Journal of Environmental Science Current Research Category: Environmental science Type: Research Article

Characterization of Groundwater in the Basaltic Fractured Rock Aquiferous Formations of the Limbe Coastal Region of Mount Cameroon, SWR-Cameroon

Akoachere RA1*, Eyong TA1, Egbe SE1, Yaya OO2, Mbua RL3 and Nwude MO4
1 Department Of Geology, University Of Buea, Buea, Cameroon
2 National Water Resources Institute, Kadun, Nigeria
3 Department Of Environmental Science, University Of Buea, Buea, Cameroon
4 National Water Resources Institute, Kaduna, Nigeria

*Corresponding Author(s):
Akoachere RA
Department Of Geology, University Of Buea, Buea, Cameroon
Tel:+237 690153887,
Email:r.akoachere@ubuea.cm

Received Date: Mar 11, 2019
Accepted Date: Jun 13, 2019
Published Date: Jun 24, 2019

Abstract

Limbe is the capital of Fako Division in the South West Region of Cameroon. It is an economic centre for Agro-industries, the Limbe deep seaport and has the only oil refinery (SONARA) in the Cameroon. It is an area where the groundwater is becoming increasingly the most important water resource as the exponential population growth imposed by urban migration towards the Agro-Oil-Port city has out-paced the development of water supply infrastructure. There are little or no funds allocated to carry out research on the rock/groundwater interactions and groundwater quality of the phreatic aquiferous formations in Limbe Coastal Region of Mount Cameroon. This study was carried out to characterize the groundwater solute chemistry and groundwater domestic-agro-industrial quality using hydrogeochemical tools and physicochemical parameters; Ionic ratios, Gibbs’s diagram’s, Piper’s diagrams, Durov’s diagrams and water quality indices. From field physicochemical parameters; wet season, pH range from 6.1-8.5; Temperature, 22.3-27.9°C; EC, 38-963µS/cm; TDS, 25.46-645.21 mg/L and dry season pH values ranged from 6.4- 7.8; temperature, 24.1-30.4oC; EC, 59-1515µS/cm; TDS, 39.53-1015.05 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 sequence of abundance of major ions are Ca2+>K+>Mg2+>Na+>NH4+, HCO3- >Cl->HPO42->SO42->NO3-in wet season and Ca2+>K+>Mg2+> Na+,HCO3->Cl->SO42->HPO42->NO3- dry season. Groundwater ionic content was as a result of ion exchange fromrock weathering reactions. Water types are MgHCO3, CaCl and NaCl in wet season MgHCO3, CaHCO3, CaCl and NaCl in dry season. The hydrogeochemical facies are CaMgHCO3, Ca-Mg-Cl-SO4 and Na-K-HCO3 in the wet season and Ca-Mg-HCO3 in dry season. Ion exchange, Simple dissolution and uncommon dissolution determined groundwater character. HT wet season 5.04-531.95mg/L, 91.50-570.61mg/L dry season;WQI wet season 2.85-307.31, dry season 1.04-272.43; RSC wet season -7.79-2.33, dry season -2.99-0.82; %Na wet season 8.56-53.56, dry season 1.67-13.41; KR,

Keywords

Basaltic Fractured rock; Characterization; Groundwater-quality; Hydrogeochemical-facies; Limbe-Cameroon

INTRODUCTION

Limbe the Fako Divisional headquarters of the Southwest Region in Cameroon is situated between 3.96-4.06N and 9.15-9.24E shown in figure 1. As an urban town with over 130,000 inhabitants [1] and insufficient water supply by the national water supply company CAMWATER, the inhabitants are obliged to turn to other sources of water such as natural springs, community water project catchments, hand-dug wells and boreholes. The inhabitants of parts of Limbe have always complained of low quality of drinking water from wells during the rainy season as such, there was a need to carry out this research to determine the root cause of this loss of quality during the rainy season. With the population explosion and the increased need for groundwater for economic development such as agriculture, industrial and domestic activities, understanding the groundwater chemical character and the aquiferous formations through which this water flows is of great importance for the future development of this region and sustainable use and protection of this aquifer system.

Figure 1: Map of the Study Area, Limbe-Southwest Region Cameroon.


Climate
Limbe experiences a subequatorial climate (hot and humid throughout) with mainly two distinct seasons; a rainy season between April to October and a dry season from November to March with a mean annual rainfall of about 3,100 mm, ±1,100 standard deviation [2]. The annual rainfall is high, with yearly precipitations varying from 1,500 to 6,000 mm in the last 34 years for different stations. Peak rainfall is recorded from June to August or September. June and July are characterized by intense and short-lived rainfall usually lasting less than 5hours a day. Whereas, August and September tend to experience less intense but more prolonged rainfalls that can last for 4 to 5 days in a row. Monthly rainfall totals frequently attain over 500 mm and sometimes up to 1,000 mm in June, July and August. The mean annual temperature is ~26 °C and shows only limited variations of ~4° throughout the year. Humidity is generally above 85%. These characteristics correspond to the Tropical Monsoon Climate according to the Köppen climate classification scheme [3,4].

Drainage
Limbe and Ejengele are the two main rivers in Limbe with the former being the largest. Limbe River takes its rise from Mt Cameroon, through Mile 4, Mile 2, Middle farms, Botanic Garden and into the Atlantic Ocean. Limbe River has a trellis drainage pattern with all its numerous tributaries running parallel down the slopes of Mount Cameroon. The slopes are steep, up to 43%, causing the streams to flow with high velocities. Other smaller streams consist of: Mange, Sange-Mile 4, Grand Lake-Mile 2, Konkikar, Balimba-Mabeta New layout, Motowoh water and Ndiba water. There are numerous springs notably: Likomba, Toma-Mile 4, Busumbu spring-mile 2, Cold source Mile 1 and Crystal garden. Gravity catchments have been constructed around some other smaller springs for additional water supply; Mile 4, Mabeta, Mawoh, Motowoh, Batoke. The serivers empty into the Atlantic Ocean.

Geology
Limbe sits on the plains and south eastern slopes of the ridge of Mount Cameroon separating the Rio del Rey and Douala basins; the Cameroon Volcanic Line presented in figure 2. The geology of Limbe is volcanic being consequent of the eruptive events of Mount Cameroon. The geology is of tertiary basaltic rocks composed of multiple porphyritic basaltic lava flows, punctuated by several strombolian pyroclastic cones to the West and North West and lahar deposit to the East seen in figure 2. These rocks either lie exposed at the surface or are covered by extremely fertile dark brown, reddish brown, yellowish and/or pale yellow sticky, clay, silt and silty clayey soils derived from intense in situ weathering. Soil thicknesses range from a few centimeters to more than 10 m [2]. The mineral content of basalts in Limbe consists mainly of Clinopyroxene (Ca(TiMgAl) SiAl)2O6), Hematite (Fe2O3), and Goethite (FeO(OH) and the soils comprises mainly of Anatase (TiO2), Annite (KFe3AlSiO10(OH,F)2), Augite (Ca2(Al-Fe)4(MgFe)4Si6O24), Goethite, Hematite, and Kaolinitic (Al2Si2O5(OH)4) minerals.

Figure 2: Sedimentary basins sandwiching Mount Cameroon and location of Limbe b. Geologic Map of Limbe and Environs, indicating the main geomorphologic characteristics, trend of the hydrographic network and morphology of some pyroclastic cones [2].

