1

Original Research Article 1

Delineating Aquifer Systems Using Dar Zarouk 2

Parameters Determined From Surface Geoelectric 3

Survey: Case Study of Okigwe District, 4

Southeastern Nigeria . 5

6

Leonard I. Nwosu*, Anthony S. Ekine and Cyril N. Nw ankwo 7 Depart ment of Physics, University of Port Harcourt , Nigeria. 8 9 10

ABSTRACT 11 This study is aimed at delineating the aquifer systems in the study area and hence determine the 12 parts with higher aquifer yield. To achieve this, 120 Vertical Electrical Soundings (VES) were 13 carried out in Okigwe District of Imo State of Nigeria, using the Schlumberger electrode array and 14 a maximum electrode spread of 900m. Twelve of the VES stations were sited near existing 15 boreholes to enhance interpretation. The resistivity of the aquifers zones varied across the study 16 area. The highest thickness of 104.4m for the aquifer is recorded at Amonze (VES 95). Using an 17 average transmissivity of 1032.0848 m2/day determined from pumping test data of the boreholes 18 in the area, a mean conductance value of 91.222 m/day was obtained for the area. The hydraulic 19 conductivity values obtained from the VES results varied from 9.8854 to 115.9646 m/day. The 20 transmissivity values range from 992.04 to 10388m2/day. The storativity or storage coefficient 21 determined for the area ranged from 1.59x10-4 to 7.80x10-3 while the specific capacity is fairly 22 uniform with magnitude of about 877 m3/day. Using Dar Zarrouk parameters determined for the 23 area, three aquifer systems have been identified viz: the Coastal plain sands aquifer, the Bende-24 Ameki sandstone/shale aquifer and the Ajali sandstone aquifer. The coastal plain sands aquifer 25 serves the Southern part of Isiala Mbano, Ehime Mbano, Ihitte Uboma and the entire Obowo 26 Local Government Areas (LGAs). The Bende-Ameki sandstone aquifer serves part of Ihitte 27 Uboma, the Northern part of Ehime Mbano and Isiala Mbano. The Ajali sandstone aquifer serves 28 the Northwestern part of the study area in Okigwe. Groundwater potential evaluation of the district 29 based on the survey and borehole hydrogeological information revealed that the Southern part of 30 the district is the most prolific in terms of groundwater exploitation and thus the most promising 31 for siting productive boreholes. 32

33 Key Words: Hydraulic conductivity, Aquifer Systems, Aquiferous Zones, Dar Zarouk, VES. 34 35

1. INTRODUCTION 36 Okigwe District is in Imo State of Nigeria. The area lies between Latitudes 5o30′N to and 5o57′N of 37 the Eequator and Longitudes 7o04′E to 7o26′E of the Greenwich Meridian (Fig. 1) covering a land 38 area of about 1,824 km2. The dDistrict is made up of six LGAs namely, Isiala Mbano, Ihitte 39 Uboma, Ehime Mbano, Onuimo, Obowo and Okigwe. Scarcity of potable water in both the rural 40 and urban areas of Okigwe dDistrict especially during the dry seasons has become a concern to 41 Government and the citizens of the area. Governments’ intervention by of siting the Okigwe 42 regional water scheme in Onuimo LGA hasdid not yielded the desired results of supplying making 43

Comment [DTA1]: check.

Comment [DTA2]: Pls check this grammar. I hope we are not expecting a new round of grammatical errors.

