is not frequently encountered and indicates probable mixing or uncommon dissolution influences. | 0 | 1 | 0 | 2.2 |
samples, 24.4%; Class 4 simple dissolution: 2 samples, 4.4%; Class 5 simple dissolution or mixing: 1 sample, 2.2%; Class 6probable mixing or uncommon dissolution influences: 1 sample, 2.2% respectively. There are no Classes 5 - 9 in the rainy season and no Classes 7 - 9 in the dry season samples in groundwater from 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
season. Chloride-rich water: 10 samples 22.2% in the rainy and 28 samples, 62.2% in dry season. The dominance of Calcium bicarbonate and Magnesium bicarbonate waters types in both seasons indicate freshly recharged groundwater that has not moved a long distance from its source (Hounslow, 1995).
1) Piper’s Hydrogeochemical facies
From the Piper’s diagram, Field (I): Ca-Mg-Cl-SO4 hydrogeochemical facies has10 samples, 22.2% in the rainy and 16 samples, 35.6% in the dry season. This facies is characteristic of stagnant groundwater zones or of groundwater at some distance along its flow path (Eelco 2014). Field (IV), Ca-Mg-HCO3 hydrogeochemical facies has 35 samples, 77.8% in the rainy and 29 samples, 64.4% in the dry season. 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 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-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.
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 (
| Yaoundé Durov’s | samples | % | |||
|---|---|---|---|---|---|
| SI | Water type | Wet | Dry | Wet | Dry |
| Diamond Field | |||||
| A | Normal earth alkaline water; prevailing HCO 3 − | 29 | 10 | 64.4 | 22.2 |
| B | Normal earth alkaline water; prevailing HCO 3 − , SO 4 2 − or Cl− | 2 | 12 | 4.4 | 26.7 |
| C | Normal earth alkaline water; prevailing SO 4 2 − or Cl− | 10 | 23 | 22.2 | 51.1 |
| D | Earth alkaline water; increased portions of alkalis; prevailing HCO 3 − | 4 | 0 | 8.9 | 0 |
| Cation Field | |||||
| 1 | Ca-rich waters | 33 | 19 | 73.3 | 42.2 |
| 2 | Mg-rich waters | 12 | 26 | 26.7 | 57.8 |
| Anion Field | |||||
| 4 | HCO 3 − waters | 37 | 17 | 82.2 | 37.8 |
| 6 | Cl− waters | 8 | 28 | 17.8 | 62.2 |
earth alkaline water with prevailing bicarbonate. Category B; 3 samples, 4.4% are characterized by normal earth alkaline water with prevailing sulfate or chloride and Category C; 10 samples, 22.2% are characterized by alkaline earth water with increased portions of alkalis with prevailing bicarbonate. In the dry season: Category A; 11 samples, 22.2%; Category B, 11 samples, 22.2% and Category C; 23 samples, 51.1%. The dominant water types are Category A, 64.4%; Normal earth alkaline water; prevailing HCO 3 − in the rainy season and Category C, 51.1%; Normal earth alkaline water; prevailing SO 4 2 − or Cl− in the dry season. This groundwater has a modern age and is being rapidly recharged by precipitation.
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). For the rainy season 43 samples, 95.6% have WQI 0 - 25; Excellent. 2 samples, 4.4% have WQI 26 - 50; Good, while in the dry season; 15 samples, 33.3% have WQI 0 - 25; excellent. 9 samples, 20% have WQI 26 - 50; Good; 8 samples, 17.7% have WQI 51 - 75 poor; 7 samples, 15.5% have WQI 76 - 100; very poor and 6 samples, 13.3% have WQI > 100; unsuitable
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
groundwater. The general acceptance level of hardness is 300 mg/L, although WHO has set an allowable limit of 600 mg/L (
To assess the overall irrigational water quality of the samples collected, six computed water quality parameters have been considered; namely, Sodium Percentage (SSP), Sodium Adsorption Ratio (SAR), Soluble Permeability Index (P.I.), Residual Sodium Carbonate (RSC), Magnesium Adsorption Ratio and Kelly’s Ratio.
