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The increasing demand for water occasioned by harsh climatic conditions, population explosion, and increasing urbanization has necessitated more attention and reliance on groundwater resources, particularly in water-limited regions. Thus, judicious management of available groundwater resources becomes crucial to meet the freshwater requirements in such zones. In this study, remote sensing and geographic information system were deployed to delineate groundwater recharge zones in semi-arid geological transition zones of Bauchi, northeastern Nigeria.Seven thematic layers comprising elevation, slope, land use/land cover, drainage density, lithology, magnetic lineament density, and hydraulic head were integrated based on their degree of influence in groundwater recharge. Normalization of the weights was achieved through the analytical hierarchical process (AHP), while the overall integration of the thematic maps was actualized by overlay analysis in ArcGIS 10.3.1 environment to generate a final groundwater recharge zone map of the area.The resulting map was delineated into three different zones: low, moderate, and high, occupying 20% (627 km 2 ), 77% (2352 km 2 ), and 3% (80 km 2 ) respectively. The overall assessment of recharge zones revealed that a high percentage of the southwestern part of the study are characterized dominantly by poor groundwater recharge potential, attributable to high elevations and impervious rock outcrops with associated steep slopes, thus limited infiltration owing to the high velocity of runoff. The northwestern and parts of the southwestern zones, underlain by migmatite/gneiss, and partly granites, however, exhibited medium-low recharge potential, owing to the occurrence of considerably lower altitudes and occupied by thicker regoliths and underlying rocks of mediumhigh lineament densities. The sandstones revealed predominantly medium-high recharge potential, attributable to high permeability, lower hydraulic heads, and relatively flat geomorphology which enhances meteoric recharge and base flow processes.

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Sustainable management of groundwater resources in geological transition zones is essential due to their complex geology, increasing population, industrialization and climate change. Groundwater quality monitoring and assessment represents a viable panacea to this problem. Therefore, there is a great need to investigate groundwater resources in terms of their chemistry and pollution, to ascertain their quality and implement robust pollution abatement strategies. This study focused on the characterisation of groundwater in a typical geological transition zone in Northeastern Nigeria. Eighty-seven (87) groundwater samples were collected. pH, conductivity, total dissolved solids (TDS) were measured in-situ using a multiparameter probe, while major cations and anions were measured using atomic absorption spectrometry and ion chromatography, respectively. TDS ranged between 95 and 1,154 mg L− 1 in basement terrains and between 49 − 1,105 in sedimentary areas. pH ranged between 6.8 and 7.7 mg L− 1 in basement terrains and between 5.0 and 6.5 in sedimentary areas, suggesting a moderately acidic to alkaline low mineralized groundwater. Calcium (2.6–128.0 mg L− 1) was the dominant cation in the basement areas, suggesting silicate weathering/dissolution, while sodium (1.9–106.0 mg L− 1) dominated the sedimentary zones due to base exchange reactions. The water quality index revealed that the basement setting was predominated by poor to unsuitable groundwater, while the sedimentary terrain was characterised by potable groundwater. The dominant hydrochemical facie in the basement areas was Ca2+–(Mg2+)–HCO3− characteristic of recharge meteoric water. The Na+– (K+)–HCO3− facie characterised the sedimentary zones, indicative of cation exchange reactions, while the mixed water facie typifies the geological contact zones. The shallow nature of the basement groundwaters makes them more susceptible to geogenic and anthropogenic pollution compared to the sandstone aquifers. However, the basement aquifers have better irrigation indices (Kelly ratio and Soluble sodium percent) as compared to the sandstone aquifers which exhibits poor Kelly ratios (< 1) and Soluble sodium percent (> 50) ratings.

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