Hydrogeologic framework of the shallow aquifers in the Ikom–Mamfe Embayment, Nigeria using an integrated approach

Journal of African Earth Sciences 92 (2014) 25–44

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Hydrogeologic framework of the shallow aquifers in the Ikom–Mamfe Embayment, Nigeria using an integrated approach q Aniekan Edet ⇑, C.S. Okereke Department of Geology, University of Calabar, UPO, POB 3609, Calabar, Nigeria

a r t i c l e

i n f o

Article history: Received 1 April 2013 Received in revised form 9 January 2014 Accepted 17 January 2014 Available online 30 January 2014 Keywords: Lineaments Geoelectrical Weathering Quality Vulnerability Nigeria

a b s t r a c t A detailed hydrogeological investigation was carried out in the Ikom–Mamfe Embayment of Nigeria using lineaments, geological, geoelectrical, and hydraulic parameters. The objective was to assess aquifer framework and resource potential of the area. The study was carried out because the aquifers are of particular importance as they are more or less the only source of water supply available for the rural population. In addition, expanding communities will trigger increase in water demand that will translate to more dependence on groundwater. The study identified four major hydrostratigraphic units: Mamfe (oldest), Ezillo, Amaseri and intrusives (youngest). A comprehensive investigation of the basin revealed its lateral and vertical dimensions and hydrogeological characteristics. Moreover, study of lineaments, aquifer parameters, water level fluctuations confirmed the heterogeneity of the aquifers and their potentials to rural water supply. Water rock interactions, mainly silicate weathering, explain the groundwater compositions which are Ca–HCO3, Ca–HCO3–Cl and Ca–Na–HCO3. The water quality is good for domestic and agricultural uses. However, in terms of vulnerability of the aquifers to pollution, 80% of the Ikom– Mamfe Embayment has been classified as medium to high vulnerability. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The evaluation and subsequent development of groundwater resources requires an understanding of the hydrogeologic properties of the aquifers. The aquifers of the Ikom–Mamfe Embayment are of particular importance as they are in most cases the only alternative source of water supply available for the rural population. Earlier studies in the area focused on geological, tectonic and sedimentological investigations of the basin (Amajor, 1987; Banerjee, 1980; Ebong, 1989; Ojoh, 1990; Petters et al., 1987; Ekwueme et al., 1995; Reyment, 1965). Despite these investigations, very few studies have been done on the hydrogeology of the basin. For example, the work of Edim (1998) applied geoelectric surveys to delineate aquiferous layers in Obubra, in the northwestern sector of the basin, while the work of Ugbaja (2007) included the groundwater quality and vulnerability assessment in the western part of the basin. Edet et al. (2009) dealt mostly with the processes controlling the chemistry of groundwater in the western parts of the basin. Other studies in the basin include geophysical reports

on site investigations for rural water supply by different water-related agencies. However, the development of the groundwater resources in the basin has not been very successful despite the huge amount of human, financial and material resources committed to the project by the Local, State and Federal Governments on one hand and by donor agencies such as UNICEF on the other hand. In addition, the expansion of rural communities would require an increase in water demand that will translate to more dependence on groundwater. Thus there is therefore the need for a better understanding of the hydrogeologic processes of the basin. The objective of the present work was to describe the hydrogeologic framework of the Ikom– Mamfe Embayment including delineation of aquifers, occurrence of groundwater, productivity of wells and the groundwater quality and vulnerability. This would allow for the effective management and future development of the groundwater resources of the basin.

2. General features of the Ikom–Mamfe Embayment 2.1. Physiography and climate

q

This MS was presented at the 24th Colloquium of African Geology (CAG24) to be held in Addis Ababa, Ethiopia on 8–14 January 2013. ⇑ Corresponding author. Tel.: +234 8036667216. E-mail addresses: [email protected], [email protected] (A. Edet). http://dx.doi.org/10.1016/j.jafrearsci.2014.01.004 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved.

The study area, Ikom–Mamfe Embayment spans five local government areas (Abi, Etung, Ikom, Obubra, Ikom) of Cross River State, southeastern Nigeria (Fig. 1). This area has a total population

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Fig. 1. Map of study area including sample locations.

of 756,276 based on preliminary 2006 Census figures. This represents about 26.18% of the entire population of Cross River State. The embayment lies between latitudes 5° 450 and 6° 100 N and longitudes 8° 00 and 8° 450 E covering an estimated area of about 5700 km2. The basin is dominated by relics of tectonic events which shaped the landscape overtime (Petters, 1989). The relief is characterized by undulations, demarcating the lowlands from the highlands due to sandstone ridges and igneous ridges. The basin enjoys a tropical climate with mean annual temperatures of between 21.9 and 31.7 °C. Rainfall occurs almost exclusively between April and October. Long term average rainfall varies between 2018 and 2465 mm at Ikom and Obubra, Fig. 1 (CRBDA, 1982). 2.2. Geology and hydrogeology The Ikom–Mamfe Embayment has an estimated thickness of about 3000–4000 m based on gravity studies (Petters et al., 1987). The relation between stratigraphic and hydrogeologic units is presented in Fig. 2. The area is underlain by the Cenomanian– Turonian Ezillo Formation and Turonian–Coniacian Amaseri Sandstone both of the Eze Aku Group, and the Mamfe Formation of the Asu River Group (Ekwueme et al., 1995). The oldest formation is the Albian–Cenomanian Mamfe Formation. The rock sequence here consists of conglomeritic, immature, arkosic, cross-bedded coarse to medium grained-sandstones and mudstones (Ekwueme et al., 1995). This forms the Mamfe Hydrostratigraphic Unit (MA) and covers the eastern part of the embayment, extending from Alesi (KM 12) through Ikom (KM 4) to Emangbe (KM 6) and Alok (KM 1), Fig. 1. The sandstones are weathered and fractured. Groundwater is tapped mostly by shallow boreholes (average depth, 60 m) and hand dug wells (average depth, 15 m). Recharge to this unit is through rainfall and baseflow from the Cross River. The MA covers approximately 37.3% of the study area.

Overlying the Mamfe Formation is the Late Cenomanian–Turonian Ezillo Formation. The Ezillo Formation consists of marginal marine to marine shales with sandstones, siltstones and limestones intercalation. At Ohana (OB 18), the shales are black, and are baked and slaty due to heat from intrusion by doleritic sills (Fig. 3), while at Appiapum (OB 10), the rock unit consist of finegrained sandstones. The lower part of this formation is a sequence of alternating highly bioturbated silty shale and shale with siltstone, ironstone and mudstone intercalations (Ekwueme et al., 1995). The Ezillo Formation constitutes the Ezillo Hydrostratigraphic Unit (EA). Groundwater development is through shallow hand dug wells. The EA extends from Ofodua (OB 3) through Iyametet (OB 12) to Ohene Eda (OB 13) covering about 27.8% of the embayment. Recharge to the aquifer is through rainfall and the Cross River. The Amaseri Sandstone of late Turonian–Coniancian age constitutes the Amaseri Hydrostratigraphic Unit (AA) and covers about 24.6% of the basin. Lithologically, the unit consists of a basal shaly and fossiliferous part through a calcareous middle part to highly bioturbated sandstones. As observed at Ediba (AB 9), the sequence consists of intercalation of biotubated sandstone and fissile shale (Fig. 4). At Abini (YR 13), the rock is fine to medium-grained, fairly sorted, and highly bioturbated calcareous sandstone, while at Adim (BA 1), the sequence consists of a lower mudstone, middle fissile shale and sandstone and upper massive shale. The sequence at Ugep (YR 4) consists of lower burrowed sandstone; middle medium grained highly bioturbated sandstone and upper shale. Groundwater abstraction here is through hand dug well. Series of post-Turonian dark coloured basic to intermediate igneous bodies of different sizes and shapes mostly concordant to the country rock outcrops as discontinuous ridges in various parts of the basin. Most of the igneous bodies are minor sills, which are parallel to the regional strike direction and show vertical and horizontal joints (Hossain, 1981). The emplacement of these

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Fig. 2. Stratigraphic and hydrogeological units of the Mamfe–Ikom Embayment (modified from Petters, 1982; Ekwueme et al., 1995).

Fig. 4. Intercalations of sandstone and shales at Ediba (AB 9). The location of Ediba is shown in Fig. 1.

In the eastern part of the basin, Tertiary-Recent basic intrusives are exposed near Ikom (KM 4) and Agbokim Water Falls (ET 1). These constitute the Volcanic Hydrostratigraphic Unit (VA). The intrusives are hard where groundwater typically occurs within shallow weathered and fractured layers exposed at the surface. Within the Volcanic Hydrostratigraphic Unit (VA), groundwater development is through shallow boreholes. This forms the most important source of water in the rural communities in the basin. Recharge to this unit is through rainfall. The VA covers about 10.3% of the basin. Fig. 3. Shales intruded by (A) dolerite sill and (B) fractured at Ohana (OB 18). The location of Ohana is shown on Fig. 1.

intrusive had caused baking of the adjoining country rocks resulting in fracture permeability of the Eze Aku Group as observed at Ohana (OB 18).

3. Methods of study Radar imagery, aerial photographs and the groundwater geologic map were used visually to delineate lineaments (Edet, 1993). Geological logs based on drilled data were used to describe the lithology. Special attention was given to grain size and hence

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the hydraulic conductivity. Vertical electrical soundings (VESs) were conducted using the Schlumberger array at various locations within the Ikom–Mamfe Embayment (Table 1, Fig. 1). The maximum current electrode separation was kept at between 300 and 500 m (AB/2 = 150–250 m). The acquisition of resistivity data was carried out between 1992 and 2007 by the authors. The main

purpose of the interpretation of resistivity data was to determine the resistivity and thickness of the different layers. These results were subsequently used to obtain a picture of the geological framework. Therefore qualitative, quantitative and geological methods were applied in the interpretation. The VES data were complemented with boreholes drill data in some cases.

Table 1 Sample locations and type of data obtained from each location. No.

