The deposition of trace elements and major nutrients in dust and rainwater in northern Nigeria

Abstract

The deposition of trace and major elements from the atmosphere was measured throughout 1976 at a rural site in the Nigerian savanna. Separate measurements were made of the deposition in rainwater and dry deposition, and of the dry deposition velocity that is related to particle size. The total (wet and dry) deposition of many elements showed marked seasonal dependence, increasing by an order of magnitude when, in April, the northerly winds from the Sahara desert are replaced by a tropical maritime air stream which remains predominant until October. The concentration in rainwater and deposition of elements associated with soil dust in Nigeria exceeded those in the U.K., e.g. for Al, Fe and Sc. However, the reverse was true for potentially toxic pollutants (As, Pb, Se, V). These differences were emphasised by a comparison of enrichment factors (ratio of element to Sc in total deposition relative to ratios of the same elements in average soil). In most cases the element/Sc ratios in total deposition and in locally grown sorghum grain were very similar though the ratios for some elements were higher in total deposition than in local and “average” soil. The input of elements from the atmosphere exceeded their removal in the grain.