Evaluation of aquifer hydraulic characteristics using geoelectrical sounding, pumping and laboratory tests: A case study of Lokoja and Patti Formations, Southern Bida Basin, Nigeria

2.1 Geology of the area

The study area lies within the Mid-Niger or Nupe Basin. The stratigraphic succession of the Mid-Niger Basin, collectively referred to as the Nupe Group [30] comprises of Northern Bida Basin (Sub-Basin) and Southern Bida Basin or Lokoja Sub-basin (Figure 2).

Figure 2

Stratigraphic successions in the Mid-Niger Basin [Redrawn from [6].

Stratigraphically, in the Southern Bida Basin, the Campanian – Maastrichtian Lokoja Formation non-conformably overlies the Pre-Cambrian to Lower Paleozoic Basement gneisses and schists. This is overlain by the Maastrichtian Patti Formation and succeeded by the Maastrichtian Agbaja Ironstone Formation. The Lokoja and Patti Formations constitute the focus of this study.

2.2 Patti Formation

The lithologic units within Patti Formation include sandstones, siltstones, claystones and shales interbedded with bioturbated ironstones [7]. The sandstone unit of this formation is more mineralogically mature compared to the Lokoja Formation in the southern Bida Basin [31]. There is predominance of argillaceous units especially siltstone, shales and claystones in this Formation [32]. The water-bearing bodies of interest are therefore the sandstone units and where the clays or siltstone are with a major amount of sand (i.e. clayey sand). However, where shale or clay is the only geo-material or has minor amount of sand (e.g. sandy clay), it is not of primary interest. The outcrops of this Formation are exposed within Koton-Karfe and Abaji [6] which are largely the northern part of the study area (Figure 1). Thus study localities (Akpogu,Gegu Beki, Gegu Ega, Etegi, Girinya, Abaji, Koton-karfi, Ozi, Ozahi) are underlain by Patti Formation and also have similar geology. [33] noted that the coarser Lokoja Formation becomes more and more buried and the finer members of the Patti Formation becomes more exposed as one moves into the basin. Also, the work of [7] shows that argillaceous (fine grained) sediments predominate in the central parts of the basin. Thus it is logical to say there is a general finning upward and coarsening downward of the rock units. Generally, the rock units of Patti and Lokoja Formations have different grain sizes, porosity and permeability [26]. Hence, making it possible to unravel the hydraulic characteristics by the use of geophysical, pumping test and laboratory methods.

3.1 Pumping Test

A total of five (5) pumping test data were used in this study. The Pumping tests data used in this study were carried out by self and some were obtained from Lower Niger River Basin Authority, Ilorin, Kwara State and Kogi State Water Board, Lokoja Nigeria. Three (3) of the pumping tests were carried out in Felele, Odah and Okofi penetrating the Lokoja Formation while the remaining two (2) were carried out at Akpogu town and Ozahi, penetrating the Patti Formation (Figure 1). The pumping test data were evaluated using constant pumping rate discharge method. The pumping rates were within 0.2 l/ sec – 2 l/ sec, and the time of pumping varies from 1 – 6 hrs. The principle of a pumping test involves the application of a stress to an aquifer by extracting groundwater from a pumping well and measuring the aquifer response to that stress by monitoring drawdown as a function of time. The obtained pumping test data were incorporated into an appropriate well flow equation (i.e. Theis recovery formula, Jacobs-Cooper drawdown-time formula) to determine in-situ aquifer characteristics. The transmissivity of the aquifers from pumping tests were determined by the Theis recovery method and Jacob Cooper’s drawdown method [34, 35].

For Jacob Cooper’s method,

While for Theis recovery method,

Where Q is discharge rate in m³/day, Δs’= change in the residual drawdown in metre per log cycle of t/t’ (t is time since pumping started and t’ is time since pumping stopped). Change in residual draw down is actually the difference between the drawdown component due to continuous pumping and the recovery component due to recharge. The S’ is plotted on the arithmetic scale of the y-axis, the t/t’ is plotted on the logarithmic scale of the x-axis.

