Characterization of Mercury Sources in a Coastal Environment Using Mercury Stable Isotopes

Abstract

Mercury (Hg) stable isotope has shown to be a significant indicator for source identification and distribution of contaminant source in the natural environment. Hg concentration (THg) and stable isotope analyses were conducted on the sediments collected from three sites: the Pohang old port (OP), the Pohang new port (NP), and the Yeongil bay (YB) in a coastal region of Pohang, South Korea, to characterize the major sources and its contribution to the coastal environment. THg concentration of the surface sediments in the OP, NP, and YB were 0.36±0.25 mg/kg, 0.18±0.09 mg/kg, and 0.18±0.21 mg/kg, respectively, with the OP having distinctively higher THg concentration compared to the other two sites. TOC and grain size, which are known to be affiliated with the concentration, displayed significant correlation only with the OP sediments. The average THg concentration of sediment core at three sites of OP (OPC.2, OPC.9 and OPC.14) were 0.52 mg/kg, 0.46 mg/kg, and 0.21 mg/kg, respectively. The THg concentration of sediment core at two sites of the YB (YBC.7 and YBC.14) were 0.19 mg/kg and 0.08 mg/kg, respectively. The general decreasing trend of THg concentration with depth indicates recent Hg input to the system. The isotope ratio of the surface sediments were compared with the isotopic compositions of potential Hg source end-members: Hg derived from atmospheric deposition (δ202Hg; -0.12±0.34‰, Δ199Hg; 0.12±0.25‰, n=36) and riverine inflow (δ202Hg; -0.18±0.25‰, Δ199Hg; -0.05±0.03‰, n=4) which represents Hg released from Gumu Creek. The result shows high contribution of Gumu Creek’s influence exists at the OP. Finally, THg concentration and Hg stable isotope analyses were performed at varying sediment fractions at three sites from the OP (OP.4, OP.13, and OP.20). The δ202Hg from the OP.4 exhibited minor variation, while other sited showed a significant increasing trend with particle sizes. The Δ199Hg showed an opposite pattern of decreasing Δ199Hg with increasing particle size. The high δ202Hg and low Δ199Hg in large sediment fraction match well with Hg transported from the Gumu Creek. The size fractional measurement of Hg isotopes appears to be a useful tool for evaluating transport pathways of varying Hg sources. This study implies the effectiveness of Hg stable isotopes for specifying the contamination source, by offering contribution fractions of Hg sources in the environment under observation.