2

portable water from the Imo River readily available largely due to pollution and paucity lack of 44 funds for extensive project execution. 45 46 Many failed boreholes projects exist have been experienced in manymost parts of the dDistrict in 47 the recent past. This borehole failures may have resulted from the nature of the geological setting 48 of the area especially in around the northern part and inadequate or lack of geophysical 49 information to guide during the drilling phase. Hence proper geophysical investigation of the area 50 is required as the area is fast experiencing increase in commercial and industrial activities owing 51 to rapid urbanization. This e study is aimed at therefore, attempts to mapping the aquifer systems 52 in an geologically complicated environment that has with high borehole failure rate. This will help 53 in generating information that will aid borehole developers in the successful drilling of water 54 boreholes. 55 56 Electrical resistivity of rocks is a variable property that depends on a number of factors including 57 the resistivity of the pore electrolyte and is inversely related to the porosity and degree of 58 saturation of the formation (Ekine, 2010) ). The resistivity of rock layers vary considerably from 59 Fformation to Fformation as well as within a particular layer (Ekine, 2010). This variation can be 60 very large wide for any particular layer. Table 1 gives approximate ranges for of the electrical 61 resistivity of rocks and soils, which was used in the interpretation of VES data in this work. 62 63 64

Table 1: Approximate ranges for the electric resistivity of 65

rocks and soils (Vingoe, 1972). 66

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69

70

71

72

73

74

75

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77 78 79 80 Dar Zarouk parameters derived from layer resistivity and thickness of a geoelectric layer 81 generated from the modeling of VES dataduring Vertical Electrical Sounding (VES) survey has 82 been proved to be very useful in understanding the spatial distribution of aquifer hydraulic 83 parameters (Zhody, et al., 1965). Niwas and Singhal (1981) derived analytically the relationship 84 between aquifer transmissivity and transverse resistance, and that between transmissivity and 85 longitudinal conductance. The results of their study showed that in areas where the geologic 86 setting and water quality do not vary greatly, the product of hydraulic conductivity K and aquifer 87 conductivity σ, product (Kσ), will remains fairly constant. Thus if the value of hydraulic 88

Comment [DTA3]: Again?

Comment [DTA4]: How did you know? I think a reference is necessary to support this claim since none of the authors is working with the agency.

Comment [DTA5]: You still need to mention some of the affected communities in your work but they MUST be CLEARLY shown in your location map.

Comment [DTA6]: Check for grammatical error.

Comment [DTA7]: Complete the sentence by inserting the techniques that you intend to use.

Comment [DTA8]: Yet this Table 1 is not still looking original. I am very sure that nothing stops our dear authors from giving their esteemed readers something better.

3

98

Fig. 1: Map of study area showing sounding stations and interpretative Geoelectric Cross-Section (IGCS) traverse.

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Sounding Station

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37

conductivity K is known from the existing boreholes and the electrical conductivity σ, from the 89 sounding interpretation around the boreholes are available, it is possible to estimate the 90 transmissivity and its spatial variation from place to place by determining the transverse 91 resistance or longitudinal conductance for the aquifer. This is the basis of this geophysical survey. 92 93

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115 116 117 118 1.1 Literature Review 119 120 A review of the geology of the district ( (Fig. 2) reveals the following stratigraphic units; the Benin 121 Formation, the Ogwashi - Asaba Formation, the Bende - Ameki Formation, Imo Shale Formation, 122 Nsukka Formation and Ajali Formation (Akaolisa and Selemo, 2009). 123 124

Comment [DTA9]: Seen but still lacks tint of originality with the same old crowded information. Even at a magnification of over 200, the entries are still very difficult to read. Nothing stops the authors from improving the quality of this map. In your map legend, some fonts are bolded while others are faint. Any reason for this?

Comment [DTA10]: I am finding it difficult to understand why the authors removed the titles and subtitles that were proper in my assessment. Now they have bundled up geological and theoretical basis of their study together. Even if they are to be subtitles under Literature Review, they could have been allowed to remain. I strongly suggest that they should be brought back.