Geomorphology
The topography is marked by ridges and deeply incised ravines with a general W-E orientation, at high angle to the general NE-SW orientation of Mt Cameroon and gently sloping foot slopes of Mt Cameroon. These ridges form part of the Limbe-Mabeta volcanic massif, made up of degraded and deeply weathered Tertiary basaltic lava flows [5] The Limbe-Mabeta massif is an eroded volcanic massif SSE of Mt Cameroon. It is characterized by a series of sub-parallel E-W oriented valleys bordered by the 170°-striking Om be structure on its Eastern side [6]. Individual ridges are separated by asymmetric V-shaped valleys occupied by perennial and/or ephemeral streams. These streams either empty themselves directly into the ocean or into the delta around Mabeta.

Hydrogeology
Very little work has been done on the hydrogeology of Limbe with sparse data to correlate. Some groundwater baseline hydrogeochemical studies have been carried out around Mount Cameroon [7-10]. The Limbe area is made up mostly pyroclastic and of jointed weathered fractured and columnar basalt resulting in volcanic fractured rock aquifers where saturated. In addition, Limbe being a coastal area in contact with the sea may experiences seawater intrusion.

From geophysical sections and bore well drill cuttings, the lithostratigraphy is sub-divided into three layers as in figure 3; a soil cover formation topmost which constitutes weathered rocks, recent degradation of lava flows, transported materials, black soils and volcanic sands. This formation has a smaller volume as compared to the underlying basalts and has received only minor alterations by tectonic activities. The intermediate layer is made up of younger fractured columnar basalts and volcanic sand that constitute the aquifer. The basaltic rocks show traces of grinding, crushing, fracturing in a SW-NE direction, these fractures are the major fractures determining the storage of groundwater in the aquifers in the basal to-andesitic formation. The Ancient massive basalts bottom Formation make up the oldest formation and these rocks are the oldest in age. This formation has no fractures and as such do not contain water.

Figure 3: Geophysical section of formations in, Limbe

MATERIALS AND METHODS

Materials

The field materials and equipment used in the study are listed in table 1; they were calibrated as per manufacturer’s specifications.

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 to 200°C)

To measure temperature of water

Total Dissolved Solid meter

Hanna HI 96301 with ATC

To measure Total dissolved solids in water

Water sampler

Gallenkampf 1000ml

To collect well water sample from well

Sample bottles

Polystyrene 500ml

To hold sample for onward transmission to laboratory

ArcGIS

Version 10.1

GIS Drawing sampling/Tests location maps

Global Mapper

Version 15

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

Table 1: Field Equipment, Software’s, their specifications and functions.

Methods

An extensive field program of bore well; data acquisition, field hydrogeological measurement/tests, sampling and laboratory analysis of collected water was conducted in Limbe according to protocols; ISO 5667 1 [11], ISO 5667-11 [12], ISO 5667-3 [13] and Barcelona et al., [14].

A field visit was done in August 2016 by with hydrogeological traverse field mapping to determine appropriate hand-dug wells, boreholes, springs and streams. Field measurements and sampling started from Moliwethrough to Down Beach. The city of Limbe was divided into zones and work was carried out in two seasons; wet season September 2016 and dry season February 2017. GIS platforms were used to analyze field data for the creation of sample location, drainage and water level contour maps.

Measurements were carried out for longitudes, latitudes, Surface elevation, Wells: Well water level and Well depths Groundwater tests were carried out as follows: 897 in situ tests were carried out in 154 wells (147 hand dug wells and 7 boreholes) for Temperature (°C), pH, Electrical Conductivity (EC) and Total Dissolved Solids (TDS). Nine9 selected wells went dry during dry season. Forty 40 groundwater samples were collected; 20 samples 13 hand-dug wells, 5 boreholes, 1 river and 1 spring for each season. Samples were collected in 500ml containers, sealed and sent to Institute of Agricultural Research and Development-I.R.A.Dusing the standard methods APHA [15] to analyze for:

• Major cations in mg/L: Ca2+, Mg2+, Na+, K+ and NH4+
• Major anions in mg/L: HCO3-, Cl- , SO42-, HPO42- and NO3- 

To fully understand the relationship between the geology of the area, groundwater, hydrogeochemical tools were used such as:

Ionic ratio for indicative elements is a useful hydrogeochemical tool to identify source rock of ions and formation contribution to solute hydrogeochemistry [16].These were used in this study.

Gibbs Diagram is a plot of Na+/ (Na++HCO3- Ca2+) and Cl-/ (Cl+HCO3-) 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+K apices , 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 [sulphate chloride]/? anions and cations [Na+K]/? cations. This plot is a useful hydrogeochemical tool to compare water samples, determine water type and hydrogeochemical facies Langguth [17]. This has been used here for these purposes.

Durov diagram is a composite plot consisting of two ternary diagrams where the mill equivalent 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 [18] which give the hydrogeochemical processes determining the character of the water types in the aquiferous formation Langguth [17].

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-, SO42-, NO3-, NH4+) in order to assess the degree of groundwater contamination and suitability listed in table 2.

Indices

Formula

Reference

Percentage Sodium

Wilcox (1955) [21]

Kelly’s Ratio

Kelly (1940) [22]

Magnesium Adsorption Ratio

Palliwal  (1972) [23]

Total Hardness

TH (CaCO3) mg/L = 2.5 Ca2+ + 4.1Mg2+

Todd (1980) [24]

Residual Sodium Carbonate

Eaton (1950) [25]

Sodium Adsorption Ratio

Richard (1954) [26]

Permeability Index

Doneen (1962) [27]

Water Quality Index

Sisodia and Moundiotiya (2006) [20]

Table 2: Formulae for the determination for indices/parameters for water quality assessment.

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 listed in table 2.

The following software’s; Surfer 12, Global mapped 11 and AqQA 1.5 AGIS 10.3 were used for data presentation, interpretation and analysis.

RESULTS AND INTERPRETATION

Physicochemical parameters
The field measured physicochemical parameters of groundwater in Limbe are: Temperature, pH, EC and TDS for selected wells in each quarter shown in table 3. The summary statistics for all 154 tested wells in Limbe is shown in table 4. The EC values (WEC; DEC) increase with decrease in the Distance to Shoreline (DS) in both seasons. Wet season ECs (WEC) are lower than dry season ECs (DEC) as in figure 4.
 
Figure 4: Seasonal variations in field measured groundwater electrical conductivities with distance from shoreline in Limbe: The EC values (WEC; DEC) increase with decrease in the distance to shoreline (DS) in both seasons. Wet season ECs (WEC) are lower than dry season ECs (DEC).