Location

Aquifer unit

Site code

Coordinates N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Agbokim Water Fall Etomi Effraya Mfum Ajassor Bendighe Ekiem Yawunde Obubra Ababene Ofodua Ovukwa Ogada Ovonom Okokori Iyamoyong Apipum Iyamitet Ohene eda Oderiga Akpantere Imabana Ofunbonga Ohana Ofat NYSC Camp Onyeadama Usumutong Adadama Ekureku Itigidi Afafanyi Ebijakara Ebom Igbo Ikpalegwa Ediba Adim Inyima Nko Idomi Ugep Assiga Ekori Ekpenti Mkpani Agoi Ekpo Ajere beach Ochikpo Onyekpeden Isabang Ochon Edondon Alok Edor Nde Ikom Okunni Emangbe Akparabong Nkum Nkarasi Okangha Ekunkunela Alesi Adinjikpor Nwang

ET 1 VA

AA

EA

MA

5° 54.644 ET 2 ET 3 ET 4 ET 5 KM 14 KM 15 OB 1 OB 2 OB 3 OB 4 OB 5 OB 7 OB 8 OB 9 OB 10 OB 12 OB 13 OB 14 OB 15 OB 16 OB 17 OB 18 OB 20 OB 21 OB 22 AB 1 AB 2 AB 3 AB 4 AB 5 AB 6 AB 7 AB 8 AB 9 BA 1 YR 1 YR 2 YR 3 YR 4 YR 5 YR 6 YR 7 YR 8 YR 9 YR 10 YR 11 YR 12 OB 6 OB 11 OB 19 KM 1 KM 2 KM 3 KM 4 KM 5 KM 6 KM 7 KM 8 KM 9 KM 10 KM 11 KM 12 KM 13 KM 14

Data E

8° 5° 5° 5° 5° 6° 5° 6° 5° 5° 5° 6° 5° 5° 5° 5° 5° 6° 5° 5° 6° 6° 5° 5° 6°

52.879 56.195 51.778 52.878 50.117 01.114 57.582 05.064 57.210 58.404 55.324 03.066 59.391 49.275 57.908 59.773 51.530 10.817 57.400 54.450 01.000 05.483 57.275 57.248 03.066

VES 8° 49.223 8° 43.820 8° 53.520 8° 52.005 8° 51.748 8° 44.900 8° 19.808 8° 15.589 8° 15.503 8° 15.264 8° 20.854 8° 16.207 8° 25.724 8° 21.144 8° 17.258 8° 20.298 8° 21.633 8° 15.600 8° 16.900 8° 18.100 8° 22.233 8° 21.787 8° 15.285 8° 20.854

VES, SWL VES, SWL VES, SWL VES VES, SWL SWL VES, Litholog, VES, Litholog, VES, Litholog, VES, Litholog VES, Litholog VES VES VES, Litholog, VES, Litholog VES, Litholog, VES, Litholog, VES, Litholog, VES, Litholog VES, Litholog, VES, Litholog, VES, Litholog, VES, Litholog VES, Litholog

5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 5° 6° 6° 6° 5° 5° 6° 6° 6° 6° 5° 5° 5° 6° 6°

50.196 55.679 56.831 53.860 52.717 50.600 49.783 56.204 53.155 43.770 55.212 52.673 45.351 48.743 56.319 52.546 53.318 49.371 50.327 55.990 57.909 57.361 54.986 56.065 52.006 19.063 12.125 04.424 56.805 56.116 19.748 01.862 12.112 13.702 56.406 55.345 55.135 00.002 20.944

8° 8° 8° 8° 7° 7° 7° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8° 8°

VES, Litholog, SWL, Aquifer parameters VES VES, Litholog, SWL, Aquifer parameters VES VES, Litholog, SWL, Aquifer parameters VES, Litholog, SWL, Aquifer parameters VES, Litholog, SWL, Aquifer parameters VES, Litholog, SWL, Aquifer parameters VES, Water quality VES, Litholog, Water quality VES, Litholog VES, Litholog, SWL, Aquifer parameters, Water quality VES, Litholog, SWL, Aquifer parameters VES, Litholog, SWL, Aquifer parameters, Water quality VES, Litholog VES, Litholog, SWL, Aquifer parameters VES, Litholog VES, Litholog, SWL, Aquifer parameters VES, Litholog VES, Litholog VES, Litholog VES, Litholog VES, SWL VES, Litholog, Aquifer parameters, Water quality VES, Litholog, SWL VES VES, Litholog, SWL, Water quality VES, Litholog, SWL SWL VES VES VES, Litholog, SWL, Water quality VES VES Litholog Litholog Litholog, SWL Litholog, Water quality VES, Water quality

00.708 05.293 02.319 01.511 59.300 58.650 58.916 07.343 02.027 02.519 12.213 11.423 05.204 04.676 09.770 07.232 07.507 09.599 15.755 06. 032 11.528 11.472 26.680 26.493 25.668 38.979 38.038 40.907 43.343 38.150 39.205 44.910 41.200 40.410 35.348 30.811 30.298 44.610 40.101

SWL, Aquifer parameters, Water quality SWL SWL, Aquifer parameters

Aquifer parameters SWL SWL, Aquifer parameters SWL, Aquifer parameters SWL, Aquifer parameters SWL, Aquifer parameters SWL

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

Fig. 5. Rose diagram of lineaments from lineament and photogeological maps.

Three hand dug wells within the study area at Edor, Obubra and Ugep (Fig. 1) were used to monitor the groundwater level fluctuations and quality. Aquifer parameters were estimated from pumping test and VES data. In addition, the vulnerability of the various aquifers was assessed using the DRASTIC (Aller et al., 1987) and GOD (Foster, 1987) methods. The sample locations for the study are presented in Fig. 1 and Table 1. 4. Lineaments and lineament density One of the objectives of the present work was to delineate areas with high groundwater potentials. The mapped lineaments were therefore plotted on the rose diagram shown as Fig. 5. This plot indicates that the longer lineaments accounted for the dominant N 10–40°E and N 00–50°W trends. For the different hydrostratigraphic units, MA, EA, AS and VA the average lineament directions

29

were respectively, 35°NE/27°NW, 23°NE/70°NW, 58°NE/61°NW and 42°NE/56°NW. The lengths of the lineaments ranged between 1.25 and 11 km for the entire embayment. The average length for the different hydrostratigraphic units were 3.48 km, 4.78 km, 2.99 km and 3.95 km respectively for MA, EA, AA and VA. Lineament density map was produced by preparing a 2 km  5 km grids and then measuring the total length of lineament within each grid as outlined by Edet (1993) and Okereke et al. (1998). The lineament density values were obtained using the method of Edet et al. (1994) and plotted at the centre of each grid. The values were then joined by isolines to prepare the lineament density map (Fig. 6). Lineaments are the main features that control the occurrence of groundwater in some parts of the basin. The average lineament density values were 0.44  102 km1, 1.20  102 km1, 0.29  102 km1 and 0.52  102 km1 respectively, for the MA, EA, AA and VA hydrostratigraphic units. These lineaments, in terms of joints and fractures, impact secondary permeability. Studies have shown that boreholes/wells located on or near fractures are generally more transmmisive (Lattman and Matzke, 1961; Mabee et al., 1994; Edet, 1993; Edet et al., 1994; Edet and Okereke, 1997). Hence, in the study area, the EA and VA hydrostratigraphic units having greater values of lineament length density have potentials for high yield for groundwater compared to the MA and AA hydrostratigraphic units, indicating moderate and low groundwater potentials. The higher lineament density values were attributed to the post-Santonian igneous activity within EA unit and to the Tertiary igneous activity that affected VA unit. Available limited yield data from the study showed that yield increased with increase in lineament density as earlier reported by Edet (1993) and Edet et al. (1994).

Fig. 6. Lineament density map of Ikom–Mamfe Embayment.

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10000

ρa Ωm

1000 Nde (MA) Obubra (EA) Nko (AA) Effraya (VA)

100 10 1 1

10

100

1000

Electrode spacing, AB/2,(m) Fig. 7. Typical geoelectric curves for the different hydrostratigraphic units at Nde (MA), Obubra (EA), Nko (AA) and Effraya (VA). The VES locations and units are shown in Fig. 1.

5. Hydrogeoelectrical characterization and profiles Forty-one (41) VES measurements were made, covering all the hydrostratigraphic units (MA, EA, AA and VA), Fig. 1. Quantitative interpretations of the geoelectrical data were made by considering the variations in the apparent resistivity for each electrode separation while the point of interest was kept constant. Master curves were used to obtain the resistivity (q) and thickness (h) of each

geoelectric layer (Zohdy et al. (1974). This was complimented by computer-aid interpretation, forward modeling, which helped in obtaining better a better resolution of the thicknesses and resistiviies of the different geologic layers (Edet and Okereke, 1997). Representative geoelectrical curves for the different hydrostratigraphic units are presented as Fig. 7, while summary of the layer resistivity values and thicknesses are listed in Table 2. Fig. 8 shows the correlation of inferred geoelectrical sections and borehole logs which revealed the lateral and vertical hydro-lithological variations in the Ikom–Mamfe Embayment. The compilations showed that the unsaturated topsoil is composed of lateritic, clayey, silty, sandy or gravelly material and characterized by very low to moderate resistivity (20–1000 X m). This material is distinguished from the saturated zone with very low to high resistivity (22–3000 X m). The lower part of the resistivity range (<50 X m) represented shaly-clayey horizons, while the moderate part (50–500 X m) represents sandstone, siltstone, fractured and baked shale, or weathered basaltic intrusive. On the other hand, the higher resistivity (>500 X m) represents fractured, coarse-grained conglomeritic sandstone, baked shale, intrusive or limestone. Thin layers of some horizons as seen from the lithological logs were not reflected in the VES data but are thought to have contributed to the variability of the bed rock resistivity.

Table 2a Geoelectrical layer apparent resistivity and thickness obtained from interpretation of geoelectrical sounding. No.

Geological formation

Aquifer unit

Location

Code

Layer resistivity (O m)

q1 1 2 3 4 5

Intrusives

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Amaseri Sandstone

26 27 28 29 30 31 32 33 34 35

Ezillo

36 37 38 39 40 41 42

Mamfe

VA

AA

EA

MA

q2

q3

Agbokim Water Fall Etomi Mfum Bendighe Ekiem Effraya

ET 1 ET 2 ET 4 KM 14 ET 5

800 280 550 500 550

350 650 1400 230 250

2700 360 500 1610

Usumutong Adadama Ekureku Igbo Ikpalegwa Ediba Inyima Nko Idomi Ugep Assiga Ekori Ekpenti Mkpani Agoi Ekpo Ajere beach Ochikpo Onyekpeden Isabang Ochon Edondon

AB 1 AB 2 AB 3 AB 8 AB 9 YR 1 YR 2 YR 3 YR 4 YR 5 YR 6 YR 7 YR 8 YR 9 YR 10 YR 11 YR 12 OB 6 OB 11 OB 19

500 20 1200 62 130 150 100 50 480 300 30 460 14 40 14 110 220 15 700 150

230 250 150 550 55 400 3000 100 150 1500 120 1200 110 160 120 550 70 450 460 320

1610 50 650 84 180 800 1500 780 800 1200 70 500

Obubra Ababene Ofodua Ovukwa Ogada Ovonom Okokori Iyamoyong Apiapum Ohene Eda

OB OB OB OB OB OB OB OB OB OB

1 2 3 4 5 7 8 9 10 13

96 100 350 15 500 360 80 90 44 20

360 30 1500 400 150 750 110 340 34 400

16 110 20

Alok Edor Nde Okunni Emangbe Akparabong Nkum

KM KM KM KM KM KM KM

1 2 3 5 6 7 8

450 235 400 1900 125 270 250

80 75 220 400 450 50 68

360 27 30 35 138 110

9.4 250 770 15 44 1500

1100 600 700 25

Layer thickness (m)

q4

q5

75

336 36

75 880

19 132

81

140 24 30

18.8 50 60 22

21

h1

h2

h3

4 1 0.55 0.5 1

40 10 2.2 9.5 10

a a

0.5 1 4 1.5 0.44 1.5 1 1 1 1 0.5 1.6 3 2 0.58 1 1 1.3 0.7 0.6

9 15 50 50 7.5 70 60 21 11 10 20 23

a a a

0.85 1 1 4 0.64 0.5 1 0.7 0.13 6

6 5.5 5

1 0.55 5 5 1 1 10

a a

15

h4

a

a

150 60

a a

a a a a a 50 28

110 130

a 88 7.8 5 5

a 40

a

a a

a 12 5

a 7.5

160

160 45

a a

a

a 2.3 2.2 14 7.7 4 20

25 45 20 69

a

17 10

a a

a 227.7 17

a

a a a a

a

31

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44 Table 2b Statistics of geoelectrical data for the different hydrostratigraphic units. Geological formation

