1 Introduction

Groundwater is among the valuable natural resources that support human health, economic development and ecological diversity [7, 28]. It plays a key role in nature due to its natural quality, fairly well distributed and continuous availability. It has become major source of water supply for domestic, industrial and agricultural uses in both rural and urban settlements all over the nations [30]. However, groundwater has been accounted for more than 50% of humanity’s freshwater for everyday uses and about 30% of agricultural and industrial development [3]. Groundwater potential zone can be said to be a zone of saturation where water can be stored and transmitted in a large quantity. The occurrence and movement of groundwater in a hard rock terrain are controlled by degree of weathering and extent of fracturing of the bed rocks [11], while the quantity of groundwater in aquifer is governed by many factors, viz., topography, lithology, geological structures, lineament density, aperture and connectivity as well as primary and secondary porosity [20]. The identification of groundwater potential zone requires direct or indirect analysis of some observable hydrogeomorphic features such as geomorphology, slope, drainage density, land use/land cover, lineament density, geology and soil type.

Currently, groundwater is gaining more attention due to unplanned growth of urbanization, industrialization and ever-growing population which has led to increased demand for groundwater for domestic, industrial and agricultural uses. This alarming plight calls for cost-efficient approach suitable for groundwater exploration for sustainable budgeting, management and development to eliminate water scarcity [15, 31]. However, overexploitation of groundwater resources due to increasing population can lead to decline of groundwater levels and deterioration of groundwater quality.

Lacking a proper management of water resources, the focus solely on the identification of groundwater potential zones leads to increased overexploitation.

Consequently, the revolution in efficient, powerful and high-speed computers, the advent of remote sensing (RS) and geographic information system (GIS) offers a great platform and scope for integrating different hydrogeomorphic themes that can be used to identify groundwater potential zones of the present area. A combination of remote sensing (RS) and geographic information system (GIS) has proven to be an efficient tool in water resource studies [14, 17].

In the past, many researchers (from Nigeria and other countries) have used RS and GIS techniques to determine groundwater potential zone with successful results [6, 8, 10, 12, 24]. In these studies, the commonly used thematic layers are geology, geomorphology, slope, soil type, land use/land cover, drainage density and lineament density.

In the study area, there is little or no existing work published on the use of integrated GIS and RS techniques in identification of groundwater potential zone. The existing work published centered on conventional methods such as geological, hydrogeological and geophysical approaches were employed for siting boreholes for water supply [2, 5, 16, 18, 21,22,23]. Population growth in Kogi State has raised concerns about the allocation of surface water flow and the increased demands for groundwater resources. To address these concerns, there is a need for improved methods for accurately accessing regional groundwater resources. Hence, the aim of this study is to identify groundwater potential zones of Kogi State, North Central Nigeria using GIS and RS techniques for proper management, development and planning with the following objectives: A; to prepare thematic layers, viz., geology, geomorphology, soil, slope, drainage density, land use/land cover, lineament density and rainfall, B; to delineate groundwater potential zones through integration of several thematic layers with GIS and RS techniques.

2 The study area

Kogi State lies between latitudes 6°30′N and 8°40′N and longitudes 5°10′E and 8°10′E, encompassing the twenty-one local government area of the state (Fig. 1). According census 2006, the population of the state has reached up to 3,314,043 inhabitants and has an area of approximately 29,833 km2. The adjacent states are Federal Capital Territory, Nasarawa and Niger to the north, Edo, Anambra and Enugu to the south, Benue to the east and Kwara, Ekiti and Ondo State to the west.

Fig. 1
figure 1

Location map of the study area

Full size image

2.1 Drainage, topography, climate and vegetation

The topography of the area consists of a hilly plateau dipping gently in the northeastern and eastern directions down to the Niger River valley, while in the southwestern part of the area many hills overlooking the plateau are made up of Precambrian gneisses and granites and long ridges with gentle to steep slopes. The ground elevations range between 140 and 300 m above sea level.

The Kogi State climate is characterized by two seasons, namely rainy and dry seasons. The area records about 1839 millimeters average rainfall. The climate is generally cool during the raining season which lasts from March to October and relatively hot during the dry season. The dry season commences in November and lasts till April (Fig. 2). Average temperature is 28 °C. December and early January are coldest as a result of dry dusty harmattan winds [19]. The area has derived guinea savannah vegetation. The highlands are of semitropical rain forest type. It is characteristically green and is complemented by typical grassy vegetation. Fresh water swamp forests also occur in the area.

