Environmental Research 105 (2007) 53–66 Decadal mercury trends in San Francisco Estuary sediments Christopher H. Conaway a,Ã , John R.M. Ross b , Richard Looker c , Robert P. Mason d , A. Russell Flegal a a Department of Environmental Toxicology, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA b San Francisco Estuary Institute, 7770 Pardee Lane, 2nd Floor, Oakland, CA 94621-1424, USA c San Francisco Bay Regional Water Quality Control Board, TMDL Policy and Planning Division, 1515 Clay St. Suite 1400, Oakland, CA 94612, USA d Department of Marine Sciences, University of Connecticut, 1080 Shennecossett Road, Groton, CT 06340, USA Received 28 April 2006; received in revised form 17 October 2006; accepted 20 October 2006 Available online 11 December 2006 Abstract Monitoring sediment quality and total mercury concentrations over the period 1993–2001 at 26 stations in San Francisco Estuary has shown the seasonal cycling of mercury sediment concentrations, as well as a significant (Po0:05) decrease in those concentrations at eight stations across the estuary. This decrease in sediment mercury concentrations is attributed to the transport of relatively cleaner sediment to the estuary from the Sacramento River and San Joaquin River watersheds. Despite the decreases observed in some parts of the estuary, no corresponding trend has been found in concurrent studies on sport fish and bivalves in the estuary. r 2006 Elsevier Inc. All rights reserved. Keywords: San Francisco Bay; Mercury; Sediment 1. Historical context The history of mercury contamination and regulation in San Francisco Estuary spans more than 150 years (Table 1). In the 1970s, when mercury was recognized as an environmental health problem, it was already known that the rocks and sediments of the area were rich in mercury (D’Itri, 1972). However, it was not known at that time what the magnitude or relative contributions of natural versus anthropogenic sources were to mercury in the region. Nevertheless, mercury concentrations in some fish exceeded existing federal guidelines, and fish consumption advisories were issued by the State Department of Health for striped bass and catfish in the Sacramento-San Joaquin Delta and the San Francisco Bay area (NRC, 1978). As mercury use was reduced in the 1970s and into the 1990s due to legislation and concerns about its toxic nature, a gradual understanding of the sources of mercury to the estuary developed, including natural sources, urban runoff, atmospheric deposition, and mercury from historic gold and mercury mining (Phillips, 1987). Later studies demon- strated that the contribution of natural mineralization was relatively small, and attributed the bulk of the mercury contamination in the area to historic gold and mercury mining in the watershed (Hornberger et al., 1999; Marvin- DiPasquale et al., 2003; Conaway et al., 2004). Further, these studies showed that mercury concentrations in sediment were generally decreasing with time. Mercury in sediment is a source of mercury to fish through bacterially-mediated mercury methylation (Com- peau and Bartha, 1984; Gilmour et al., 1992) and biomagnification (Boudou and Ribeyre, 1997), and sedi- ments have proven to be an important source of mono- methylmercury (MMHg) in the San Francisco Bay-Delta (Choe et al., 2004), and in other estuary systems (Mason et al., 1999; Covelli et al., 2001; Sunderland et al., 2004; Mason et al., 2006). However, due to the many factors involved in the transformation of inorganic mercury to methylmercury (Benoit et al., 2003; Marvin-DiPasquale and Agee, 2003; Marvin-DiPasquale et al., 2003; Sunder- land et al., 2006) and involved in biomagnification (Boudou and Ribeyre, 1997), the relationship between mercury in sediment and mercury in fish is complex, and ARTICLE IN PRESS www.elsevier.com/locate/envres 0013-9351/$ - see front matter r 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2006.10.006 Ã Corresponding author. Fax: +1 831 459 2088. E-mail address: conaway@etox.ucsc.edu (C.H. Conaway).