Impact of fluvial deposition on potential toxic metals burden of selected floodplains in Southwestern Nigeria

Abstract

Rivers in many parts of the world are receptors for all kinds of wastes, including those containing trace metal contaminants. This is particularly so in many African countries where environmental laws are poorly enforced. A common phenomenon in those countries is the overflowing of rivers to the adjacent lands (so-called floodplains), thereby encouraging the cultivation of food crops on the plains, as artificial irrigation is often not necessary. However, toxic trace metals may be deposited when polluted river waters overflow to the plains. Here, we carried out an initial investigation into the impact of flood water deposition on chromium (Cr), cobalt (Co) and lead (Pb) concentrations of three river floodplains in Southwestern Nigeria. Using a sequential extraction technique that fractionates metals in soil into their different species, we show that fluvial deposition contributes more significantly to the lead and cobalt contents of the floodplain soils than the chromium content. Contribution of fluvial deposition to total concentrations of the metals was estimated to be (79.3–99%) for Pb, (67.2–85.7%) for Co, and (37.9–50.4%) for Cr. Clay and organic matter composition of the floodplains were up to 25.6% and 12.8%, respectively, compared to the maximum of 15.9% and 4.7% measured in adjacent non-flooded lands which served as controls. Pb and Cr were detected at values ranging from 0.01 to 0.03 mg/kg in an edible vegetable (Amaranthus hybridus) grown on all the floodplains, thus revealing a route of chronic human exposure to these potentially toxic metals.