Assessment of ground water potentiality in semi-arid area of central Tanzania. implication from geology and geomorphology of the dodoman supergroup

ABSTRACT

Semiarid areas face challenges in the availability of water for domestic, agricultural and industrial use. The freshwater availability in semi-arid is limited due to low periods of rainfall and high evaporation rate. Groundwater resource is the only promising source of freshwater in semi-arid regions. The increased demand for freshwater in the semiarid region has increased the demand for groundwater exploration. The availability of groundwater in the subsurface is influenced by the subsurface geology, geomorphological properties and climatical condition of the region. This review aimed to analyse and combine pieces of available information on groundwater potential assessments in the central part of Tanzania. Central TANZANIA is within the craton basement rocks, where most aquifers are crystalline and fractured crystalline rocks. The groundwater origin, movement and existence rely upon numerous geomorphological and geological factors. Because of the terrain's geology and the compartmentalization of aquifers, determining the groundwater potential is difficult in basement terrain. Studying the geomorphology and geology of groundwater recharge zones is the key to exploring groundwater availability. The integration of geomorphological, geological and geophysical information yields a promising groundwater potential zone for placement of a well. The selection of the geophysical methods depends on the nature and geology of the area. Electrical Resistivity Tomography (ERT) can be utilized in combination with other geophysical methods in fractured and weathered crystalline rocks in the semiarid environment, as in the Dodoma region, because it is the best geophysical tool for groundwater exploration in the fractured aquifer.

KEYWORDS:

• Semiarid areas
• fractured aquifer
• geomorphology
• geology
• recharge zones

1. Introduction

Water is one of the essential factors that supports all forms of life in plants and animals (Adetunde et al., 2014; VanLoon & Duffy, 2017). It is generally obtained from two principal natural sources: surface water (e.g., freshwater lakes, rivers and streams) and groundwater (e.g., borehole water and well water; Maduka et al., 2018; McMurry & Fay, 2004). With the ever-increasing population, the provision of sustainable water for domestic and economic uses is a challenge for most cities, especially in developing countries such as Tanzania (Maduka et al., 2018). This is because as the human population increases, there is a parallel expansion in domestic and economic activities such as urbanization, industrialization, mining, agricultural and fishing activities (Adetunde et al., 2014), all of which require water. The scarcity of water is more observable in arid and semiarid regions such as Dodoma in Tanzania because these regions are associated with a limited availability of surface water sources, which to a large extent are subjected to the effects of natural processes, pollution and climate change (Herrera-Pantoja & Hiscock, 2015). Surface water is more sensitive to climate variability than groundwater (Shemsanga et al., 2018). Thus, in arid and semiarid regions, groundwater is the most important natural resource, and groundwater constitutes approximately 30% of the world’s available freshwater resources (Plessis, 2017), of which more than 25% of Tanzanians depend on groundwater for their daily domestic and economic activities (Elisante & Muzuka, 2017). The quantity, quality and accessibility of groundwater to a large extent depends on geology, among other factors such as topographical factors, drainage density, soil, LULC, normalized difference vegetation index, mean annual rainfall and distance to the stream (Al-Abadi et al., 2016, 2021). The geology controls the storage, transportation and quality of groundwater (Elisante & Muzuka, 2017).

The Dodoma region, which is the central official capital and government city of Tanzania, is a drought-prone, semiarid region of Tanzania. The Dodoma region has limited rains and surface water sources (Shemsanga et al., 2018); as a result, the region depends on shallow and deep aquifers for freshwater needs. Dodoma city is currently the fast-growing city in Tanzania (Wawa, 2020). The growth has been stimulated by the government decision to shift her capital city from Dar es salaam to Dodoma. Regional growth has been marked with population growth, which results in an increase in domestic and economic activities such as horticulture, agriculture, construction and industrialization. These activities are increasing pressure on water sources (Mdee & Tembo, 2021). The local people of the Dodoma region have decided to opt for shallow aquifer exploration at depths of less than 15 m; however, the groundwater dynamics of the Dodoma region are not well understood and seem to be of no interest to local researchers, policymakers and water resource managers (Shemsanga et al., 2018).

The main source of water supply in the Dodoma region is groundwater extracted from 24 deep boreholes located in the Makutupora Basin, 27 km north of Dodoma city (DUWASA, 2015; Seddon et al., 2021; Shemsanga et al., 2018), which is supplied by the government through the Dodoma Urban Water Supply and Sanitation Authority (DUWASA). In 2015, the water production capacity by DUWASA was 61,500 m3 per day (DUWASA, 2015; Seddon et al., 2021), which was distributed to approximately 300,000 inhabitants of Dodoma city. Maurice et al., 2019 observed a decline in the hydraulic groundwater head in the Makutupora aquifer. The gradual decline in the groundwater head is due to the high demand for water caused by the increased number of people in the region. This has resulted in a significant shortage of water in the region since the surface techniques are no long in use for domestic use.

