Essay: Estimation of aquifer hydraulic characteristics using surface geo-sounding data

ABSTRACT
Detailed knowledge of the hydraulic characteristics of an aquifer system is very essential for the management of groundwater resources. Conventionally these characteristics are determined through the conventional pumping test method often carried out in water wells. However, there is a dearth of accurate and reliable pumping test data in the study area as carrying out pumping tests at a number of sites may be costly and time consuming. The application of geophysical methods in combination with pumping test using the Dar-zurrock parameters therefore provides a cost effective alternative to the estimation of hydraulic parameters from wells. For this reason, surface geosounding data have been used for evaluation of the hydraulic characteristics of the aquifer system of the Middle Imo River Basin, Southeastern Nigeria. Forty (40) Schlumberger Vertical Electrical Soundings (VES) were carried out in various parts of the study area using a maximum electrode separation of 1000metres. Four of the soundings were made at existing boreholes for parametric purposes. The VES data were processed using a combination of curve matching techniques and computer iterative modeling using the OFFIX software. Aquifer hydraulic characteristics were estimated using the concept of Dar-Zarrock parameters. Results of the study revealed that the hydraulic conductivity ranges from 1.08m/day to 37.04m/day in areas underlain by the Benin Formation; and 0.409m/day to 5.21m/day in areas underlain by the Ameki Formation. Transmissivity values on the other hand ranges from 61.9m2/day to 1558.4m2/day in areas underlain by Benin Formation: and 1.41m2/day to 413.89m2/day in areas underlain by the Ameki Formation.
Keywords: Hydraulic characteristics, geo-sounding data, Dar- zurrock parameters, aquifers, Nigeria,
INTRODUCTION
Over the years, access to safe drinking water has posed a major challenge to developing countries of the world. More than half of the rural populace in Nigeria owes their source of drinking water to untreated rain and surface water sources. Groundwater however remains the only realistic and affordable source of potable water across much of the country.
The sedimentary sequences of Southeastern Nigeria including those of the Imo River basin are known to contain several aquiferous units (Uma, 1989). However several boreholes in the area have become unproductive due to complexity in the geology of the area. Although numerous boreholes abound in the Imo River Basin, there has not been any systematic and comprehensive study to establish the nature and distribution of the aquifers beneath the basin (Uma, 1989).
An accurate groundwater resource assessment and a quantitative description of aquifers have become imperative in other to address several hydrological and hydro-geological problems associated with groundwater management. Fluid transmissivity, transverse resistance, longitudinal conductance, hydraulic conductivity and aquifer depth are fundamental properties describing subsurface hydrology. As a result, many investigation techniques are commonly employed with the aim of the estimation of spatial distribution of the above mentioned hydraulic parameters. Field estimations of the above parameters are not always available. Hydraulic conductivity appears to be the most problematic to obtain because of either the great range of observed values or the unsatisfactory laboratory measurements.Traditionally, one of the most effective ways of hydraulic conductivity calculation is the pumping tests obtained from wells. Nevertheless a probable sparse spatial distribution of the available boreholes gives rise to significant problems in modeling the hydrogeological systems. In such cases, drilling new boreholes has proved to be rather expensive as well as time consuming.
Geophysicists have realized that the integration of aquifer characteristics calculated from the boreholes locations and surface resistivity parameters extracted from surface resistivity measurements can be highly effective, since a correlation between hydraulic and electrical aquifer properties can be possible as both properties are related to the pore space structure and heterogeneity (Huntley, 1986; Boerner et al, 1996; Rubin and Hubbard, 2005; Niwas et al, 2006).Several authors have earlier attempted the estimation of aquifer hydraulic properties from surface electrical soundings (Kelly, 1977; Keller and Frischneichk, 1979; Niwas and Singhal, 1981). In the same vein several authors have successfully evaluated aquifer hydraulic characteristics from Dar Zarrock parameters using data obtained from Vertical electrical Resistivity sounding (Onuoha and Mbazi, 1988; Mbonu etal, 1991; Onu and Ibezim, 2004; Ekwe etal, 2006, Opara etal 2013).
