A new gold ore deposit was found in the Talawaan region, North Sulawesi, Indonesia, in 1996, which was followed by establishment of a great number of small-scale gold ore smelters from 1998. In order to evaluate mercury behavior in the environment after release from the smelters, we determined the methylmercury (MeHg) as well as total mercury (THg) levels in sediments at more than 10 stations along the Talawaan River in 2010 and 2011. Furthemore, we conducted to catch some species of fishes along the stream to clarify the bio-accumulation of mercury.

Geometric means of sediment THg and MeHg concentrations in the river were 0.31 µg/g and 0.92 ng/g, respectively. The maximum THg and MeHg levels were both detected near site of smelters, as 3.25 µg/g and 9.20 ng/g. MeHg distributions were highly correlated with those of THg, which implies that MeHg was generated from the elemental mercury derived from the artisanal gold mining waste materials. In addition, MeHg concentrations in sediment samples were correlated with amounts of ignition loss.

Some carnivorous fish showed high value of Hg, and the accumulation of MeHg in fishes in this area in the food web was observed. Thus, we infer that metallic mercury diffusion from mining into the environment causes bio-accumulation of MeHg in the Talawaan watershed.