Efforts to combat water shortages in Dodoma have been made since and since. The construction of dams such as Imagi, hombolo, Makalu and others aimed to harness surface water during the rainy period and use it in the future during the dry season. This project did not work well for two major reasons: 1) climatological and meteorological reasons and 2) the discovery of the Makutupora aquifer. The rainfall characteristics and high evaporation rate in the region caused the harnessed surface water to dry early during the dry season and left a region with a shortage of water for a longer period. The Makutupora aquifer promised in terms of the quantity to sustain the population in Dodoma and resulted in abandonment of the use of surface reservoirs or dams created within the area. Considerably when the Makutupora aquifer was discovered, the population of Dodoma was less than the current population.

Conflict of water users has taken up in the Dodoma region (Kusiluka et al., 2004; Namwata et al., 2015; Shemsanga et al., 2018). Some water users have been restricted from using the water supplied by DUWASA. Currently, the supply and availability of water in Dodoma is full of uncertainty (Mayaya et al., 2015). The supply of water is limited to infrastructures and the availability of water. Even when infrastructure is available, tap water is not frequently obtained. In view of this, there is an urgent need to explore new sources of water in the Dodoma region. Groundwater exploration has to be at the state of art for the sustainable supply of water for domestic and economic activities.

Groundwater exploration is the study of underground formations to better understand the hydrogeological cycle, determine the quality and quantity of groundwater and determine the nature and type of aquifers (Shishaye & Abdi, 2016). The groundwater condition at any location in a subsurface depends on the distribution of permeable layers (gravel, sand and fractured rocks) and impermeable or low-permeability layers (e.g., till, clay and solid rocks; Ernstson, 2006; Kirsch, 2006). Tanzania, and in particular the Dodoma region, lacks adequate data and information available for the major aquifers (Sangana et al., 2019). Even the available data and information are scattered, fragmented and incomplete. Keeping in mind that the hydrogeological setting of the aquifers varies according to their location and variations in geology, there is a need to understand the exact nature of the hydrogeological conditions. This review paper aims to gather geological information on the Dodoma region for groundwater potential assessment. It explains the favourable geological environments for groundwater potentiality, the geology of Tanzania and that of the Dodoma region in particular with groundwater potential and the implication of the geology and geomorphology to groundwater potential. This review paper further proposes a way forward for exploring groundwater and the best techniques that are used to explore groundwater based on the geomorphological and geological environment.

2. Hydrological setting of Sub-Sahara Africa (SSA)

MacDonald and Davies (2000) subdivided Sub-Saharan Africa (SSA) into four major hydrogeological provinces that are related to the geological environments of each area (Figure 1). These provinces are (i) the Precambrian basement, (ii) consolidated sedimentary rocks, (iii) unconsolidated sediments and (iv) volcanic provinces (Gustafson & Krásný, 1994; MacDonald & Davies, 2000; Morris et al., 2003). The geological characteristics are summarized in Table 1. The largest and smallest hydrogeological provinces are found in Precambrian terrains and volcanic provinces, respectively (MacDonald et al., 2008).

Figure 1. Hydrogeological environments of Sub-Saharan Africa (after MacDonald and Davies (2000)).

Table 1. Groundwater geological environment characteristics of SSA modified after MacDonald et al. (2008).

Groundwater environments	Rocks/Geological formations	Potential of groundwater	Targets for groundwater
Crystalline rocks	Deeply altered and cracked basement	Moderate	Highly weathered zone fractures
	Poorly altered and fractured basement	moderate	Weathering fractures and pockets
Consolidated sedimentary rocks	Sandstones	Moderate-high	Fractured or porous sandstone
	Limestones	Moderate	Fractured and karstic limestones
	Mudstones	Low	Fractured mudstones
Unconsolidated sedimentary rocks	Large basins	Moderate-high	Gravel and sands
	Riversides, alluvium	Moderate	Consolidated alluvium strata
Volcanic rocks	Mountainous regions	Moderate	Fractured basalts
	Plains and plateaux	Moderate	Fractured basalts

The Precambrian basement is large and extends to approximately 40% of SSA land (MacDonald et al., 2008). It is composed of crystalline rocks such as mica schist, quartzite and granite (Chilton & Foster, 1995). The basement is very old and presents some weathered areas in some places. Groundwater availability in the Precambrian basement depends largely on the degree of weathering of rocks and the presence of fractures. The rate of weathering of crystalline rocks is dependent on some factors, among which the most important are (i) the terrain geomorphology, (ii) mineralogy of the host rock, (iii) groundwater temperature and occurrence and (iv) the presence of fractures in the rock (Jones, 1985; Wright, 1992). Aquifers are not exposed to surface pollution, which makes the water of good quality but, in some cases, vulnerable to metal contamination from the host rock. Water within the basement is exploited through boreholes and wells, depending on whether it is deeper or shallow.