This present study is centered on predicting the hydro-geophysical characteristics of the aquifer systems of the middle Imo River Basin South Eastern Nigeria which includes the aquifer geometry, hydraulic conductivity and transmissivity using direct electrical resistivity method. A total of Forty (40) Schlumberger Vertical Electrical Soundings (VES) were carried out in various parts of the study area using a maximum electrode separation of 1000metres. Four of the soundings were made at existing boreholes for comparison. Aquifer hydraulic characteristics were estimated using the concept of Dar-Zarrock parameters.
STUDY AREA
The study location lies between longitude 7o 17lE to 7o 34lE and latitude 5o 26lN to 5o 38lN. The area consists of parts of Obowo, Umuahia and Ihitte Uboma Local Government areas of South Eastern Nigeria (fig 1). The study area lies within the sub equatorial climatic belt characterized by two major seasons; the wet and dry seasons. Rainfall is high with an annual average of about 2200mm. Relative humidity is also high and generally over 70%. Mean annual temperature is about 27oC while the mean evaporation rate is 3.omm/day.
The area has low-lying to moderately high plain topography. The general elevation stands at about 152m above sea level (Mbonu et al 1991) with a general slope of about 0.0014 southward (Uma 1989). The area is drained by the Imo River and its tributaries which flows in a southern direction and empties into the Atlantic Ocean.
Fig 1: Topographic map of the study area.
The area is underlain by the Benin Formation (Miocene- Recent). The sediments of the Benin Formation consists of lenticular, unconsolidated coarse to medium-fine grained sands and clayey shales (Simpson, 1955). The sands are generally moderate, sorted, poorly cemented and angular in shape (Mbonu et al, 1991). The Benin Formation overlies the Ameki Formation and dips southwest ward (fig2).
The Ameki Formation (Eocene-Oligocene) consists of medium to coarse grained white sandstone, bluish calcareous silt with mottled clay, thin limestone and abundant calcareous shale.
Fig 2: Geologic map of the study area
DATA ACQUISITION AND PROCESSING
Geophysical data acquisition was carried out using the Vertical Electrical Sounding (VES). The Schlumberger electrode configuration was adopted in the sounding. In the Schlumberger array, the current and potential and potential pairs of electrode have a common midpoint but the distances between adjacent electrodes differ.
Theoretically, the Resistivity (??) of a material is directly proportionally to the Resistance (R) of the material to current flow. Thus;
?? =KR —————————————————1
where K is the Geometric factor and is obtained thus;
K = —————————————–2
a and b being half current electrode spacing and potential electrode spacing respectively.
A total of 40 vertical electrical soundings with a maximum current electrode separation (AB) of 1000m was acquired (fig 3). The ABEM SAS 4000 Terrameter was used to obtain the VES data. The observed data was converted to apparent resistivity values by multiplying with the Schlumberger Geometric factor K such that:
(Keller and Frischnechk).——————-3
Modeling of the VES data was done using the OFFIX software. Analysis of the resulting apparent Resistivity versus half current electrode separations yielded earth models composed of individual layers of specified thickness and apparent resistivity.
2.2 Aquifer Hydraulic Characteristics from Geoelectrical data.
The Dar-Zarrock parameters of transverse resistance and longitudinal conductance have been found useful to the estimation of aquifer hydraulic characteristics. Niwas and Singhal (1981) established an analytical relationship between transmissivity and transverse resistance on one hand and between transmissivity and longitudinal conductance on the other hand as follows:
KS/??————————————————–4
Where T= aquifer Transmissivity, K= hydraulic Conductivity, ?? = Electrical Conductivity (inverse of resistivity), R = Transverse Resistance = h??, S = Longitudinal Conductance =h/??, h =aquifer thickness,
and ?? = aquifer resistivity.
In areas of similar geologic setting and water quality, the product K?? remains fairly constant (Niwas and Singhal 1981). Thus knowledge of K from existing boreholes and ?? from VES data can be used to estimate K?? values for areas without boreholes. This relationship was applied in this study.
