The behavior of metals/metalloids during natural weathering: A systematic study of the mono-lithological watersheds in the upper Pearl River Basin, China

The Pearl River Basin is a region with elevated geochemical baseline levels due to high metals/metalloids content in the sediments and soils. In this paper we systematically analyzed the behavior of these metals/metalloid (Cr, Ni, Cu, Zn, Cd, As, and Pb) in bedrock, riverbed sediments, soils, and river water in three different mono-lithological areas (carbonate, basalt, and mud-shale). The results show that the content of transition elements (Cr, Ni, Cu, and Zn) in carbonate rocks are much lower than for shales and basalts, but these metals produce higher enrichment levels in sediments and soils through rapid weathering. Furthermore, Cd, As and Pb are significantly enriched in the upper soils of the carbonate profile and the watersheds dispersed with black shales have distinctly higher Cd enrichment levels. This secondary enrichment of metals through the weathering and pedogenesis of carbonate rocks, and the discharge of metal elements by the weathering of the black rock series, leads to metal pollution in the Pearl River. Most of the small watersheds in the upper reaches of the Pearl River exhibit low or moderate ecological risk but considerable ecological risk exist in the watersheds dispersed with these black shales. Among all the trace elements, Cd generate the highest ecological risk in the individual small watersheds in the upper reaches of the Pearl River. (C) 2019 Elsevier B.V. All rights reserved.