Fig. 2
figure 2

Trend of mean annual rainfall within the area for 19 years

Full size image

2.2 Geology and hydrogeology of the study area

The geology and hydrogeology of the area have been extensively studied by various researchers. Previous works on the stratigraphy, petrography and hydrogeology of the formations were carried out by Reyment [1, 4, 9, 13, 25, 26, 29]. The geology of Kogi State is made up of two major litho-petrological components, namely the Precambrian basement complex of North Central Nigeria, and sedimentary basins. The Precambrian basement complex of North Central Nigeria is made up of the migmatite–gneiss–quartzite complex (migmatite, gneisses, quartzite and quartz-schist and small lenses of calc–silicate rocks), the schist belts (migmatised to unmigmatised paraschist and meta-igneous and charnockitic rocks), the older granites (granodiorite to granite and potassic syenite) and unmetamorphosed dolerite dykes. The sedimentary basins, containing sediment fill of Cretaceous ages, comprise the Mid-Niger (Bida/Nupe) and Anambra basins. The sedimentary basins unconformably overlie the basement complex and consists of shale, siltstone and fine-grained sandstones.

The distribution of groundwater is controlled by rock lithology, texture and structures and by hydrological (stream inflow) as well as meteorological factors such as rainfall.

In the study area, two types of aquifer are identifiable; one is deep unconfined condition, while the other is shallow semi-confined but locally confined in some places. The deep unconfined condition is most widespread and occupies about 80% of the land area underlain by sedimentary basins, while the shallow semi-confined and confined aquifers occur in the weathered mantle, fractured and/or faulted traces and are limited to small portions of the area. Depth to water table in the area varies from 3.4 m to above 60 m.

3 Data used and methodology

Remote sensing data which comprise of 90-m interval Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM) and Landsat Enhanced Thematic Mapper plus (ETM+) alongside with conventional data such as geological, pedological and lineament, as well as meteorological and field observation data were used in this study (Fig. 3).

Fig. 3
figure 3

Flowchart of the methodology adopted in the study

Full size image

The Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM) and Landsat Enhanced Thematic Mapper plus (ETM+) retrieved from earthexplorer.usgs.gov website has been used for the preparation of thematic layers of geomorphology, drainage density, slope and land use/land cover using ArcGIS 10.5 software platform.

However, the analogue maps of geology, lineament and soil obtained from Nigeria Geological Survey Agency (NGSA) and (FAO/UNESCO/IS-RIC) with a scale of 1:250,000 have been converted to digital format by scanning in Tiff Format and geo-referenced into Universal Transverse Mercator (UTM-Zone 32 N) and datum WGS-1984. These maps were further updated using satellite data. Furthermore, the thematic layers of geology, geomorphology, soil, lineament density, slope, land use/land cover and drainage density were used for the evaluation of groundwater potential zone of the study area. All these thematic layers were converted into raster layers and superimposed by weighted overlay in ArcGIS environment. Each layer was assigned a rank and weightage of 1–4 according to their availability of groundwater occurrence. Moreover, the normalized weights of each of the thematic layer were calculated with the aid of pair-wise comparison matrix as adopted by [27] analytical hierarchy process (Tables 1, 2).

Table 1 Pair-wise comparison matrices of the different thematic layers
Full size table
Table 2 Weights of a thematic map for groundwater potential zones
Full size table

The groundwater potential zone (Fig. 6) was mathematically calculated using ArcGIS raster analysis as follows:

$${\text{GWP}} = \left( {{\text{GG}}_{w} {\text{GG}}_{\text{Wi}} + {\text{ GM}}_{w} {\text{GM}}_{\text{Wi}} + {\text{ ST}}_{w} {\text{ST}}_{\text{Wi}} + {\text{ SL}}_{w} {\text{SL}}_{\text{Wi}} + {\text{ LD}}_{w} {\text{LD}}_{\text{Wi}} + {\text{ DD}}_{w} {\text{DD}}_{\text{Wi}} + {\text{ LULC}}_{w} {\text{LULC}}_{\text{Wi}} } \right)$$

where GG = geology, GM = geomorphology, ST = soil type, SL = slope, LD = lineaments density, DD = drainage density, LULC = land use/land cover, w = normalized weight of a theme and Wi = normalized weight of individual classes.

Finally, the results were validated with existing borehole yield data. Figure 2 shows the illustration of the methodology of the study.

