Electrical resistivity method was conducted at Kahe-Mtakuja basin aimed at appraising the potential of the basin as a source of groundwater by establishing shallow stratigraphy and delineating aquifer formations. A total of fifty-eight vertical electrical sounding data (VES) were acquired using Schlumberger array and the data were analyzed to obtain apparent resistivity and layer depth. The interpretation of resistivity data revealed three main geoelectric layers. The first layer has resistivity values ranging from 40 to 230 Ωm with thickness ranging from 0.4 to 2 m. The second layer has resistivity values in the range of 2 to 10 Ωm and thickness of 2 to 25 m. The third layer has slightly high resistivity values ranging from 10 to 60 Ωm and thickness in the range of 30 to 70 m. This layer is mainly dominated with sand. The resistivity cross-sections constructed from the interpretation of VES data indicate that the Kahe-Mtakuja basin has shallow stratigraphy consisting of 3 layers. The layers are composed mainly of top red soil, clay (sometime alternating with sand) and sand formation holding the groundwater. These findings are consistent with lithological logs of the borehole drilled near Kahe-Mtakuja that indicate two to five layers composed of alluvial deposits alternating with different lithological thicknesses. The high correlation between the VES results and borehole lithological logs near Kahe-Mtakuja suggests that the area is potential for aquiferous formation. Based on constructed stratigraphy, the aquifer formation of the basin is found in alluvial deposits composed of mainly sand. The potentiality of this area for aquiferous formation is vital for providing additional baseline data on the aquifer characteristics and will assist in reducing water scarce in the area.
Groundwater is an important and limited resource in many parts of Tanzania and the world at large, and is approximately 50 to 70 times more plentiful than surface water [
The lack of required data in various parts of Tanzania has hindered thorough study of the hydrogeology leading to lack of ground water resources quantification and owing to that existing borehole data have been the only information available in many areas [
Due to limited extensive studies on groundwater occurrences in Tanzania, the government has established nine basins based on the water utilization (control and regulation) Act No. 42 of 1974 and its subsequent amendments. The roles of the established basins are to undertake water resource monitoring and assessment, water allocation, water control and management. The Pangani basin is one of the big established basins, which comprises Kilimanjaro, Arusha and Tanga regions and 5% of its area being located in Kenya [
The Kahe basin area is formed as a large-scale structural basin. The basin is filled by Quaternary superficial deposits including alluvium deposits chiefly composed of sand, gravel and clay, along with calcareous deposits with some lava and pyroclastic volcanic rocks. The pyroclastic are inserted between new alluvial deposits, which consist of gravel, sand, silt and clay [
To the northwest of the basin, a N-S trending fault is observed that marks a boundary between alluvial and Neogene volcanic rocks. Also there is NW-SE fault trending to the west that slightly changes to NNW-SSE striking direction a few meters before entering the basin. At the eastern side there is spread of boulder bed and conglomerates as well as brown tuffuraceous and agglomerate deposits. Most of the areas surrounding the basin are largely covered by red soil derived from volcanic rocks and locally calcareous. At the northern and northwestern parts are Neogene volcanic rocks and volcanic sediment, which include rhomb porphyry and lahar (
The Kahe-Mtakuja basin is filled by alluvium deposits, which seem to become thicker toward northeast. There are two major types of geological materials, which constitute the alluvial deposits in the basin. The first type consists of basalt flows and pyroclastic products from Kilimanjaro volcanic [
In understanding the recharge of the basin, water balance calculations were obtained after simulation using the ZONEBUDGET programmed. The computation shows that a total recharge of 170 × 106 m3/day enter the Kahe basin from the mountains at an average infiltration rate of 515 mm/year [
Groundwater is mainly controlled by the geology of the area. Porosity and geological structures such as fractures, faults, and shear zones control groundwater occurrence and distribution [
basins such as Kahe-Mtakuja basin, groundwater is mainly controlled by porosity and permeability of the rock.
