Distribution of total mercury and methylmercury around the small-scale gold mining area along the Cikaniki River, Bogor, Indonesia

This study investigates the distribution of total mercury (T-Hg) and methylmercury (MeHg) in the soil and water around the artisanal and small-scale gold mining (ASGM) area along the Cikaniki River, West Java, Indonesia. The concentration of T-Hg and MeHg in the forest soil ranged from 0.07 to 16.7 mg kg⁻¹ and from <0.07 to 2.0 μg kg⁻¹, respectively, whereas it ranged from 0.40 to 24.9 mg kg⁻¹ and from <0.07 to 56.3 μg kg⁻¹, respectively, in the paddy field soil. In the vertical variation of the T-Hg of forest soil, the highest values were observed at the soil surface, and these values were found to decrease with increasing depth. A similar variation was observed for MeHg and total organic carbon content (TOC), and a linear relationship was observed between them. Mercury deposited on the soil surface can be trapped and retained by organic matter and subjected to methylation. The slope of the line obtained for the T-Hg vs. TOC plot became larger near the ASGM villages, implying a higher rate of mercury deposition in these areas. In contrast, the plots of MeHg vs. TOC fell along the same trend line regardless of the distance from the ASGM village. Organic carbon content may be a predominant factor in controlling MeHg formation in forest soils. The T-Hg concentration in the river water ranged from 0.40 to 9.6 μg L⁻¹. River water used for irrigation can prove to be a source of mercury for the paddy fields. The concentrations of Hg⁰ and Hg²⁺ in river water showed similar variations as that observed for the T-Hg concentration. The highest Hg⁰ concentration of 3.2 μg L⁻¹ can be attributed to the waste inflow from work sites. The presence of Hg⁰ in river water can become a source of mercury present in the atmosphere along the river. MeHg concentration in the river water was found to be 0.004–0.14% of T-Hg concentration, which was considerably lower than the concentrations of other Hg species. However, MeHg comprised approximately 0.2% of the T-Hg in paddy field soil. Mercury deposited from the atmosphere and the river water can be subjected to methylation. Paddy fields are very important ecosystems; therefore, the effect of MeHg on these ecosystems and human beings should be further investigated.