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saturated zone (Karanth, 1987; Appelo and Postma,. 2005). The high PCO2 in groundwater .... Planet Earth. Springer Science + Business Media B.V. pp. 21-33.
Vol. 7(6), pp. 518-530, June 2013 DOI: 10.5897/AJEST2013.1456 ISSN 1996-0786 © 2013 Academic Journals http://www.academicjournals.org/AJEST

African Journal of Environmental Science and Technology

Full Length Research Paper

Hydrochemistry of shallow groundwater and surface water in the Ndop plain, North West Cameroon Mengnjo J. Wirmvem1*, Takeshi Ohba1, Wilson Y. Fantong2, Samuel N. Ayonghe 3, Justice Y. Suila 4, Asobo Nkengmatia E. Asaah 5, Gregory Tanyileke 2, and Joseph V. Hell 2 1

Department of Chemistry, School of Science, Tokai University, Hiratsuka, 259-1211, Japan. 2 Institute of Mining and Geological Research, P.O. Box 4110, Yaounde, Cameroon. 3 Department of Environmental Science, University of Buea, Box 63, Buea, Cameroon. 4 G.B.H.S. Atiela, Nkwen, P.O. Box 5147, Bamenda, Cameroon. 5 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan. Accepted 20 May, 2013

This study was conducted on the hydrochemistry of shallow groundwater and surface water in the Ndop plain, North West Cameroon. The objectives were to determine the physico-chemical characteristics of water, controls on water chemistry and suitability for drinking and irrigation. Forty-six shallow groundwater and 26 surface water samples were investigated. Field measurements of physical parameters were preceded by chemical analyses of the samples for major ions and F-. About 69% of the water sources had pH values below 6.5, classifying the water as barely acidic. Electrical conductivity (Mg2+>K+ for cations and HCO3>>NO3-≥Cl->SO42- for anions. Major ion concentrations were low, and within the WHO guidelines for drinking water, but F- concentrations were much lower (100 μS/cm were diluted 5 or 10 times to acquire an absorbance within the range of the used standards for Na+, K+ and Ca2+. The anions: F-, Cl-, NO3-, SO42-, PO43-, NO2- and Br- were determined by using an Ion Chromatography (Dionex ICS-900) while dissolved Silica (Si(OH) 4), hereafter considered as SiO2, was determined by using an ICP-AES. The reliability of the chemical analyses was verified by using an ionic balance error (IBE) equation (Appelo and Postma, 2005). The values were within ±5 %; hence, suitable for geochemical interpretations.

RESULTS AND DISCUSSION Physico-chemical composition of groundwater and surface water Table 1 presents results of the 72 water samples with a statistical summary shown in Table 2. A statistical summary

Wirmvem et al.

of rain data is presented in Table 3. Mean depth to the water table (K for cations and HCO3 >>NO3 ≥Cl >SO42for anions. The small range in their concentrations and higher dispersions around their respective mean values as reflected by the small STD values, except for NO3- and Cl- (Table 2), suggests a unique source of most ions and a single aquifer system. Mean concentration of SiO2 was higher than that of individual cations but less than the mean of HCO3- (Table 2). This suggests that SiO2 was released more than the cations into solution. The high SiO2 suggests the abundance of silicate minerals (Srinivasamoorthy et al., 2008) in agreement with the dominant granitic basement (Figure 1) and as observed in similar terrains by Njitchoua and Ngounou-Ngatcha (1997) and Fantong et al. (2009). As expected, the mean TDS increased progressively from a minimum in rainwater (20 mg/l) to surface water (30 mg/l) and a maximum in groundwater (44 mg/l). Similarly, mean relative concentrations of dissolved ions showed a progressive increase from the lowest in rainwater, to surface water, and the highest in groundwater (Figure 2a). These results suggest chemical evolution from rain to surface water and groundwater. The relatively high TDS in groundwater indicated the influence of the unconsolidated sediments and granitic bedrock material in chemically enriching the water as it percolates through the under-saturated zone (or flows through the aquifer). The small difference in mean TDS of groundwater and rainwater by only 53 % is likely because these are early rains, usually enriched in dissolved ions (Takounjou et al., 2011). The low major ion concentrations in groundwater depict low water-rock interactions in the granitic basement, short residence time, the shallow nature of the aquifer and its acidic nature (Edmunds and Smedley, 1996). Similar low pH and major ions have been reported from granitic basement complex aquifers in Ghana (Adomako et al., 2011) and Nigeria (Edet et al., 2011). Hydrochemical facies The meq/l concentrations of major ions plotted on a pie + chart (Figure 2b), showed that Na , HCO3 and to some 2+ extent Ca were the major ionic species. Low K+ may be