Consolidated sedimentary rocks are another type of province that accounts for 32% of the land of SSA. These sedimentary rocks are mainly sandstone, limestone, mudstone and siltstone. A large quantity of water is found in cracked and friable sandstone aquifers (MacDonald & Davies, 2000). Limestone groundwater is largely related to karstification, where water accumulates in karts (Bonacci, 2015). Groundwater potentiality, on the other hand, is rarely found in siltstone and mudstone because of low effective porosity (Davies & MacDonald, 1999). Consolidated aquifers are mainly located in large sedimentary basins of arid regions, such as the Kalahari and Karoo sediments in southern Africa, Somali sediments in East Africa and Benue Trough in west Africa (Selley, 1997).

Groundwater availability and flow in consolidated sediments are controlled by the sediment type, the nature of the cementing material and matrices. Boreholes and dug wells are useful for the exploitation of these aquifers for deep and shallow aquifers, respectively. The quality of groundwater is good, but in some exceptional cases, groundwater may be exposed to depth salinity in shallow aquifers.

Unconsolidated terrains contain a large quantity of water and are providers of water in many regions around the world for domestic, industrial and agricultural uses. (Morris et al., 2003). They form aquifers that account for 22% of the land in the SSA regions (MacDonald & Davies, 2000). The geological formations of these aquifers include stratified permeable gravel (or sand), silt and clay, and most of them are unconfined aquifers.

Sand and gravel provide a large quantity of this type in SSA countries and are good aquifers due to their highly effective porosity and permeability (Morris et al., 2003). However, this high permeability increases their vulnerability to pollution of urban wastewaters, agricultural chemical products and sometimes natural pollution related to geogenic arsenic, iron and nitrates (Bretzler et al., 2017; Ligate et al., 2021; Morris et al., 2003). The exposure of this type of aquifer to pollution is higher when the aquifer is shallow. The exploitation of groundwater is mainly done through wells when the aquifer is shallow and boreholes when it is deep.

Volcanic provinces occupy a small area, which accounts for 6% of the SSA land (Davies & MacDonald, 1999). Rocks of these provinces are dominated by basalts, pyroclastic rocks and volcanic ash deposits. Most of the volcanic provinces in Africa are located within the Eastern African Rift System (EARS), which extends from Ethiopia to Mozambique over a distance of 6000 kilometres (Chorowicz, 2005; Ponte et al., 2019), as shown in Figure 1.

Fractures are the main characteristics that control the presence and availability of groundwater in volcanic aquifer systems. However, these aquifers and surface water in volcanic areas are highly exposed to fluoride contamination and make it of poor quality (Kut et al., 2016). High fluoride concentrations in drinking water led to health problems such as dental fluorosis. Dental fluorosis has been observed in many volcanic regions located within the EARS, including Ethiopia, Kenya, Tanzania and the Democratic Republic of Congo (Balagizi et al., 2018; Chacha, 2020; Kut et al., 2016; Olaka et al., 2010, 2016).

3. Hydrogeology of Tanzania

The geology of Tanzania (Figure 2) comprises rocks of mainly Precambrian and Phanerozoic formations. The oldest known formations in the country are of Archean age and form predominant granite-greenstone terrains (Borg & Shackleton, 1997; Manya et al., 2006). The central portion of Tanzania is covered by the Tanzanian Craton (2.5 Ga), which extends to the southern and eastern parts of Lake Victoria (Schlüter, 2008).

Figure 2. Hydrogeological setting of Tanzania.

The craton is composed of igneous and metamorphic crystalline basement rock and is bordered by the Proterozoic Ubendian mobile belt, Mozambique Belt, Usagaran mobile belt and Karagwe-Ankole orogenic Belt. The Archean craton contains vestiges of the Dodoman Supergroup in central Tanzania and the Nyanzian Supergroup and Kavirondian Supergroup in the north. The remnants of these two orogenic belts are present as lenses of sedimentary and volcanic rocks largely visible as granites and migmatite (Borg & Shackleton, 1997). The main aquifer systems in Tanzania are unconsolidated and consolidated sedimentary rocks and fractured basement rock complexes (Bakari et al., 2012a, 2012b).