3.2 Geophysical investigation

A total of twenty (20) Vertical Electrical Soundings using Schlumberger configuration were carried out at nineteen (19) different localities within the area. The AB/2 (half-current spacing) was between 40 m to 80 m, because of the knowledge of borehole depths within the area. The ABEM Terrameter Geo-Pulse was used which performs automatic recording of both voltage and current, stacks the results, computes the resistance in real time and digitally displays it. Apparent resistivity values were obtained by multiplying resistance with appropriate geometric factor. The values were plotted against half electrode spacing on logarithmic coordinate to obtain the sounding curves from which, geo-electric layers, resistivities and thicknesses and depths of the layers were determined using IPI2WIN software. The resulting curves generated from the software produced a low Root Mean Square Error of not more than an approximate 1%. From the curve, resistivity and thickness and depth to water table were obtained.

3.3 Dar-Zarrouk Parameters

The term Dar-Zarrouk parameters (Transverse resistance (R) and the longitudinal conductance (S)) were first used by [36]. These parameters are derived from layer resistivity and thickness obtained during VES survey. [37] derived the analytical relationship between aquifer transmissivity and transverse resistance, as well as the relationship between transmissivity and longitudinal conductance. The estimation of aquifer parameters – transmissivity and hydraulic conductivity from a pumping test is time consuming and expensive especially when it involves performing a number of them in an area. The geophysical method (VES) has been proven by many authors to be an efficient, cost effective and less time consuming alternative. In this study, attempt has been made to estimate aquifer parameters (transmissivity and hydraulic conductivity) using Dar-Zarrouk parameter (Transverse Resistance) from VES. The relationship between transmissivity values from five (5) pumping test (at Felele, Odah, Okofi, Akpogu and Ozahi) and transverse unit resistance was derived by performing a regression analysis between both parameters (Table 1 and Figure 3). The method used is called “grand mean centering”. This was used because it helps to avoid negative constant term when the negativity does not make interpretable sense. When this method is used, it does not alter the estimates of the other independent variables. The relationship derived between transmissivity from pumping test and transverse resistance is given as: T = 13.39 + 0.06R. Where R is transverse unit resistance measured in ohm-m² and T is transmissivity measured in m²/day.

Figure 3

Relationship between pumping test transmissivity (TM) and Transverse Resistance (TR) (from mean centering).

Using this relationship, transmissivity is calculated for the 20 VES points in the area of study. Values of hydraulic conductivity (K_p) and transmissivity (T_p) gotten from locations where pumping test results were correlated with the values of hydraulic conductivity (K_c) and T_c (Transmissivity) from Dar-Zarrouk parameter to see whether they are in strong agreement (strongly correlated) or weak agreement (weakly correlated).

3.4 Laboratory method (Constant and Falling head permeameters)

Twenty-One (21) samples extracted from aquiferous units of surface outcrops belonging to Lokoja and Patti Formations were subjected to Constant Head and Falling Head Permeameter test in order to determine the coefficient of permeability (hydraulic conductivity). The samples were named after the localities from which they have been taken. For twelve samples extracted from aquiferous units belonging to Lokoja Formation, the samples are labeled Ohono (1, 2 and 3), Agbaja Hill (1, 2 and 3), Jamata, Before Niger Bridge, Felele (1 and 2), Okumi and Mount Patti 1. While for nine samples extracted from aquiferous units belonging to Patti Formation, the samples are labeled Agbaja Hill (4, 5 and 6),Mount Patti (2 and 3), Ozi, Gegu Beki, Girinya and Gegu Ega. Five (5) coarse grained sandy samples belonging to the Lokoja Formation were analyzed using the Constant head, while sixteen (16) samples which are the fine grained sandy fractions from Lokoja Formation and the Patti Formations were analyzed using Falling head permeameter. The hydraulic conductivity values (coefficient of permeability) were obtained using the Darcy’s law of liquid flow.

3.4.1 Procedure for the Constant Head Permeameter

The samples were placed in between porous stone in a cylindrical jar (Figure 4). The water supply at the inlet was adjusted such that the difference of head between the inlet and outlet remains steady. The setting was left in this condition for minimum of 24 hours to establish a constant rate of flow. The water which flowed through the sample during a known duration was collected in a graduated flask.

Figure 4

Set-up for Constant head permeability test.

The procedure was repeated for other samples and the coefficient of permeability for each of the sample was calculated using the relationship given below.

Coefficient of Permeability,

(3)K=QLAht

Where, Q = Total quantity of water flowing through in elapsed time (cm³), L = Length of sample in the Permeameter (cm), A = Area of cross- section of the sample (cm²), h = Hydraulic head (cm).