Aquifer unit

Statistics

Layer rsistivity (O m)

Mamfe

VA

Mean Min Max SD

518.57 125.00 1900.00 618.65

191.86 50.00 450.00 169.50

606.25 25.00 1100.00 443.65

Mean Min Max SD

237.25 14.00 1200.00 301.62

497.25 55.00 3000.00 698.54

495.78 27.00 1610.00 521.83

258.00 36.00 880.00 322.80

Mean Min Max SD

165.50 15.00 500.00 171.42

407.40 30.00 1500.00 441.54

303.82 9.40 1500.00 511.08

34.13 18.80 60.00 16.87

1.58 0.13 6.00 1.88

Mean Min Max SD

532.50 280.00 800.00 213.44

657.50 230.00 1400.00 525.57

1292.50 360.00 2700.00 1092.32

75.00 75.00 75.00

1.51 0.50 4.00 1.67

q1

Amaseri Sandstone

Ezillo

Intrusives

AA

EA

MA

q2

Layer thickness (m)

q3

q4

q5

57.25 18.75 132.00 64.75

h1

h2

3.36 0.55 10.00 3.51

a

1.26 0.44 4.00 0.88

a

h3

h4

h5

a a

20.00

a a 5.00

7.50

110.00

a

a

a

2.20

10.00

a

a

15.43 2.20 40.00 16.77

79.50 15.00

a

Fig. 8a. Vertical electrical profiles and borehole logs across the Ikom–Mamfe Embayment in NW-SE direction.

5.1. Mamfe Hydrostratigraphic Unit, MA (conglomeritic sandstone, siltstone, shale) The geoelectric interpretation indicated two layers of Q-type at KM 1, KM 2 and KM 8 and three layers of H-type at KM 3 and Q-type at KM 7. The resistivity for the first layer for all the locations

varied between 125 and 19,000 O m while the thickness in the range from 4 to 10 m. From the lithological logs (Fig. 8), this layer represents unsaturated lateritic, silty clay, sand and sandy clay. The second geoelectric layer has resistivity which range from 50 to 450 O m and represents clayey sandstone, silty sandstone, fractured shale and coarse grained sandstone. This layer constitutes

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Fig. 8b. Vertical electrical profiles and borehole logs across the Ikom–Mamfe Embayment in SW-NE direction.

Table 3 Estimates of aquifer parameters for the different hydrostratigraphic units. Porosity values for the entire aquifer were estimated from rock specimens obtained during the field work. Hydrostratigraphic unit/ Parameter Hydraulic conductivity, K Porosity, n Hydraulic gradient, i Velocity, V = Ki/n Discharge, Q = vA Length Width Thickness Transmissivity, T Area Extractable amount of water

Unit m/d % m/d m3/d  106 km km m m2/d km2 106  m3

Volcanics VA 30.00 0.0017 0 22.5 12.5 35.3 281.25 2978.44

the major aquiferous zone of the Mamfe hydrostratigraphic unit. The thickness varied between 20 m at KM 6 and infinity while the static water level (SWL) varied from 6 to 8 m. The third geoelectric layer with resistivity in the range of 25–1100 O m at KM 7 and KM 3 represents saturated coarse gravelly sandstone. The wide range of resistivity within this could result from result of significant variability in clay content or water due to variable grain size of strata. However, the lithological log for KM 17 did not show any distinct difference in grain size between low resistivity zone (25 O m) and the high resistivity zone (1100 X m) at KM 3.

Amaseri AA

Ezillo EA

Mamfe MA

123.12 7.08 0.0024 0.0414 2563.94 47.5 20 57 7017.84 950.00 3833.82

161.28 14.23 0.0042 0.0480 3775.90 50 20 30 4838.4 1000.00 4270.00

216.66 15.11 0.0025 0.0358 4349.27 55 20 42 9099.9138 1100.00 6167.34

However, at a nearby location KM 13, the lithologic data showed alternations of saturated sands, clayey sand, which represented lower resistivity (25–50 X m). 5.2. Ezillo Hydrostratigraphic Unit, EA (baked and fractured shale, sandstone) The VES data for the Ezillo Hydrostratigraphic Unit showed two (OB 4), three (OB 3, OB 8, OB 13) and four (OB 1, OB 2, OB 5, OB7, OB9, OB 10) geoelectric layers (Table 1). The thickness of the first

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T resistivity (m2/day)

5000 4000 3000 2000

y = 0.95x + 121.98

1000

r 2 = 0.76

0 0

2000

4000

T field (m2/day) Fig. 9. Relationship between field transmissivity and electrical resistivity derived transmissivity. Data obtained from Table 2.

layer varied between 0.13 and 6 m while the resistivity values ranged from 44 to 360 O m. This layer represents dry silty sand, clay and shale. The second layer is saturated with resistivity values of 30–1500 O m, and thickness which ranged from 2.2 m to a. The layer represented the major aquiferous zone and is composed of silty sand, limestone, fractured shale and sandstone. The SWL varied between 2 and 4 m. The third layer has resistivity in the range of 9.4–1500 O m and with thickness in the range 10 m–a. The aquifer materials are composed of fractured shale and fractured sandy shale. The fourth layer at OB 2 and OB 10 represents fractured and baked shales, with resistivity values of 22–60 O m.

5.3. Amaseri Sandstone Hydrostratigraphic Unit, AA (sandstone, siltstone, shale) The geoelectrical measurements within the Amaseri Sandstone Hydrostratigraphic Unit generally showed three geoelectrical layers. The resistivity of first layer ranged from 14 to 1200 O m. Its thickness varied between 0.44 and 4 m. From geologic logs, this layer represented dry, gravelly, conglomeritic sandstone, silty sand, sandy clay, clay and shale. The second layer had resistivity values and thicknesses, which ranged from 55 to 3000 O m and from 5 m to a. This layer represented saturated silty, clayey coarse sandstone, silty fractured shale, basaltic intrusive, coarse grained

33

sandstone and fractured shale. The SWL for the second layer varied from 6 to 12 m. The third layer resistivity ranged from 27 to 1610 O m. The thickness of the layer was not resolved by majority of the VES measurements. The lower resistivity values (<50 O m) represented fractured shale and weathered basalt. Otherwise, the layer generally, represents fractured coarse sandstone, fractured shale and basalt. 5.4. Volcanic Hydrostratigraphic Unit, VA (basalts, sandstone, shale) Geoelectric models for Volcanic Hydrostratigraphic Unit generally, showed three geoelectrical layers and a 4-layer in one case. The first layer is characterized by resistivity values in the range from 280 to 800 O m and thicknesses of between 0.5 and 4 m. These resistivity values represented dry silty clay and sandy clay resulting from various degrees of weathering of the basaltic intrusive. The second layer is marked by resistivity values of between 350 and 1400 O m and thickness of 2.2 to 40 m. It corresponds to fractured, silty sandstone and conglomeritic, gravelly sandstone. The layer is aquifereous layer with an average SWL of 5 m. The resistivity of the third layer varied from 360 to 2700 O m. This layer represents saturated fractured silty sandstone. The variability of resistivity values in the VA is attributed to the variability in grain size and degree of weathering and fracturing. The high resistivity values (>1000 X m) are due to either country rock (coarse grained conglomeritic sandstone) and/or fresh basalt. The fourth layer is characterized with a resistivity of 75 O m and a thickness of a. 6. Groundwater hydraulics and aquifer parameters 6.1. Hydraulic conductivity, porosity The hydraulic conductivity K was determined from results of grain size analysis using the Kozeny–Carmen formula (Carmen, 1939) as follows:

K K-Carmen ¼ ðqg=tÞ½n3 =ð1  nÞ2 ðD2m =180Þ cm=s

Fig. 10. Groundwater flow map for the different hydrostratigraphic units in Mamfe Embayment. R and D – recharge and discharge areas.

ð1Þ

R ¼ q  h and C ¼ h=q

ð6Þ

Niwas and Singhal (1981) have established an analytical relationship between aquifer transmissivity (T) and transverse resistance (R) on one hand and between transmissivity (T) and longitudinal conductance (C) on the other as follows:

T ¼ K rR ¼ KC

ð7aÞ

where r = 1/q is the electrical conductivity in lS/cm; R = h  q and C = h/q. Niwas and Singhal (1981) also noted that in areas of similar geological conditions and groundwater quality, the product Kr remains fairly constant. Thus, knowing K values from existing boreholes and r values extracted from electrical sounding interpretation of the aquifer at different borehole locations, it was possible to determine aquifer transmissivity and its variation from place to place where no boreholes are available. Eq. (7a) was rewritten in terms of the modified aquifer resistivity, which took into account the ratio of average water resistivity (q-) to the water resistivity (q ) at the measuring point (Tizro et al., 2010). Thus m

T ¼ K r 0 R0 0

ð7bÞ 0

where r = rq /q- and R = q-/q are the modified electrical conductivity and transverse resistance of the aquifer. Transmissivity values calculated using Eq. (7b) ranged between 177.12 m2/day for the AA and 3913.47 m2/day in MA (Table 3).

729.00 714.38 3480.00 3240.00 1592.52 1418.45 3743.83 3913.47 38.10 216.00 49.77 58.41 26 37.5 30 32 64

20.50 85.00 52.50 6.00 17.00 18.75 15.00 28.50 67.00 30 110 30

3616.74 0.00670.005670.05760.0489711 2865679.380.00030.000030.00470.0004260 866.24 0.01820.060610.98183.2727775 4406.09 0.00220.001360.00000.00000 12 5731.36 0.00250.002970.00000.00000 8 7499.91 0.00250.002500.09530.09525 1266.95 0.01330.011842.88002.55733 7090.99 0.00450.004020.22620.2000325 23529.48 0.02000.002851.16820.1663270 3075 255000 2887.5 2760 6800 7500 1125 6270 3350 176.43 0.137 33713.880.028 16.50 0.955 734.35 0.013 337.14 0.043 400.00 0.047 84.46 0.200 248.81 0.130 351.19 1.340

Kr

EC, electrical conductivity of water; qw, water resistivity qb, bulk resistivity; q0b , modified bulk resistivty; FF, formation factor; Rw, water resistance; Rw average, average water resistance; C, longitudinal unit conductance; R, transverse unit resistance; R0 , modified transverse unit resistance; r, electrical conductivity of aquifer; r0 , modified electrical conductivity of aquifer; ET, aquifer thickness from electrical resistivity data (VES); BT, aquifer thickness from drill data; AT, average aquifer thickness; k, hydraulic conductivityl; T, transmissivity and T0 , modified transmissivity.