Porosity in sedimentary rocks can be either primary or secondary porosity. Primary porosity is the porosity formed during deposition of sediments while secondary porosity is the porosity that formed after the deposition of sediments mainly due to digenesis, cementation or deformation such as fracturing and faulting of the rocks. Clay tends to have high porosity but poor permeability hence poor groundwater reservoir. Sand, gravel and boulders tend to have high porosity and permeability forming good groundwater reservoirs. Materials, which are well sorted and free from silt and clay, are good for aquifer formation [
The stratigraphy of an area can be known using data from borehole lithological
logs. In Kahe basin many boreholes have been drilled and six of the boreholes were drilled for groundwater exploration, with maximum depth of penetration < 110 m as shown in
The stratigraphic sequence of one of the boreholes at TPC farms with their depth range in brackets intersects unconsolidated clay and silt (0 to 20 m), then boulders, pebbles and cobbles (20 to 30 m), clay (30 to 33 m), basalt (33 to 70 m) and clay (70 to 80 m) as shown by borehole R3 (
The borehole at Kahe village intersected silt from 0 to 2 m depth followed by clay, gravel and sand that are alternating with different thickness from 2 to 68 m depth. The borehole at Mikocheni intersected clay from 0 to 6 m depth, and then boulders, pebbles and cobbles from 6 to 14 m depth, followed by clay and sand from 14 to 44 m and 44 to 60 m depths respectively. The borehole drilled at Samanga intersected silty loam from 0 to 6 m depth followed by sand from 6 to 62 m depth. Lastly the borehole at Mawala intersected clay and sand from 0 to 38 m and 38 to 94 m depths respectively.
Generally, lithological logs from most of the boreholes drilled in the basin in-
dicate alternation of clay, silt, fine sand to courser sand, boulders, pebbles to cobbles, pyroclastic and basalt. These materials were deposited in the basin from volcanic weathering and lava flow from Mount Kilimanjaro, and their ages are of Neogene to Quaternary [
This study uses data from Pangani Basin Water Office, which was collected in August-September 2008 using the ABEM SAS 300C Terrameter. The instrument measures and displays the resistance of the subsurface materials. Other instruments used include; metal electrodes, measuring tape, labelled tag (used in locating station position), hammer (used in driving the electrodes into the ground), compass, and connecting cables. The ABEM Terrameter is capable of controlling external transmitter with the output of 1 - 1000 mA, maximum output voltage of 400 V or an external 12 V D.C source and output power up to 100 W. The instrument was easily operated using four electrodes. The first two electrodes are for transmitting currents and the other two electrodes are for measuring the resistance of the ground.
Choice for the Sounding StationThe quality of Vertical Electrical Sounding (VES) data is highly controlled by the nature of sounding station. The VES data were carried out in the location identified by magnetic profile, which were the points with low magnetic anomaly among the linear magnetic value. The linear magnetic profiles were designed to strike the groundwater flow in order to detect the linear features that tend to control groundwater flow. This was designed by considering the topography that result in an uneven and linear distribution of VES station as shown in
1) Resistivity Measurements
In this study Electrical Resistivity Method has been employed in data acquisition. The method has been widely adopted in groundwater exploration [
The resistivity measurements were collected using Schlumberger configuration with a maximum current electrode spacing ranging from 150 to 250 m. Schlumberger configuration is based on four electrode array where the four electrodes A, B, M and N were arranged in AMNB sequence. The electrical current I, is applied to the outer A and B electrodes and potential difference ∆V is measured between the inner M and N electrodes [
The measurements were done by keeping the distance between potential electrodes small and increasing progressively the current electrodes separation until the potential difference becomes too low/weak to be measured. Then, in order to increase the values/strength of the potential difference, the MN distance was increased and thereafter the distance AB was again increased progressively until the maximum current electrodes separation is 150 - 250 m.
2) Sources of Errors
The resistivity measurements were conducted during the dry season, the signal to noise ratio was low due to leakage of current that caused by poor electrodes contact with the ground. Leakage of primary current into the MN-circuit caused cusps on sounding curves and in some cases led to general distortion of the sounding curve as shown by most of sounding curves presented in Supplementary
From resistivity measurements, the apparent resistivity values were calculated by multiplying the resistance with corresponding geometrical factor. Then the apparent resistivity, ρa, values were plotted against the electrode spacing (1/2 AB) on a log-log scale to obtain the VES sounding curves using an appropriate computer software ix1Dv3CD.
The lateral variations of apparent resistivity in the study area show that there is decrease in resistivity from 30 Ωm to 10 Ωm. The top most layer has slightly high resistivity as compare to the layer beneath it. The apparent resistivity values slightly rise from 10 to 60 Ωm as the depth of penetration increases.