4

5 4 5 N05 4 5 N0

5 3 0 ’N0

7 0 0 E0

7 0 0 E0

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7 0 5 E0

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5 0 1 5 ’N05 1 5 ’N0

125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154

Fig. 2: Geological map of Imo State showing the study Area (Adapted from Akaolisa 155 and Selemo, 2009). 156

157 The Benin Formation is overlain by lateritic overburden and underlain by the Ogwashi - Asaba 158 Formation, which is in turn underlain by the Ameki Formation of Eocene to Oligocene age (Mbonu 159 et al., 1990). The Benin Fformation consists of coarse - grained gravelly sandstones with minor 160 intercalations of shales and clay. The sand units are mostly coarse to fine grained, pebbly and 161 poorly sorted (Onyeagocha, 1980; Short and Stauble, 1967). The southern part of the study area 162 covering Obowo, Ssouthern part of Ehime Mbano and Isiala Mbano fall within this Fformation. 163 The Ogwashi - Asaba Formation is made up of variable succession of clays, sands and grits with 164 seams of lignite. It also forms part of the study area. The Bende-Ameki Formation consists of 165 greenish - grey clayey sandstones, shales and mudstones with interbedded limestones. This 166 Fformation in turn overlies the impervious lmo Shale Group. 167 168 The Ajali fFormation consists of thick friable, poorly sorted sandstones typically white in colour but 169 sometimes iron- stained. A marked banding of coarse and fine layer is displayed (kogbe, 170 1989).The Ajali fFormation is often overlain by a considerable thickness of red earth, which 171 consists of red, earthy sands formed by the weathering and ferruginisation of the 172 formation(Kogbe, 1989). 173

Comment [DTA11]: Imo State or parts of Imo State? Confirm that your legend entries matches with what you have on the map.

Comment [DTA12]: What are you reserving this space for?

Comment [DTA13]: Check.

Comment [DTA14]: Separate

5

174 175 Using the Logan’s approximation of Theim’s equation (Logan, 1964), aquifer parameters from 176 pumping test data can be determined for steady state aquifer flow as follows: 177

Q= 1 178

Wwhere Q = well discharge in m3 / day, K = average hydraulic conductivity in m / day, 179 h = thickness of the aquifer in m, Smw = maximum drawdown in the pumped well in m, 180 rw = radius of the pumped well in m and rmax = radius of influence of the pumping in m 181

182 K = average hydraulic conductivity in m / day 183

h = thickness of the aquifer in (m) 184 Smw = maximum drawdown in the pumped well in (m). 185 rw = radius of the pumped well in (m). 186 rmax = radius of influence of the pumping in (m). 187 188 The area circumscribed by rmax is the area in the borehole environment that contributes water to 189 the well. Equation (1) was modified by Logan (1964) for steady state flow in confined aquifer as 190

mw

rwr

S

QKh

π2

log3.2 max

= 2 191

And since the logarithm of the ratio of rmax to rw is very small, assuming average condition of radii, 192 rmax = 500 m, rw= 0.3048 m, then 193

21.3log max =wr

r 194

The equation further reduces to 195

mwS

QKh

18.1= 3 196

∴ mwhS

QK

18.1= 4 197

198 Table 3 shows the pumping test data obtained for some boreholes in the study area. 199 Computation of the aquifer parameters using equations 1 to 4 gave the values displayed in Table 200 2 for parameters 1 to 6. 201 202 Dar Zarouk Pparameters equations are derived by considering a column of unit square cross-203 sectional area cut out of group of layers of infinite extent. The total transverse unit resistance R, 204 and total longitudinal unit conductance S, are respectively given as: 205

206

ni ,....3,2,1= 5 207 208 The total longitudinal unit conductance, 209

.,...3,2,1 ni = 6 210

211

Comment [DTA15]: I suggest that a subtopic should be here.

Comment [DTA16]: You still mean ‘aquifer flow’

6

where pi and hi are the resistivity and thickness of the i-th layer. The average longitudinal 212 resistivity, The parameters R and S are called the Dar Zarouk parameters. 213 214

= 7 215

where 216

∑=

=n

iihH

1 ni ,...3,2,1= 8 217

and the average transverse resistivity 218

= 9 219

220 Transmissivity, Tri, used in groundwater hydrology (Mbonu et al., 1990), is given by 221

iiri hKT = 10 222

where Ki is the hydraulic conductivity of the i-th layer of thickness hi. The parameters R and S are 223

called the Dar Zarouk parameters. 224

The relationship between aquifer transmissivity, Tr and transverse resistance, R and that between 225