Fig 3: VES location map of the study area
PRESENTATION OF RESULTS
Results of the estimated aquifer parameters as deduced from the sounding interpretations are shown in table 1
VES NO Depth to water table (m) Aquifer Thickness h (m) Apparent Resistivity ?? (ohm-m) Aquifer Conductivity
Transverse Resistance
( ?? x l) Longitudinal Conductance
(l/ ??) K Values from wells K
Value Hydraulic Conductivity
(m/day) Transmissivity
(m2/day) Storativity
1. 56.2 41.7 4640 0.000216 193488 0.00899 37.04 1547.9 1.25E-04
2. 0.8 2.8 191 0.00524 534.8 0.01466 0.503 1.410 8.4E-06
3. 26.7 79.3 1980 0.00051 157014 0.0401 5.36 0.00271 5.21 413.89 2.38E-04
4 34.9 18.6 1210 0.00083 22506 0.01537 9.68 0.00799 9.64 179.3 5.58E-05
5. 10.1 12.6 1120 0.00089 14112 0.01125 8.99 113.3 3.78E-05
6 7.1 42.2 1080 0.00093 45576 0.03907 2.77 0.00256 2.85 120.14 1.27E-04
7 11.4 64.2 735 0.0014 44187 0.08735 5.71 366.6 1.93E-04
8 19.7 57.9 774 0.0013 44814.6 0.07481 2.04 118.13 1.74E-04
9 90.5 108.5 207 0.0048 22459.5 0.5242 1.67 181.2 3.26E-04
10 68.2 49.8 636 0.00157 31672.8 0.0783 5.09 235.5 1.47E-04
11 68 65 155 0.00645 10075 0. 4193 0.409 26.56 1.95E-04
12 55 63 206 0.0048 12978 0.3058 1.67 105.2 1.89E-04
13 0.6 3.2 644 0.0016 2060.8 0.004969 1.698 3.50 9.6E-06
14 31.1 36.1 876 0.0011 17183.6 0.07584 8.24 0.0173 7.27 262.4 108E-04
15 24.7 25.9 1730 0.00058 44807 0.01497 4.56 118.11 7.77E-05
16 10.6 12.6 1110 0.0009 13986 0.01135 2.93 36.87 3.78E-05
17 47.9 15.1 734 0.001362 11083.4 0.02057 5.88 88.79 4.53E-05
18 42.7 17.7 920 0.00109
16284 0.01924 6.11 0.00666 7.33 129.7 5.31E-05
19 37.5 57.3 136 0.0074 7792.8 0.4213 1.08 61.9 1.72E-04
20 56.7 64.3 1710 0.00058 109953 0.0376 13.79 886.7 1.92E-04
21 75.4 39.6 900 0.0011 35640 0.044 7.27 287.9 1.19E-04
22 39.2 21.9 1220 0.00082 26718 0.01795 9.75 213.5 6.57E-05
23 38 66 345 0.0029 22770 0.1913 1.72 0.00499 2.75 181.5 1.96E-04
24 90.8 126.2 175 0.0057 22085 0.7211 0.5 0.00285 1.4 176.7 3.79E-04
25 1.1 6.9 202 0.0049 1393.8 0.03415 0.53 3.674 2.07E-05
26 24.1 99.9 457 0.0022 45654.3 0.2186 3.64 363.6 3.0E-04
27 0.7 1.7 980 0.00102 1666 0.001737 2.583 4.39 5.1E-06
28 6.7 15.5 254 0.004 3937 0.061 0.67 10.37 4.65E-05
29 91.8 34.2 2270 0.00044 77634 0.01507 18.2 622.4 1.03E-04
30 82.1 74.9 194 0.0051 14530.6 0.3861 1.57 117.6 2.25E-04
31 80.1 21.9 4210 0.00024 92199 0.005202 33.3 729.3 6.57E-05
32 45.9 46.8 4190 0.00024 196092 0.01117 33.3 1558.4 1.4E-04
33 50 71.5 1620 0.00062 115830 0.04414 12.9 922.3 2.15E-04
34 46.6 20.4 1280 0.00078 26112 0.01594 10.26 209.3 6.12E-05
35 46.7 25.2 1730 0.00058 43596 0.01457 13.79 347.5 7.56E-05
36 61.6 39.4 2100 0.00048 82740 0.01876 16.67 656.8 1.18E-04
37 94.5 22.5 1980 0.00051 44550 0.01136 5.22 117.43 6.75E-05
38 40.3 76.7 1250 0.0008 95875 0.06136 3.30 252.73 2.3E-05
39 1.8 2.0 850 0.0012 1700 0.002353 2,24 4.48 6.0E-06
40 12.6 8.5 752 0.0013 6392 0.0113 1.982 16.85 2.55E-05
Table 1 : Summary of the aquifer Hydraulic parameters interpreted from the Geo-electric section
Aquifer Resistivity in the study area
The apparent resistivity across the study area was deduced from the geo electric sounding. The apparent resistivity of the study area indicates the western axis which comprises towns in Obowo having the highest resistivity of 4640’m. This corresponds to the region having red colors. The lowest resistivity is observed in the Eastern axis comprising areas such as Umuire Ibeku having the least value of 191’m. This corresponds to the region colored grey to blue.