The main components of the unconsolidated sedimentary aquifers in Tanzania are the unconsolidated sediments of the alluvial deposits, fluvial deposits, and volcano-pyroclastic sediments (Kashaigili, 2010). The alluvial deposits are largely restricted to the coastal delta areas and along river valleys. Unconsolidated sediments are found close to coastal plain areas and are made up of beach sands, dunes and salt marshes. The occurrence of some consolidated limestone deposits is uncommon (Mjemah & Walraevens, 2015; Van Camp et al., 2013; Walraevens, 2008). Volcano-pyroclastic sediments occur in proximity to previously active volcanoes (Brown & Sparks, 2010).

Due to the varying lithologies in this form of the aquifer system, the borehole yields differ greatly (Kashaigili, 2010). However, the most prospective aspect of the unconsolidated sedimentary aquifer system is found within the volcano-pyroclastic and alluvium deposits of the Kahe Basin and Sanya Plain near Kilimanjaro (Lwimbo et al., 2019a, 2019b). Recorded yields range between 0.2 and 2 L/s (Mlangi & Mulibo, 2018). Although the thickness of unconsolidated aquifers is poorly defined, the depth to the water table falls between 10–20 m. The average depth of boreholes in this system is between 100–200 m (Kashaigili, 2010), and groundwater quality around coastal plain deposits is vulnerable to seawater intrusion (Sappa et al., 2017).

Consolidated sedimentary aquifers in Tanzania are broadly segregated into two types: coastal sedimentary aquifers and the Karoo aquifer system (Kashaigili, 2010). The main material components of the coastal sedimentary aquifer system are sandstones, limestones, marls and shales. The most prospective of these materials are sandstones and limestones, with the least prospective being marls and shales. The coastal sedimentary aquifer system is generally unconfined. The highest borehole yields in coastal sedimentary aquifers have been recorded within limestones (1–6 L/s) and to a lesser extent within sandstones (1–2.5 L/s).

The aquifer thickness varies from 5–30 m, with the surface to water table depth recorded between 10–35 m. The average depth of boreholes in this system generally does not exceed a depth of 80 m (Kashaigili, 2010). The main material components of the Karoo sedimentary aquifer system are sandstones and conglomerates. These materials are characterized by intergranular flow and storage, which are locally improved by secondary fracture permeability. Karoo sedimentary aquifer systems are predominantly unconfined, with recorded borehole yields between 0.1–5 L/s, although yields as high as 15 L/s have been encountered. The thickness of aquifers within this system is similar to that of the coastal aquifer system.

The main material components of the basement aquifer system consist of schist gneiss and migmatite (Rwebugisa, 2008). The occurrence of groundwater in this aquifer system is primarily structurally controlled and limited to secondary permeability, such as weathered zones, joints, fractures, faults or dissolution features (Nkotagu, 1996). The groundwater potential in weathered zones within this system is controlled by the degree and depth of weathered zones and related fractures as well as the saturated thickness. Basement aquifer systems are generally discontinuous and confined (Nkotagu, 1996).

Boreholes within zones that produce higher yields are often located in restricted bands of gneissic and sedimentary rocks whose permeability has been greatly impacted by fracturing due to their proximity to fault zones. These zones can commonly be found in the Pangani Basin (Shemsanga et al., 2017). Borehole yields in the basement aquifers range from 3 L/s and are largely controlled by lithologies. The highest yield reported from the Pangani Basin is 17 L/s (Komakech et al., 2012) Significant variability occurs within the transmissivity values reported from the Pangani Basin, with hydraulic conductivity values ranging between 1–16 m/day.

The average aquifer thickness within the basement aquifers is 50 m. There are no significant recorded water quality or quantity issues, although the reported values of fluoride have been as high as 180 mg/L (Kilham & Hecky, 1973). These anomalous zones of fluoride are often found in boreholes situated in proximity to the rift zone as well as the crystalline bedrock and have been interpreted by Davies (2010) as a result of mixing with fluids from hot springs and volcanic gases, which can contain concentrations of fluoride of several tens to hundreds of milligrams per litre. Recharge generally occurs through fracture zones, faults or lineaments.

4. Geological environment for groundwater potential

Groundwater availability is primarily determined by the geological environment of the area (Yeh et al., 2009). The groundwater is stored in the pore space and fractures found within the rock (Rahmati et al., 2016). The flow of water within the rock depends on the connectivity of the pore space and fractures (De Vries & Simmers, 2002), which makes some geologic formations more preferential for groundwater than others. Highly fractured crystalline rocks and sedimentary rocks with high effective porosity are preferred for ground water potentiality (Ejepu et al., 2015; Singhal & Gupta, 2010).

