Quality and hydrochemical assessment of groundwater in geological transition zones: A case study from N.E. Nigeria

Abstract 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 ( 50) ratings.