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due to its low geochemical mobility (Hem, 1989; 2+ Srinivasamoorthy et al., 2008) while low Mg is possibly due its to low content in rocks of the study area. On a Piper diagram (computed using GW_Chart Version 1.23.7.0), most samples plotted in the field of alkali metals (Na+K) (Figure 3). This indicates the dominance of alkali metals in the groundwater aquifer system. The water types and their proportions are shown (Figure 3). The observed spatial distribution of the water types (Figure 1) showed no discernible pattern from Ca-HCO3 to Na-HCO3. Given the gentle relief in the plain, this suggests a mixed aquifer system as revealed by the mixed Na-Ca-HCO3 water. Similarly, no clear pattern, in water type, was observed from surface water to groundwater (Figure 3). Nonetheless, there is chemical evolution from Ca+Mg+SO4+Cl (rain), to Ca-HCO3 (largely groundwater and lakes), and to Na-HCO3 (basically groundwater, streams, rivers, and dam waters) as indicated by the dotted arrow (Figure 3). It is probable that the dissolution of Na+ into surface waters from alkali-rich trachytic rocks in the surrounding highlands tends to mask the chemical evolution. The presence of Ca-HCO3 water in groundwater within the plain, which is sporadic, in surface waters (streams and rivers) discharging from the highlands suggests that the main recharge of groundwater may not necessarily occur at higher altitudes but rather through direct precipitation to the surface and subsequent percolation through the under-saturated zone into the aquifer. Origin of solutes and hydrochemical controls The major ion chemistry of groundwater and compositional relations among ionic species can reveal the origin of solutes and processes that generated an observed water composition (Hounslow, 1995; Demlie et al., 2007; Cendon et al., 2011; Tay, 2012). Table 4 shows correlation coefficients between the ionic species in water. The good positive correlation of NO3 with Cl suggests their anthropogenic origin (Demlie et al., 2007; Fantong et al., 2009). Main potential sources of NO3- are numerous shallow pit toilets and oxidation of organic matter given the proximity (>15 m) to shallow groundwater and agricultural dominance, respectively. Fonge et al. (2012) identified relatively high concentrations of NO33and PO4 in soil water (0.3 mg/l) was detected in OW22, OW32, BH29, BH41 and LK16; IB, ionic balance.

Table 2. Statistical summary of physico-chemical data of groundwater and surface water in the study area.

Parameter Alt. (m) Dtw (m) Dow (m) Tw (oC) pH EC (μS/cm) TDS (mg/l) + Na (mg/l) + K (mg/l) Ca2+ (mg/l) Mg2+ (mg/l) HCO3- (mg/l) NO3- (mg/l) Cl (mg/l) SO42- (mg/l) F- (mg/l) SiO2 (mg/l)

Surface and ground water (n=72) Min Max Mean Std 1135 2272 1235 193 18.5 27.30 22.79 2.04 5.20 8.41 6.23 0.71 9.0 282 59.75 54.62 5.85 183.30 38.84 35.5 0.24 21.54 3.37 3.28 0.47 24.69 2.94 4.48 0.11 20.25 1.80 3.02 0.0 6.74 1.04 1.31 2.45 91.12 18.69 17.53 0.0 27.92 1.80 4.51 0.04 26.87 1.13 3.31 0.0 12.76 0.77 1.91 0.0 0.39 0.08 0.07 2.18 20.54 9.04 5.10

Min 1163 1.60 0.30 19.50 5.20 9.00 5.85 0.24 0.47 0.11 0.12 2.45 0.00 0.05 0.00 0.01 2.18

Groundwater (n=46) Max Mean 1260 1188 27.00 8.81 8.40 1.57 25.70 23.65 6.74 5.90 282 67.16 183.3 43.65 21.54 3.21 24.69 3.53 20.25 2.50 6.74 1.03 91.12 19.13 27.92 2.41 26.87 1.53 12.76 1.12 0.39 0.08 18.97 8.93

STD 19.24 5.06 1.36 1.32 0.37 62.32 40.51 3.26 5.45 3.58 1.31 18.80 5.05 4.07 2.32 0.08 4.80

Min 1135 18.5 5.48 12.0 7.8 0.41 0.66 0.14 0.0 3.83 0.00 0.04 0.0 0.0 2.26

Surface water (n=26) Max Mean 2272 1317 27.3 21.29 8.41 6.83 175 46.65 113.75 30.33 17.34 3.67 5.24 1.89 1.77 0.56 5.34 1.06 78.65 17.92 16.07 0.73 1.88 0.42 0.68 0.17 0.3 0.08 20.54 9.23

Std 302 2.22 0.76 33.47 21.76 3.29 1.05 0.41 1.31 14.97 3.07 0.42 0.21 0.06 5.58

Max, maximum, Min, minimum; STD, standard deviation; Alt, Altitude; Dtw, depth to water (m); Dow, depth of water (m); Tw, wat er temperature; EC, electrical conductivity; TDS, total dissolved solids.

agreement with the geology. Fluoride showed a distinct + 2+ positive correlation with Na relative to Ca (Table 4).

The likely source of F- in the granitic basement is fluorite, CaF2, the most common F -bearing mineral (Edmunds

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Afr. J. Environ. Sci. Technol.

Table 3. Physico-chemical data of January 2012 sporadic rainfall events in the Ndop Plain and its Environs.