5. Geomorphological environmental for groundwater potential

Geomorphology is the study of landforms, their origin and evolution, and the processes that shape them. Geomorphic units are critical in determining the relative groundwater potential of different areas (Rani et al., 2015). The geomorphology of the area influences the quality and availability of groundwater (Ballukraya & Kalimuthu, 2010). It determines the surface flow direction and controls the amount and rate of infiltration (Alsharhan & Rizk, 2020). The geomorphological features that play a great role in groundwater availability and quality include slope, elevation, drainage density, lineament density, soil, land use and land cover (LULC).

Elevation is an important factor in the occurrence of groundwater because weather and climatic conditions vary greatly at different elevations, resulting in differences in soil and vegetation. Slope is defined as the rise or fall of land surface (Al-Abadi et al., 2016) delivered from elevation (Ettazarini & El Jakani, 2020). The slope has a significant impact on the amount of water to be recharged to the aquifer (Rajaveni et al., 2017; Yeh et al., 2009). It controls the amount of water accumulated on the surface, and low slope land has the potential for groundwater availability because precipitation will have more time to stay on the ground and finally percolate into the ground and recharge the saturated zone compared to high slope areas (Yeh et al., 2009). The rate of change of slope is called curvature, or sometimes referred to as slope of the slope (Kimerling et al., 2016) and represents the morphology of the topography.

The curvature is interpreted into three types: total curvature, profile curvature and plan curvature. All these factors have a significant impact on the availability of groundwater. The profile curvature is parallel to the maximum slope direction and primarily affects the acceleration and deceleration of flow across the surface (Al-Abadi et al., 2016). The calculated value of the profile curvature is negative, positive or zero. A negative profile curvature indicates that the surface is upwardly convex. A positive curvature implies that the surface is upwardly concave at that cell. A value of zero indicates that the surface is linear (Lee & Evangelista, 2005; Staley et al., 2006). The plan curvature primarily determines the divergence and convergence of the flow across the surface.

It is always perpendicular to the direction of the maximum slope. During the calculation, a cell is assigned a negative, positive or zero value, which implies that the surface is sideward concave, convex and linear, respectively (Kimerling et al., 2016). The combination of the profile curvature and the plan curvature is called the total curvature and enables us to understand the accurate flow of water on the surface (Al-Abadi et al., 2016; Yeh et al., 2009). Aspect is the orientation of the slope, measured clockwise in degrees from 0° to 360° and indicates where the slope faces at that location. It identifies the downslope direction of the maximum rate of change in value from one raster cell to its neighbours. Aspect has a strong influence on hydrologic processes through evapotranspiration and the direction of frontal precipitation and thus on weathering, vegetation, and root development, particularly in drier environments (Al-Abadi et al., 2016; Dai et al., 2001; Golkarian & Rahmati, 2018).

Drainage density is defined as the total length of the stream per unit area occupied (Dragičević et al., 2018; Yeh et al., 2009). It is an important factor in the identification of suitable areas for groundwater potentiality. Areas with high drainage densities are not suitable for groundwater potentiality. High-level drainage density areas reduce infiltration and increase runoff (Khodaei & Nassery, 2013; Yeh et al., 2009). Drainage density is influenced by geology, soil-water absorption capacity, canopy cover and climate (Khodaei & Nassery, 2013).

Land use/land cover (LULC) generally refers to the categorization or classification of human activities and natural elements on the landscape within a specific time frame based on established scientific and statistical methods of analysis of appropriate source material. These include urban or built-up land, forestland, barren land, and agricultural land. LULC is a significant factor affecting groundwater availability and quality (Rajaveni et al., 2017; Scanlon et al., 2005). It affects groundwater recharge processes, surface runoff, and the infiltration rate (Ghosh & Jana, 2018; Rajaveni et al., 2017; Yeh et al., 2009) and modifies the groundwater chemistry through surface processes (Scanlon et al., 2005).

Lineament is a very important feature in groundwater exploration, especially in crystalline igneous rock (Ballukraya & Kalimuthu, 2010; Mseli et al., 2021; Saraf & Choudhury, 1998). They are defined as mappable, simple or composite linear features of a surface, whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differ distinctly from the patterns of adjacent features and presumably reflect a subsurface phenomenon (Khodaei & Nassery, 2013). They are characterized by a length greater than 300 m, and those whose length is longer than 10 km have the potential for groundwater, as joints and faults serve as conduits for the movement of groundwater (Saraf & Choudhury, 1998; Teeuw, 1995). Lineament, such as faults and other deeper fractures, can cause an interchange between the surface and groundwater (Yeh et al., 2009). Studying the lineament in an area can be done using both image and nonimage data, where image analysis is the best due to oblique constant illumination, suppression of spatial detail and regional coverage (Waters et al., 1990). Lineament density is obtained by dividing the lineament length by the catchment area. Areas with high lineament density are favoured for groundwater potentiality (Mdee & Tembo, 2021).