The resistivity contour maps at depths of 5, 50 and 150 m were prepared to show lateral variations of apparent resistivity (Figures 4(a)-(c)). A resistivity contour map at a depth of 5 m shows that the resistivity values range from 0 to 90 Ωm. Generally a large part has resistivity values between 0 to 30 Ωm as shown on
At a depth of 50 m below the earth’s surface, the resistivity ranges from 0 to 40 Ωm, and a large part on the map is dominated with resistivity values between 10 to 30 Ωm (
depth of 50 m, indicate that the layer is likely to be homogeneous. A resistivity contour map at depth of 150 m below the earth’s surface shows that the resistivity values range from 0 to 25 Ωm and a large area is being dominated by resistivity between 5 to 25 Ωm (
The Vertical Electrical Sounding (VES) curves for this study are presented in
AB 2 = 10.25 , 50 and 100 m, respectively. The displacement is due to increase in current electrodes spacing ( AB 2 ) , which decreases the current signal as the
depth of penetration increases. The degree of displacement caused by the lateral effects differs from one MN-segment to the other and from one VES station to another depending on the heterogeneity of the formation. The sounding curves of most VES are smooth to slightly smooth (
The resistivity value at the depth of <4 m is higher in most of the sounding curves presented. This is due to the fluctuation in soil moisture within the study area that indicates the uppermost ground is dry giving rise in resistivity values. However, this effect varies differently depending on the rock type. The effect is high on clay sediments due to the nature of clay, which tend to absorb moisture and retain, the situation, which is different when comparing to sand and silt.
However, at the northern part on the location of VES 40 to VES 50 of the study area, the upper ground has higher resistivity values due to presence of gravel and sand while at the southern part on location of VES 1 to VES 29, the resistivity values are low due to the presence of silt and clay as intersected by borehole lithological logs. The resistivity values at a depth of 4 m to 25 m range from 5 to 15 Ωm; at the depth range of 25 to 100 m the resistivity values slightly rise from 15 to 30 Ωm for most of the VES (
The geological sections were drawn to show the variation of apparent resistivity
VES No. | No. of layers | Layer resistivity (Ωm) | Depth (m) | VES No. | No. of layers | Layer resistivity (Ωm) | Depth (m) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
ρa1 | ρa 2 | ρa 3 | ρa4 | ρa5 | ρa1 | ρa2 | ρa3 | ρa4 | ρa5 | ρa6 | ||||||
1 | 5 | 63.56 | 4.29 | 13.50 | 4.98 | 15.02 | 6.27 | 30 | 4 | 82.11 | 4.78 | 20.21 | 26.84 | 75.81 | ||
2 | 4 | 80.02 | 3.27 | 15.22 | 12.27 | 48.36 | 31 | 5 | 104.62 | 2.34 | 3.96 | 35.81 | 13.32 | 51.22 | ||
3 | 4 | 33.05 | 39.66 | 13.06 | 26.50 | 49.17 | 32 | 4 | 51.24 | 5.18 | 20.78 | 26.50 | 58.06 | |||
4 | 4 | 16.94 | 11.01 | 6.97 | 68.93 | 68.05 | 33 | 5 | 133.41 | 2.75 | 15.33 | 3.43 | 19.93 | 9.22 | ||
5 | 5 | 30.31 | 9.43 | 3.32 | 11.10 | 20.21 | 79.43 | 34 | 5 | 78.92 | 3.10 | 26.50 | 6.42 | 61.89 | 54.34 | |