SN

Location

E

N

SE (m)

DS(m)

WD (m)

WSL(m)

WT (0C)

WpH

WEC (µS)

WTDS (mg/L)

DSL
(m)

DT (0C)

DpH

DEC (µS)

DTDS (mg/L)

1

Motowoh

9.2183

4.0027

9

574.59

2.68

1.64

27.2

7.6

119

79.73

1.86

28.3

6.7

130

87.1

2

New Town

9.2146

4.0158

29

1113.61

42

25

27.2

7.8

208

139.36

29

26.2

6.9

295

197.65

3

Karrata 2

9.1896

4.0242

70

1511.49

62

40

26.2

7.9

217

145.39

45

28.7

6.9

212

142.04

4

Sokolo

9.1739

4.0208

109

1011.36

36

25

25.7

8

225

150.75

30

28.5

6.9

144

96.48

5

Mawoh

9.2207

4.0093

32

1144.73

5.83

3.11

27.1

6.3

162

108.54

3.45

28.7

6.6

248

166.16

6

Clerks Qters

9.2102

4.0099

4

776.86

2.45

1.5

25.1

8.2

245

164.15

2.25

27.8

6.9

227

152.09

7

Ngeme

9.1512

4.0147

34

669.06

45

16

25.6

7.8

238

159.46

20

25.8

7

224

150.08

8

Karrata

9.1842

4.0231

77

1200.3

55

30

25.6

8

282

188.94

30

27.1

6.9

231

154.77

9

Church Str.

9.2096

4.0164

30

861.33

2.69

1.79

27

7.4

334

223.78

2.18

28

6.9

217

145.39

10

Mbonjo

9.2146

3.998

30

900.23

1.86

1

26.6

7.6

180

120.6

0.71

27.8

6.9

121

81.07

11

Mabeta

9.2261

4.0129

39

1778.22

7.59

2

26.3

7.2

46

30.82

5.6

30.4

7

166

111.22

12

Mile 4

9.2269

4.0552

240

5512.49

2.73

1.64

25.5

8

221

148.07

2.19

27.5

6.7

259

173.53

13

West end

9.2165

4.0092

18

742.41

3.59

1

26.9

7.7

378

253.26

2.02

28.4

7.1

544

364.48

14

Mile 1-2

9.2205

4.0249

79

2311.69

2.85

1.93

26.1

8.5

138

92.46

2.37

27.5

6.8

187

125.29

15

Kulu

9.2142

4.0044

24

217.83

2.65

1.07

26.3

7.9

330

221.1

1.9

28.1

7.2

414

277.38

16

Mile 2

9.2097

4.0342

49

2674.45

2.86

1.6

25.4

7.2

196

131.32

1.6

27.8

7

452

302.84

17

Dockyard

9.2125

3.9998

1

46.57

2.82

1.17

27.1

7.7

963

645.21

1.85

28.8

7.2

1240

830.8

18

Mile 1

9.2154

4.0241

60

1889.36

3.83

3.45

26.3

8

335

224.45

3.75

27.3

6.8

334

223.78

19

Sokolo 2

9.1743

4.0213

81

1120.28

40

25

25.8

8.2

150

100.5

25

29

6.9

144

96.48

20

Dockyard 2

9.2126

3.9998

14

92.24

2.65

1

26.3

7.7

947

634.49

1.73

28.6

7

1515

1015.05

21

Kulu 2

9.2138

4.0029

8

124.36

2.89

1.04

26.8

8

284

190.28

1.79

28.6

7.3

742

497.14

22

Mbonjo 2

9.2152

3.9975

22

466.78

3.88

2.4

26.2

7.8

142

95.14

3.39

27.7

6.7

218

146.06

23

Motowoh 2

9.2186

4.0008

20

669.06

6.48

2.64

27.2

7.3

101

67.67

6.23

27.5

6.7

156

104.52

24

Mawoh 2

9.2251

4.0126

52

1811.56

11.38

3.74

26.5

7.2

98

65.66

11.17

27.3

6.8

404

270.68

25

Mabeta 2

9.2252

4.0131

37

1794.89

6.09

4.65

26.6

6.6

81

54.27

5.48

27.5

6.7

156

104.52

26

Westend 2

9.2163

4.0101

22

761.3

4.6

2.45

27.7

7.5

502

336.34

2.45

28.2

7.1

466

312.22

27

Church Str. 2

9.2091

4.0171

21

865.77

3.81

2

26.2

7.7

237

158.79

3.7

28

6.8

260

174.2

28

Mile 2-2

9.2108

4.0337

78

2622.88

4.34

4.16

26.1

7.1

226

151.42

4.16

27.1

7

254

170.18

29

Mile 4-2

9.2266

4.0522

245

5223.53

4.14

3.5

25.3

7.3

124

83.08

3.5

26.6

7

167

111.89

Table 3: Field measured physicochemical parameters of 29 representative wells during two seasons in Limbe.
Note: SN=Sample Number, WD=Well Depth, DS=Distance to Shoreline, WDW= Wet season Static water level, WT= Wet Season Temperature, WpH= Wet Season pH, WEC= Wet Season EC, WTDS=Wet Season TDS, DWL= Dry season Static water level, DT= Dry Season Temperature, DpH= Dry Season pH, DEC= Dry Season EC, DTDS= Dry Season TDS
 

Parameters

Wet

Dry

 

Min

Max

Mean

Std.

Min

Max

Mean

Std.

T(oC)

22.3

27.9

26.43

0.71

24.1

30.4

28.04

0.88

PH

6.1

8.5

7.28

0.51

6.4

7.8

6.84

0.23

EC (µS/cm)

38

963

213.87

160.35

59

1515

282.64

208.51

TDS (mg/L)

30.15

645.21

166.93

107.4

81.07

830.8

204.25

139.70

Table 4: Basic Statistics of the physicochemical tests for all 154 field tested wells in Limbe; min, max, mean and standard deviation of these parameters in two seasons.

The wells are at surface elevations of 1m to 245m with well-depths ranging from 1.8 to 6.2m and at distances from shoreline of 46.67 to 5223.53m.

Surface elevation: Surface elevations range from -2m.a.m.s.l at lower Motowoh to 256 a.m.s.l at Mile 4 shown in figure 5. Limbe is low lying on the shorelines of the Atlantic Ocean at the foot hills of Mount Cameroon. The low elevation areas are prone to floods in the rainy season.

Figure 5: Surface elevation profile of Limbe. Areas of low elevations are Church Street, Clerks Quarters, Mabeta, Kulu, Dockyard and highest elevation area is Mile 4. This elevation depicts discharge into the ocean.

Well parameters: The wells have depths ranging from 0.75m at Church Street to 13.17m at Upper Mawoh for hand-dug wells and from 36m at Sokolo (Bota New layout) to 62m at Lake Restaurant (Sokolo old road) for boreholes, depth to water levels ranged from 0.22m at Church Street to 8.17m at upper Mawohfor hand-dug wells and 20m at Lake Restaurant to 40m at Karrata shown in figure 6. 

Figure 6: Groundwater level contours for (a) wet season and (b) dry season; with least value at Church Street (0.22m) and highest value at Karrata (40m) in both seasons. Groundwater can be found at shallow levels like Church unlike Karrata.

Groundwater level contours: From elevation and depth to water level, the groundwater contours were drawn with equipotential vectors simulating groundwater flow lines and flow direction presented in figure 7. Groundwater levels mimic the surface topography from high areas Mile 4 to low areas Church Street, Mbonjo. The groundwater flows into the Atlantic Ocean. 
 
Figure 7: Spatial variation of Flow direction for (a) wet season and (b) dry season. Note: water moves away from peak values area (Mile 4) and moves towards lower value areas (Church Street, Clerk quarters, Mabeta, Dockyard) and into the ocean.

