Historical artisanal gold mining poses long-term risk of mercury to river ecosystems: A case study in Tongguan area of northwestern China.

Artisanal and small-scale gold mining (ASGM) practice is an important historical source of mercury (Hg) release worldwide, which could affect the local ecosystem even after many years of inactivity. In this study, sampling and analysis of water, sediment, periphyton and nearby farmland soil were conducted to assess the current levels of total mercury (THg) and methylmercury (MeHg), and the potential impact of legacy Hg on river system in Tongguan, a typical historical ASGM area of northwestern China. Results showed that 97 % of sediments and 40 % of soils are at extremely high-risk level of Hg, and approximately 32 % of river water samples exceed the Class III guidance limit (100 ng/L of Hg) of the environmental quality standards for surface water in China. Concentration of MeHg in sediment was even higher than that reported in 2006, suggesting that legacy Hg can still be methylated and further released. Positive correlations between MeHg in water and sediment/periphyton indicated that in-situ Hg methylation in these matrices was the main source of MeHg in river system, while periphyton plays a more important role than sediment in this river system with rocky riverbeds. High concentration and bioaccumulation factor of MeHg were observed in periphyton, implying a substantial burden on local aquatic biota, which could pose potential health risks to local residents due to the bioaccumulation and biomagnification of MeHg through the aquatic food web. Besides, the river system in Tongguan was approximately estimated to transport 2.06 kg of THg and 0.013 kg of MeHg annually into the Yellow River. This study demonstrates that ASGM can pose a long-term risk of Hg to river ecosystem even after the mining activities ceased for nearly three decades.