6 | 5 | 38.09 | 6.88 | 17.67 | 4.53 | 16.50 | 20.14 | 35 | 4 | 43.57 | 4.18 | 15.22 | 33.30 | 69.76 | ||
7 | 4 | 72.85 | 2.48 | 8.19 | 15.04 | 25.15 | 36 | 5 | 57.12 | 29.49 | 8.41 | 20.51 | 24.08 | 70.69 | ||
8 | 5 | 53.36 | 5.18 | 14.43 | 4.46 | 24.08 | 13.56 | 37 | 3 | 147.77 | 9.83 | 22.95 | 13.44 | |||
9 | 4 | 59.48 | 3.70 | 15.22 | 18.92 | 80.87 | 38 | 4 | 247.51 | 92.56 | 16.23 | 25.35 | 27.09 | |||
10 | 5 | 85.51 | 3.80 | 15.63 | 8.19 | 40.44 | 49.93 | 39 | 5 | 20.21 | 8.08 | 17.44 | 10.30 | 26.50 | 22.44 | |
11 | 4 | 115.88 | 4.84 | 15.04 | 19.93 | 66.07 | 40 | 5 | 70.73 | 13.33 | 6.97 | 24.12 | 41.31 | 56.06 | ||
12 | 5 | 90.20 | 3.19 | 18.37 | 3.55 | 29.11 | 16.62 | 41 | 5 | 118.14 | 33.05 | 9.83 | 16.98 | 25.42 | 76.20 | |
13 | 5 | 43.01 | 4.29 | 14.43 | 2.61 | 21.64 | 12.39 | 42 | 6 | 115.41 | 10.65 | 15.98 | 8.19 | 15.04 | 24.39 | 66.23 |
14 | 5 | 59.37 | 10.02 | 7.11 | 11.79 | 24.39 | 73.31 | 43 | 5 | 204.17 | 7.25 | 23.92 | 11.57 | 42.97 | 74.65 | |
15 | 5 | 111.17 | 6.04 | 9.83 | 12.29 | 12.29 | 63.88 | 44 | 5 | 74.75 | 15.22 | 4.40 | 14.83 | 214.39 | 72.48 | |
16 | 5 | 64.51 | 4.12 | 23.92 | 8.03 | 8.03 | 62.57 | 45 | 4 | 89.13 | 58.24 | 12.02 | 29.27 | 29.27 | 26.47 | |
17 | 5 | 97.81 | 172.50 | 18.92 | 14.04 | 14.04 | 51.94 | 46 | 5 | 177.34 | 56.03 | 9.83 | 28.16 | 15.04 | 43.55 | |
18 | 5 | 40.25 | 4.23 | 8.41 | 14.64 | 14.64 | 72.45 | 47 | 4 | 163.23 | 18.76 | 6.42 | 22.51 | 15.69 | ||
19 | 5 | 61.94 | 11.63 | 21.64 | 10.59 | 10.59 | 16.83 | 48 | 5 | 107.25 | 30.59 | 13.83 | 20.44 | 45.29 | 69.40 | |
20 | 4 | 60.48 | 27.11 | 16.14 | 27.11 | 72.48 | 49 | 4 | 104.62 | 22.08 | 15.98 | 27.02 | 56.61 | |||
21 | 4 | 69.70 | 46.47 | 15.39 | 41.76 | 78.85 | 50 | 5 | 124.51 | 38.02 | 110.26 | 15.82 | 32.75 | 54.61 | ||
22 | 4 | 54.05 | 7.37 | 16.08 | 39.08 | 80.91 | 51 | 5 | 41.84 | 2.10 | 7.46 | 56.39 | 56.39 | 119.13 | ||
23 | 5 | 37.60 | 5.47 | 14.06 | 5.25 | 38.09 | 28.00 | 52 | 5 | 118.14 | 1.87 | 7.88 | 16.98 | 14.15 | 74.14 | |
24 | 5 | 70.73 | 33.24 | 16.08 | 7.76 | 95.32 | 57.61 | 53 | 5 | 184.33 | 3.10 | 4.75 | 13.06 | 50.58 | 66.69 | |
25 | 5 | 72.04 | 4.86 | 6.72 | 78.13 | 14.67 | 75.32 | 54 | 5 | 85.51 | 2.24 | 6.43 | 13.86 | 17.44 | 72.48 | |
26 | 5 | 136.40 | 7.41 | 12.02 | 42.97 | 14.43 | 72.28 | 55 | 5 | 35.65 | 1.52 | 9.76 | 13.86 | 22.82 | 72.11 | |
27 | 5 | 92.90 | 10.65 | 38.05 | 5.93 | 25.92 | 8.38 | 56 | 4 | 312.46 | 2.70 | 13.06 | 72.85 | 66.47 | ||
28 | 5 | 26.50 | 32.39 | 12.78 | 26.84 | 17.19 | 67.64 | 57 | 5 | 33.30 | 9.50 | 36.11 | 7.57 | 16.08 | 17.04 | |
29 | 5 | 16.61 | 31.10 | 8.87 | 38.90 | 13.06 | 70.21 | 58 | 4 | 90.87 | 22.08 | 5.35 | 14.71 | 8.60 |
with depth as a function of rock type change and structural features (Figures 6(a)-(f)). The sections are drawn for one profiling to another profiling; while others are combined due to one profile differ from another by a few meter distances. Six sections are drawn: first section involves VES 1 to VES 8, second section concerns VES 9 to VES 19, third section displays lithology for VES 18 to VES 29,
fourth section shows lithology for VES 30 to VES 39 and VES 45, fifth section indicates lithology for VES 40 to VES 44 and VES 46 to VES 50 and the last section is for VES 51 to VES 58.