Temperature: The temperature values ranged from 22.3 - 27.9°C in the wet season and 24.1 - 30.4°C in the dry season seen in figure 8. The temperatures of groundwater in Limbe and environs are relatively low. There is a general increase from wet to dry season.

Figure 8: Spatial variation of Temperature values for both a) wet and b) dry seasons. There is an increase from wet to dry season. The lowest values come from areas along the Atlantic Ocean. Highest values are found in the North, Northwest a1nd Southeast of Limbe.

pH: pH values ranged from slightly to alkaline 6.1-8.5 in the wet season and slightly acidic to peralkaline 6.4-7.8 in the dry season as in figure 9.

Figure 9: Spatial variation of pH values for a) wet and b) dry seasons. Highest values are found North, Northwest through South in the wet season and Southeast in the dry season. Note: pH values are more elevated in the wet season than dry season.

Electrical Conductivity (EC): The values ranged between 38-963 μS/cm in wet season and 59-1515 μS/cm in the dry season shown in figure 10. The higher values of electrical conductivity are due to high solute concentration in water.

Figure 10: Spatial variation of Electrical Conductivity EC values for: a) wet and b) dry seasons. EC is maximum in the dry season and peaks Southeast of Limbe for both seasons. The highest value is 963 μS/cm in the wet season, and 1515 μS/cm in dry season, was recorded in Dockyard, Limbe.

Total dissolved solids: TDS ranged from 25.46 to 645.21 mg/L in the wet season and 39.53 to 830.8 mg/L in the dry season seen in figure 11. This indicates a freshwater area except for Dockyard.

Figure 11: Spatial variation of Total Dissolved Solids for: a) wet and b) dry seasons. TDS is maximum in the dry season and peaks Southeast of Limbe for both seasons but the TDS shows a more distributed load of higher concentration in the wet season from Northwest to Southeast.

Chemical properties of groundwater
The results of the chemical analysis varied in both seasons. In the wet season Ca+>K+>Mg2+>Na+>NH4+- HCO3- >Cl->HPO42->SO42->NO3- and Ca+>K+>Mg2+>Na+- HCO3- >Cl->SO42->HPO42->NO3- dry season. Cl-is widespread in the wet season than the dry season and peaks at Dockyard for both seasons. Most samples indicated a lack or decrease in Cl- concentration in the dry season with the exception of Dockyard which experiences an increase in Cl- concentrations presented in tables 5, 6, and figures 12, 13.

Figure 12: Spatial distribution of Cations a. Ca2+ b. Mg2+ c. Na+ c. K+ d. NH4+ for Wet and dry seasons, Limbe 

Figure 13: Spatial distribution of Anions a. HCO3- b. Cl- c. SO42- c. HPO42- d. NO3- for Wet and Dry seasons, Limbe. 

Wet (mg/L)

SN

Names

Na+

K+

Ca2+

Mg2+

NH4+

HCO3-

NO3-

SO42-

Cl-

HPO42-

1

Motowoh

0.04

0.88

0.00

1.83

2.61

34.16

0.14

11.41

10.00

1.36

2

St Ann

0.04

0.88

0.00

1.53

2.88

142.74

0.66

8.09

8.00

2.29

3

Elegance

0.25

3.86

3.10

2.71

0.27

107.97

1.13

3.98

8.00

3.56

4

Sokolo

0.34

4.21

3.10

2.45

0.54

88.45

0.00

3.32

6.00

2.63

5

Mawoh

0.21

2.28

0.00

1.51

0.18

18.91

0.00

5.43

22.00

1.70

6

ClerksQuarters

0.39

4.39

6.20

4.51

1.35

121.39

0.00

3.13

8.00

1.52

7

Limbe River

0.3

3.51

3.10

2.99

0.00

127.49

0.00

2.67

6.00

2.89

8

Ngeme

0.26

3.16

3.10

3.11

0.18

116.51

0.00

2.2

6.00

3.23

9

Karrata

0.30

3.86

6.20

4.22

0.00

101.87

0.43

2.25

7.00

23.87

10

Church Street

0.21

1.93

0.00

1.23

0.00

99.43

0.63

4.63

25.00

2.63

11

Mbonjo

0.30

2.46

0.00

2.1

0.00

83.57

2.58

3.6

10.00

6.11

12

Mabeta

0.00

0.53

0.00

1.76

0.00

0.61

0.02

3.32

10.00

0.08

13

Mile 4

0.26

2.81

3.10

2.67

0.00

48.19

0.00

5.33

18.00

0.00

14

Westend

0.86

10.00

2.79

1.46

0.00

100.65

0.00

10.66

32.00

0.00

15

Towe

0.09

1.05

0.00

1.63

0.00

57.95

0.00

3.23

9.00

0.00

16

Kulu

0.30

3.69

3.1

2.71

0.00

165.31

0.00

6.03

23.00

9.85

17

Mile 2

0.09

1.76

0.00

1.61

0.99

71.37

0.00

3.65

14.00

0.00

18

Dockyard

4.11

60.37

161.2

31.45

0.00

173.45

0.00

16.14

77.00

5.95

19

Spring

0.13

2.98

3.00

1.90

0.00

112.85

0.08

2.11

5.00

0.60

20

Rain

0.00

0.70

0.00

1.65

0.00

0.00

0.00

4.91

8.00

0.00

 

Min

0.00

0.53

0.00

1.23

0.00

0.00

0.00

2.11

5.00

0.00

 

Max

4.11

60.37

161.20

31.45

2.88

173.45

2.58

16.14

77.00

23.87

 

Mean

0.42

5.77

9.90

3.75

0.45

88.64

0.28

5.30

15.60

3.41

 

Std

0.89

13.02

35.67

6.58

0.87

49.78

0.62

3.65

16.32

5.45

Table 5: Basic Statistics of the physicochemical tests for all 154 field tested wells in Limbe; min, max, mean and standard deviation of these parameters in two seasons.
 

Dry (mg/L)

 

Names

Na+

K+

Ca2+

Mg2+

NH4+

HCO3-

NO3-

SO42-

Cl-

HPO42-

1

Motowoh

0.30

1.6

14.00

13.78

0.00

41.48

0.00

2.31

2.00

0.12

2

St Ann

0.25

1.25

14.00

22.16

0.00

103.70

0.00

0.60

0.00

0.41

3

Elegance

0.62

6.28

32.40

20.82

0.00

124.44

0.01

0.00

0.00

0.20

4

Sokolo

0.55

5.85

27.80

16.26

0.00

76.86

0.00

0.00

0.00

0.20

5

Mawoh

0.3

4.29

23.20

18.85

0.00

84.18

0.01

0.00

9.00

0.24

6

Clerks Quarters

0.74

7.53

37.20

18.14

0.00

136.64

0.00

1.86

0.00

0.14

7

Limbe River

0.58

6.28

32.40

21.76

0.00

135.42

0.00

0.15

0.00

0.16

8

Ngeme

0.58

5.46

32.40

20.93

0.00

123.22

0.00

0.91

0.00

0.16

9

Karrata

0.55

6.28

27.80

20.71

0.00

115.90

0.01

0.15

0.00

0.12

10

Church Street

0.37

2.5

23.20

18.85

0.00

109.80

0.00

1.66

25.00

0.20

11

Mbonjo

0.32

2.33

18.60

14.5

0.00

43.92

0.00

1.11

0.00

0.10

12

Mabeta

0.25

1.25

14.00

14.5

0.00

18.30

0.00

1.86

3.00

0.24

13

Mile 4

0.64

5.85

32.40

19.48

0.00

101.26

0.01

0.91

8.00

0.08

14

Westend

1.59

18.72

74.20

21.96

0.00

153.72

0.00

10.06

17.00

0.39

15

Towe

0.07

1.99

23.20

16.56

0.00

87.84

0.01

0.91

0.00

0.31

16

Kulu

0.62

5.03

32.40

31.92

0.00

208.62

0.00

3.12

9.00

0.31

17

Mile 2

0.94

7.89

46.40

24.96

0.00

145.18

0.00

5.68

10.00

0.61

18

Dockyard

3.77

62.74

155.20

44.54

0.00

602.68

0.00

16.8

67.00

0.12

19

Spring

0.58

5.58

37.20

23.3

0.00

125.66

0.00

0.60

0.00

0.20

20

Alpha Club

0.37

1.79

23.20

29.16

0.00

163.48

0.00

0.20

6.00

0.18

 