4 Results and discussion

4.1 Geology

The study area is underlain by the Precambrian basement complex rocks of North Central and Cretaceous–Quaternary sedimentary rocks of Southeastern and Northwestern Nigeria, respectively (Fig. 4). The basement complex rocks of North Central Nigeria comprise igneous and metamorphic rock units (migmatite–gneiss complex, meta-sedimentary and volcanic series, pan-African older granitoids and per-alkaline younger granitoids). However, the sedimentary rock units of Southeastern and Northwestern Nigeria consist of Anambra and Mid-Niger (Bida/Nupe) Basins and associated with shale, siltstone, claystone, mudstone, limestone, ironstone and sandstone.

Fig. 4
figure 4

(Modified from NGSA, 2006)

Geologic map of the study area.

Full size image

In accordance with the intricacy of the tectonic evolution of the basement complex and sedimentary rocks, several studies have identified the geology and categorization of the Basement and sedimentary rocks of North Central Nigeria. The research conducted by [26, 30] on the geology of North Central Nigeria classified the basement complex rocks into five major rock types and the sedimentary rocks into seven geological formations:

  1. (a)

    Tectonic units (silicified, sheared rocks, large quartz veins).

  2. (b)

    Migmatite–gneiss complex.

  3. (c)

    Meta-sediment/volcanic series.

  4. (d)

    Pan-African older granitoids.

  5. (e)

    Per-alkaline younger granitoids.

  6. (f)

    Gundumi formation.

  7. (g)

    Nupe formation.

  8. (h)

    Nkporo formation.

  9. (i)

    Bassange formation.

  10. (j)

    Nsukka formation.

  11. (k)

    Imo formation.

  12. (l)

    Ogwashi formation.

In the northwestern portion of the study area, the massive unfractured lithological units in the basement complex have little control on the occurrence of groundwater except in a case where secondary porosity has taken place through the development of fissures, water-filled pores, weathered overburden or fractured rock units which in turn form the groundwater potential zones. Meanwhile, in the northeastern and southeastern parts of the area which is made up of sedimentary rocks has more influence on groundwater availability especially, in cases of primary and secondary porosity.

Hence, based on groundwater availability, appropriate weights are assigned to different rock units of the study area. Sandstone was assigned high weightage due to its highest porosity followed by quartzite, shale and limestone which was assigned moderate weights due to their moderate porosity as a result of fracturing, faulting and jointing. Low value was assigned to migmatite/gneisses, biotite granite and schist/metasediments due to their resistance to weathering, faulting and jointing in the area. The lithological units were reclassified according to weights generated (Table 2).

4.2 Geomorphology

According to [12], hydrogeomorphology is the study of various landform and drainage characteristics that could have a direct control on the occurrence and flow of groundwater. The geomorphologic units of the area can be divided into lowlands, plains, plateau and inselbergs (Fig. 5). Among these, lowlands and plains covering about 9946 km/km2 (33.34%) and 7855 km/km2 (26.33%) are very good locations for groundwater targeting; therefore, they were assigned high weightage due to its moderate to low runoff. In the study area, western and eastern sides which constitute about 7543 km/km2 (25.28%) are covered by inselberg hills. The shallow aquifer in the inselberg hill area has moderate to poor groundwater potentials; hence, it was assigned low weightage. Ridges are seen in the northwestern and southeastern sections of the area covered about 4489 km/km2 (15.05%).

Fig. 5
figure 5

Geomorphologic map of the study area

Full size image

4.3 Soil

As indicated in Fig. 6, four types of soils are generally seen in the study area, namely hydromorphic, ferruginous, ferralsols and hydromorphic and ferralsols. Ferralsols soil constitutes about 14,879 km/km2 (49.87%) and is the predominant soil in the study area. However, the ferruginous soils are seen in the western portion of the area and spread over 9975 km/km2 (33.44%). Hydromorphic soil which covered an area extent of about 2986 km/km2 (10.01%) is seen all along the banks of Niger and Benue Rivers. According to their influence on groundwater occurrence, ferralsols and hydromorphic soils have high water holding capacity; therefore, they were assigned high weightage.