The results of the surface resistivity measurements (VES) were employed to calculate the transverse unit resistance, R and the longitudinal unit conductance, C from the aquifer resistivity (q in X m) and aquifer thickness (h in metres) as follows (Bhattacharya and Patra, 1968; Edet and Okereke, 1997):

0.8502 0.0890 3.3334 0.6264 1.1865 1.0000 0.8880 0.8842 0.1424

6.3. Transmissivity

14.82 1.176 2832.0011.238 1.39 0.300 61.69 1.596 28.32 0.843 33.60 1.000 7.10 1.126 20.90 1.131 29.50 7.024

where t is the saturated thickness of the aquifer obtained from geoelectric sounding and water level measurements in m. The data presented in Table 3 show that, comparatively, the VA has higher values of porosity compared to the other hydrostratigraphic units.

150 3000 55 460 400 400 75 220 50

ð5Þ

10.12 1.06 39.68 7.46 14.12 11.90 10.57 10.53 1.69

GR ¼ Atn ðm3 Þ

98.8 944 25.2 134.1 70.8 84 94.6 95 590

where i is the hydraulic gradient, n is the porosity and A is the cross sectional area of the of the aquifer. The total amount of groundwater reserve (GR) for the hydrostratigrahic unit was determined as follows:

r0

ð4Þ

q0b (X m) C (X1)R (X m2)R0 (X m2) r

Q ¼ KiA

average/Rw Rw/Rw average

ð3Þ

Rw

Va ¼ Ki=n ðm=dÞ and

Site codeHydrostratigraphic unitEC (lS/cm) qw (X m) qb (X m)FF

ð2Þ

Table 4 Aquifer parameters obtained from VES results and available hydraulic parameters for some locations in the hydrostratigraphic units MA, EA and AA.

Vd ¼ Ki ðm=dÞ

Kr0

The calculations of actual groundwater velocity, Va (intergranular microscopic flow that describes the velocity through the actual pore spaces of the aquifer), specific discharge velocity, Vd (the velocity through given cross sectional area, A) and total discharge (Q) were based on Darcy’s law as follows:

AA AA AA EA EA MA MA MA MA

6.2. Groundwater velocity and reserve

Ugep YR 4 Nko YR 2 Ediba AB 9 Ochon OB 11 Obubra OB 1 Ekunkunela KM 11 Edor KM 2 Nde KM 3 AkparabongKM 7

ET (m)BT (m)AT (m)k (m/d)T (m2/d)T0 (m2/d)

where q = density of fluid, g = acceleration due to gravity (980 cm/ s2), t = viscosity of the fluid water (0.01 g/cm s), n = porosity and Dm representative grain size. The K values varied between 123.12 and 216.00 m/day, with lower values in the AA relative to EA and MA in that order (Table 3). The highest and lowest porosity values for the embayment, 30% and 7.08% were recorded in the VA and AA, and this indicated low to very high porosities.

8.64 259.20 177.12 14.00 1606.18 1190.00 54.00 1620.00 2835.00

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

Location

34

m

m

35

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

Table 5 Aquifer parameters and hydraulics for the different hydrostratigraphic units based on pumping test data. Hydraulic conductivity values in some cases were estimated from grain size analysis. Q (m3/d)

t (m)

K (m/d)

13.05 10.50 7.57 19.09 5.59

125.40 115.00 49.00 210.00 71.88

26.55 30.91 16.95 34.50 8.85

0.58 0.53 0.06 1.15 0.55

45.19 47.00 27.00 58.00 7.99

5.36 4.39 0.00 14.22 4.13

613.78 123.53 50.00 5184.00 1422.96

39.53 41.20 22.09 56.30 8.93

Mean Med Min Max SD

50.41 50.00 47.30 56.40 2.42

5.29 4.90 1.40 9.50 2.60

96.28 90.00 53.59 180.00 48.07

Mean Med Min Max SD

37.47 38.79 30.25 43.36 6.65

5.77 5.66 3.30 8.36 2.53

Aquifer unit

Statistics

BH depth (m)

SWL (m)

VA (PT 5)

Mean Med Min Max SD

39.60 41.00 26.00 50.00 9.18

AA (PT 28 & GS 2)

Mean Med Min Max SD

EA (PT 7 & GS 2)

MA (PT 3 & GS 3) GS data 3

T (m2/d)

SC (m3/d/m)

SCI

16.16 8.97 1.05 38.46 19.71

88.25 49.00 5.75 210.00 107.63

3.17 2.89 0.34 6.28 2.98

3.67 0.19 0.00 48.24 9.70

129.53 8.91 0.44 1427.00 310.52

77.12 30.63 2.38 312.50 93.96

2.17 0.72 0.06 9.58 2.93

45.12 45.10 40.50 52.60 3.51

6.29 0.04 0.01 37.58 15.33

330.67 1.73 0.33 1976.89 806.48

7.80 8.14 1.80 12.00 4.04

0.17 0.17 0.04 0.27 0.09

33.35 35.00 24.59 35.49 4.29

17.10 0.05 0.03 53.24 24.53

598.57 1.15 0.92 1863.49 858.70

5.80 6.09 5.00 6.30 0.70

0.19 0.17 0.14 0.26 0.06

PT – pumping test data and GS – grain size data (Kozeny–Carmen formula, Carmen, 1939).

High values of transmissivity were recorded in MA ranging from 714.38 m2/day at KM 11 to 3913.47 m2/day at KM 7. The result is consistent with findings that MA is composed of coarse gravelly sandstone marked with resistivity in the range 657.5 and 2700 X m. These findings make MA attractive for drilling with high yield expectations; hence groundwater development should be concentrated within the parts of the basin underlain by this hydrostratigraphic unit. Comparing the values of transmissivity calculated using the electrical resistivity (Eq. (7)) with field measured transmissivity showed a good result (Fig. 9). Table 3 gives a summary of the interpreted VES results, thicknesses, hydraulic conductivity and transmissivity values for the entire aquifer. 6.4. Groundwater flow and fluctuation A general overview of the spatial aspects of groundwater flow in the area is shown on the contour map of measured groundwater levels with respect to mean sea level within the different aquifers (Fig. 10). The groundwater is deduced to flow generally from the north towards the south within MA hydrostratigraphic unit, from west to east within the EA, and is east/west with a divide in the north/south direction along Ekori–Ugep axis, within the AA hydrostratigraphic unit. Groundwater level fluctuations were monitored during the study period in wells at Edor, KM 2 (MA), Obubra, OB 1 (EA) and Ugep, YR 4 (AA). The fluctuations with respect to the ground level during the wet and dry seasons for the three locations varied between 0.39 m in EA and 2.36 m in MA. The higher fluctuation in groundwater level in the hydrostratigraphic unit MA is attributed to high porosity of the aquifer material. The AA hydrostratigraphic unit had the least fluctuation due mainly to the intercalations of fine materials within the aquifer. These differences indicated that hydrostratigraphic unit MA is more sensitive to environmental factors such as fluctuation in precipitation and discharge of groundwater to the Cross River, the main sources of recharge to the unit. 6.5. Aquifer parameters based on drilled data Table 4 is a compilation of values of aquifer parameters for the hydrostratigraphic units based on pumping test data and estimates

from grain size distribution analysis. The yield was obtained as the volume of water discharge per unit time and pumping test. The productivity of a well is measured by its specific capacity (SC), defined as the volume of water pumped per unit time per unit drawdown (Freeze and Cherry, 1979). Walton (1962) introduced the concept of specific capacity index (SCI) and defined as specific capacity divided by the penetrated aquifer thickness. The ranges of SC and SCI values are also included in Table 5. The order of decreasing productivity of aquifers is VA – weathered and jointed basalts and sandstones; AA – baked and fractured shales, basalts and sandstones; EA fractured sandstones, shales and mudstone; MA – conglomeritic, coarse-medium grained sandstones and mudstones.

7. Groundwater chemistry Groundwater samples were collected and measured for ions, using the methods described by Edet and Worden (2009) and Edet et al. (2012). Field measurements of temperature, conductivity, total dissolved solids (TDS), pH, Eh and dissolved oxygen (DO) were made using standard field equipment. These equipment included Hanna HI 9835 Conductivity/TDS meter (temperature, conductivity, TDS); Hanna HI 8314 pH/Eh meter (pH, Eh) and Hanna HI 9142 DO meter for dissoloved oxygen. The statistical summary of the physicochemical parameters are listed in Table 6a. The electrical conductivity (EC) ranged from 25.2 to 944.0 lS/cm (average, 238.71 lS/cm). The pH of the groundwater ranged from 5.61 to 7.85 with an average of 6.23. TDS values ranged from 12.75 to 465 mg/l (average of 118.49 mg/l). These results showed that, on the average, groundwater of the Ikom–Mamfe embayment are acidic (pH < 7.0), fresh (TDS < 1000 mg/l) and soft (TH < 75 mg/l). The major anions constituted 37% of the measured TDS, chloride was the dominant dissolved anion and accounted for almost 22% of the TDS. The concentration of Cl varied between 0.44 and 99.62 mg/l. On equivalent basis, chloride accounted for 45% of the total anions and was followed by bicarbonate which accounted for 44% of the total anions, with a concentration of between 2.23 and 60 mg/l. Concentration of sulphate varied between 0.41 and 55.32 mg/l and contributed 11% of total anion. The concentration of nitrate varied between 0.07 and 103.99 mg/l. Nitrate levels higher than the WHO (1993) maximum acceptable standard of 10 mg/l

36

Table 6a Physicochemical parameters of representative groundwater samples obtained from boreholes for the MA, EA and AA hydrostratigraphic units. Location

Code

Aquifer unit

Temp. (°C)

EC (lS/cm)

TDS (ppm)

pH

Eh (mV)

DO (mg/l)

Na+ (mg/l)

K+ (mg/l)

Ca2+ (mg/l)

Mg2+ (mg/l)

Cl (mg/l)

HCO 3 (mg/l)

SO2 4 (mg/l)

NO 3 (mg/l)

1 2 3 4 5 6 7 8 9 10

Adim Ugep Ediba Nko Ochon Obubra Edor Nwang Akparabong Adijinkpor

BA 1 YR 4 AB 9 YR 2 OB 11 OB 1 KM 2 CR 29 KM 7 KM 13

AA AA AA AA EA EA MA MA MA MA

29.8 29.3 28.8 30.9 28.8 30.1 30.1 29.5 30.9

88.0 98.9 25.2 944.0 134.1 70.8 94.6 79.3 590.0 262.2

44.1 49.4 12.8 465.0 66.1 35.4 47.3 39.7 294.0 131.1

6.08 5.68 6.53 7.85 6.06 5.94 5.86 5.61 6.37 6.30

106 119 91 48 106 110 113 123 96 98

0.7 2.8 3.1 3.4 2.8 2.7 3.3 3.0 3.0 3.3

3.08 9.76 2.06 65.09 5.78 13.55 1.15 51.07 29.79

2.16 4.64 0.94 108.93 0.56 1.28 3.40 1.60 56.81 8.32

11.41 5.33 2.43 68.79 4.40 4.76 4.20 1.15 24.68 5.71

0.54 0.38 0.82 8.74 0.60 1.79 0.70 0.21 5.77 2.93

0.98 7.75 0.73 99.63 1.23 7.09 7.77 0.44 97.15 25.33

8.10 10.10 2.49 60.00 4.13 6.91 9.75 2.23 48.10 24.70

2.01 4.69 0.41 55.33 1.26 – – – 17.90 2.47

0.07 14.79 0.45 51.08 3.20 5.41 25.48 0.42 103.99 34.89

Mean Median Minimum Maximum Std. dev.

