Mercury distributions and mercury isotope signatures in sediments of Dongjiang, the Pearl River Delta, China

The Pearl River Delta (PRD) is one of the most industrialized and urbanized regions in China. In order to assess the pollution status of mercury (Hg) in PRD river system, the distribution of total mercury (HgT) and methylmercury (Me-Hg) in sediment from a large river named Dongjiang (DJ) was for the first time investigated. HgT concentrations in sediment increased from the upstream to downstream of Dongjiang area and at most sites of DJ were significantly higher than the background values, which suggested that the DJ was contaminated with Hg, especially in the downstream of DJ. Me-Hg concentrations in sediment of DJ ranged from 0.56 to10.62 ng/g, and were significantly correlated with HgT and organic matter. In order to determine the potential Hg contamination sources, typical sediments from different parts of DJ were chosen for Hg isotope analysis. The results showed that the mass-dependent fraction (MDF) in the sediments varied significantly (δ202Hg: −2.35 to− 0.60‰), and the mass-independent fraction (MIF) in the sediments also varied considerably (Δ199Hg: −0.02 to − 0.27‰). The samples with the highest HgT located in the industrial area had the highest δ202Hg (− 1.14‰ to − 0.60‰) measured values and insignificant MIF (Δ199Hg: −0.04 to − 0.01‰). Meanwhile, the samples with the lowest HgT located in the background area had the lowest δ202Hg (− 2.16‰ to − 1.55‰) and Δ199Hg (− 0.20 to − 0.27‰) measured values. Unlike the above two cases are the samples located in the urban area, which have relative lower MDF (δ202Hg: −2.35‰ to − 1.96‰) and small but significant MIF (Δ199Hg: −0.10‰ to − 0.08‰) with relative higher HgT. We demonstrated that the dominant Hg sources in DJ sediments could be categorized as the regional background, urban and industrial sources. In our study, we demonstrated that Hg stable isotope method could serve as an effective tool for tracing mercury contamination sources in the environment.

Research highlights
► Hg distribution and Hg isotope ratios in sediment of Dongjiang was investigated.
► The river sediment was contaminated with Hg due to human activities.
► A triple source mass balance using both MDF and MIF signatures was establised.
► Different sources of mercury contaminations have distinct mercury isotope signatures.
► The contributions of mercury from different sources were calculated.