GROUNDWATER CHARACTERIZATION IN PARTS OF THE LOWER BENUE TROUGH, NIGERIA
Groundwater constitutes the main sources of water supply in part of lower Benue Trough (mainly Gboko area). Sustainable development and management of the resources require an understanding of its holistic characteristics. In this study, an attempt has been made to study the groundwater quantitative potential distributions, its geochemical evolution, geochemical characteristics, and quality. The methods of investigation employed included pumping tests, static water level measurement, groundwater and subsurface soil samplings. The results of the pumping tests carried out within Gboko metropolis showed that the Gboko Central has the lowest groundwater potential with transmissivity value of 8.208m2/day while Gboko Low Cost area has the highest groundwater potential with transmissivity value of 281.52m2/day. The variation in the groundwater potential is attributed to varying degree of weathering. Nitrate concentration in the studied groundwater ranged from 0mg/l (below detection limit of 0.02) to 156.85mg/l and its concentration has a negative correlation (correlation coefficient of -0.0997) with water well depth. Nitrate was also found to pollute the groundwater of the area with its contamination observed to result from both agricultural practices and wastes, and also from domestic wastes. The groundwater from the urban rland use (especially the poor income earners with poor environmental sanitation) have more dissolved nitrate than those from the rural agricultural land use. The concentration of each heavy metal tested in groundwater of the area varied spatially throughout the area. Co, Fe, Pb, Ba and Mn were found to be above the stipulated WHO 2008 guideline values. The sources of heavy metals in the groundwater were found to be both geogenic origin (weathering of host rocks) and anthropogenic (agricultural wastes and practices, domestic wastes and urbanization effects). The groundwater of the area is not generally potable as 84% of the groundwater samples tested was found to have heavy metal content. Principal component analysis revealed 12 controlling processes that are affecting the hydrochemical characteristics of the groundwater. Some of these processes include weathering of host rocks REE minerals, geogenic and anthropogenic contamination, uranyl complex factor, alkaline earth metal and heavy metal factor, redox potential factor, mixed or diverse contamination sources, metallurgical material wastes factor, and arsenic contamination factor. Cluster analysis grouped the groundwater samples of the area into 16 groups with various groups falling under high or low pollution loading. The results of the rare earth elements (REEs) showed that the groundwater of the area is characterised with predominance of light rare earth elements (LREEs) formed under an oxidising condition over heavy rare earth elements (HREEs) formed under a reducing condition. Also, the groundwater of the area all showed positive Ce and Eu anomalies. The positive Ce anomaly may be attributed to oxidation of Ce3+ to Ce4+ and incorporation of Ce into Mn and Fe oxyhydroxides phases while positive Eu may be attributed to weathering of zircon rich minerals. The δ18O composition of the groundwater samples of the study area ranged from -3.14 o/oo to -28.45 o/oo, δ2H ranged from -28.45o/oo to -14.53o/oo,and the groundwater showed a general depletion in both δ18O and δ2H. Also, 85% of the groundwater of the area was formed or recharged under a cooler climate than the present day. The aquifer vulnerability GOD model classified the aquifers of the area into three zones namely, the high vulnerability, the moderate vulnerability and the low vulnerability zones. The moderate and low vulnerability zones predominate in the area. The results of the subsurface soil samples analysis revealed that the soils in the area have been polluted by As, Cd, Ag, Au, Mn, Mo, Ni, Pb, Co, Cr, Cu, Hg, Sb and V. It was observed that both geogenic sources and anthropogenic activities mainly agricultural activities and urbanization are contributing to the contamination and pollution of the soils.