1) Sodium Adsorption Ratio (SAR)
SAR values are plotted against EC values (in µS/cm) on the U.S. Salinity diagram to categorize analysed water samples according to their irrigational suitability quotient. SAR values range from 0.005 - 0.075 in rainy seasons and 0.01 - 0.14 during Dry season
2) Soluble Sodium Percentage (SSP) or Na%
Soluble Sodium Percentage ( SSP ) is used to evaluate sodium hazard. SSP values range from 1.50% - 15.00% in rainy season and 3.00% - 26% during the dry season. Water with a SSP greater than 60% can result in sodium accumulations that will cause a breakdown in the soil’s physical properties
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
4) Residual Sodium Carbonate (RSC)
Residual sodium carbonate (RSC) index of irrigation soil water indicates the alkalinity hazard of a soil. In Yaoundé, groundwater RSC values range between 0.5 Simbok - 1.75 Nkondengi in the rainy season and 0.25 Mvogbesi, Nsimeyong - 1.45 Tamtam, Madagascar in the dry season. In the rainy season, 42 samples 93.4% have RSC < 1.25; Good, 2 samples (4.4%) have RSC > 1.25 - 2.5 <; Doubtfuland 1 sample (2.2%) has RSC > 2.5; Unsuitable (
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). In the rainy season in Yaoundé, 25 samples (56%) are <50; suitable and 20 samples (44%) >50 are unsuitable. In the dry season, 33 samples (73%) are <50; suitable and 12 samples (27%) > 50 are unsuitable
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
KR > 1 is unsuitable. During rainy season, KR values vary between 0.003 - 0.026 and during the dry season the values vary between 0.005 - 0.07 (
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 (
In this model, precipitation infiltrates and flows through;
1) Sandy clayey weathered basement regolith (with relics of fissures and fractures)
2) Fractured/fissured unweathered basement
3) Deep large regional fractures with larger flow volumes.
Groundwater is stored in;
1) The fracture/fissure insular network infrastructure of unconnected joints, fissures and fractures; the phreatic local subsystem with only limited local storage and can only supply dugwells in the wet seasons. Here storage is directly impacted by seasonal variations in precipitation.
2) The fracture/fissure network infrastructure of connected joints, fissures and fractures; the intermediate subsystem. Here only intermediate flow and storage 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.
Groundwater contours are concentric around areas of high elevation and mimic the surface in all seasons; indicative of topography controlled piston flow in a phreatic aquiferous formation.
The results of laboratory analysis of groundwater together with hydrogeochemical tools have elucidated the processes and mechanisms affecting the groundwater chemistry in Yaoundé: Chemical weathering of rock-forming minerals and atmospheric precipitation are the major and minor factors influencing the processes of groundwater ions acquisition from the matrix of the aquiferous formation while ion exchange, dissolution and mixing are the mechanisms by which the groundwater equilibrates and develops its chemical character over its flow domain. The dominant water types are Ca-rich waters and Mg-rich waters. The dominant Hydrogeochemical facies are; Ca-Mg-HCO3 in the rainy season and Ca-Mg-Cl-SO4 in the dry season. All samples are “fresh water” type during all seasons.
The Ca-Mg-HCO3 hydrogeochemical facies is characteristic of freshly recharged groundwater that has equilibrated with CO2 and soluble carbonate minerals under open system conditions in the vadose zone in a single hydraulic phreatic aquiferous fractured rock formation; the Yaoundé migmatite gneiss.
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.
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 CaHCO3, MgHCO3 and MgCl− freshwater types low in ionic content that vary with seasons and soft, typical of phreatic aquifers in basement aquifers.
Groundwater chemical quality indices indicate the water is good for domestic and agro industrial use in both seasons.
The Yaounde gneiss fractured rock aquifer is made up of three flow fields: a) The local flow field with stagnant areas where infiltrated water is almost stationary and connected to the other flow regimes only during the peak of the rainy season, with the dominant hydrogeochemical facies being Ca-Mg-Cl-SO4 at all seasons; b) The intermediate flow field where some fissures are interconnected to other fissures in the meter scale during the dry season; and c) The regional flow field where the fissures are interconnected in the kilometer scale in all seasons. The dominant hydrogeochemical facies in the intermediate and regional flow is Ca-Mg-HCO3.
The weathered regolith, the local fissures and intermediate fissures/fractures and the regional fracture networks form a single chemically continuous unit indicating a single phreatic weathered basement fractured-rock aquifer of variable depth up to 83 m.
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.
We sincerely thank all the field workers, Geofor-Cameroon and the private well owners for access and well data.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
None.
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