Groundwater potential is heavily influenced by soil texture. Soil texture directly affects infiltration, aquifer conditions (Pal et al., 2020) and runoff (Chakrabortty et al., 2018). Permeability and porosity vary significantly with the soil texture (soil texture directly influences the infiltration rate as the porosity and permeability vary with the texture (Das & Pal, 2019). In comparison to fine-grained soil, coarse-grained soil has a higher infiltration capacity based on porosity and permeability (Pal et al., 2020).

6. Geological and geomorphological setting of Dodoma

The Dodoma area is situated in the central part of Tanzania within the Hombolo subbasin of the Wami basin, which falls within the basement crystalline rocks of the Archean Dodoma Supergroup (Nkotagu, 1996), as shown in Figure 3.

Figure 3. Geological setting of the study area.

The rock basement is covered with unconsolidated material with a thickness ranging between 50 m and 100 m and an average thickness of 60 m (Nkotagu, 1996). The interior of the basin contains schist gneiss and migmatites with metasedimentary and granitic protoliths (Thomas et al., 2016). The associated metasedimentary rocks within the craton are primarily composed of ferruginous quartzites, ironstones, micaceous quartzites and quartzo-feldspathic schist. The hydrological areas of the basin are characterised by a series of interconnected normal faults defining downthrown grabens (Thomas et al., 2013a, 2013b) with hydraulic conductivity of 10⁻⁵ m sec⁻¹ (Nkotagu, 1996). Research by Rwebugisa (2008) on the weathered fractured aquifer of the Makutupora Basin revealed a transmissivity value of 670 m²/day within the basement rocks, with a value of 490 m²/day recorded for the overlying sandstone and gravel deposits.

The lithologies within the Dodoma area are predominantly dense and relatively resistant to weathering. The principal mechanism of recharge is through secondary porosities, that is, the fractured zones (Elisante & Muzuka, 2017). The average aquifer thickness within the basement aquifers is 50 m, and the normal borehole depth in the Dodoma system ranges between 70 m and 120 m (Kashaigili, 2010).

The digital elevation map with a resolution of 30 m obtained from the United States Geological Survey (USGS n.d) was used to analyse the slope, aspect, total curvature and drainage density of the study area. The Dodoma region is situated at an elevation between 595 m and 2338 m, with an average elevation value of 1146 m measured above sea level (Figure 4). Using the regional district map (Figure 5), highly elevated areas in Dodoma are found within Kondoa, Chemba, Mpwapwa and Kongwa, and low elevation areas are found in Bahi, Dodoman Urban and southern Chamwino. The slope map developed depicts that the slope ranges between 0° and 21.8° with an average value of 1.56°, as shown in Figure 6. Highly sloped areas are found in the southeastern part of the region, and the rest of the region has a normal slope, with a class of 0° to 1.11° being dominant. The orientation of the slope (aspect) has been analysed, and its map presented in Figure 7 indicates that the western part (Bahi district) is flat.

Figure 4. Elevation map.

Figure 5. Regional district boundaries.

Figure 6. Slope.

Figure 7. Aspect.

Figure 8 shows the drainage density in the region. The drainage density ranges between 0 and 67.12 km/km² with an average value of 7.02 ^km/km2. The higher density of streams is located in the south and southwest of the region. The profile curvature and plan curvature were combined, and a map of the total curvature is presented in Figure 9. The value ranges between −0.12 and 0.13, with an average value of 0.00 correct to two decimal places. Using the digital soil map of the world developed by FAO (2003), the soil texture in the study area was developed and is presented in Figure 10. Sandy loam texture is the dominant soil texture in the region occupying nearly the whole central part from north to south. Due to the grain size, clay loam is situated in the western part where depositional likely take place revived by the slope and aspect in Figures 6&7, respectively. The lineament density in the study area was developed and is presented in Figure 11. The lineaments were extracted with the help of Landsat 8 OLI/TIRS images downloaded from the United States Geological Survey (USGS n.d) with row/path values of 168/063, 168/064, 168/065, 169/063, 169/064 and 169/065. The Landsat images were imported into Geomatica software for processing the lineament. The obtained lineaments were further analysed in an arc GIS environment to obtain the lineament density. The lineament density in the study area ranges between 0 and 0.71 km per square km with an average value of 0.08 km per square km.

Figure 8. Drainage density.

Figure 9. Total curvature.

Figure 10. Soil texture.

Figure 11. Lineament density.