Tr and S have been derived analytically by Niwas and Singhal (1981) as 226

11 227 228 Thus, it is possible to estimate the transmissivity and its spatial variation from place to place from 229 values the determinations of R or and S for of the aquifer. 230 231 Parameters 7 to 15 of Table 3 were determined using the above equation. Notice the close 232 agreement between parameters 2 and 15 for Madonna II Ihitte Uboma. 233 234

2. MATERIAL AND METHODS 235 A total of 120 Vertical Electrical Soundings (VESs) were carried out in the study area using the 236 Schlumberger electrode configuration with a maximum electrode spacing of 900 m. The ABEM 237 Terrameter (SAS) 300B was used to acquire the data. It has a liquid crystal digital read-out and 238 an automatic signal averaging microprocessor. Four stainless steel non-polarizable electrodes 239 were made up of used, two current electrodes and two potential electrodes were used. A 12V dc 240 source was used to power the instrumentsupply current. 241 242 The current electrode spacing was increased symmetrically about the centre, keeping the 243 potential electrode constant until it became necessary to increase the potential electrode spacing 244 as the strength of the recorded signal diminished. The apparent resistivity values computed were 245 plotted against half of the current electrode spacing (L/2) on a log-log graph (Fig. 3(i)). The 246 sounding curves obtained were subjected to conventional partial curve matching using the Rijks 247

Comment [DTA17]: Check sir.

Comment [DTA18]: Cross check with earlier comment

Comment [DTA19]: Is this your ‘t’ subscripted

Comment [DTA20]: Space.

Comment [DTA21]: Are you sure that this your ‘t’ is subscripted.

Comment [DTA22]: Please check this equation again

Comment [DTA23]: Pls specify the equation that you are referring to as above.

Comment [DTA24]: ??

Comment [DTA25]: Once you have determined an acronym for some words or phrase, you do not need to be defining thema again.

Comment [DTA26]: Which of the electrodes are you referring to?

7

Waterstaat (1975) master curves to obtain the initial model parameters (resistivities and 248 thicknesses) for computer aided interpretation. The field measurements were inverted using the 249 Schlumberger automatic analysis version 0.92 software package (Hemkler, 1985) to determine 250 the true resistivitiesy and depths of subsurface formations. The resulting model curves have three 251 to five interpretable geoelectric layers with less than 5% RMS errors. 252 253

3. RESULTS AND DISCUSSION 254 The survey revealed that the study area is multi-layered exhibiting variation in resistivity with 255 depth. The geoelectric section comparesd favourably with the borehole lithology and the 256 resistivity model gave the resistivity of the probable aquifer, the depth to aquifer and the aquifer 257 thickness. Figures 3, 4 and 5 show typical modelled VES curves, the geoelectric section and 258 boreholes lithology of VES 79 near Isinweke borehole in Ihitte Uboma LGA; VES 32 near Anara 259 borehole and VES 110 near Okigwe borehole respectively. The curves are combinations of H- 260 type and K-type (Ekine, 2010) and HKH-type and KQH-type (Oseji, 2005). 261

