Redistribution of trace elements upon the weathering of volcanic ash soils in Costa Rica

Abstract

Redistribution of trace elements upon weathering over Al/Fe-humuscomplexes and amorphous minerals was studied in a sequence of soils derived from andesitic ash on the Turrialba volcano in Costa Rica. All profiles are located on the northern slope where the mean annual precipitation reaches up to 8000 mm (Nieuwenhuyse, not published). The profiles were described, and both undisturbed core samples and field moist samples were taken. A part of the samles was analyzed, the analyses done include selective dissolution and total chemical analysis with XRFS. Dry bulkdensities increase from 0.25-0.40 g/cm3 in the profiles at lower altitudes to 0.90-1.40 g/cm3 in the profiles at higher altitudes. At the same time moisture contents decrease from about 300 percent in the profiles with low altitudes to about 90 percent in the profiles with high altitudes. There is a large difference between the measured CEC and the derived effective CEC. The measured CEC is too high due to adsorption of Mg2+ along with SO subíndice 4 exponente 2- by amorphous minerals. Total C contents reach up to 20 percent. The top of each profile, except of profile T3, is pure non-allophanic, while the bottom is pure allophanic. With two sets of graphs the interaction between trace elements and organic matter and allophane was studied. Zr was used as reference. The first set shows the contents of trace elements versus the organic matter content. To exclude the diluting effect of organic matter a second set was prepared showing the interaction between trace elements and allophane. Both sets show that the profiles are devided into two groups (T1, T2 and T3 versus T4-T8) and that important processes in the partitioning of trace elements are physical and chemical weathering. This approach, together with the other results, has led to the following conclusions. CEC determination using MgSO subíndice 4 gives CEC values which are too high. Soils at lower altitudes (640-950 m) show more progressive weathering than soils at higher altitudes. Co, Cu, Ga, La, Pb, V and Zn form complexes with organic matter. Cr, Co and V are bound to allophane. Since V is bound to organic matter and allophane, V would be of limited use as a reference element. The present study is based on bulk data only, therefore further research is neccesary.