(4)K=cmsec100=1cm×1sec×86400=m/day

3.4.2 Procedure for the Falling Head Permeameter

For the fine sands and clays, the constant head permeability test shown in Figure 5 is not suitable, because only very small quantity of fluid flows through the soil, and it would take very long time to collect an appreciable volume of water. For such soils a test set up as illustrated in Figure 6 is more suitable.

Figure 5

Falling head Permeability test.

Figure 6

Pumping Test Graph for Felele (Pumping rate (Q) is 2 l/s = 172.8 m3/day).

In the apparatus, a sample is enclosed by a small circular ring, placed in a container filled with water. The lower end of the sample is in open connection with the water in the container, through a porous stone below the sample. At the top of the sample it is connected to a thin glass tube, in which the water level is higher than the constant water level in the container. Because of this difference in water level, water will flow through the sample, in very small quantities, but sufficient to be observed by the lowering of the water level in the thin tube. In this case the head difference h is not constant, because no water is added to the system, and the level h is gradually reduced. This water level is observed as a function of time. The procedure for calculation of hydraulic conductivity is given by:

(5)K=2.3026aLA×Log10H1Log10H2t2−t1

Where a, is the area of stand pipe tube.

(6)a=πd24=π(3)24=

L= Length of sample = 130 mm

A = Cross Sectional Area of the sample =πd24=3.142(100)24=7857.143 mm2

k=K1+K22;mm/secK=1mm1000×1sec×86400=m/day

4.1 Vertical Electrical Sounding (VES)

The results of VES for areas underlain by Lokoja and Patti Formations revealed 4-5 geo-electrical layers (Table 1-5). The resistivity and thickness values of the first layer varies from 9.5-2303 ohm-m and 0.82-529 m, respectively,which is diagnostic of the top soil of variable composition. The resistivity values of the second layer are within 2.5-637 ohmm with thickness values between 0.19-8.38 m. The resistivity and thickness values of the third layer are within 5.273110 ohm-m and 2.59-15.7 m, respectively. The fourth layer has resistivity and thickness values varying from 3.86-1677 ohm-m and 8.19-32.3 m, respectively. The fifth layer has resistivity value of 14.9-513 ohm-m and thickness values from 7.37-13.4 m. Each geo-electrical layer has geological and hydrogeological significance. The first is the top soil, the second is clay or shale in the case of Patti Formation, and this is an aquiclude (impermeable layer). The third layer is sandy clay which is not desirable for optimum groundwater yield. The fourth layer is clayey sand which in many places has significant groundwater yield depending on the thickness. The fifth layer is the sand which is significant for optimum groundwater yield. The fourth or the fifth layer constitute the target layer for groundwater potential evaluation. The interpretation generally agrees with the sedimentological characteristics of the rocks in the basin with finning upward or coarsening downward sequence.

4.2 Depths to water table and aquifer thickness

The depths to water table for Lokoja Formation are within 5.91 – 40.8 m, while those of Patti Formation fall within 13.7 – 40.3 m (Table 8). The variation in depths to water table can be attributed to various factors, one of such is differences in elevation. The increase in depths to water table towards the Lokoja area may be due to higher concentration of groundwater exploitation because of higher population as being the capital of the Kogi state. The thicknesses of aquiferous units of Lokoja Formation range from 7.37 – 27.3 m, while those of Patti Formation range between 10.8 – 20.1 m (Table 7). The higher values of aquiferous units of Lokoja Formation over Patti Formation support the findings of [7, 31, 33] that there is the predominance of argillaceous rocks (siltstones, shales and claystones) in the Patti Formation.

4.3 Aquifer characteristics from VES

The pumping test result (Figures 678910), calculated hydraulic conductivity and transmissivity results are presented in Tables 9 and 10, for Lokoja and Patti Formations. The hydraulic conductivity (K) values of aquiferous units of Lokoja Formation using Dar-Zarrouk parameter range from 1.92-91.7 m/day, whereas those of the Patti Formation varies between 2.15-31.8 m/ day. Transmissivity values for Lokoja Formation from Dar-Zarrouk parameter range from 24.97-2117 m²/day, while those of Patti Formation range from 27.9-456.91 m²/day.

Figure 7

Pumping Test Graph for Odah (Pumping rate (Q) is 0.2 l/s = 17.28 m3/day).

Figure 8

Pumping Test Graph for Okofi (Pumping rate (Q) is 0.48 l/sec. = 41.47 m3/day).

Figure 9

Pumping Test Graph of Akpogu (Pumping rate (Q) is 0.38 l/s = 32.382 m3/day).