Min

0.07

1.25

14.00

13.78

0.00

18.30

0.00

0.00

0.00

0.08

 

Max

3.77

62.74

155.20

44.54

0.00

602.68

0.01

16.80

67.00

0.61

 

Mean

0.70

8.02

36.06

21.66

0.00

135.12

0.00

2.44

7.80

0.22

 

Std

0.79

13.44

31.11

7.09

0.00

118.70

0.00

4.14

15.49

0.13

Table 6: Basic statistics of results from chemical Analysis of groundwater for dry season, Limbe. The values of rainwater, springs, streams, Rivers and groundwater are similar indicating connectivity typical of phreatic aquifers in fractured rock aquifers.

MECHANISMS CONTROLLING WATER CHEMISTRY

Ionic Ratios of Groundwater: 18 ionic ratios in groundwater were used to deduce formation inputs in the coastal town of Limbe, as presented in tables 7, 8 and 9.

SN

SO 
/CL

Na 
/Cl

Mg 
/Cl

Na 
/HCO

Ca 
/HCO

Ca 
/SO

Ca/Mg

Ca+Mg 
/Na+K

HCO3/ 
∑An

NO3 
/∑An

SO4 
/∑An

Cl 
/∑An

Na+K+Cl 
/Na+ 
K+Cl+Ca

Na 
/Na+Cl

Mg 
/Ca+Mg

Ca 
/Ca+SO4

Ca+Mg 
/SO4

Mg/Ca

1

1.16

0.15

6.89

0.01

0.34

6.06

1.02

14.62

0.90

0.00

0.05

0.04

-0.01

0.13

0.50

0.86

12.03

0.98

2

0.00

0.00

0.00

0.00

0.14

23.33

0.63

24.11

0.99

0.00

0.01

0.00

0.10

1.00

0.61

0.96

60.27

1.58

3

0.00

0.00

0.00

0.00

0.26

0.00

1.56

7.71

1.00

0.00

0.00

0.00

0.18

1.00

0.39

1.00

0.00

0.64

4

0.00

0.00

0.00

0.01

0.36

0.00

1.71

6.88

1.00

0.00

0.00

0.00

0.19

1.00

0.37

1.00

0.00

0.58

5

0.00

0.03

2.09

0.00

0.28

0.00

1.23

9.16

0.90

0.00

0.00

0.10

-0.23

0.03

0.45

1.00

0.00

0.81

6

0.00

0.00

0.00

0.01

0.27

20.00

2.05

6.69

0.99

0.00

0.01

0.00

0.18

1.00

0.33

0.95

29.75

0.49

7

0.00

0.00

0.00

0.00

0.24

216.00

1.49

7.90

1.00

0.00

0.00

0.00

0.17

1.00

0.40

1.00

361.07

0.67

8

0.00

0.00

0.00

0.00

0.26

35.60

1.55

8.83

0.99

0.00

0.01

0.00

0.16

1.00

0.39

0.97

58.60

0.65

9

0.00

0.00

0.00

0.00

0.24

185.33

1.34

7.10

1.00

0.00

0.00

0.00

0.20

1.00

0.43

0.99

323.40

0.74

10

0.07

0.01

0.75

0.00

0.21

13.98

1.23

14.65

0.80

0.00

0.01

0.18

-20.68

0.01

0.45

0.93

25.33

0.81

11

0.00

0.00

0.00

0.01

0.42

16.76

1.28

12.49

0.97

0.00

0.02

0.00

0.12

1.00

0.44

0.94

29.82

0.78

12

0.62

0.08

4.83

0.01

0.77

7.53

0.97

19.00

0.78

0.00

0.08

0.13

-0.12

0.08

0.51

0.88

15.32

1.04

13

0.11

0.08

2.44

0.01

0.32

35.60

1.66

7.99

0.92

0.00

0.01

0.07

-0.05

0.07

0.38

0.97

57.01

0.60

14

0.59

0.09

1.29

0.01

0.48

7.38

3.38

4.73

0.85

0.00

0.06

0.09

0.04

0.09

0.23

0.88

9.56

0.30

15

0.00

0.00

0.00

0.00

0.26

25.49

1.40

19.30

0.99

0.00

0.01

0.00

0.08

1.00

0.42

0.96

43.69

0.71

16

0.35

0.07

3.55

0.00

0.16

10.38

1.02

11.38

0.94

0.00

0.01

0.04

-0.12

0.06

0.50

0.91

20.62

0.99

17

0.57

0.09

2.50

0.01

0.32

8.17

1.86

8.08

0.90

0.00

0.04

0.06

-0.03

0.09

0.35

0.89

12.56

0.54

18

0.25

0.06

0.66

0.01

0.26

9.24

3.48

3.00

0.88

0.00

0.02

0.10

0.00

0.05

0.22

0.90

11.89

0.29

19

0.00

0.00

0.00

0.00

0.30

62.00

1.60

9.82

0.99

0.00

0.00

0.00

0.14

1.00

0.39

0.98

100.83

0.63

20

0.03

0.06

4.86

0.00

0.14

116.00

0.80

24.24

0.96

0.00

0.00

0.04

-0.20

0.06

0.56

0.99

261.80

1.26

Min

0.00

0.00

0.00

0.00

0.14

0.00

0.63

3.00

0.78

0.00

0.00

0.00

-20.68

0.01

0.22

0.86

0.00

0.29

Max

1.16

0.15

6.89

0.01

0.77

216.00

3.48

24.24

1.00

0.00

0.08

0.18

0.20

1.00

0.61

1.00

361.07

1.58

Mean

0.19

0.04

1.49

0.01

0.30

39.94

1.56

11.39

0.94

0.00

0.02

0.04

-0.99

0.53

0.41

0.95

71.68

0.75

Std

0.32

0.05

2.07

0.00

0.14

61.31

0.73

6.10

0.07

0.00

0.02

0.05

4.64

0.48

0.10

0.05

109.24

0.31

Table 7: Ionic ratios of groundwater ions: Summary statistics for wet season.
 

