Concentration and distribution of mercury in drainage catchment sediment and alluvial soil of China

•Hg geochemical baselines of catchment sediment/soil were established in whole China.•The low baselines are 13 and 11 μg/kg in top and deep soils respectively.•The median value is at 26 and 18 μg/kg in top and deep soils respectively.•The map of ratios of top to deep soils better visualises human introduction.•6.86% of top and 3.52% deep samples have Hg concentrations exceeding 150 μg/kg.

The China Geochemical Baselines (CGB) project provides nation-wide catchment sediment/alluvial soil geochemical baseline data for 76 elements including Hg from 3382 top (0–25 cm) and 3380 deep sediment/alluvial soil samples (under a depth of 100 cm) at 3382 sampling sites, corresponding to a sampling density of about 1 site per 3000 km2. Mercury was determined by cold vapour generation atomic fluorescence spectrometry (AFS) under strict quality control using field duplicates, standard reference materials and analytical replicate samples. The 25th percentile is at 13 and 11 μg/kg in top and deep sediment/alluvial soil samples, respectively; these concentrations are close to the crustal abundance in China. The median Hg value is at 26 and 18 μg/kg in top and deep sediment/alluvial soil samples, respectively; the 75th percentile is 56 μg/kg in top and 36 μg/kg in deep sediment/alluvial soil samples. Mercury concentrations at the 50th (26 μg Hg/kg) and 75th (56 μg Hg/kg) percentiles exhibit systematic enrichment in the near-surface material. The Hg distribution maps show distinct increasing trends from northern to southern and from western to eastern China, which are primarily related to the distribution of parent rocks, including rocks associated with mineral resources of Hg, Sb, Au, As, Pb–Zn, and climate, resulting in different soil types, as well as to population density with accompanying industrial development. Anomalous Hg concentrations in top sediment/alluvial soil samples above the 85th percentile (87 μg/kg) occur mainly in south and south-west and sparsely in north and east China; they most likely have a dual origin, natural and human-induced, with the latter having a greater contribution because of the intense industrial activities, and particularly mercury emissions from Hg, Sb, As, Au, Pb–Zn mining, coal combustion, and production of batteries, fluorescent lamps, thermometers and cement. The spatial distribution map of Hg ratios of top to deep sediment/alluvial soil samples displays higher values, suggesting that top samples are most likely polluted from human activities in the highly populated urban areas of eastern China. About 6.86% and 3.52% of top and deep sediment/alluvial soil samples, respectively, have Hg concentrations higher than the soil contamination limit of 150 μg/kg set by the National Environmental Standards for Heavy Metals of the People's Republic of China.