A Hypothesis on the Origin of Warm Groundwater 50KM NW of Benin City, Southern Nigeria

Abstract

The discovery of warm groundwater (about 52⁰C) in Ofumwengbe community, near Benin City, Nigeria provides a strong impetus for this study, which seeks to provide a geological explanation of its origin. Eighty-Four (84) ditch cutting samples were obtained from three (03) groundwater wells drilled into aquifers within the Imo Formation, which underlies the study area. The methodology employed involves lithologic description, acid test to estimate calcite content as well as bulk geochemistry (25 samples analysed with CNS analyser) and elemental analysis (20 samples analysed with sodium peroxide fusion and ICP-OES and ICP-MS techniques). The results obtained show that the Imo Formation is dominated by light grey mudstones and subordinate sand units with varying degrees of effervescence. The average Uranium (U) and Thorium (Th) concentration in the samples are 4.37ppm and 9.4ppm respectively. The mean Total Carbon, Total Inorganic Carbon Total Organic Carbon (TOC) are 2.54 wt. %, 1.45 wt. % and 1.15 wt. % respectively. The observed lithological and geochemical characteristics suggest a proximal shallow marine paleoenvironment with oxic bottom waters, which agrees with data published by other researchers. Furthermore, the observed TOC, U, and Th data, the negative covariation between TOC vs. U, and Th indicate that the mudstones are not radioactive, and cannot by itself generate the heat observed in the warm groundwater. We hypothesize that a deeper heat source (perhaps the basement) transmitted heat through the fault systems of the Benin hinge line, which was subsequently trapped by the thick low thermal conductivity mudstones of the Imo Formation, ultimately warming up the water within the sand aquifer system of the Imo Formation.