 

SO 
/CL

Na 
/Cl

Mg 
/Cl

Na 
/HCO

Ca 
/HCO

Ca 
/SO

Ca 
/Mg

Ca+Mg 
/Na+K

HCO3 
/∑An

NO3 
/∑An

SO4 
/∑An

Cl 
/∑An

Na+K+Cl
/Na+ 
K+Cl+Ca

Na 
/Na+Cl

Mg 
/Ca+Mg

Ca 
/Ca+SO+

Ca+Mg
/SO+

Mg 
/Ca

1

1.16

0.15

6.89

0.01

0.34

6.06

1.02

14.62

0.90

0.00

0.05

0.04

-0.01

0.13

0.50

0.86

12.03

0.98

2

0.00

0.00

0.00

0.00

0.14

23.33

0.63

24.11

0.99

0.00

0.01

0.00

0.10

1.00

0.61

0.96

60.27

1.58

3

0.00

0.00

0.00

0.00

0.26

0.00

1.56

7.71

1.00

0.00

0.00

0.00

0.18

1.00

0.39

1.00

0.00

0.64

4

0.00

0.00

0.00

0.01

0.36

0.00

1.71

6.88

1.00

0.00

0.00

0.00

0.19

1.00

0.37

1.00

0.00

0.58

5

0.00

0.03

2.09

0.00

0.28

0.00

1.23

9.16

0.90

0.00

0.00

0.10

-0.23

0.03

0.45

1.00

0.00

0.81

6

0.00

0.00

0.00

0.01

0.27

20.00

2.05

6.69

0.99

0.00

0.01

0.00

0.18

1.00

0.33

0.95

29.75

0.49

7

0.00

0.00

0.00

0.00

0.24

216.00

1.49

7.90

1.00

0.00

0.00

0.00

0.17

1.00

0.40

1.00

361.07

0.67

8

0.00

0.00

0.00

0.00

0.26

35.60

1.55

8.83

0.99

0.00

0.01

0.00

0.16

1.00

0.39

0.97

58.60

0.65

9

0.00

0.00

0.00

0.00

0.24

185.33

1.34

7.10

1.00

0.00

0.00

0.00

0.20

1.00

0.43

0.99

323.40

0.74

10

0.07

0.01

0.75

0.00

0.21

13.98

1.23

14.65

0.80

0.00

0.01

0.18

-20.68

0.01

0.45

0.93

25.33

0.81

11

0.00

0.00

0.00

0.01

0.42

16.76

1.28

12.49

0.97

0.00

0.02

0.00

0.12

1.00

0.44

0.94

29.82

0.78

12

0.62

0.08

4.83

0.01

0.77

7.53

0.97

19.00

0.78

0.00

0.08

0.13

-0.12

0.08

0.51

0.88

15.32

1.04

13

0.11

0.08

2.44

0.01

0.32

35.60

1.66

7.99

0.92

0.00

0.01

0.07

-0.05

0.07

0.38

0.97

57.01

0.60

14

0.59

0.09

1.29

0.01

0.48

7.38

3.38

4.73

0.85

0.00

0.06

0.09

0.04

0.09

0.23

0.88

9.56

0.30

15

0.00

0.00

0.00

0.00

0.26

25.49

1.40

19.30

0.99

0.00

0.01

0.00

0.08

1.00

0.42

0.96

43.69

0.71

16

0.35

0.07

3.55

0.00

0.16

10.38

1.02

11.38

0.94

0.00

0.01

0.04

-0.12

0.06

0.50

0.91

20.62

0.99

17

0.57

0.09

2.50

0.01

0.32

8.17

1.86

8.08

0.90

0.00

0.04

0.06

-0.03

0.09

0.35

0.89

12.56

0.54

18

0.25

0.06

0.66

0.01

0.26

9.24

3.48

3.00

0.88

0.00

0.02

0.10

0.00

0.05

0.22

0.90

11.89

0.29

19

0.00

0.00

0.00

0.00

0.30

62.00

1.60

9.82

0.99

0.00

0.00

0.00

0.14

1.00

0.39

0.98

100.83

0.63

20

0.03

0.06

4.86

0.00

0.14

116.00

0.80

24.24

0.96

0.00

0.00

0.04

-0.20

0.06

0.56

0.99

261.80

1.26

Min

0.00

0.00

0.00

0.00

0.14

0.00

0.63

3.00

0.78

0.00

0.00

0.00

-20.68

0.01

0.22

0.86

0.00

0.29

Max

1.16

0.15

6.89

0.01

0.77

216.00

3.48

24.24

1.00

0.00

0.08

0.18

0.20

1.00

0.61

1.00

361.07

1.58

Mean

0.19

0.04

1.49

0.01

0.30

39.94

1.56

11.39

0.94

0.00

0.02

0.04

-0.99

0.53

0.41

0.95

71.68

0.75

Std

0.32

0.05

2.07

0.00

0.14

61.31

0.73

6.10

0.07

0.00

0.02

0.05

4.64

0.48

0.10

0.05

109.24

0.31

Table 8: Ionic ratios of groundwater ions: Summary statistics for dry season.
 

Ionic Ratio

Wet

Dry

Comments

Interpretation

SO4/CL

0.19 - 1.14

0 - 1.16

Very High

Suggests additional sources of Sulphate.

Na/Cl

0 - 0.06

0 - 0.15

Low

Silicate weathering, some marine water.

Mg/Cl

0.05 - 0.6

0 - 6.89

Very High

Depict a silicate weathering environment

Na/HCO

0 - 0.02

0 - 0.01

Very Low

Low weathering of Na-silicates.

Ca/HCO

0 - 0.93

0.14 - 0.77

Low

Ca-silicate weathering from the basalts of Limbe.

Ca/SO4

0 - 9.99

0 - 216

Very High

There is no gypsum dissolution in volcanic coastal regions

Ca/Mg

0 - 5.13

0.63 - 3.48

High

Typical of coastal regions due to cation-exchange

Mg/Ca

0 - 1

0.29 - 1.58

Very Low

Silicate rock weathering

(Ca+Mg)/(Na+K)

0.39 - 3.32

3 - 24.24

Very High

Occurrence of silicate weathering.

HCO3-/∑Anions

0 - 0.94

0.78 - 1

Low-High

Silicate weathering reactions and some seawater

NO3/∑Anions

0 - 0.02

0 -(2.38E-05)

Low

No anthropogenicactivities.

SO4/∑Anions

0.02 - 0.38

0 - 0.08

Low

No oxidation of sulphides.

Cl-/∑Anions

0.04 -0.71

0 - 0.18

Low

Rock weathering

-4.96-4.92

-20.68-0.20

Low-High

Plagioclase weathering unlikely

0 - 0.05

0.01 - 1.0

Low-High

Some halite Solution; Reverse softening and seawater; Sodium source other than halite-albite, ion exchange

0.16 - 1.0

0.22 - 0.61

Low-High

Silicate weathering. Ferromagnesian minerals but no evidence of granitic weathering

0 - 0.91

0.86 - 1.0

Low-High

Ion exchange/Calcium removal and Calcium source from silicates

0.16-11.94

0 - 361.07

Low-High

No dolomite at all, Dedolomitization

Table 9: Interpretation of Ionic Ratios for wet and dry seasons with determined formation characteristics.

