The influence of rainfall on metal concentration and behavior in the soil

The effect of rainfall on the behavior of Fe, Mn, Cu, Zn, Pb, and Ti in mat ure soils developed on same-age volcanic bedrock in Hawaii was studied. Con centrations of the mentioned metals in total digested and weak acid-leached soil and bedrock samples were analyzed. Mass transport calculations yielde d the quantity of metal enrichment or depletion in the soils relative to th e bedrock. In addition, elemental mobility and the distribution between the labile and the residual fraction were examined. Based on mass transport calculations there is no significant effect of rain fall amount on the enrichment or depletion of any elements (except Cu) with in soils receiving less than 570 mm/yr (4 sites). In the intermediate sites (4 sites, 930-1380 mm/yr) there is a variable effect of rainfall on all el ements in the soil, but the extent of variation differs from one metal to t he other. In the rainy site (2500 mm/yr) there is a large degree of enrichm ent or depletion of certain metals. Manganese and Zn are highly depleted an d Pb is strongly enriched, whereas Fe and Cu do nor. show a significant mas s change. Among all elements, Pb is the only one that has a continuous effe ct of rainfall throughout the transect. Lead is enriched in all the sites a nd this enrichment is well correlated with increasing rainfall. Iron is imm obile in the soil, reflecting the relative stability of the Fe-rich phases and its high correlation with Ti. The mass change of Mn throughout the tran sect is remarkably correlated with Al (R-2 = 0.87). The behavior of the tra ce metals Pb, Zn, and Cu in the Kohala soils is different because of the ph ases with which these metals are likely to be associated. Lead is probably adsorbed and coprecipitated mostly with Fe oxides and hydrous oxides. Zinc is presumably associated with Mn and Al oxides and hydrous oxides and with organic matter, whereas Cu is most likely complexed by organic matter. Copy right (C) 1999 Elsevier Science Ltd.