29.80 29.80 28.80 30.90 0.79

238.71 96.75 25.20 944.00 297.13

118.49 48.35 12.75 465.00 146.61

6.23 6.07 5.61 7.85 0.64

101.00 106.00 48.00 123.00 21.15

2.81 3.00 0.70 3.40 0.78

20.15 9.76 1.15 65.09 23.47

18.86 2.78 0.56 108.93 35.97

13.29 5.04 1.15 68.79 20.64

2.25 0.76 0.21 8.74 2.85

24.81 7.42 0.44 99.63 39.48

17.65 8.93 2.23 60.00 20.40

12.01 2.47 0.41 55.33 20.02

23.98 10.10 0.07 103.99 33.00

Statistics

Table 6b Seasonal variations of physicochemical parameters of groundwater samples from Ugep, YR 4 (AA), Obubra, OB1 (EA) and Edor, KM 2 (MA) hydrostratigraphic units. S/No. Location Code Aquifer unit Season SWL (m) Temp. (°C) EC (lS/cm) TDS (ppm) pH

 2 Eh (mV) DO (mg/l) Na+ (mg/l) K+ (mg/l) Ca2+ (mg/l) Mg2+ (mg/l) Cl (mg/l) HCO 3 (mg/l) SO4 (mg/l) NO3 (mg/l)

1 2 3 4 5 6

Ugep Ugep Obubra Obubra Edor Edor

120 70 87 77 134 69.00

7 8

Average Average

YR 4 YR4 OB 1 OB 1 KM 2 KM 2

AA AA EA EA MA MA

Wet Dry Wet Dry Wet Dry

2.30 4.05 4.55 5.40 2.75 7.90

28.6 29.5 30.0 29.7 29.5 30.50

98.3 53.8 89.0 131.8 163.4 122.60

49.1 26.8 44.5 65.9 81.9 62.10

5.64 6.09 6.67 5.91 5.21 6.11

Wet Dry

3.20 5.78

29.37 29.90

116.90 102.73

58.50 51.60

5.84 113.67 6.04 72.00

2.1 2.3 3.3 2.7 3.0 3.00 2.80 2.67

10.06 4.28 4.66 12.23 5.36

3.88 3.49 0.95 0.87 2.39 2.19

5.70 3.13 7.74 10.48 4.38 4.60

0.38 0.31 4.27 5.48 1.00 0.49

6.59 3.04 4.70 4.00 13.26 1.90

9.56 5.22 9.75 12.60 8.34 21.10

6.15 1.42 0 0 1.66 0.43

13.57 5.19 14.03 22.33 19.78 7.08

8.98 3.21

2.41 2.19

5.94 6.07

1.88 2.10

8.18 2.98

9.22 12.97

2.60 0.62

15.80 11.53

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

S/No.

37

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44 Table 6c Quality assessment parameters of groundwater samples from Ugep, YR 4 (AA), Obubra, OB1 (EA) and Edor, KM 2 (MA) hydrostratigraphic units.

a

S/No.

Location

Code

Aquifer unit

THa (mg/l)

ECa (lS/cm)

SARa (%)

Naa (%)

RSCa (%)

1 2 3 4 5 6 7 8 9 10

Adim Ugep Ediba Nko Ochon Obubra Edor Nwang Akparabong Adijinkpor

BA 1 YR 4 AB 9 YR 2 OB 11 OB 1 KM 2 CR 29 KM 7 KM 13

AA AA AA AA EA EA MA MA MA MA

30.75 14.88 9.48 208.41 13.51 19.38 13.43 3.78 85.77 26.47

88 98.9 25.2 944 134.1 70.8 94.6 79.3 590.0 262.2

0.24 1.10 0.29 1.96 0.66 0.57 1.61 0.26 2.40 2.52

23.55 64.62 37.52 57.43 48.75 42.28 71.58 54.58 68.19 74.02

0.48 0.13 0.15 3.18 0.20 0.27 0.11 0.04 0.93 0.12

TH – Total hardness, EC – electrical conductivity, SAR – sodium adsorption ratio, Na – sodium, RSC – residual sodium carbonate.

Table 6d Suitability of groundwater samples for irrigation from Ugep, YR 4 (AA), Obubra, OB1 (EA) and Edor, KM 2 (MA) hydrostratigraphic units. Quality parameter

Reference

Total Hardness as CaCO3 (mg/l), TH

Electrical Conductivity, EC (lS/cm)

Range

Sawyer and McCarty (1967)

Ragunath (1987)

Classification

AA

2

3

MA

<250 250–750 750–2000 2000–3000 >3000

Excellent Good Permissible Doubtful Unsuitable

2 2

4

2

4

100

100

100

1 3

2

2 1 1

25 75

100

50 25 25

4

2

4

100

100

100

Excellent Good Doubtful Unsuitable

Percent Sodium, %Na

Ragunath (1987)

<20 20–40 40–60 60–80 >80

Excellent Good Permissible Doubtful Unsuitable

<1.25 1.25–2.5 >2.5

None Slight-Moderate Severe

100

AA

3 1

<10 10–18 18–26 >26

75 25

EA

Soft Moderately hard Hard Very Hard

Richards (1954)

Ragunath (1987)

% of samples

EA

<75 75–150 150–300 >300

Sodium Adsorption Ratio, SAR

Residual Sodium Carbonate, RSC

No of samples MA

1 2

75 25

3

50 50

100

1

75 25

Table 7 Results of Pearson’s correlation between the physicochemical parameters.

SWL Temp. EC TDS pH Na+ K+ Ca2+ Mg2+ Cl HCO 3 SO2 4 NO 3

SWL

Temp.

EC

TDS

pH

Na+

K+

Ca2+

Mg2+

Cl

HCO 3

1.000 0.514 0.625 0.621 0.872 0.475 0.666 0.831 0.602 0.368 0.502 0.767

1.000 0.757 0.754 0.762 0.708 0.794 0.810 0.718 0.586 0.680 0.874

1.000 1.000 0.924 0.983 0.997 0.943 0.998 0.943 0.985 0.950

1.000 0.921 0.984 0.996 0.940 0.998 0.945 0.986 0.948

1.000 0.838 0.945 0.996 0.908 0.752 0.849 0.976

1.000 0.968 0.866 0.988 0.985 0.998 0.885

1.000 0.964 0.990 0.914 0.969 0.972

1.000 0.925 0.779 0.872 0.991

1.000 0.959 0.992 0.930

1.000 0.986 0.794

1.000 0.884

1.000

0.102

0.226

0.659

0.664

0.336

0.780

0.600

0.372

0.696

0.871

0.778

0.399

for drinking and domestic uses were recorded in AA and MA hydrostratigraphic units. The major cations constituted approximately 63% of the TDS. Calcium and sodium are the dominant cations. Calcium concentration ranged from 1.15 to 68.78 mg/l. The average concentration of sodium was 20.15 mg/l (1.14–65.09 mg/l). The concentrations of potassium and magnesium in the groundwater ranged from 0.56 to 108.91 mg/l and 0.21 to 8.74 mg/l respectively.

SO2 4

NO 3

1.000

7.1. Hydrochemical facies Because of the variations in the concentrations of ions, it was not possible to establish well-defined hydrochemical facies for each aquifer. However, from the concentrations, the majority of the groundwater samples classified as mixed, consisting of a blend of waters with different chemical characteristics. This was probably due to the fact that the wells sampled interacted with different

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

Compositional relations among dissolved species can revealed the origin of solutes and the process that generated the observed water composition (Jalali, 2005). Statistical analysis indicated a positive correlation between some pairs of physicochemical parameters (Table 7). The Na–Cl relationship has often been used to identify mechanisms for acquiring salinity and saline intrusions (Dixon and Chiswell, 1992; Garcia et al., 2001). The relatively high Na+ and Cl contents of some samples suggested the dissolution of chloride salts. A simultaneous enrichment in both ions indicated dissolution of chlorite salts or concentration by evaporation process (Jalali, 2005). The dissolution of chloride salt is the process which dominates some parts of the Benue Basin (Ekwere and Ukpong, 1994; Umah et al., 1990; Tijani et al., 1996) in which the study area is located. Fig. 11a shows the plot of Na+ as a function of Cl. The dissolution of halite in groundwater releases equal concentrations of Na and Cl in the solution but analytical results for some samples deviated from the expected 1:1 relation, indicating that a fraction of Na is associated with another anion. A Na+/Cl ratio greater than

+

Na (meq/l)

A

AA EA MA

1.00 0.10 0.01

0.01

0.10

1.00

10.00

-

Cl (meq/l)

B

1.00

AA EA MA

2+

Ca + Mg (meq/l)

10.00

2+

0.10

0.01 0.01

0.10

1.00

10.00

-

HCO 3 (meq/l) 10.00

C

1.00

AA EA MA

2+

7.3. Sources of ions

10.00

2+

Data on the seasonal variation of the static water level showed shallower levels with respect to the ground surface in the wet season (average water depth, 3.2 m) compared to deeper water levels in the dry season (average water depth, 5.7 m). This is attributed to higher amount of precipitation in the wet season (average precipitation, 283 mm) relative to the dry season (average precipitation, 0.2 mm). Also important is the higher air temperature recorded in the dry season (average temperature, 35.9 °C) compared to the temperature in the wet season with average temperature of 30.2 °C. The groundwater temperature also showed higher values in the dry season (30.3 °C) as against lower values for the wet season (29.16 °C). The average EC/TDS values were minimum in the dry season (102.73lS/cm/51.60 ppm) and maximum in the wet season (116.90 lS/cm/58.50 ppm). Calcium, sodium, potassium, magnesium, chloride, bicarbonate and sulphate showed similar trends (Table 6b). Spatial variations of physicochemical parameters in the area showed higher EC/TDS concentrations (>100 lS/cm/>50 mg/l) at Ochon, OB 11 (134.1 lS/cm/66.1 mg/l), Akparabong, KM 7 (590 lS/cm/294 mg/l) and Nko, YR 2 (944 lS/cm/465 mg/l). The other locations exhibited low concentrations (<100 lS/cm/ <50 mg/l). The relatively high EC/TDS could be attributed to the occurrence of brine in parts of the study area (Ekwere and Ukpong, 1994; Umah et al., 1990; Tijani et al., 1996). The major ions followed the same trend as EC/TDS. High nitrate (>10 mg/l) was recorded at Ugep (14.79 mg/l), Nko (51.08 mg/l), Edor (25.48 mg/l), Adijinkpor (34.89 mg/l) and Akparabong (103.99 mg/l). The high concentration of nitrate is attributed to poor waste management and bad practices near the wells. Those wells that were covered are characterised with high nitrate concentration which resulted from infiltration of used water and/or surface runoff into the wells. In contrast, wells with low nitrate concentration were noticed to be those that are well constructed and covered.

