Groundwater quality of Benin City urban aquifer of the Pleistocene-Oligocene Benin Formation, Nigeria

Abstract

A general investigation of the groundwater quality of the Benin City Urban Aquifer of Benin Formation for the effects of athropogenic activities shows the following results: lead (Pb) has mean value of 0.044 mg/l from 25 sampled boreholes.  Seven of these borehole waters show anomalous Pb values of over 0.050mg/l.  For the 25 boreholes, Pb mean value of 0.044mg/l, 0.055mg/l standard deviation and variance of 0.003mg/l are indicative of high Pb concentration compared with 0.05mg/l W.H.O. limit.  Although there is low concentration-n of NO₃ with a mean value of 0.7mg/l, nitrate shows a general increase towards the more densely populated city centre.  Comparatively, specific site values of nitrate concentrations of 5.65mg/l, 3.19mg/l, 1.74mg/l, 1.12mg/l, although still with safe limits; become significant as they occur within the same target seven of the 25 sampled borehole waters, two of which show 2 and 3 coliform counts (CFU/ML) respectively.  These seven boreholes are found to be located near refuse dumps, breweries, cattle and livestock markets, petrol stations, and automobile (motor mechanic) repair grounds; suggesting an impact.  Furthermore, a trilinear plot shows a Ca-Mg (HCO₃) water chemistry indicative of a shallow groundwater type and of short-recent geologic history.