262 The Iinterpretative Ggeoelectric Ccross-Ssections (IGCSs) constructed for the area along profiles 263 CD, EF and GH (Fig. 1) reveal that there are variations in resistivity and thickness of the 264 aquifersous zones across the entire study area. The top layers generally show large variations in 265 layer resistivity. These layers correspond to the brown to reddish lateritic overburden interspersed 266 with sandy soils, sandy clay with humus. The aquifersous layer consists of fine to medium coarse 267 sands, sandstones and sandy shales. The aquifers is are thicker in the southern part of the study 268 area. The aquiferous layer is underlain by conductive layer composed of clay-sand/lignite and 269 shale. The geoelectric sections CD, EF and GH (Figures 6, 7 and 8) indicate that the main 270 aquifers are confined in some parts of the traverses. The Northeastern part and the South-South 271 area indicate prospective area for siting water boreholes with high yield expectations. Similar 272 observation is recorded by profile GH in the Southern and South-South zone with high aquifer 273 thickness (Maduagwu, 1990; Njoku, 1991). 274 275 Table 3 compares aquifer characteristics determined from pumping test data with those 276 calculated on the basis of VES data. Parameters 1 to 6 were determined from pumping test data 277 while parameters 7 to 15 were computed from VES results. The computation was based on the 278 screened sections of the aquifer in the borehole. The hydraulic properties of the aquiferous zone 279 horizon determined for the VES stations 61 to 80 are displayed in Table 4. The table shows 280 variation in transmissivity and storativity values as well as other hydraulic properties of the 281 aquiferous zones. 282 283 The hydraulic conductivity values obtained from the VES results varied from 9.8854 to 115.9646 284 m/day. The storativity values determined for the area varied from 1.59x10-4 to 7.80x10-3 while the 285 specific capacity is fairly uniform with a maximum value 877 m3/day. The transmissivity value of 286 992.04 m2/day was recorded in the Northern part of the study area. The value increased towards 287 the southern part with corresponding higher values of aquifer thickness and storage coefficient. 288 289 290 3.1 Evaluation of Aquifer Systems in the Study Area 291 On the basis of the diagnostic electrical and hydraulic conductivity product (Kσ) values, three 292 aquifer systems have been identified (Fig. 9). These are: 293 (i). The coastal plain sand aquifer (Zone A) is relatively thick and unconfined with high thickness 294 and transmissivity values as well as storage coefficient. The aquifer materials are fine to medium 295 coarse, unconsolidated sand and sandstone with seams of sandy clay (Ekwe et al., 2006). This 296 zone covers the southern part of Isiala Mbano, Ehime Mbano, Ihitte Uboma and the entire Obowo 297 LGA. The Kσ values range from 0.0093 to 0.00921.This close range suggests little variation in the 298 water quality of the aquifers. The relatively lower resistivity value recorded at Okwuoma (VES 81) 299 could be linked to Abadaba lLake and that recorded at VES 95 to the presence of Onuinyi River. 300 The aquifer is relatively deep in thise zone. However, the lower resistivity values that VES 91 has 301 over VES 81 could be linked to the nature of the depositional environment The depth to the 302

Comment [DTA27]: In stack contrast to what is indicated beside the curves.

Comment [DTA28]: There nust be consistence in writing figure captions. I am seeing Figures in one page and Fig. in other pages and even within the same page.

Comment [DTA29]: The term ‘aquiferous’ does not exist in the English Dictionary. It appears to be a jargon and as such should not be used. I suggest that you change them to aquifers.

Comment [DTA30]: All these sentences begin with the same words/phrases thereby rendering them monotonous. How I wish this was to be a piece of poetic or dramatic writing where they could have conveyed a different meaning to the writing.

Comment [DTA31]: Which table?

Comment [DTA32]: I do not think that it is possible to achieve this level of accuracy from geophysics. I will rather suggest that you reduce them to a maximum of 2 dec pls.

Comment [DTA33]: Recorded or observed?

8

K-type ( < >

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40 131

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0ft

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120

Dep

th(m

)

Resistivity (ohm - metre) x 10

ii

ii i

i

i Model curve ii Geoelectric section iii Lithology

Fig. 3.11 Model curve, geoelectric section and lith ology ofVES 79 near Isinweke Borehole.