Ca + Mg (meq/l)

7.2. Seasonal and spatial variations

0.10

0.01 0.00

0.50

1.00 -

1.50

2.00

2.50

2-

HCO3 + SO4 (meq/l)

2+

AA aquifer: Ca–HCO3, Ca–Na–HCO3–Cl, Na–HCO3–Cl and Ca– Na–Cl. EA aquifer: Ca–HCO3–Cl and Ca–Na–HCO3–Cl. MA aquifer: Ca–Na–HCO3, Na–HCO3–Cl and Na–Cl.

1 is typically reflective of Na+ released by silicate weathering reaction (Meybeck, 1987). Thus silicate weathering is thought to be partly probable source of Na+ in groundwater of the study area. The Ca2þ þ Mg2þ =HCO 3 ratio marks the upper limit of bicarbonate input from weathering of carbonate rock (Stallard and Edmond, 1983). In the Ikom–Mamfe Embayment, some samples showed ex2+ cess of HCO + Mg2+ indicating some extra source of Ca2+ 3 over Ca and Mg2+ and that part of the excess positive charge has to be bal anced by anions, such as SO2 4 and/or Cl (Fig. 11b). The excess of 2+ 2+ HCO over Ca + Mg requires that part of the HCO 3 3 be balanced by alkalis, such as Na+ and K+. Fig. 11c is a plot of Ca2+ + Mg2+ ver2 sus HCO which revealed that most of the samples were 3 þ SO4 above the 1:1 equiline, thereby requiring that a portion of the 2 HCO 3 þ SO4 be balanced by the alkalis. Furthermore, the plot of 2+ 2+ Ca + Mg versus total cations (TC), Fig. 11d, showed that the plotted points fell below the equiline which is an indication of increased contributions of Na+ and K+. From the foregoing, it is clear that the groundwater chemistry is likely due to dissolution of silicate and carbonate minerals. For water draining wholly carbonate

10.00

D

1.00

AA EA MA

0.10

2+

rock types thus justifying the widely spread mixed character. Based on the classification of Deutsch (1997), the different water types were as follows:

Ca + Mg (meq/l)

38

0.01 0.01

0.10

1.00

10.00

Total cations (meq/l) Fig. 11. Cross plots between (a) Na+ and Cl, (b) Ca2+ + Mg2+ and HCO 3 , (c) 2 Ca2+ + Mg2+ and HCO and (d) Ca2+ + Mg2+ and total cations (TC) for 3 þ SO4 groundwater samples from the AA, EA and MA hydrostratigrahic units.

39

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44 þ terrain, the ratio of Ca2+/Na+, Mg2+/Na+ and HCO 3 =Na are of the order of 50, 10 and 120 respectively (Negrel et al., 1993; Meybeck, 1987; Stallard, 1980). However, the chemical composition reflecting these ratios for drainage silicate rocks is Ca2+/Na+  0.35, þ Mg2+/Na+  0.24 and HCO 3 =Na  2 (Gaillardet et al., 1999). The observed average ratios for the study area were Ca2+/Na+ = 1.01, þ Mg2+/Na+ = 0.16 and HCO 3 =Na ¼ 1:22. These values are much lower than those for waters draining both carbonate and silicate lithology indicating that the chemistry is controlled by both carbonate and silicate weathering.

7.4. Quality assessment Agriculture is the main occupation of the people of the Ikom– Mamfe embayment. Hence some parameters such as total hardness (TH), sodium adsorption ratio (SAR), percent sodium (%Na) and residual sodium carbonate (RSC) were estimated to assess the suitability of groundwater from the area for irrigation purposes (Tables 6c and 6d). The TH of water was computed using the formula of Sawyer et al. (2003) as follows:

TH ðas CaCO3 Þ mg=l ¼ ðCa2þ þ Mg2þ Þ meq =l  50

ð8Þ

The hardness values ranged between 3.78 and 208.41 mg/l with an average of 42.59 mg/l. These values satisfy the WHO maximum allowable and most desirable limits of 500 and 100 mg/l respectively. Electrical conductivity (EC) and sodium concentration are very important in the classification of irrigation water, i.e. the salinity. The total concentration of soluble salt in irrigation water can be classified as low (EC < 250 lS/cm), medium (250–750 lS/cm), high (750–2250 lS/cm) and very high (2250–5000 lS/cm) salinity (Richards, 1954). Salinity/alkali hazard in the use of water for irrigation is determined by the absolute and relative concentrations of

cations and is expressed in terms of sodium adsorption ratio (SAR) which was estimated by the Eq. (9) (Prasanna et al., 2011):

SAR ¼

Na

ð9Þ

½ðCa þ MgÞ=20:5

There is a relationship between SAR values of irrigation water and the extent to which sodium is adsorbed by the soils. If water used for irrigation is high in sodium and low in calcium, the ion exchange complex may become saturated with sodium. This can destroy the soil structure owing to dispersion of clay particles (Singh et al., 2005). The calculated value of SAR in the area ranged from 0.24 to 2.52. On the US Salinity diagram (Richards, 1954) in which EC is used as the salinity hazard and SAR as the alkalinity hazard, most of the data fell in the C1–S1 field which indicated low salinity and low alkalinity hazard; two samples fell on C2–S1 field with indication of medium salinity and low alkalinity hazard while one sample in the C3–S1 field indicated high salinity and low alkalinity hazard. These results suggested that 70% of groundwater in the area can be used for irrigation in most soil and crops with little danger of development of exchangeable sodium and salinity. The sodium percentage (%Na) in the groundwater of Ikom– Mamfe study area ranged between 23.55% and 74.02%, with an average of 54.25%. On the Wilcox (1955) diagram relating EC and sodium percent, the data showed that the water is of excellent to good quality and can be used for irrigation purposes. To quantify the effect of carbonate and bicarbonate, residual sodium carbonate (RSC) was also computed. A high value of RSC leads to an increase in the adsorption of sodium on soil (Eaton, 1950). Irrigation water having RSC > 5 meq/l are considered harmful to the growth of plants; water with RSC value >2.5 meq/l but <5.0 meq/l is not considered suitable for irrigation while RSC < 2.5 meq/l is considered suitable for irrigation. The RSC values for the area varied between 3.18 and 0.04 meq/l. This indicates that the groundwater is safe for irrigation purpose.

Table 8 Weights and ratings assigned to each DRASTIC parameter. Parameter

Unit

Code

DRASTIC weight

Agricultural weight

Range

Rating

Depth to water table

m

D

5

5

<2.5 25–5.0 5.0–7.5 7.5–10.0 >10.0

10 7 5 3 1

Net recharge

mm

R

4

4

<80.0 80.0–100.0 100.0–120.0 120.0–140.0 >140.0

3 5 7 9 10

Aquifer media

A

3

3

Sandstone Sandstone/Basalt Sandstone/Shale Weathered Basalt

10 9 8 5

Soil media

S

2

5

Loamy sand Sandy loam Sandy clay loam Clay

10 7 5 3

T

1

3

<0.5 0.5–1.0 1.0–1.5 >1.5

10 7 5 3

I

5

4

Sand Clay/Sand Clay

10 7 1

C

3

2

<50.0 50.0–100.0 100.0–150.0 >150

3 5 7 10

Topography

%

Impact of vadose zone

Hydraulic conductivity

m/day

40

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

8. Aquifer vulnerability Aquifer vulnerability to pollution mapping enables the identification of areas prone to the risk of contamination. Hence, this section contains results of evaluation of the vulnerability of the Table 9 DRASTIC and GOD index intervals and classes of vulnerability. DRASTIC generic

DRASTIC agricultural

GOD

Vulnerability class

Symbol

<80

<115

<0.35

VL

80–100

115–125

100–120

125–150 150–160

Medium vulnerability High vulnerability

M

120–140 >140

>160

0.35– 0.40 0.40– 0.45 0.45– 0.50 >0.50

Very low vulnerability Low vulnerability

Very high vulnerability

VH

L

H

major aquifers of the Ikom–Mamfe Embayment using a documented method, the DRASTIC (Aller et al., 1987). The approach was used for comparison and reliability and because they are widely used for vulnerability mapping. The DRASTIC method is based on the evaluation of seven parameters which determine the infiltration of water and its movement into the aquifer. These parameters are the depth to water (D), net recharge (R), aquifer media (A), soil media (S), topography (T), impact of vadose zone (I) and hydraulic conductivity of the aquifer (C). Each parameter was weighted to reflect the relative importance to vulnerability (Table 8). The most significant parameters had a weight of 5 and the least significant a weight of 1. In addition, each parameter was assigned a rating of between 1 and 10 depending on the local conditions (Table 9); the conditions of low vulnerability provide low rating and vice versa. Next, the vulnerability index (VI) was computed as sum of products of relative weights and ratings as follows:

VIDRASTIC ¼ Dw Dr þ Rw Rr þ Aw Ar þ Sw Sr þ T w T r þ Iw Ir þ C w C r

ð10Þ

Table 10a Input data and computation of vulnerability index for DRASTIC. A Aquifer media

S Soil media

DRASTIC Index generic

Vulnerability classa

DRASTIC Index agricultural

Vulnerability classb

1.5

Clay

Clay

80

L

90

L

10.00

0.5

Clay

Clay

80

L

96

L

14.00 85.66

0.5

L

131

M

10.00

0.5

Clay

Loamy sand Clay

78 80

L

96

L

4.00

4.5

Clay

Clay

89

L

104

L

Nko

KM 14 KM 15 YR 2

8.00

0.5

Sand

Weathered basalt Weathered basalt Weathered basalt Weathered basalt Weathered basalt Sandstone

121

M

188

H

7

Okokori

OB 8

6.00

1.5

Sand

121

M

186

H

8

Ediba

AB 9

9.10

111

M

188

H

9

Ebom

AB 7

101

M

172

H

10

Ekureku

121

M

177

H

11

Loamy sand Loamy sand Loamy sand Loamy sand Sandy Clay Loamy sand Loamy sand Loamy sand Loamy sand Clay

218

VH

182

H

121

M

193

H

121

M

181

H

121

M

168

H

120

M

127

M

Code Aquifer unit

D SWL (m)

1

Etomi

ET 2

10.00

2

Effraya

ET 3

3

Mfum

ET 4

4 5

Bendeghe Ekiem Yawende

6

0.5

43.3

151.2

Clay

Sand

Sandstone/ shale Sandstone

Sand

Sandstone

AB 3

4.80

1.5

Sand

Sandstone

Ugep

YR 4

8.30

1.5

Sand

Sandstone

12

Ovukwa

OB 4

3.20

1.5

Sand

13

Oderiga

3.80

14

Iyamitet

15

Obubra

OB 14 OB EA 12 OB 1

16

Ohana Ede

Shale/ basalt Shale/ basalt Sandstone/ shale Sandstone/ shale Sandstone/ shale Sandstone

19 20 21 22

OB 13 Edor KM 2 Nde KM 3 Ikom KM 4 Nkarasi KM 9 Okanga KM 10 Ekunkunela KM 12

AA

76.85

T Topography (%)

1.5

18

a

VA

R Recharge (mm)

16.60

17

b

C Kmean (m/ day)

I Vadose zone

S/ Location No. name

MA

DRASTIC generic based. DRASTIC agricultural based.