4.3.1 Hydraulic conductivity-K and Transmissivity-T from VES

Based on the hydraulic conductivity classification scheme of [26], the hydraulic conductivities obtained for the aquifers in both formations are in the range of Permeable to High (Figure 11a). These results are slightly different from that obtained by [26] for the same formations within the Southern Bida Basin because of differences in methodology adopted. While this study adopts the use of Dar-Zarrouk Parameter, their study adopted the use of empirical relationship of Shepherd from grain size distribution. Their results gave permeable to high range for Lokoja Formation and within permeable range for Patti Formation (Figure 11b). However, because of higher thickness values of the aquifers of Lokoja Formation, they have higher transmissivity than those of Patti Formation, since it is the product of hydraulic conductivity (K) and thickness (h), thus, making the aquifers of Lokoja Formation more prolific for groundwater exploration than those of Patti Formation (Table 11). Based on the transmissivity classification scheme of [38], the transmissivity values of the aquifers of Lokoja Formation are within Low to High, while those of Patti Formation fall within Low to Moderate potential (Table 12).

Figure 10

Pumping Test Graph of Ozahi (Pumping rate (Q) is 0.9 l/sec = 78.6 m3/day).

Figure 11

Evaluation of hydraulic conductivity of aquiferous units of: (a) Lokoja Formation and Patti Formation (current study). b) Lokoja and Patti Formation [26].

Table 9

Pumping Test Result of different locations within Lokoja and Patti Formations.

Lokoja Formation					Patti Formation
Felele		Odah		Okofi		Akpogu		Ozahi
Time ratio t/t’ (mins)	Residual Drawdown s’ (m)	Time (mins)	Drawdown, s (m)	Time (mins)	Drawdown, s (m)	Time ratio t/t’ (mins)	Residual Drawdown s’ (m)	Time (mins)	Drawdown, s (m)
1.5	0	4	7	5	6	2	0	4	1
2.5	0.05	10	12	10	10.9	2.3	0.08	10	1.5
6	0.1	20	15	20	13.9	2.86	0.08	20	4.8
12	0.2	40	17	40	16.9	3.6	0.12	40	7.8
30	0.2	60	19.2	60	18	5.3	0.15	60	8
40	0.2	75	20.5	75	19	7.5	0.25	75	19.8
50	0.2	90	21.2	90	21.4	14	0.12	90	20.4
60	0.4	120	21.5	120	21.8	17.25	0.19	120	20.4
80	0.4	125	22	125	23.8	33.5	0.53	150	21
110	0.8	150	22.2	150	25	66	0.77	180	21
140	1.25	180	22.8	180	25.4	2	0	210	21
160	1.9	210	22.8	210	25.4	2.3	0.08	250	21
180	2.8	225	22.8	250	25.4	2.86	0.08	260	21
		250	22.8	260	25.4			300	21
		300	22.8	300	25.4			360
		320	22.8	320	25.4
		350	22.8	350	25.4

Table 10

Hydraulic conductivity and Transmissivity from Dar-Zarrouk parameter.

VES Location	p(Ωm)	h(m)	R=hp (Ωm2)	Tc = 13.39 + 0.06R (m2/d)	Kc = Tc/h (m/d)	*Tp	*Kp
Lokoja Formation
*Felele	134	7.37	987.58	72.64	9.86	50.18	6.7
Karara	81.9	18.6	1523.34	104.79	5.6
Okumi	112	17.4	1948.8	130.32	7.5
Nataco	316	18.1	5719.6	356.57	19.7
Banda	135	27.5	3712.5	236.14	8.6
Edeha	1518	23.1	35065.8	2117.34	91.7
Akpaya- Gbayi	50.5	18.6	939.3	69.75	3.8
*Odah	14.9	13	193.7	24.97	1.92	0.27	0.023
Jamata	75.9	16.9	1282.71	90.35	5.3
*Okofi	16.8	12.5	210	25.99	2.08	0.54	0.045
Patti Formation
*Akpogu	50.4	13	655.2	52.70	4.1	15.0	1.51
Girinya	420	17.6	7392	456.91	26.0
Abaji-Naharati	513	13.4	6874.2	425.84	31.8
Ozi	149	11.5	1713.5	116.2	10.1
Koton-karfe Market Sq	98.3	10.8	1061.64	77.09	7.1
Koton-karfe LGEA	149	16.5	2458.5	160.9	9.8
*Ozahi	18.6	13	241.8	27.90	2.15	1.11	0.09
Adabo	142	13	1846	124.15	9.6
Gegu	149	11.5	1713.5	116.2	10.1
Etegi	316	14.4	4550.4	286.4	19.9

1. *Tp and Kp, Tc and Kc are transmissivity and hydraulic conductivity values derived from pumping test and Dar-Zarrouk Parameter respectively.