12 of the 18, 66.7% ionic ratios calculated gave indices indicating silicate weathering of geologic formations in Limbe as a source of solute concentration in the groundwater while nitrate ratio indicates no anthropogenic contribution and sulfate indices indicates no oxidation of sulfides. Sulphate and Sodium indices indicate alternative sources of ions such as silicate weathering and ion exchange. Ca and Mg are sourced from silicate weathering. Sodium, Chloride and Bicarbonate indices indicate seawater intrusion; this is prominent in Dockyard and to some extent in Church Street, Mabeta, Mawoh, Motowoh, West End and Mbonjo. There is no plagioclase weathering, gypsum dissolution nor dolomitization.

Rock-Groundwater Interaction in Limbe: From Gibbs diagram for cations; 20 samples 100% are of rock-weathering dominance and for anions; 18 samples 90% are of rock weathering dominance and 2 samples 10% are of the atmospheric precipitation dominance during the wet season. During the dry season for cations and anions, all 20 samples 100% are controlled by rock weathering as in figure 14 and table 10. This reveals the weathering of the aquifer matrix as the primary dominant process in the acquisition of ions and atmospheric precipitation as the secondary process controlling the hydrogeochemistry in Limbe.

Figure 14: Gibbs Diagram indicating the interaction between aquifer formation and groundwater samples from Limbe. Almost all samples plot in the rock -weathering field for all the seasons.

Type of Rock-water Interaction

TDS mg/L

Wet

Dry

Cation

%

Anion

%

Cation

%

Anion

%

Rock - Weathering dominance

50-1000

20

100

18

90

20

100

20

100

Atmospheric Precipitation dominance

1-50

-

-

2

10

-

 

-

 

Table 10: Wet and Dry seasonal variations in rock/groundwater interaction from Gibbs diagram, Limbe.

Groundwater types: The diamond field of Piper’s diagram was divided into seven classes A-G classifying water types and designated with alphabets from A to G are shown in figure 15. Using this Classification, water from Limbe is distinguished falls into A, B, C, D, E, G categories presented in table 11. There is no category F in the wet season and no B, C, D, E, F, and G in the dry. In the wet season: Category A; 5 samples, 25 %; 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; 2 samples, 10 % are characterized by Normal earth alkaline water; prevailing chloride. Category D; 8 samples 40%; are characterized by earth alkaline water, with increased portions of alkalis and prevailing bicarbonate. Category E; 1 sample 5%; characterized by earth alkaline water, with increased portions of alkalis with prevailing chloride and Category G; 1 sample, characterized by alkaline water with prevailing bicarbonate. There are no categories B to G in the dry season and no Category F in the wet season. In the dry season: Category A; 20 samples, 100%. In the wet season, the dominant water types are Category A, 25%; and Category D, 40% while in the dry season Category A; 100%. From table 11, MgHCO3 is the dominant water type, followed by CaCl, MgCl, Na+ Cl in the wet season and MgHCO3 is the dominant water type, followed by CaHCO3, in the dry season.
 
Figure 15: Piper’s diagram for 3 water types and 3 groundwater hydrogeochemical facies in Limbe; in the wet season; Field (I): Ca-Mg-Cl-SO4 has 3 samples 15%, Field III: Na- K-HCO3 1 sample 5% and Field IV: Ca-Mg-HCO3 has 16 samples, 80%. In the dry season; Field (IV) Ca-Mg-HCO3 20 samples 100%. No samples plotted on Fields II in the wet or I and III in the dry season. Water types include; MgHCO3 90%; CaCl 5% and NaCl 5% in Wet seasons and MgHCO3 70%, CaHCO3 (30%) dry season.

 

Piper-Langguth Classification Limbe

Wet

Dry

Class

Characteristic-Water type

No

%

No

%

Diamond Field

A

Normal earth alkaline water ; prevailing HCO3-

5

25

20

100

B

Normal earth alkaline water ; prevailing HCO3-or Cl-

3

15

-

-

C

Normal earth alkaline water; prevailing Cl-

2

10

-

-

D

Earth alkaline water ; increased portions of alkalis; prevailing HCO3-

8

40

-

-

E

Earth alkaline water with added portions of alkalis with prevailing chloride

1

5

-

-

G

Alkaline water with prevailing bicarbonate

1

5

-

-

Cation Field

1

Ca-rich waters

1

5

6

30

2

Mg-rich waters

18

90

14

70

3

Na+K

1

5

-

-

Anion Field

4

HCO3- waters

17

85

20

100

6

Cl- waters

3

15

-

-

Table 11: Categories of Limbe groundwater.

Piper’s hydrogeochemical facies: From the Piper’s diagram in figure14 the hydrogeochemical facies were determined and presented in table 12, Field (I): Ca-Mg-Cl-SO4 hydrogeochemical facies has 3 samples, 15% in the wet and 0 samples and 0 % in the dry season. This facies is characteristic of stagnant at some distance along its flow path possibly from the slopes of Mount Cameroon. Field (III): Na-K-HCO3hydrogeochemical facies has 1 sample, 5% in the wet and 0 samples, 0 % in the dry season. This facies is characteristic of stagnant groundwater zones commonly zones of mixing in seawater encroached coastal regions. Field (IV), Ca-Mg-HCO3 hydrogeochemical facies has 16 samples, 80% in the wet and 20 samples, 100% 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 in the wet season and Field II and III in the dry season. The high contribution of alkaline earth elements 80% in the wet season and 100 % in the dry season is due to direct ion-exchange processes which enrich groundwater with alkaline earth elements. The dominance of Ca-Mg-HCO3hydrogeochemical facies in this area could be due to dissolution of gases and minerals, particularly CO2and CO2-related compounds from the atmosphere dissolved in precipitation and during groundwater infiltration through the vadose zone.

Fields

Hydrogeochemical facies

Wet

Dry

No

%

No

%

Field I

Ca2+ - Mg2+ - Cl- -SO42-

3

15

-

-

Field III

Na+ - K+ - HCO3-

1

5

-

-

Field IV

Ca2+ - Mg2+ - HCO3-

16

80

20

100

Table 12: Classification of hydrogeochemical facies based on Piper diagram.

Hydrogeochemical character of Limbe groundwater: Based on the Durov’s diagram, the classification by Lloyd and Heathcoat [18] presented in figure 16 for Limbe groundwater shows four classes occur in the wet season; Class-2; Ion exchange; 1 samples, 5%; Class-4Recharge, 3 samples 15%; Class-5Simple dissolution or mixing, 4 samples 20% and Class-6 Mixing and uncommon dissolution influences, 12 samples 60%. Three Classes occur in the dry season: Class-3 ion exchanged water, 10 samples, 50%; Class-5 Simple dissolution or mixing, 1 sample, 5% and Class-6 Mixing and uncommon dissolution influences, 9 samples 45% respectively. There are no Classes 1,3,7,8 and 9 in the wet season and no Classes 1, 2, 4, 7, 8 and 9 in the dry season in Limbe. In the wet 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 precipitation 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 presence of samples showing Na+ and Cl- as dominant cation /anion, Classes 2, 4, 5, 6 in the wet season and Classes 3, 5, 6 in the dry season, indicates that the groundwater in Limbe is related to ion exchange, simple dissolution and reverse or inverse ion exchange of NaCl waters or end-point down gradient waters such as seawater; indicative of seawater intrusion.