76.85

1.5

149.0

6.00

1.5

5.10

4.5

5.80

1.5

5.05

1.5

Clay/ sand Clay/ sand Clay/ sand Clay/ sand Sand

6.00

1.5

Sand

11.50 85.66

4.5

5.00

119

M

168

H

131

H

200

H

Sandstone

Loamy sand Loamy sand Loam

131

H

175

H

Sand

Sandstone

Clay

122

H

139

H

1.5

Sand

Sandstone

131

H

210

VH

5.00

1.5

Sand

Sandstone

131

H

210

VH

6.00

1.5

Sand

Sandstone

Loamy sand Loamy sand Loamy sand

131

H

200

H

254.0

41

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44 Table 10b Input data and computation of vulnerability index for GOD. No.

Location name

Code

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Etomi Effraya Mfum Bendeghe Ekiem Yawende Nko Ovukwa Oderiga Iyamitet Okokori Ediba Ebom Ekureku Ugep Obubra Ohana Ede Edor Nde Ikom Nkarasi Okanga Ekunkunela

ET 2 ET 3 ET 4 KM 14 KM 15 YR 2 OB 4 OB 14 OB 12 OB 8 AB 9 AB 7 AB 3 YR 4 OB 1 OB 13 KM 2 KM 3 KM 4 KM 9 KM 10 KM 12

Hydrostratigraphic unit

VA

AA

EA

AA

SWL (m)

G

O

D

Vulnerability index

Aquifer vulnerability class

10.00 10.00 14.00 10.00 4.00 8.00 3.20 3.80 6.00 6.00 9.10 16.60 4.80 8.30 5.10 5.80 5.05 6.00 11.50 5.00 5.00 6.00

0.6 0.6 0.6 0.6 0.6 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.7 0.7 0.9 0.9 0.9 0.9 0.9 0.9

0.55 0.55 0.55 0.55 0.55 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.55 0.55 0.7 0.7 0.7 0.7 0.7 0.7

0.8 0.8 0.7 0.8 0.9 0.8 0.9 0.9 0.8 0.8 0.8 0.7 0.9 0.8 0.8 0.8 0.8 0.8 0.6 0.9 0.9 0.8

0.264 0.264 0.231 0.264 0.297 0.416 0.468 0.468 0.416 0.416 0.416 0.364 0.468 0.416 0.308 0.308 0.504 0.504 0.378 0.567 0.567 0.504

L L L L L M M M M M M M M M M M H H M H H H

where the subscripts w and r represent weight and rating for each DRASTIC parameter. The minimum DRASTIC index for the Ikom–Mamfe Embayment was 78 while the maximum was 131. For the DRASTIC agriculture, the minimum and maximum indices were 115 and 161. The classes of vulnerability are presented in Table 9. The GOD method is based on the evaluation of three parameters: groundwater occurrence (G), overall aquifer class (O) and depth to water table (D). The vulnerability index was obtained using the equation below.

VIGOD ¼ Gr  Or  Dr

ð11Þ

where r represents rating for the specific attribute of GOD (Foster, 1987). The classes of vulnerability for the area are indicated in Table 9. The GOD vulnerability index for the area varied between a low of 0.231 and a maximum of 0.567. 8.1. Vulnerability assessment The data for this study were obtained between 1993 and 2010 and composed into a data base. The depth of water level (SWL) was obtained from the records of water development agencies and by direct measurements from open hand-dug wells in the Ikom–Mamfe Embayment. Data for 22 locations across the 4 hydrostratigrahic units were used in the study (Table 10). The SWL varied between 3.20 m in EA hydrostratigrahic unit and 16.60 m in AA. The values for each of the 22 locations were entered according to the ratings for DRASTIC. The estimation of the recharge was based on the precipitation data measured by the Nigerian Meteorological Agency (NIMET) at Ikom (KM 4). The recharge was estimated from the equation:

RD ¼ P C p =C s

ð12Þ

where RD is the recharge in mm, P* precipitation in mm, Cp average chloride concentration in precipitation in mg/l and Cs average chloride concentration in groundwater in mg/l. The results of the estimation indicated a total recharge of between 76.85 and 85.66 mm (Table 10). The aquifer media and vadose zone characteristics were obtained from the descriptions of lithological logs. Aquifer media consists of sandstone, fractured and baked shale, weathered basalt,

and fractured fresh basalt. The vadose zone lithology composed mainly sand and clay with variations as clayey sand and sandy clay. The soil map (CRBDA, 1982) was used to identify the different soil types which were redefined according to DRASTIC classification. The soil types, after re-classifications were sandy clay, loamy sand, clay, and loam. The topographical map (1/250,000) was used to evaluate the slope. It showed that the slope varied between 0.5% and 4.5%. The data for hydraulic conductivity (K) were obtained from sieve analysis and pumping test. The K varied between 43.32 and 254.02 m/d. The results of vulnerability index computation are also listed in Table 10. 8.2. Vulnerability maps The vulnerability maps based on DRASTIC and GOD techniques are presented as Fig. 12. There is good agreement in the classes of vulnerability revealed by these techniques applied to the Ikom– Mamfe Embayment. The classes ranged from very low (VL) to very high (VH) and are distributed within the three broad zones referred to as very low (VL) to low (L), medium (M), and high (H) to very high (VH). The very low to low vulnerability zone covers about 16% of the basin area and is restricted to the eastern parts, extending between Ikom (KM 4) and international border with the Republic of Cameroun (Fig. 1). In this part of the basin dominated by the VA hydrostratigraphic unit, the depth to water level ranged between 4 and 14 m (mean 9.60 m) and the average hydraulic conductivity (K) was estimated at 43.32 m/day. The weathered and fractured basaltic intrusive forms the aquifer media while clay characterizes the vadose zone. The main soil type was clay. The medium vulnerability zone covers about 44% of the Ikom–Mamfe Embayment in two sectors. The main sector dominates the western part of the basin, between its western limits and a line drawn from Ugep (YR 4), through Apiapium (OB 10), to Emangbe (KM 6), Fig. 1. It is underlain by the EA and AA hydrostratigraphic units. The second sector is a strip of territory about 20 km wide adjoining the western side of very low to low vulnerability zone and underlain by the MA hydrostratigraphic unit. In these sectors, the depth to water level ranged between 3.2 and 16.6 m (mean 6.48 m) and the K was 150.12 m/day. Sandstone,

42

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

Fig. 12a. Generic based DRASTIC vulnerability map for Ikom–Mamfe Embayment.

Fig. 12b. Agricultural based DRASTIC vulnerability map for Ikom–Mamfe Embayment.

A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

43

Fig. 12c. GOD vulnerability map for Ikom–Mamfe Embayment.

weathered–fractured basaltic intrusives or shale constitutes the aquifer media while sand or clayey sand characterizes the vadose zone. The main soil type was loamy sand with clay or sandy clay at few locations. The concentration of nitrate for these locations ranged from 0.45 to 51.08 mg/l for the zone. The high to very high vulnerability zone occupies roughly 40% of the Ikom–Mamfe Embayment and is situated between the two sectors of medium vulnerability zone. As with the latter zone, the high to very high vulnerability zone is underlain by the MA, EA and VA hydrostratigraphic units. Here the depth of water level ranged between 5.0 and 11.5 m (mean 6.43 m) and K was 254.02 m/day. Sandstone constitutes the aquifer media while sand characterizes the vadose zone. The main soil type is loamy sand. The concentration of nitrate for three locations in this zone ranged from 14.79 to 103.99 mg/l.

9. Conclusions Primary and secondary data were analysed to gain an insight into the hydrogeologic framework of the Ikom–Mamfe Embayment which is dominated by four major hydrostratigrahic units: MA, EA, AA and VA. Lineament analyses were applied to produce a lineament density map from which the potential areas of high yield for groundwater were delineated. Geoelectrical and geological cross sections revealed the lateral and vertical hydro-lithological variations of the different aquiferous layers in the Ikom–Mamfe embayment. A general overview showed an upper unsaturated topsoil composed of lateritic, clayey, silty, sandy or gravelly material characterized by very low to moderate resistivity (20–1000 X m). The underlying saturated zone

was considered to comprise two layers. Thus the second layer is a zone of very low to high resistivity (22–3000 X m) while the third layer is marked by a low resistivity (<50 X m). This last layer represents shaly/clayey horizons. In some locations, moderate resistivity (50–>500 X m) represents fractured/coarse grained/ conglomeritic sandstone, sandstone, siltstone, fractured/baked shale or weathered basaltic intrusive. Geoelectric parameters were also applied to estimate aquifer parameters. The variations in lithology were responsible for the wide ranges of porosity, hydraulic conductivity and transmissivity of the aquifer. The groundwater flow direction varied within the hydrostratigraphic units : north towards the south in MA; west to east in EA; and east/west, with a N-S divide in AA. High fluctuation in groundwater level has been attributed to high porosity of the aquifer material (sandstone). This indicates that the units with high groundwater fluctuations are more sensitive to environmental factors such as fluctuation in precipitation and discharge to the nearby rivers. The groundwater chemistry was shown to be controlled by halite dissolution and carbonate and silicate weathering. Estimation of HCO 3 contribution from weathering of carbonate and silicate showed that over 95% of the HCO 3 was contributed by silicate weathering while less than 5% contribution was due to carbonate weathering. The hardness values ranged between 3.78 and 208.41 mg/l with an average of 42.59 mg/l. The average value satisfied the WHO maximum allowable and most desirable limits. The values of SAR, EC, Na% and RSC showed that most of the water was of low salinity and low alkalinity hazard, excellent to good quality and safe for irrigation purpose. The evaluation of the vulnerability to pollution maps based on DRASTIC and GOD revealed that a large part of Ikom–Mamfe Embayment (up to 80% of the area) possess medium to high risk

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A. Edet, C.S. Okereke / Journal of African Earth Sciences 92 (2014) 25–44

potential to surface pollution hazard. The low risk part of the basin is marked by clayey soil type, clayey nature of the vadose zone, deep water level and low hydraulic conductivity compared to the high risk part.

