Elemental enrichment of sediments in an unprotected shallow groundwater of Lagos and Ogun States, Nigeria.

Sediments quality is a good indicator of pollution in a water body where various elements were concentrated. Limited information is available on sediments from hand-dug wells. The present study evaluates sediment samples collected from groundwater of Lagos and Ogun States, Southwest Nigeria. Twenty sediment samples were collected from shallow groundwater, and ten rock samples were also collected from the vicinity of recently dug wells. Trace elements were determined in both the sediments and the rock samples using inductively coupled plasma-mass spectrometry. The enrichment factor was calculated for different elements using Fe, Ti, Mn and Cu as normalizing elements. The order of sediment contamination with each normalizing elements are Cu > Mn > Ti > Fe. The geo-accumulation (Igeo) indexes for Cr, Pb, Cu and Ni are 1.31, 1.05, 1.94 and 1.85, respectively. The Igeo for Lagos sediments is in the order Cr > Pb > Ni > Cu, while in Ogun sediments the order is Cu > Ni > Cr > Pb. The results were compared with Canadian Council of Ministers of Environment values of threshold effect level and probable effect level, which shows the sediments are not toxic. The pollution load index and ecological risk index values are 2.463 and 0.0014, respectively, which further indicates the sediments are not toxic in nature. The major source of most elements in sediments is the host rocks found in the vicinity of the groundwater while high level of some elements recorded in sediments are from the anthropogenic sources.