7. Groundwater exploration in crystalline igneous rock

Groundwater exploration techniques come with a considerable financial cost. Most of the cost of groundwater exploration in Tanzania is borne by the government and donor agencies (Komakech et al., 2012). The methods applicable for the terrain can be grouped into surface and subsurface techniques. Surface techniques generally come with a lower cost and sometimes provide inconclusive results. Subsurface techniques are generally pricier and have a greater chance of success. Due to the prohibitive cost of subsurface techniques, they are primarily undertaken by government agencies to verify the results of surface techniques. The higher level of accuracy of these techniques permits the assessment of the geophysical attributes of the rock materials as well as borehole lithological logs from core samples. An example of such a project in SSA is the Ghana Water Resources Commission’s HAP project in the Voltaian sedimentary basin of Ghana (Carrier et al., 2008).

Some nonconventional methods that were quite popular in the 17^th century include esoteric methods, which are now seen as more of pseudoscience (Vogt & Golde, 1958). There, however, is an argument that these methods have some scientific underpinnings and work in certain areas. The belief behind these methods is that the flow of groundwater can induce some detectable currents above the Earth’s surface. These currents are detectable via a rotating wet plant twit or forked stick when moved over such areas (Bakari et al., 2012a; Vogt & Golde, 1958; Vogt & Hyman, 2000).

Another often employed technique is geomorphological techniques. These techniques take advantage of the physical attributes of the terrain in siting boreholes (Mundalik et al., 2018). Surface drainage is one of the techniques that is reliant on the basement rock of the terrain. The nature of groundwater flow through the crystalline basement rocks of the terrain depends mainly on secondary porosity due to its dense and crystalline nature. Ordinarily, groundwater flow tends to align with surface drainage (Toth, 1963). This makes this method a useful technique in siting boreholes in the most successful parts of the area. In certain areas, the paths of water bodies are influenced by the underlying structures, which gives a clue as to areas that are prospective. In such areas, the intersections of streams downhill are very successful spots for groundwater.

The formation of surface geomorphological features is reliant on several geological processes, such as weathering and erosion (McAuliffe, 1994). These processes result in landforms that have the required permeability and porosity to hold and transmit groundwater. The high content of silicate materials in the rocks of the study area may result in weathering to form clay-rich regolith materials, which are sometimes favorable sites for groundwater accumulation (Klint & Gravesen, 1999). Such techniques are useful in areas within the Pangani Basin, where local permeability has been greatly impacted by fracturing due to their proximity to fault zones (Shemsanga et al., 2017).

Towards the fringes of the area to the south and west in the Tertiary-Quaternary unconsolidated materials, the study of landforms could point to the more permeable strata liable to promote groundwater accumulation. These areas occur around the coastal delta areas and along river valleys in Tanzania and are made up of alluvial deposits, unconsolidated sediments and volcano-pyroclastic sediments (Kashaigili, 2010). Such materials are favourable groundwater reservoirs and indicators of groundwater prospects.

The nature of the topography and drainage of an area can be used to evaluate the possible occurrence of groundwater in an area (Rao, 2006). Areas where water bodies converge as well as small watersheds are often viable locations for groundwater recharge. The gradient of groundwater flow is known to also align closely with the topographic gradient. This could be applied as an ineffective technique in siting boreholes. However, this must be done in cognizance of the previous success rate of drilled wells in the area. In areas known to have several dry wells, this technique may only be useful in selecting sites for geophysical prospecting. It is, however, known that experienced drillers often employ this technique in drilling private domestic boreholes as a way of cutting cost. Drainage density is also used as a means of detecting groundwater availability. This provides a useful metric for indicating the potentiality of groundwater in an area.

These discussed methods are inexpensive and may be more practical in selecting sites for drilling domestic wells. However, in the search for commercial or industrial wells, more robust methods must be applied. The application of geological methods provides a comparatively higher level of possible success and may involve moderate cost. The review of the geology of an area may serve as a good starting point in groundwater prospecting. The nature of the Dodoma area will require the collection of aerial photographs, an evaluation of the structural features of the area, such as faults and fractures, and the assessment of all possible hydrogeological data in the area. This may give possible indications of the areas more likely to prove successful for boreholes. The geological structural methods that may be applied in the most igneous and metamorphic terrain of the Dodoma area include the presence of dikes, which serve as effective barriers for arresting the flow of groundwater. Taking the attitude of such dikes has been known to prove useful in picking the groundwater potential zones upstream.