References Aller, L.T., Bennet, J.H., Lehr, R.J., Petty, G., Hackett, G., 1987. DRASTIC: A Standardised System for Evaluating Groundwater Pollution Potentials Using Hydrogeological Settings. EPA/600/2-87/035, US Environmental Protection Agency, Washington DC, 455 pp. Amajor, L.C., 1987. The Eze-Aku Sandstone ridges (Turonian) of southeastern Nigeria: a reinterpretation of their depositional origin. Nigerian J. Min. Geol. 23, 17–26. Banerjee, K., 1980. A subtidal bar model for Eze-Aku sand bodies, Nigeria. Sed. Geol. 25, 291–309. Bhattacharya, K.P., Patra, H.P., 1968. Direct Current Geoelectric Sounding. Elsevier, Amsterdam. Carmen, P.C., 1939. Permeability of saturated sands, soils and clays. J. Agric. Sci. 29, 263–273. Cross River Basin Development Authority (CRBDA), 1982. Inventory of Natural Site Conditions – Soil, Slope, Hydrology, Landuse and Vegetation Throughout the Area of Operation of the Authority. Progress, Report No. 4, 145 pp. Deutsch, W.J., 1997. Groundwater Geochemistry. Fundamentals and Applications to Contamination. Lewis, New York. Dixon, W., Chiswell, B., 1992. The use of hydrochemical sections to identify recharge areas and saline intrusions in alluvial aquifers, southeast Queensland, Australia. J. Hydrol. 130, 299–338. Eaton, F.M., 1950. Significance of carbonates in irrigation waters. Soil Sci. 39, 123– 133. Ebong, U.E., 1989. The Sedimentology of the Cretaceous Highway Transect from the Calabar Flank across the Mamfe Embayment. M.Sc Thesis, University of Calabar, Nigeria, Unpublished. Edet, A.E., 1993. Hydrogeology of Parts of Cross River State, Nigeria: Evidence from Aero-Geological and Surface Resistivity Studies. PhD Thesis, University of Calabar, Calabar, Nigeria, Unpublished, 316 pp. Edet, A.E., Teme, S.C., Okereke, C.S., Esu, E.O., 1994. Lineament analysis for groundwater exploration in Precambrian Obudu and Oban massif southeastern Nigeria. J. Min. Geol. 30 (1), 87–95. Edet, A.E., Okereke, C.S., 1997. Assessment of hydrogeological conditions in basement aquifers of Precambrian Oban massif, southeastern Nigeria. J. Appl. Geophys. 36, 195–204. Edet, A., Worden, R., 2009. Monitoring of physical parameters and evaluation of the chemical composition of river and groundwater in Calabar (Southeastern Nigeria). Environ. Monit. Assess. 157, 243–258. Edet, A., Ugbaja, A., Offiong, O., 2009. Groundwater geochemistry in the Mamfe Embayment of Cross River Basin, Nigeria. In: Proceedings Groundwater Conference 15–18 November, 2009 Somerset West, Western Province, South Africa, p. 28. Edet, A.E., Worden, R.H., Mohammed, E.A., Preston, M.R., 2012. Hydrogeochemical processes in a populated shallow Coastal Plain Sand Aquifer southeastern Nigeria. Environ. Earth Sci. 65, 1933–1953. Edim, M.O., 1998. Geoelectrical Study of Shallow Aquifers in the Cretaceous Sediments of Obubra and Environs. M.Sc Thesis, University of Calabar, Calabar, Nigeria, Unpublished, 194 p. Ekwere, S.J., Ukpong, E.E., 1994. Geochemistry of saline water in Ogoja, Cross River State of Nigeria. J. Min. Geol. 30 (1), 11–15. Ekwueme, B.N., Nyong, E.E., Petters, S.W., 1995. Geological Excursion Guide Book to Oban massif, Calabar Flank and Mamfe Embayment, Southeastern Nigeria. Decford Publishers Ltd, Calabar, Nigeria, 36 pp. Foster, S.S.D., 1987. Fundamental concepts in aquifer vulnerability, pollution risk and protection strategy. In: van Duijvenbooden, W., van Waegeningh, H.G. (Eds.), Vulnerability of Soil and Groundwater to Pollutants, Processing and Information, vol. 38. TNO Committee on Hydrological Research, The Hague, pp. 69–86. Freeze, R.A., Cherry, J.A., 1979. Groundwater, second ed. Prentice Hall, Eaglewood, Cliff, New Jersey, USA, 604 pp. Gaillardet, J., Dupre, B., Louvat, P., Allegre, J.C., 1999. Global silicate weathering and CO2 consumption rates deduced from chemistry of large rivers. Chem. Geol. 159, 3–30. Garcia, M.G., Hidalgo, M.V., Blesa, M.A., 2001. Geochemistry of groundwater in alluvial plain of Tucuman province, Argentina. Hydrogeol. J. 9, 597–610.

Hossain, M.T., 1981. Geochemistry and petrology of the minor intrusives between Efut Eso and Nko in the Ugep area of Cross River State. J. Min. Geol. 18 (1), 42– 51. Jalali, M., 2005. Major ion chemistry of groundwaters in the Bahar area, Hamadan, western Iran. Environ. Geol. 47, 763–772. Lattman, L.H., Matzke, R.H., 1961. Geological significance of fracture-trace mapping in Appalachian Plateau. Am. Assoc. Petrol. Geol. 27 (3), 435–440. Mabee, S.B., Hardcastle, K.C., Wise, D.U., 1994. A method of collecting and analyzing lineaments for regional-scale fractured-bedrock aquifer studies. GroundWater 32, 884–894. Meybeck, M., 1987. Global chemical weathering of surficial rocks estimated from river dissolved loads. Am. J. Sci. 287, 401–428. Negrel, Ph., Petelet-Giraud, E., Barbier, J., Gautier, E., 1993. Surface water– groundwater interactions in an alluvial plain: chemical and isotropics systematics. J. Hydrol. 277, 248–267. Niwas, S., Singhal, D.C., 1981. Aquifer transmissivity of porous media from resistivity. J. Hydrol. 82, 143–153. Ojoh, K.A., 1990. Cretaceous geodynamic evolution of the southern part of the Benue Trough (Nigeria) in the equatorial domain of South Atlantic: stratigraphy, basin analysis and paleo-oceanography. Bull. Centre Res. Explor. Prod. ElfAquitaine 14 (2), 419–442. Okereke, C.S., Esu, E.O., Edet, A.E., 1998. Determination of potential groundwater sites using geological and geophysical techniques in the Cross River state, southeastern Nigeria. J. Afr. Earth Sci. 27 (1), 149–163. Petters, S.W., Okereke, C.S., Nwajide, C.S., 1987. Geology of Mamfe Rift, SE Nigeria. In: Matheis, G., Schandelmerer, H. (Eds.), Current Research in African Earth Sciences. Balkema, Rotterdam, pp. 299–302. Petters, S.W., 1982. Central west African Cretaceous-Tertiary benthic foraminifera and stratigraphy. Paloeontographica, A 179, 1–104. Petters, S.W., 1989. A Regional Hydrogeological Study of Rural Water Supply Options for Planning and Implementation of the Phase II Rural Water Programme in Cross River State. DFRRI, Cross River State, 97 pp. Prasanna, M.V., Chidambaram, S., Gireesh, T.V., Jabir Ali, T.V., 2011. A study on hydrochemical characteristics of surface and sub-surface water in and around Perumal Lake, Cuddalore district, Tamil Nadu, South India. Environ. Earth Sci. 63, 31–47. Ragunath, H.M., 1987. Groundwater. Wiley Eastern Ltd., New Delhi. Reyment, R.A., 1965. Aspects of the Geology of Nigeria. Ibadan University Press, Ibadan, Nigeria. Richards, L.A., 1954. Diagnosis and Improvement of Saline Alkali Soils. US Department of Agriculture, Handbook 60, p. 160. Sawyer, G.N., McCarty, D.L., 1967. Chemistry of Sanitary Engineers, second ed. McGraw Hill, New York. Sawyer, G.N., McCarty, D.L., Parkin, G.F., 2003. Chemistry of Environmental Engineering and Science, fifth ed. McGraw Hill, New York. Singh, A.K., Mondal, G.C., Singh, P.K., Singh, S., Singh, T.B., Tewary, B.K., 2005. Hydrochemistry of reservoirs of Damodar River Basin, India: weathering processes and water quality assessment. Environ. Geol. 48, 1014–1028. Stallard, R.F., 1980. Major Element Geochemistry of Amazon River System. PhD Thesis, Massachusetts Institute of Technology. Stallard, R.F., Edmond, J.M., 1983. Geochemistry of the Amazon River. The influence of geology and weathering environment on dissolved load. J. Geophys. Res. 88, 9671–9688. Tijani, M.N., Loehnert, E.P., Umah, K.O., 1996. Origin of saline groundwaters in the Ogoja area, Lower Benue Basin, Nigeria. J. Afr. Earth Sci. 23 (2), 237–252. Tizro, A.T., Voudouris, K.S., Salehzade, M., Mashayekhi, H., 2010. Hydrogeological framework and estimation of aquifer hydraulic parameters using geoelectrical data: a case study from West Iran. Hydrogeol. J. 18, 917–929. Umah, K.O., Onuoha, K.M., Egboka, C.E., 1990. Hydrochemical Facies, Groundwater Flow Pattern and Origin of Saline Waters in Parts of the Western Flank of the Cross River Basin, Nigeria. In The Evolution of the Benue Basin (Eds COfoegbu), pp. 115–134. Ugbaja, A.N., 2007. Hydrogeochemical Studies and Vulnerability Assessment of Sandstone–Shale–Intrusive Aquifers, Mamfe Embayment, Southeastern Nigeria, PhD Thesis, University of Calabar, Calabar, Nigeria, Unpublished, 168 pp. Walton, W.C., 1962. Selected Analytical Methods for Well and Aquifer Evaluation. III State Water Survey, Bulletin 49. Wilcox, L.V., 1955. Classification and Use of Irrigation Waters. Circ 969. US Department of Agriculture, Washington, DC. World Health Organisation, WHO, 1993. Guidelines for Drinking Water Quality. Revision of the 181 Guidelines Final Task Group Meeting, 21–25 September, 1992. WHO, Geneva, Switzerland. Zohdy, A.A.R., Eaton, G.P., Mabey, D.R., 1974. Application of Surface Geophysics to Groundwater Investigations. United States Geological Survey Book 2 (Chapter D1).