Fig 4: Contour map of apparent resistivity of the study area
Aquifer depth and Thickness of the study area
The map of Depth to water table shows varying distribution of aquifer depth. Shallow aquifer (blue color) trend the North-East and South-Eastern axis of the study area with the Ibeku areas having the least values with depth as low as 0.6m. Deeper aquifer is distributed along the central part of the study area with Umukabia having the deepest aquifer with depth of 91.8m.
The isopach map indicates the distribution aquifer of variable thickness in the study area. The central zone and the North Eastern axis appear to have the thickest aquifer with Ezeleke Umuekwule having the highest thickness of 108m. Areas within Ibeku have the thinnest with thickness as low as 1.7m in Ukomo Ibeku.
Fig 5a: Contour map of depth to water table across study area
Fig 5b: 3-D Map of Depth to water table (m) across study area
Fig 6: Contour map of Aquifer Thickness across study area
Longitudinal conductance and Transverse resistance
The Longitudinal Conductance was estimated by dividing the Aquifer Thickness by the Aquifer Apparent Resistivity. The distribution of the longitudinal conductance across the study area indicates maximum values across the central part of the study area. Lower values were distributed on the other remaining parts of the study area.
The Transverse Resistance across the study area was estimated by taking the product of Aquifer Apparent Resistance and Aquifer Thickness. The distribution of the Transverse Resistance shows and increasing trend from South-East to the North-West of the study area as indicated from grey to red coloration. The minimum value of Transverse Resistance is about 534’m2 near Umuire Ibeku while the maximum value is about 196092’m2 near Umulogho Obowo.
Fig 7: Contour map showing distribution of longitudinal conductance across study area
Fig 8: Contour map of Transverse Resistance across study area
Aquifer Conductivity
The Eastern axis of the study area having Umuire Ibeku and Okwoyi Ibeku has the maximum aquifer conductivity 0.0048mhos. This corresponds to the red color as shown in the plot in figure 9. Low conductivity values are seen distributed within the western and southern axes of the study area. Minimum conductivity of 0.000216mhos was recorded in Umulogho. This corresponds to the grey color as shown in the plot.
Fig 9: contour map of aquifer conductivity (mhos) of the study area
Hydraulic Conductivity
The Hydraulic Conductivity (K) describes the ability of a material to conduct fluids under a unit hydraulic gradient (Fetter, 1994). In this study the Hydraulic Conductivity was estimated from the product of the diagnostic constant and the Aquifer Apparent Resistivity. The map indicates the areas within the Eastern part of the study area which falls under the Ameki Formation exhibiting low conductivity ranging from 0.4m/day to 5.2m/day with an average of 2.44m/day. The Western part of the study area which falls under the Benin Formation exhibits high Hydraulic Conductivity ranging from 1.4m/day to 37.04m/day with an average value of 10.8m/day. The areas in the map with blue to purple color may be bad prospect for groundwater because of its low conductivity. Other areas colored green to red may be good prospect for groundwater because they contain conductive aquifers.