Location Bamenda Ndop Ndawara Kumbo Max. Min. Mean

pH 6.26 6.57 6.45 6.53 6.57 6.26 6.45

EC 36 30 28 32 36 28 31.5

TDS 23.4 19.5 18.2 20.8 23.4 18.2 20.5

+

Na 0.36 0.26 0.29 0.23 0.36 0.23 0.29

+

K 0.90 0.88 0.87 1.09 1.09 0.87 0.94

2+

Ca 1.39 1.62 1.30 1.37 1.62 1.3 1.42

2+

Mg 0.37 0.43 0.36 0.27 0.43 0.27 0.36

HCO3 3.5 3.77 3.97 4.12 4.12 3.5 3.84

-

-

NO3 2.97 2.58 2.47 3.73 3.73 2.47 2.94

-

Cl 2.3 1.56 1.43 1.22 2.3 1.22 1.63

2-

-

SO4 1.33 1.25 1.12 2.03 2.03 1.12 1.43

F 0.02 0.03 0.01 0.03 0.03 0.01 0.02

Na/Ca 0.43 0.27 0.37 0.28 0.43 0.27 0.34

EC = uS/cm, TDS and all ions = mg/l, Na/Ca

Figure 2. A, Relative mean concentrations of silica and cations (mg/l) in various water sources of the study area including rain. Si was not measured in rain; Si>Na>K>Ca>Mg. B, Pie chart of mean concentrations of ions (meq/l), showing high Na+ and dominant HCO3- in water sources of the study area. 2+

and Smedley, 1996). However, the released Ca is pos+ sibly exchanged for Na on clay surfaces, resulting in a 2+ relative increase in F and a decrease in Ca , hence, the 2+ weak correlation between Ca and F . High F- concentrations in groundwater have been identified in North Cameroon, along the CVL, in a graniticalkaline environment from fluorite, amphiboles and micas (Fantong et al., 2009). The low concentrations of F- in groundwater of the study area may be due to its acidic nature which renders F immobile (Hem, 1989; Edmunds and Smedley, 1996). For example, the highest value of F(0.39 mg/l) was identified in BH29 (‘borehole’) with a pH 2+ of 6.4. The strong positive correlations of HCO3 with Ca , 2+ + 2Mg , K and SO4 further indicate similar origin of ions. Given the Na-HCO3 dominance (Figure 3), the weak posi+ tive correlation of Na with HCO3 suggests that varied processes accounts for its concentration in solution. Table 5 shows a summary of computed ratios between

+

-

some ionic species. The observed Na /Cl molar ratios were high, relative to that of sea water, 0.86 (Millero et + al., 2008). A Na /Cl molar ratio approximately equal to 1 is usually attributed to halite dissolution, whereas >1 is typically interpreted as reflecting Na+ released from silicate weathering reactions (Fisher and Mullican, 1997; Cendon et al., 2010). Therefore, the excess Na+ as further shown (Figure 4) is likely from silicate weathering, possibly feldspar mineral dissolution. On a plot of total + + 2+ 2+ cations, Na +K +Ca +Mg (TC) versus alkalinity, most of the 72 samples (94 %) plotted on or near the 1:1 line (Figure 5a). The high mean HCO3-/∑anions ratio in the water (Table 5) and the 1:1 relationship (Figure 5a) suggest silicate or carbonate weathering (Hounslow, 1995; Fisher and Mullican, 1997; Kim, 2002), but acid waters (as prevailed in the study area) usually results from dissolution of non-carbonate rocks such as granites (Edmunds and Smedley, 1996), the dominant rock in the

Wirmvem et al.

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Figure 3. Piper’s diagram showing the water types and proportions in the Ndop Plain. Alkali metals (Na+K) exceed alkali earth metals (Ca+Mg). Weak acids (CO 3 +HCO3) greatly exceed strong acids (SO4+Cl). The dash line in the diamond section indicates the path of chemical evolution from Ca-MgSO4-Cl (rain), to Ca-HCO3 (groundwater, lakes and dam water), and to Na-HCO3 (groundwater, streams and rivers). Ca-Na-HCO3 represents a transition (mixing zone) from Ca-HCO3 to Na-HCO3 water.

Table 4. Pearson correlation coefficients of all analysed water sources (n=72).

Parameter Na+ K+ Mg2+ Ca2+ HCO3ClNO3SO42FpH

Na+ 1.00 0.12 0.35 -0.01 0.33 0.69 0.34 0.05 0.48 0.17

K+

Mg2+

Ca2+

HCO3-

Cl-

NO3-

SO42-

F-

pH

1.00 0.52 0.73 0.81 0.12 -0.01 0.87 0.07 0.10

1.00 0.27 0.72 0.10 0.25 0.40 0.36 0.23

1.00 0.70 0.06 0.09 0.77 0.04 0.08

1.00 -0.03 -0.08 0.68 0.35 0.33

1.00 0.52 0.11 0.07 -0.16

1.00 0.04 0.01 -0.21

1.00 0.17 0.08

1.00 0.21

1.00

area. To further determine the likely influence of silicate dissolution, the molar ratio of HCO3 /SiO2, saturation indices for calcite and aragonite, and Ca2++Mg2+ versus SO42+HCO3- relationship were examined. If HCO3-/SiO2 ratio is greater than 10, (that is, HCO3->>SiO2), carbonate weathering predominates. However, if the ratio is low and especially