CHARACTERISATION OF AQUIFERS IN BAUCHI-ALKALERI-KIRFI AREAS, NORTHEASTERN NIGERIA

Abstract

Geological Transition Zones (GTZ) are hydrogeologically problematic, owing to poorly defined basal and lateral contacts between the lithological units that make up the zones. Attempts have been made to delineate the complex lithological contacts in GTZ using geophysical and geological methods. However, the quality, hydrochemical and isotope characteristics of groundwater in GTZ, which are very significant to the sustainable management of the groundwater resources of GTZ, have not received much attention.Therefore, this study focused on characterisation of the aquifers in a typical GTZ in Bauchi-Alkaleri-Kirfi areas, Northeastern Nigeria. Geological and geophysical studies(aeromagnetic and geoelectric) were carried out for lithological delineation and characterisation of the aquifer systems. Eighty Seven Groundwater samples were collected and in-situ determination pH, conductivity and Total Dissolved Solids (TDS) were undertaken using a digital meter. Hydrochemical and stable isotope analyses were undertaken using atomic absorption spectrometry, ion chromatography and mass spectrometry respectively. Data were analysed using descriptive statistics and standard hydrochemical plots. The lithological units identified were migmatite/gneiss, granite and charnockite overlain by sandstone units. Three to five geoelectric layers with aquifer thickness of 11-158m were identified in the sandstones units,whilethe Basement Complex areas revealed saprolite thicknesses of 1.3-67.3m (granite), 1.2-44.6m (charnockite)and 2.4-83.8m (migmatite/gneiss).The depth to bedrock is put at 45m below the ground surface in the basement areas and >800m in the sedimentary terrain. The overall groundwater potential map revealed three Groundwater Potential Zones (GPZ) namely: poor (charnockite and granite), moderate (migmatite/gneiss and granite), and good (dominantly sandstone).The TDS and pH values of 95-1,558mg/L and 6.4-7.7 and 15-1,105mg/L and 5.0-8.5,characterised the basement terrain and the sedimentary areas respectively, suggesting a moderately acidic to alkaline low mineralised groundwater. Calcium (2.57-216.20mg/L) is the dominant cation in the basement areas, suggesting silicate weathering/ dissolution, while Na+ (1.59-106.30mg/L) dominated the sedimentary zones, indicative of cation exchange reactions. The water quality index revealed 58.5% as potable and 41.05% as unsuitable. The dominant hydrochemical facie in the basement areas was Ca2+-(Mg2+)-HCO3- (54.2%), characteristic of recharge meteoric water. The Na+-(K+)-HCO3-facie (14.1%) characterised the sedimentary zones, indicative of cation exchange reactions. Stable isotopes revealed meteoric source characterised by kinetic evaporation with values of δ18O =−3.4‰ and δ2H=−19.6‰ for the basement area. However, values of δ18O=−4.1‰, δ2H=−25.7‰ for the sedimentary terrain are indicative of depleted groundwater due to latitude effect and longer residence time. Saprolites and medium to coarse sandstone units are the aquifer units in the Bauchi-Alkaleri-Kirfi areas. The recharge system of the saprolitic units are more influenced by precipitation as compared to the sandstone aquifers.