Distribution of trace metals in soils from Mount Meru, Tanzania:
a combined bulk and leach study

Mark Gabriel Little* and Cin-Ty Aeolus Lee

Abstract

We conducted a modified BCR (Bureau Commun Reference) sequential extraction on an andosol from Mt. Meru in Northern Tanzania to determine the relative contribution of water soluble, carbonate, exchangeable, oxide, and residual fractions to the bulk composition of the soil. Elemental compositions of the bulk soil and four extracts, water soluble, carbonate and exchangeable, reducible oxides, and organic, were determined via ICP-MS and corrected for loss on ignition. The bulk soil and water soluble fraction were marked by surface enrichments of K and Na in a similar ratio to the protolith. The carbonate and exchangeable leach was marked by a Ca rich layer above 60 cm and a Ba and Sr rich layer below 150 cm. U enrichments in the carbonate and exchangeable fraction below 150 cm appear to be associated with Sr. Additionally, Sr:Ca ratio in the carbonate and exchangeable leach suggest that the source material for the carbonate soil fraction is the bedrock above 140cm and a different, high Sr:Ca source below 140 cm. The oxide step indicated the presence of an oxide rich layer below 120 cm which sequestered Cu, Pb, and Co in addition to the more refractory Ti and Zr. The organic leach revealed that Zr, Hf, and Nb may be sequestered by organics. We found three notable observations. (1) Exogenous material was likely added to the soil column from two different sources—one local source at the surface and another foreign source below 150 cm, the latter possibly requiring lateral transport of Sr, Ca, and Ba. (2) The Sr-bearing horizon below 150 cm appears to correspond with U retention. (3) Recovery of Zr, Hf, and Ta from the ORG fraction suggests that these elements may be mobile when associated with organic material.

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