Mercury in sediment reflecting the intensive coal mining activities: Evidence from stable mercury isotopes and Bayesian mixing model analysis.

Severe environmental issues are caused by long-term coal mining activities; however, the process of mercury (Hg) response in mining subsidence area sediments (MSAS) is still unclear, and direct evidence showing the relationship between Hg accumulation mechanism in sediments and mining activities is lacking. In this study, the characteristics of total mercury (THg) content in MSAS were investigated. Moreover, Hg isotopes were obtained to determine the main sources and environmental process of mercury in MSAS, and a MixSIAR mixing model was first used to estimate the potential Hg sources. The THg content ranged from 27.5 to 113.9 ng/g, with a mean of 65.8 ± 29.4 ng/g, exceeding the local soil background value (19.7 ng/g). The Hg in MSAS was affected by clay and organic matter. The Δ199Hg and Δ201Hg in the sediments varied from - 0.05-0.05‰ (mean: -0.01 ± 0.03‰) and - 0.07-0.01‰ (mean: -0.02 ± 0.03‰), respectively, with the fitting results suggesting that a photochemical reaction occurred in some of the Hg in the sediments prior to deposition. The results of the MixSIAR mixing model revealed that the Hg in MSAS was mainly derived from gangue, soil erosion, coal, fly ash, and feed, and their corresponding percentage contribution was 51.5 ± 9.6%, 23.8 ± 13.1%, 13.9 ± 7.9%, 8.1 ± 5.4%, and 3.1 ± 1.4%, respectively. Hg isotopes can be used to trace the transport and transformation of environmental pollutants, and this may provide an important reference for the assessment and prevention of Hg pollution in typical areas such as coal mining and coal-fired.