Fig. 3:

)

SECTION SECTION

0

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Lateritic Sandy Soil

Reddish Brown Sand Reddish Brown Plastic Clay

Lignitic Sand

Medium Sand

Fine Medium Sand

Medium Sand Sandy Clay

Sandy Clay

Sandy Clay

Sandy Clay

Medium CoarseSand

Fine Medium Sand

Fine Medium Sand

Fine Medium Sand

Medium Sand

Scale 125

in = 1 ft

aquifer is about 53 m in Obowo, 88 m to 90 m in Ihitte Uboma, 52 m to 70 m in Isiala Mbano and 303 40 m to 45 m in Ehime Mbano. 304

305 306 (ii). The Bende- Ameki Sandstone/Shale aquifer (Zone B) which covers the least area in the 307 district is located around parts of Ihitte Uboma and the Nnorthern part of Ehime Mbano and Isiala 308 Mbano LGAs. The formation is a productive aquiferous though but not to the extent of the coastal 309 plain sands with transmissivity being lower due to the higher percentage of shales. Groundwater 310 exploitation is sometimes difficult in this zone (IWADA 2002). The Kσ values range from 0.00831 311 to 0.23787. This range suggests variation in the water quality inof the aquifers. The depth to 312 aquifer is about 40 m to 45 m. 313 314 315 316 317 318

319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350

Comment [DTA34]: These modeled VES curves are still being plagued by originality problems. The unit of the variables on the axes of all the VES curves are not indicated. K-type curve in a four layered subsurface? Something is terribly wrong. Why is it that in your Figs 4-6 iii, the lithological symbols in your Fig. 3iii suddenly disappear. Also, what is responsible for the change in style? Did the authors notice that in the Legend over Figs 3-6ii, the multiplier that are indicated at the top is not complete. I strongly believe that the authors can do better than this.

9

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2 4 6 8 10 12 140ft

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pth

(m)

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ii

iii

i

i Model curve ii Geoelectric section iii Lithology

Fig. 4: Model curve, geoelectric section and lithol ogy ofVES 32 near Anara Borehole.

K-type ( < > )

in=

SECTION

Red earthy Sand

Red earthy SandClayey SandClayey SandRed SandRed Sand

mixed Sand StonePlastic ClaySilty fine Sand

Mixed Sand

Mixed Sand

Medium Coarse SandGritty no SandGritsGritsPebbly Grits

Silty fine Sand

SECTION

10

20

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4050

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300

260 Pebbly GritsPlastic ClayPlastic ClayCleyey fine SandCleyey fine SandClay fine Sand

Clay fine SandGritty mixed Sand

Gritty mixed Sand

Gritty mixed Grits Grits

Sandy ClayPlastic Clay

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351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396

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K-type ( < > )

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pth

(m)

Resistivity (ohm - metre) x 10

ii

ii i

i

i Modelled curve ii Geoelectric section iii Lithology

Fig. 5: Modelled curve, geoelectric section and lit hology ofVES 110 near Okigwe Borehole.

SECTION SECTION

Red earthy Sand

Brownish Red earthy Sand

Gritty brownish Red earthy Sand Red coarse Sand

Reddish brown mediume Sand Reddish brown mediume Sand

Reddish brown coarse Sand

Reddish brown coarse Sand Brownish clayey coarse Sand

Brownish clayey coarse

Light Brownish clay Light Brownish clay

Brownish clay

Brownish white fine sand

Brown Clay Brown Clay Brack Clay Brack Clay Brack Clay

Brack Clay Brack Clay Brack Clay

Red earthy Sand

10

20260

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397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440

441 442

11

I Geoelectric Cross-Section (IGCS) along CDnterpretative profile line

Conductive Layer (31 - 580 )

(520 - 1300 )

Moderate High Resistivity Layer (3000 - 13000 )

Underlying Bed (1000 - 3000 )

6

(300 - 1500 )

Conductive Layer (31 - 580 )

(520 - 1300 )

Moderate High Resistivity Layer (3000 - 18000 )

Underlying Bed (1000 - 3000 )

Fig. :I Geoelectric Cross-Section (IGCS) along EFnterpretative profile line7

(300 - 1500 )