Fig. 6
figure 6

Soil map of the study area

Full size image

4.4 Slope

Based on geomorphologic features, slope is among the mechanisms that govern groundwater recharge and percolation; however, the nature of slope of an area alongside with other geomorphic features determines the occurrence of groundwater. Similarly, slope map of an area plays a vital role in determination of groundwater recharge as it is influenced by rate of surface water infiltration. In the area with low slope, surface water runoff is low, therefore encouraging longer residence time for rainwater to infiltrate and percolate within the subsurface material, while high slope regions favor high surface runoff with short residence time for infiltration and percolation. In the study area, slope is classified on percentage basis as < 5.25, 5.25–10, 10.1–17.5, > 17.5 designed as nearly level, gently sloping, moderately sloping and steeply sloping (Fig. 7). Most of the study area belongs to nearly level and gently sloping category (< 5.25 and 5.25–10%). The dominance of the nearly level and gently slope classes is favorable while considering the groundwater potential, and they were assigned high weightage.

Fig. 7
figure 7

Slope map of the study area

Full size image

4.5 Lineament density

Lineament density is an important hydrogeomorphic parameter in groundwater studies. The thematic layers of lineament density map of the study area revealed four main lineament density classes: 0.00–0.04 km/km2 (low); 0.05–0.09 km/km2 (moderate); 0.10–0.16 km/km2 (high) and 0.17–0.35 km/km2 (very high). It is apparent from Fig. 8 that the majority of the study area is dominated by low lineament density constituting about 21.068 km/km2 (70.62%). Meanwhile, area with very high lineament density covers only about 562 km/km2 (1.88%) and they are seen in the region underlain by zones of localized weathering which enhances permeability and porosity. Moreover, moderate to high lineament density constitutes about 7130 km/km2 (23.90%) and 1073 km/km2 (3.60%), respectively.

Fig. 8
figure 8

Lineament density map of the study area

Full size image

4.6 Drainage density

Drainage basin with low drainage density (km/km2) shows that a large quantity of the rainfall infiltrates within the subsurface lithology. Consequently, drainage density is one the important factors that determines groundwater potential zones as it give ideas on the amount of rainfall percolation relative to runoff, slope, presence of morphological features and the subsurface formations within an area. However, it has also been observed that a low drainage density is more likely to occur in area of highly permeable subsurface lithology under nearly level to gently slope where relief is low, while high drainage density is the resultant of impermeable subsurface materials, inselberg relief, low lineaments and high slope.

In the study area, the drainage density map was grouped into four classes which are 0–0.8 (low); 0.8–1.8 (moderate); 1.8–2.9 (high) and 2.9–4.7 (very high) (Fig. 9). Low drainage density values ranging from 0 to 0.8 were observed in the area where morphological features depict very lowland and plain surface and have very high infiltration capacity and yields preferable groundwater potentials. Area with very high drainage density that covers about 809 km/km2 (2.71%) was given lower weightage as compared to the area with low drainage density which was assigned high weightage and account for about 18,641 km/km2 (62.48%). Moreover, moderate and high drainage density constitutes about 8946 km/km2 (29.99%) and 1437 km/km2 (4.82%), respectively, and implies low or moderate groundwater recharge potentials for approximately 75% of the study area.

Fig. 9
figure 9

Drainage density map of the study area

Full size image

4.7 Land use/land cover

Land use/land cover map as presented in Fig. 10 reveals that the study area is made up of vast vegetation, waterlogged, build up and rock outcrop. Table 1 shows that vegetation and water bodies accounted for 22,305 km/km2 (74.21%), whereas built-up area and rock outcrop covered 7156 km/km2 (24.38%) and 372 km/km2 (0.96%), respectively. The dominance of vegetal cover could be attributed to widespread of agricultural practice in the study area. Vegetation and waterlogged areas are good for groundwater infiltration, and therefore, they were given high weightage.

Fig. 10
figure 10

Land use/land cover map of the study area

Full size image

4.8 Characterization of groundwater potential zones

Groundwater potential zones of the area were generated from assignment and normalized weighting of the separate features of the thematic layers, viz., geology, geomorphology, soil type, slope, lineament density, drainage density and land use/land cover maps (Table 3).

