Heavy Metal Speciation and Evolution in Groundwaters Within the Lower Cross River Hydrological Basin, Se-Nigeria

In recent times, a lot of work has been carried out in evaluating environmental impacts of potentially toxic metals on a wide variety of environmental materials in a wide range of different applications. This study documents the concentration levels and risk potentials of selected potentially toxic metals in shallow groundwaters across the lower Cross River hydrological basin. This is achieved by geochemical modeling using the VISUAL-MINTEQ computer program to identify ionic species, their saturation indices and possible bioavailability. Chemical analytical results show that the metals, Fe, Mn, Cu, Pb, Zn, Cd and Cr are of low concentrations, with no direct risk to water quality. Modeling indicates they are dominantly present as free mobile ions contributing major percentages to total ion concentration (approximate 60–90% of all metals in the groundwaters). Other ionic species, dominantly aqueous carbonate, sulphates, chlorides and hydroxides of these metals, exhibit lesser percentages of contributions to total ion concentrations. Estimated saturation indices shows the groundwaters are generally undersaturated with metallic species, with only Fe 2+ ionic species recording oversaturation in groundwaters within the basement sector of the study area. Mn ions also show low levels of saturation in parts of the study area. The low concentrations of these metals and their ionic species are attributed to hydrochemical processes controlled by the presence of limiting mineral phases (carbonates and sulphates). Dominant presences of goethite further inhibit mobility of ionic species of metals and consequently reduce the risk of their contamination to groundwater and possible bioavailability.