Figure 16: Durov plot of Limbe groundwater for the processes in groundwater evolution: for wet season; Field2, Ion exchange; 1 samples, 5%; Recharge, 3 samples 15%; Simple dissolution or mixing, 4 samples 20% and Mixing and uncommon dissolution influences, 12 samples 60% while in the dry season; ion exchange,1 sample, 50%; Simple dissolution or mixing, 1 samples, 5% and Mixing and uncommon dissolution influences, 9 samples 45%.

Rock-Groundwater interaction: Based on the Durov’s diagram, the classification by Lloyd and Heathcoat [18] presented in table 13 shows that, in the wet season, most water samples plotted in Class 4, 5, 6, 95% and Class 5, 6, 50% in the dry season, which are water types not frequently encountered in basaltic terrains, but common in coastal regions; indicates probable mixing or uncommon dissolution infl

DISCUSSION

The physicochemical parameters vary accordingly with season (seasonal control) indicative of a phreatic aquifer. Some values of physicochemical parameters; Temperature, pH, EC and TDS, fall above the permissible limits of [28]. Temperatures of groundwater in Limbe are relatively low and this negates any possibility of magmatic heating as proposed by [33]. The slightly acidic to alkali pH of water in the study area is possibly due to changes in physicochemical conditions that affects the carbon dioxide, carbonate-bicarbonate equilibrium. Elevated TDS above the fresh water limit at Dockyard, 1515 mg/L, indicate seawater intrusion into this coastal aquifer that could result in the groundwater being corrosive, of salty or brackish taste, could cause scale formation that interfers and decreases the efficiency of hot water heaters [34] and could cause gastrointestinal irritation.

Basalts have a primary composition of Feldspars, Ferromagnesian Minerals (Pyroxenes and Amphiboles) and Magnetite (Fe3O4) that weather to form residual Kaolinite minerals, Hematite and Goethite with Na+, Ca2+, Mg2+ being leached [16]. The Processes in basaltic weathering, Equations 1-7 are: Hydrolysis, carbonization and solution:

NaALSi3O8 + H2CO3+ 4.5H2O = Al2Si2O5 (OH)4 + Na+ + HCO3- + 2H4SiO4 …………………….……………….(1)
Albite + Hydrogen ions + water = Kaolinite (clay) + Sodium ions + silicic acid
Or 2NaAL3O8 + 4H+ + 4H2O = Al3+ + Na+ + 2H4SiO4 ………………………………………………………………(2)
Albite + Hydrogen ions + water = Aluminum ions + Sodium ions + silicic acid
2KALSi3O8 + 2(H+ + HCO3-) +H2O = Al2Si2O5 (OH)4 + 2K+ + 2HCO3- + 4SiO2 ……………………………………. (3)
Orthoclase + Carbonic acid + water = Kaolinite (clay) + Potassium ions + Bicarbonate + silica 
Or 2KALSi3O8 + 2H+ + H2O = Al2Si2O5 (OH)4 + 2K+ + 4SiO2 ………………………………………………. (4)
Orthoclase + Hydrogen ions + water = Kaolinite (clay) + Potassium ions + silica 
Orthoclase: 2KALSi3O8 + 11 H2O = Al2Si2O5 (OH)4 + Si(OH)4 + 2K+ + 2OH……………………………….. (5)
Amphibole: Ca2Mg5Si8O22 (OH)2 + 14CO2 + 22H2O = 2Ca2+ + 5Mg2+ + 14HCO3- + 8Si(OH)4…….,...………. (6)
Pyroxene: CaMg (Si2O6) + 4CO2 + 6H2O = Ca2+ + Mg2+ + 4HCO3- + 2Si(OH)4………………………………...….… (7)

The leached cations may then enrich the percolating groundwater, giving it its ionic character.

The ionic sequence of the occurrence of major cations and anions in Limbe groundwater is safe for both the wet and dry seasons but for NH4+ which is absent in the dry season. The presence of NH4+ in the rainy season is due to excess rainfall or runoff transporting sewage and other organic materials that produce ammonium. There is an interchange between HPO42- and SO42- ions in the wet and dry seasons.

From ionic ratios: SO4/Cl, Na/Cl, Mg/Cl, Na/HCO3, Ca/HCO3, Ca/SO4, Mg/Ca, Ca/Mg, (Ca+Mg)/(Na+K), HCO3/∑Anions, NO3/∑Anions, SO4/∑Anions, Cl/∑Anions, Na/Na+K; groundwater in Limbe is affected to a great extent by silicate weathering; mostly Ca-silicates and Mg-silicates from the minerals found in basalts with little weathering of Na-silicates, no Na-absorption, some sulfate from external sources, no oxidation of sulphides and no significant anthropogenic contribution of sulfate.

Gibb’s diagrams reveals 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 Limbe. The sources of groundwater ions are; weathering of basaltic rocks (Silicate weathering) that are found in the area and atmospheric precipitation similar to other studies in the basaltic aquiferous formations along the Cameroon Volcanic Line.

The Piper’s diagrams indicate the groundwater has a modern age and is being recharged by precipitation. 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. Processes controlling groundwater solute chemistry are; ion exchange, simple dissolution and Reverse ion exchange. Reverse exchange is not frequently encountered in basaltic terrains, but common in coastal regions. The groundwater in Limbe is recharged uphill on Mount Cameroon, flows through the lava-flow formations on the Mountain slopes and into the low lying shorelines which are in some places below the sea level. During the wet season, sea levels are higher at high tide and the land is submerged by the sea. This results in seawater intruding into the phreatic basaltic fractured rock aquiferous formations in the low lying areas. However, the fresh water volumes from the mountain are so great that during the dry season, the fresh groundwater flushes the seawater back into the ocean and regains its typical basaltic fractured rock aquifer character. 

From analysis and interpretation, WQI indicates that 75% of the water is portable for both seasons and 25% are unsuitable. Almost all groundwater in Limbe is soft during the wet season but moderately to very hard in the dry season in the low lying areas; this is due to an increase in the bicarbonate ions which are higher in dry season due to tidal waves coming inland with very low levels of bicarbonate. With respect to irrigation water quality: %Na, KR, RSC, MAR, SAR and PI indicate suitability of groundwater for agricultural use during the dry season. Groundwater is more unsuitable during the wet season as shown by RSC, MAR, Salinity hazard Class (USSL classification) and PI.

CONCLUSION

In the Limbe coastal area; silicate weathering is the major source of ions into groundwater by the processes of ion-exchange, simple dissolution of the country rock characteristics of groundwater in basaltic terrains and uncommon dissolution (reverse ion exchange) not frequently encountered but common in coastal regionswith seawater intrusion.

There is seawater intrusion in the Limbe coastal area: Low surface elevations, shallow depth-to-water levels, the geology of the formations being made up of mostly porous fractured weathered and un weathered basalts and their proximity to the Atlantic shoreline increases their susceptibility to seawater intrusion.

Water Quality Indices (WQI) show groundwater is within WHO guidelines for potable water at higher elevations for all seasons but unsuitable for agricultural purposes at lower elevations during the wet season`.

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Citation: Akoachere RA, Egbe SE, Eyong TA, Yaya OO, Mbua RL, et al. (2019) Characterization of Groundwater in the Basaltic Fractured Rock Aquiferous Formations of the Limbe Coastal Region of Mount Cameroon, SWR-Cameroon. J Environ Sci Curr Res: S1002.

Copyright: © 2019  Akoachere RA, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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