Mass balance of nine trace elements in two karst catchments in southwest China.

High geological background levels of trace elements (TEs) and high population density in the karst areas of southwest China have imposed environmental pressure on the fragile ecosystems in this region. Understanding the mass budget of TEs, especially the toxic ones, is of great importance to sustain future developments. This study investigates the mass balance and fate of nine TEs (cadmium, arsenic, lead, chromium, copper, nickel, zinc, thallium, and antimony) in two karst catchments (Huilong and Chenqi) in southwest China through estimation of their mass budgets in throughfall, open field precipitation, total suspended particulate matter (TSP), litterfall, fertilization, harvested crops, surface runoff, and underground runoff. The estimated net fluxes are positive, indicating a source region, for four elements (Cu, Cr, Ni, and Tl) and negative, indicating a sink region, for five elements (As, Cd, Pb, Sb, and Zn) in both catchments. The net fluxes for the nine elements in Chenqi catchment are within a relatively small range (2.6, 2.0, 1.6, 0.6, -0.05, -0.5, -0.5, -2.9, and -3.3 mg m yr for Cu, Ni, Cr, Tl, Cd, Zn, Sb, Pb, and As, respectively), but in Huilong catchment in quite a large range (15.5, 6.0, 1.0, 0.8, -0.3, -0.9, -4.5, -7.5, and -8.7 mg m yr for Tl, Cr, Ni, Cu, Cd, Sb, As, Pb, and Zn, respectively). Rainfall (12.3%-66.2%) and litterfall (18.4%-81.3%) are the major input flux pathways, while crops harvest (16%-99%) is the major output flux pathway for the TEs in both catchments, indicating that the fate of TEs is shaped by both natural factors such as precipitation and litterfall and human activities such as fertilization and crop harvesting in these forestland-farmland compound karst catchments. Results from this study suggest that restoring forests from low-yield sloping farmlands will be useful for controlling TEs pollution in these fragile karst regions with high geological background TEs.