Table 11

Comparison of the findings of [26] study of the hydraulic characteristics of the Lokoja and Patti Formations with this present study.

Results from [26]	This study
The hydraulic conductivity of Lokoja Formation is within permeable to high range	The hydraulic conductivity of Lokoja Formation is within Permeable to High range
The hydraulic conductivity of Patti Formation is within permeable range	The hydraulic conductivity of Patti Formation is within Permeable to High range
Lokoja Formation is more prolific for groundwater exploration than Patti Formation	Lokoja Formation is more prolific for groundwater exploration than Patti Formation because of higher aquifer thickness
Deeper Lokoja Formation should be concentrated upon for Groundwater exploration	Lokoja and Patti Formations may be concentrated upon for Groundwater exploration depending on the scale of intended purpose

Table 12

Aquifer Transmissivity Potential for Lokoja and Patti Formations [After, [38]].

Lokoja Formation			Patti Formation
Transmissivity			Transmissivity
VES Location	values (m2/day)	Potential	Locality	values (m2/day)	Potential
Felele	72.64	Moderate	Akpogu	52.70	Moderate
Karara	104.79	Moderate	Girinya	456.91	Moderate
Okumi	130.32	Moderate	Abaji- Naharati	425.84	Moderate
Nataco	356.57	Moderate	Ozi	116.2	Moderate
Banda	236.14	Moderate	Kotonkarfe Market Sqr. Kotonkarfe	77.09	Moderate
Edeha	2117.34	High	LGEA	160.9	Moderate
Odah	24.97	Low	Ozahi	27.9	Low
Jamata	90.35	Moderate	Adabo	124.15	Moderate
Okofi Akpaya-	25.99	Low	Gegu	116.2	Moderate
Gbayi	69.75	Moderate	Etegi	286.4	Moderate

Based on the classification and potentiality scheme proposed by [39], it can be deduced that areas of low, moderate and aquifer potential can be targets for groundwater exploration depending on the scale/magnitude of intended purpose (Tables 12, 13 and 14).

There is a strong correlation between the hydraulic conductivity calculated from Dar-Zarrouk parameter (r-squared = 0.99) and Pumping test and between the calculated Transmissivity derived from Dar-Zarrouk Parameter and Pumping test (R-square = 0.92) in Figures 12 and 13, respectively. [28] study of the complex porous aquifer system in Anthemountas Basin, Northern Greece derived a strong correlation (r = 0.708) between hydraulic conductivity values from Dar-Zarrouk parameter and pumping tests. Also, [29] in Khanewal District derived a strong correlation coefficient of 0.9 between modelled and measured hydraulic conductivities. [27] study in Northwest Bangladesh found an error or differences of 26% between calculated transmissivity and pumping tests for 15 sites. Due to higher hydraulic conductivities and transmissivity values of the Aquifers of Lokoja Formation over the Patti, they are more prolific for ground water exploration than those of Patti Formation, corroborating [26] study.

4.4 Coefficient of Permeability (Hydraulic conductivity) from Laboratory test

The results of coefficient of permeability tests using Falling head and Constant head permeameters for 21 extracted aquiferous units from surface outcrops belonging to Lokoja and Patti Formations are presented in Table 16, and are interpreted using [26] hydraulic conductivity classification. The hydraulic conductivity values for aquiferous units of Lokoja Formation are in the range of 10⁻² - 10² m/day,while those of Patti Formation range from 10⁻²-100m/day. Aquifers of Lokoja Formation thus have K values ranging from low to high range, whereas those of Patti Formation are within low to permeable range. It should be noted however that these results are for extracted aquiferous units of surface outcrops and not from subsurface. Whereas the VES results was used to estimate the result of coefficient of permeability (hydraulic conductivity) of subsurface shallow aquifers, the Constant head and Falling head permeability tests were carried out to estimate the coefficient of permeability of extracted aquiferous units from surface outcrops. Hence it is expected that there may be differences in the range of results of coefficient of permeability.