The use of geo-botany as a means of detecting the possibility of locating groundwater may be helpful in the central areas of the Dodoma region with lush vegetation. The method exploits the presence of anomalous growth of vegetation and the seeming alignment of large trees in an approximately straight path. The presence of termitarium, which has been found to use mineral exploration (Arhin, 2010), has also been used as a sign of groundwater reservoirs. According to Shemsanga et al. (2018), the use of termitarium as one of the key indigenous knowledge techniques for groundwater exploration is already popular within the Ntyuka, Ihumwa, Nzuguni, Kitelela, Nzasa, Gawaye and Mtumba suburbs of Dodoma. The so-called heritage trees, which are old deep-rooted trees, have been relied upon as a telltale sign of a conducive location to site a borehole. This technique has already been sparingly applied within some suburbs of Dodoma with some degree of success (Shemsanga et al., 2018). A further technique employed is the presence of halophytes or plants with considerable tolerance for soluble salts as an indication of the availability of groundwater with a high salt component. The latter method may find some practical use in the regions of Dodoma due to the saline nature of waters in deep and shallow aquifers (Hiji & Ntalikwa, 2014; Kashaigili, 2010).

The igneous and metamorphic rocks are considered to be hard rock aquifers (or fissured aquifers) in hydrogeology (Comte et al., 2012). They are found within the first 100 m below the ground surface. Approximately 20% of the global land surface is underlain by these rocks (Comte et al., 2012; Murty & Raghavan, 2002). They are highly heterogeneous and exhibit many contrasting behaviours. For example, one of the two neighbouring wells may yield several cubic metres per hour, while the other is of very low discharge (Comte et al., 2012). The hydrogeological properties of these rocks are quite unpredictable at both the regional and catchment scales; as a result, their aquifers are considered discontinuous aquifers (Comte et al., 2012).

Globally, the volume of groundwater contained in hard-rock aquifers is not well constrained and has often been considered negligible from a water resource perspective (Comte et al., 2012). For instance, compact, hard, crystalline rocks such as granite are not very promising groundwater sources because their aquifer potential is characterised by low porosity (less than 7%) and low permeability, which makes exploration in such lithologies a very challenging task (Briški et al., 2020; Murty & Raghavan, 2002). The quantitative occurrence of groundwater in such rocks depends on the weathered and fractured zones because porosity and yield depend directly upon the degree of weathering and fracturing, and these properties decrease with depth. Although there are several geophysical techniques used in groundwater exploration, such as electrical resistivity, gravity, seismic refraction, magnetic-induced polarization and electromagnetic methods, these techniques, except the gravity method, have not been highly successful in crystalline rocks, specifically in granitic regions (Murty & Raghavan, 2002). The gravity method has been successfully used to explore groundwater in crystalline rocks because most of the hydrogeological factors, such as lithology, structure, and grain size, which control the primary porosity, affect the effective density of the rock (Murty & Raghavan, 2002).

Recently, significant advancement knowledge of the structure and functioning of hard rock aquifers has shown that their hydrodynamic properties are mainly related to the existence of ancient weathering profiles. The fracture permeability of crystalline rocks can greatly be increased by the alteration process, which can also induce the formation of modest but locally important aquifers (Briški et al., 2020). Thus, the alteration processes significantly upset the rock total porosity and water content, causing contrasting electrical resistivity of rocks affected by the degrees of varying weathering (Briški et al., 2020). In such an environment, electrical techniques such as electrical resistivity tomography (ERT) have become the most preferable geophysical technique to be used for the exploration of groundwater (Briški et al., 2020; Gao et al., 2018). This technique is strongly and effectively used to investigate the hydrogeological characteristics of such subsurface materials (Gao et al., 2018).

In the Dodoma region, the first geophysical technique employed for groundwater exploration was vertical electrical sounding (VES) in 1978 in the Makutupora Basin using an ABEM AC terrameter (Schlumberger array; Kasonta & Kasonta, 1995). This survey resulted in drilled boreholes that form a major groundwater source for the Dodoma Area. To date, no study has shown any geophysical exploration activities in the Dodoma area (Sangana et al., 2019). It is, therefore, time now to review and understand the hydrogeology of the Dodoma region to propose the best geophysical technique for groundwater exploration to reduce exploration risk.

8. Conclusion

Considering the looming effect of climatic change and increased population in the Dodoma region, identifying artificial recharging zones of water from scarce rainfall will increase the amount of groundwater availability in fractured crystalline aquifers and weathered crystalline aquifers. To establish a potential artificial recharging zone, a detailed integrated approach consisting of the application of remote sensing, geomorphological, surface and subsurface geologic structural analysis drainage density, and stream network should be conducted within the region. Knowing the suspected potential groundwater zone, the availability of groundwater can be explored using the hydro geophysical method. Regarding the nature and the geological setting of Dodoma, the Electrical Resistivity Tomography (ERT) method coupled with other geophysical methods can be used since it is the best geophysical tool for groundwater exploration in fractured and weathered crystalline rocks of the semiarid area like in Dodoma region.

Acknowledgments

The authors send their gratitude to the University of Dodoma for providing internet facilities and access to some of the published articles that have been used in this review.

Disclosure statement

No potential conflict of interest was reported by the author(s).