Geomorphic Significance on Distribution of Heavy Metals in Soils Affected by Pb-Zn Mining in a Limestone Karst, Western Thailand

Abstract

Soils in the Pb/Zn mineralized area derived from the limestone karst in Western Thailand, principally located in complicate mountainous/upland areas. The distributions of metals are influenced by the natural geomorphic processes and their translocation or by anthropogenic activities (Pb/Zn mine). The objective of this study was to evaluate contents of metals in soils relating to their geochemistry and geomorphic settings. Soil samples were collected at three depths including Ap, base of Ap-60 and 60-100 cm section. Total concentrations of metals in the soils were determined by extraction with aqua regia. Descriptive statistics, Pearson correlation coefficients and Principal Components Analysis (PCA) were carried out to interpret metal concentrations and distribution of metals in different geomorphic units on landscape.The average concentrations of Al, Si, Ti, Fe, Mn, Ca, K and Mg were high in all soil samples. In particular, concentrations of As, Ba, Cd, Cr, Cu, Pb and Zn exceed the limit for agriculture and habitation. The highest concentrations of As, Ba, Cd, Cr, Cu, Pb and Zn were 71 mg kg^-1, 1,611 mg kg^-1, 73 mg kg-1, 71 mg kg^-1, 87 mg kg^-1, 8,177 mg kg^-1and 3,869 mg kg^-1, respectively. The distribution of Ag, Ba, Ca, Cd, Mg, Mn, Na, P, Pb, S, Sr and Zn were in the surface horizons whereas Al, As, Co, Cr, Cu, Fe, Mo, Ni, Sc, Th, V and Y increase with depth. Relationships of metals and some soil properties especially soil pH (pH H₂Oand pH KCl) are strongly positive for Ag, Ba, Be, Ca, Mn, Mg, Pb, S, Sr, Y and Zn. Closed correlations among Ca, Ag, Ba, Cd, Pb, S, Sr, Mg and Zn and soil pH are associated with valley floor, lower footslope, lower midslope, and crest near to the Pb/Zn mines reflect the higher influence of anthropogenic activities than that of karst weathering processes. On the other hand, the high concentrations of Al, V, Cr, Fe, Ti, Sc, Ni, Mn, As, Cu, Bi, Co, Be, P, Y, Th, Ce, Rb, Mo, La, K and clay content are associated with upper and lower midslope positions. The overall results obtained therefore demonstrate that the distribution of high concentrations of As, Ba, Cd, Cr, Cu, Pb and Zn is mainly influenced by mining activities but general distribution of metal concentrations in these soils are clearly influenced by tropical karst weathering of the mineralized limestone.