Table 3 Assigned and normalized weights of different parameter for groundwater potential zones
Full size table

As displayed in Fig. 11, the groundwater potential zones map of the study area shows four distinct zones, namely poor, moderate, good and excellent zones. The groundwater map of the study area revealed that western section of the area which covers about 10,634.62 km/km2 (38.4%) indicates poor to moderate groundwater recharge potentials, whereas the northern and southern portions of the study area exhibit good groundwater recharge potential zones and constituting an area extent of approximately 14,539.30 km/km2 (51.35%). Consequently, the eastern part of the area which spread about 4659.13 km/km2 (10.23%) revealed excellent groundwater recharge potential zones (Table 4). However, a closer look at the groundwater potential map of the study area revealed that excellent potential zones occurred in the area underlain by false-bedded sandstone within the Anambra Basin in addition to low drainage density, gently slope, ferralsols type of soil and plain surface. Moreover, areas underlain by migmatite and gneiss specifically the western part of the area is characterized by low rainfall, ferruginous soil type, relatively high drainage density, steep slope and inselberg geomorphology have poor to moderate groundwater potentials. On the other hand, northern and southern portions of the study area underlain by shale, clay, ironstone, mudstone, quartz-schist, feldspathic sandstone and quartzite and characterized by steep slope, hydromorphic soil type, ridges and moderate lineament density exhibit moderate groundwater potential zones.

Fig. 11
figure 11

Groundwater potential zone map of the study area

Full size image
Table 4 Distribution of groundwater potential zones, area extent and yield classification
Full size table

4.9 Verification with borehole yield data

The accuracy of the groundwater potential zone map of the study area based on RS and GIS-based model was validated by the data on existing boreholes (yield, depth and saturated thickness) and was collated for 60 boreholes in the study area (Table 5).

Table 5 Summary of existing borehole data for the different underlying bed rocks
Full size table

Generally, the depth of boreholes across the study area varied from 30.0 to 185.9 m within the area underlain by sedimentary rocks and from 25.3 to 95.0 m in the area underlain by basement complex. At the same time, saturated thickness fluctuates between 7.3 m in the magmatic/gneiss terrains, 4.8 m in the granitic rock environments and 36.7 m in the quartzite/quartz-schist settings and from 82.3 m in the false-bedded sandstone terrains to 33.2 m in the sandy-shale environments. Consequently, the borehole yields range from 26 to 226 m3/day in the migmatite/gneiss areas, 44–100 m3/day in the granitic rock terrains, 57–350 m3/day in the quartzite/quartz-schist settings, 100–530 m3/day in the false-bedded sandstone units and from 65 to 480 m3/day in the sandy-shale environments.

However, the data from existing boreholes were superimposed on the groundwater potential zones map and various boreholes with different yields varied for distinct groundwater potential zones were delineated. Figure 12 displays the frequency distribution of different groundwater depths, saturated thickness and yields in the various terrains.

Fig. 12
figure 12

Groundwater depth, saturated thickness and yields in different geological settings of the study area

Full size image

Finally, the accuracy of the groundwater potential zone map and the data obtained from existing boreholes of the study area reveal positive correlation.

5 Conclusion

Identification of groundwater potential zones in Kogi State was conducted utilizing GIS and remote sensing techniques that provided an effective methodology in the context of time and cost. Thematic layers of geology, geomorphology, soil type, lineament density, slope, drainage density and land use/land cover maps were integrated on a GIS environment employing the weighted overlay method.

The groundwater potential zones of the study area are the cumulative weightage of all the thematic layers, and the categorization comprises the pertinent availability of groundwater, the more the weight score, the higher the groundwater potential. The groundwater potential map of the study area was grouped into four classes, and their distributions are: excellent (10.23%), good (34.26%), moderate (51.35%) and poor (4.16%). The result shows that the excellent groundwater potential zone is restrained to only 10.23% of the study area, in lowlands and plains. The generalized groundwater potential map indicates the prevailing control of geology and other hydrogeomorphologic characteristics in the identification of groundwater potential zones; the areas underlain by banded migmatite/gneisses, biotite granite, porphyritic biotite/granitic and schist/metasediments are characterized by moderate to poor groundwater potentials when compared to areas with good and excellent potentials which is characterized by Cretaceous to Quaternary sedimentary bedrocks as well as fractured/weathered quartzite and quartz-schist bedrock settings.

Consequently, the verification of groundwater potential map with existing borehole data in the area showed a positive correlation. As per Table 4, it can see that the borehole yields between 156 and 255 m3/day, and above > 225 m3/day are in the areas with good and excellent groundwater potentials, whereas borehole yields between 155 and < 55 m3/day are in the areas with moderate and poor groundwater potentials simultaneously.

The results verification clearly shows the efficiency of the integrated remote sensing and GIS approaches incorporated in this research as a useful tool for identification of suitable locations for groundwater extraction, proper management and sustainable development. However, it is perceived that the current methodology can be used as a prospective guideline for groundwater exploration and exploitation in both hard and soft rock terrains.