The Kahe-Mtakuja basin has generally low resistivity values in the range of 2 to 100 Ωm. From interpretation of Vertical Electrical Sounding (VES) data shows that the area has three to five layers with different thickness and apparent resistivity values. Also from lithological logs of the borehole indicate that the area has two to five layers composed alluvial deposits alternating with different thickness of one lithology to another. It seems that these layers are unconsolidated and more porous for groundwater occurrences.
The first layer is very thin with relatively high apparent resistivity value (>30 to 100 Ωm) mostly dominated by top red soil and fine sands with thickness of <2 m, similar to an interpretation reported in South of Zahedan City, Iran [
The groundwater occurrence in some areas is found at very shallow depth due to very low apparent resistivity of <10 Ωm shown by existence of shallow well on the area. The formation is porous from 2 to 20 m depth as in some areas is characterized by alternation of clay and sand. From 25 to 80 m depth the apparent resistivity is low (10 to 60 Ωm) and the layer is composed of sand, where groundwater formation is found. Hence aquifer formation of Kahe-Mtakuja basin is unconfined mainly found in sand formation.
The electrical resistivity method has been used in the identification of shallow stratigraphy of Kahe-Mtakuja basin by estimating the thickness and apparent resistivity of the contrasting layers. The VES curves indicate that the basin is generally characterized by relatively low apparent resistivity values in the range of 0 to 100 Ωm suggesting the presence of clay, silt to sand layers. The interpretation of resistivity data using computer software (ix1Dv3CD), revealed three main geo-electric layers. The first layer has resistivity values ranging from 40 to 230 Ωm with thickness ranging from 0.4 to 2 m. This layer is mainly composed of top red soil. The second layer has resistivity values in the range of 2 to 10 Ωm and thickness 2 to 25 m, composed of mainly clay and in some area with alternating sand. The third layer has slightly high resistivity values ranging from 10 to 60 Ωm and thickness in the range of 30 to 70 m. This layer is mainly dominated with sand.
Results from resistivity cross-sections indicate that the Kahe-Mtakuja basin has shallow stratigraphy. The stratigraphy of the basin from the surface to a few meters depth is characterized by clay. Beneath the clay, there is sand formation that holds the groundwater and this layer is very thick ranging from 30 to 70 m depths. These layers differ from one VES to another, due to topography variation from one place to another as VES were carried out.
The integration of results from this study and previous studies indicates that the hydrogeology of Kahe-Mtakuja basin is found in alluvial sedimentary deposit; hence the basin is potential source for groundwater occurrence. Boreholes drilled for domestic supplies and for irrigation purposes at Kahe-Mtakuja basin show high yield of >300 m3/h [
The broad implication of this study lies to the groundwater scientific community as it advances the understanding on the subsurface deposition of the shallow aquifer systems. The potentiality of this area for groundwater occurrence is vital for providing additional baseline data on the aquifer characteristics and will assist in reducing water scarce in the area. The findings also constitute background information or useful guide to the Government agency or organization(s) dealing with groundwater drilling and development programmes in the area.
The Government of Tanzania under the practical training of students has funded this study. We extend sincere appreciation to PANGANI Basin and the University of Dar Es Salaam for assisting in the logistics and fieldwork. Figures were produced using Arc-GIS and Surfer® Golden software package (Golden Software, LLC). We also thank two anonymous reviewers for helpful comments.
Mlangi, T.M. and Mulibo, G.D. (2018) Delineation of Shallow Stratigraphy and Aquifer Formation at Kahe Basin, Tanzania: Implication for Potential Aquiferous Formation. Journal of Geoscience and Environment Protection, 6, 78-98. https://doi.org/10.4236/gep.2018.61006