Estimation of the residual total mercury in marine sediments of Minamata Bay after a pollution prevention project Takashi Tomiyasu a,d, ⁎, Shinya Takenaka a , Yukiko Noguchi a , Hitoshi Kodamatani a , Akito Matsuyamab b , Kimihiko Oki c , Yuriko Kono d , Ryo Kanzaki a , Hirokatsu Akagi e a Graduate School of Science and Engineering, Kagoshima University, Korimoto, Kagoshima 890-0065, Japan b National Institute for Minamata Disease, Minamata, Kumamoto 867-0008, Japan c Kagoshima University Museum, Korimoto, Kagoshima 890-0065, Japan d Environment and Safety Center, Kagoshima University, 1-21-35, Korimoto, Kagoshima 890-0065, Japan e International Mercury Laboratory Inc., Minamata, Kumamoto 867-0034, Japan abstract article info Article history: Received 21 May 2013 Received in revised form 15 December 2013 Accepted 17 December 2013 Available online 30 December 2013 Keywords: Mercury pollution Minamata Bay Sediment Total mercury Residual mercury To estimate the current contamination levels in Minamata Bay, the vertical and horizontal distributions of mer- cury in the sediment were investigated. Sediment core samples were collected in 2002, 2006, 2008, and 2010 at 12 locations by gravity core sampling in Minamata Bay and Fukuro Bay, which is located in the southern part of Minamata Bay. The average total mercury concentrations during each year in the surface sediment were 2.47–3.34 and 3.50–4.66 mg kg -1 for Minamata Bay and Fukuro Bay, respectively; significant variation in the values was not observed during the study period. The total mercury concentration in Fukuro Bay in- creased with increasing depth and reached a maximum at 8–14 cm from the surface and decreased with increasing depth in the deeper layer. In Minamata Bay, the total mercury concentration did not change significantly from the surface to a depth of 10 cm and the values were considerably higher than the background level. In the lower layers of the long cores taken from both areas, the total mercury concentration decreased with depth, and the deepest layers exhibited relatively uniform low values. These values can be considered to represent the background concentration in the absence of anthropo- genic influence. The depth of the sediment affected by the effluent was estimated to be 20 ± 7 cm and 33 ± 4 cm for Minamata Bay and Fukuro Bay, respectively. Furthermore, it was estimated from the aver- age concentration and volume of the contaminated layer that approximately 1 ton of discharged mercury remained in the sediment of the two bays. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The environmental impact of discharged mercury has become a world-wide concern (Nriagu et al., 1992; Akagi et al., 1995; Ikingura and Akagi, 1996), mainly because in many developing coun- tries, the use of mercury in the mining of gold has increased in recent years. Industrial activities, such as the operation of mercury-cell chlor-alkali plants, have also been identified as major sources of Hg release to the environment. Heavy contamination of the sediment in the vicinity of the plant has been reported, and it was suggested that the contaminated sediment was an important mercury source to the water column (Hissler and Probst, 2006; Ullrich et al., 2007; Yan et al., 2008). In Minamata Bay, mercury-contaminated effluent was discharged from an acetaldehyde-producing factory over a 30-year period ending in 1965 (from 1958 to 1959, this effluent was discharged into the Minamata River). The total amount of mercury discharged from the chemical plant was reported to be 70–150 t or more, which contained significant levels of methylmercury generated as by-product in the acet- aldehyde process (Kumamoto Prefecture, 1998). Methylmercury is readily accumulated by aquatic biota. As a result, mercury that bio- accumulated in fish and shellfish caused Minamata disease in humans (Harada, 1995; Ninomiya et al., 1995; Akagi et al., 1998; Harada et al., 1998). The discharged mercury was deposited and resulted in serious contamination in the bay sediment. Concentrations as high as 2010 mg kg -1 have been observed in the sediment near the outflow of the plant. Although the concentration decreased with distance from that location, the total mercury concentration in the sediment of the center area of Minamata Bay was 20–60 mg kg -1 (Kitamura et al., 1960). Mercury also dispersed to the south Yatsushiro Sea, causing an increase in the mercury concentration levels in the sediment (Kumagai and Nishimura, 1978; Sakamoto et al., 1995; Kudo et al., Marine Chemistry 159 (2014) 19–24 ⁎ Corresponding author at: Graduate School of Science and Engineering, Kagoshima University, Korimoto, Kagoshima 890-0065, Japan. Tel.: +81 99 285 8107; fax: +81 99 259 4720. E-mail address: tomy@sci.kagoshima-u.ac.jp (T. Tomiyasu). 0304-4203/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.marchem.2013.12.002 Contents lists available at ScienceDirect Marine Chemistry journal homepage: www.elsevier.com/locate/marchem