443 444

445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478

Comment [DTA35]: The IGCS looks not only disgusting but irritating especially with the colour selection. All geological considerations necessary for proper construction of cross sections seem to have been ignored while drawing these ones. The different resistivity ranges in the various colour zonations are supposed to be inserted into the IGCS. The major reason for suggesting this in my original comments was for both the authors and the readers to take a second but critical look at the wide range of the resistivity values in each colour band Some mind bugging questions that have been provoked include i. Is it geologically meaningful to group a lithologic unit with resistivity as low as 287 with 13,300 in an area with no significant geological and environmental changes? ii. How will the authors account for the difference in the resistivity values indicated in the original IGCS and the so-called ranges indicated in the LEGEND in the present form? Its like a foul play has been unleashed. A visual comparison of the ranges indicated and what it ought to be if properly transferred shows that values were either magnified or stepped down to fit some expected results in all the IGCS. What an unscientific way of data handling! I am quite convinced my dear authors cannot convincingly read what they wrote on both the vertical as well as the horizontal axes even when they are called upon to be judges in their own case.

12

8

Conductive Layer (31 - 580 )

(520 - 1300 )

Moderate High Resistivity Layer (3000 - 6000 )

(300 - 1500 )

479 480 481 482 483 484 485 486 487

488 489 490 491 492 493 494 495 496 497 498

499 500

501 502 503 504 505 506 507 508 509 510 511

(iii). The Ajali Sandstone/Shale aquifer (Zone D) covers the nNorthwestern part of the study area 512 in Okigwe LGA. This area falls within the Ajali Formation which consists of thick friable, poorly 513 sorted sandstones. The clay and shale members of this Fformation are thick which makes ground 514 water exploitation difficult (IWADA, 2002). Water is available through cracks and fissures. 515 516 Each zone is fairly homogenous in terms of geologic setting, hydraulic properties and possibly 517 water quality. This is because Niwas and Singhal (1981) showed that in areas of similar geologic 518 setting and water quality, the product, Kσ, remain fairly constant. This assumption has also been 519 adopted by Ekine and Iheonunekwu (2007) and Nwosu et al. (2011) in identifying the aquifer 520 systems in Mbaitoli and Isiala Mbano LGAs of Imo State respectively. 521 522 On the bases of longitudinal conductance and possible groundwater divides, four zones are 523 identified using the corresponding locations of Kσ product values (Fig. 10).The areas covered by 524 Zone D are underlain by relatively high resistive (low longitudinal conductance aquifer materials). 525 These areas may not be good prospects for drilling boreholes with high yield expectation (Mbonu 526 et al., 1990). The dividing lines between Zones A and C runs almost parallel to the channels of 527

Comment [DTA36]: Certainly not a finding from this study. How did you know?

Comment [DTA37]: Pls check

13

Nterere and Odioma Rivers that join to drain southwards into the Imo River. Similarly, the dividing 528 line between Zones A and B runs along the channels of Efuru River and Eze Rivers which are 529 tributaries of Imo River that terminate at Abadaba lLake. These lines indicate possible 530 groundwater divide. 531

532 4. CONCLUSION 533

On the basis of of the diagnostic conductivity product (Kσ) and longitudinal conductance values 534 distribution, three main aquifer systems has been identified for the study area. The resistivity of 535 the aquifer zones varies across the area with an average value of about 3,133 Ωm.The highest 536 thickness of 104.4 m of aquifer was recorded in the southern part of area.This work has 537 successfully delineated the aquifer systems in Okigwe Ddistrict. Based on the VES results, the 538 sSouthern and nNortheastern parts of the district are more promising for siting borehole with high 539 yield expectations than the northwestern North Western part as there is no well defined sand 540 body that constitute aquifer there. The occurrence of aquifer in this area is linked to the presence 541 of fracture in the shale members. 542 543

COMPETING INTERESTS 544 The Authors declare that there is no competing interest for this study. 545

14

(m)(m) (m) (m) (m) (m) (m)

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15

8

3

561

562

16

x10 -5 563

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F ig.9 : D is tr ib u tio n of El ec tr ic al Co nd uc ti v it y a nd H con du ct iv i ty Product

ydraulic(K ) of the Aquiferous Zone

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