Fig 10: Contour map of the study area showing hydraulic conductivity
Aquifer Transmissivity
Aquifer Transmissivity measured m2/day is a product of Hydraulic Conductivity and thickness of an Aquifer unit. With hydraulic Conductivity K estimated from pumping tests in boreholes, it is possible to estimate the transmissivity of the aquiferous units and its variation from place to place, including areas with no available boreholes. However in this work, the transmissivity was estimated by taking the product of the Dar-Zurrock parameter of transverse resistance and the diagnostic constant K??.
The distribution of aquifer transmissivity within the study area indicates that areas within the Eastern axis of the study area have low transmissivity (indicated by the purple to blue color on the map). These areas fall within the Ameki Formation and comprise the Ibeku towns as wells Etiti and Abueke. Transmissivity value within this zone ranges from 1.4m2/day to 413.9m2/day with an average value of 83.2m2/day. Aquifer Transmissivity within the Western and central axes are higher than the Eastern axis. These areas fall within the Benin Formation. Transmissivity values in these axes ranges from 105.2m2/day to 1558.4m2/day with an average value of 422.5m2/day. Based on the color codes on the map, areas that falls within purple to blue is expected to be bad prospect for groundwater while the other areas may be good prospect for groundwater.
Fig 11: Contour map of the study area showing aquifer Transmissivity
Table 2: Statistical results based on VES of the study area
Hydraulic conductivity m/day Transmissivity m2/day Formation
Range
35.96 1496.5 Benin
4.811 412.48 Ameki
Mean
10.798 422.53 Benin
2.448 83.236 Ameki
Variance
99.562 169478.71 Benin
2.4453 20377.54 Ameki
Standard Deviation
9.978 411.678 Benin
1.564 142.75 Ameki
Table 3: Statistical results showing correlation of corresponding VES points and pumping test of the study area.
PARAMETER CORRELATION
COEFFICIENT , r REMARKS
Hydraulic conductivity (m/day) 0.9776 Dependable relationship
Tranmissivity (m2/day) 0.3885 Slight relationship
DISCUSSION AND CONCLUSION
The result of the study clearly indicates the variation in aquifer thickness within the study area. The aquifer is thickest around Nkata, Umuekwule and Matei Dei Cathedral; and thinnest around Ukomo Ibeku and Ajata Ibeku. Depth to aquifer ranges from 0.6m in Ajata Ibeku to 94.5m in Abueke Uboma.
Conductivity values vary between 1.08m/day and 37.04m/day in areas underlain by the Benin Formation and 0.409m/day and 5.21m/day in areas underlain by Ameki Formation. Transmissivity values ranges from 61.9m2/day to 1558.4m2/day across areas underlain by the Benin Formation; and 1.41m2/day to 413.89m2/day across the Ameki Formation. This results indicates that areas underlain the Benin Formation hold more potentials for groundwater than areas underlain the Ameki Formation. The result of the aquifer hydraulic parameters also suggests the possibility of areas around Ibeku to pose problems for groundwater exploration and production as the transmissivity values recorded against these areas suggest poor groundwater potentials. On the contrary Umulogho Obowo appears the most productive within the study area.
The results of study have also helped to delineate the aquiferous zone within the study area. Data from the Iso-resistivity models reveals a multistory aquiferous system. This is in close agreement with work from previous authors (Uma, 1989; Mbonu et al, 1991). Information from the iso-resistivity models also revealed that the study area is divided into two distinct and distinguishable zones. The western axis of the study area is homogeneous in terms of hydraulic properties and distinct from other parts of the study area. This is in line with the geology of the area which concludes that the study area is underlain by Ameki and Benin Formations.
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